Bench PSU - buy or build?

J

James Harris

Jan 1, 1970
0
Hi. Am looking for a bench power supply that has positive and
negative supplies. Ideal would be a device that supplies +12,
+5, -5, -12 and a variable 0-30V DC, preferably with current
limiting.

I have been thinking about making such from a computer power
supply (to get the +/-12 and +/-5) and a torroidal with 2x23V
RMS to get circa 30V with full-wave rectification. I could then
use a semiconductor regulator (eg. L200x on the positive and
maybe something on the negative side also) to adjust the output.

Frankly the above is a bit daunting. I am OK with the safety but
unsure of the details of the electronics. It seems to be
expensive to get any kind of panel meter, let alone the two or
four required (V and A on both variable outputs). And a decent
case will be costly also.

Would I be better buying? I have seen this one,
http://www.cybermarket.co.uk/ishop/923/shopscr1565.html, which
is the Skytronic unit here,
http://www.skytronic.com/uk/prod/search.php?s=650.685. It has
one +5V and two variable 0-30V for UKP200. It has -, 0, and +
outputs so does that means it will supply up to (-15 to +15) on
each output? I've read the manual on the web site which is,
frankly, dismal. This is a little offputting but other devices
with similar specs are at least twice the price.....

Should I buy or build? Any guidance much appreciated.
- James

R

Robert Monsen

Jan 1, 1970
0
James Harris said:
Hi. Am looking for a bench power supply that has positive and
negative supplies. Ideal would be a device that supplies +12,
+5, -5, -12 and a variable 0-30V DC, preferably with current
limiting.

I have been thinking about making such from a computer power
supply (to get the +/-12 and +/-5) and a torroidal with 2x23V
RMS to get circa 30V with full-wave rectification. I could then
use a semiconductor regulator (eg. L200x on the positive and
maybe something on the negative side also) to adjust the output.

Frankly the above is a bit daunting. I am OK with the safety but
unsure of the details of the electronics. It seems to be
expensive to get any kind of panel meter, let alone the two or
four required (V and A on both variable outputs). And a decent
case will be costly also.

Would I be better buying? I have seen this one,
http://www.cybermarket.co.uk/ishop/923/shopscr1565.html, which
is the Skytronic unit here,
http://www.skytronic.com/uk/prod/search.php?s=650.685. It has
one +5V and two variable 0-30V for UKP200. It has -, 0, and +
outputs so does that means it will supply up to (-15 to +15) on
each output? I've read the manual on the web site which is,
frankly, dismal. This is a little offputting but other devices
with similar specs are at least twice the price.....

Should I buy or build? Any guidance much appreciated.
- James

Watch EBAY for power supplies. You will occasionally find an old bench
supply for a reasonable price.

Building linear power supplies is pretty easy, at least the fixed voltage
ones. Just get some transformers of the right size, and use the appropriate
78xx regulator (with bridge, caps, etc.)

The variable supply is also fairly easy using something like an LM317.
However, you waste lots of power (and thus generate lots of heat) when using
something like this at 3VDC and 2A.

EBAY occasionally has variacs on sale, which is a good solution to the
waste/heat issue; I've seen them for as low as $16 US (which I lost to a sniper, drat!) One thing is that unless you are a craftsman, your enclosure won't look as nice. Also, the displays, current limiting, control knobs, etc that you get with a typical bench supply are features you'll eventually want. Regards, Bob Monsen C Chris Oates Jan 1, 1970 0 I have been thinking about making such from a computer power supply Frankly the above is a bit daunting. I am OK with the safety but unsure of the details of the electronics. Years ago I built all my own equipment it looked like crap & barely functioned but it was so much fun. You won't learn anything buying 'off the shelf' D dg Jan 1, 1970 0 If you want to build it yourself, go ahead and do it, but be warned that you will probably be better off in the long run with buying a pre-built supply. I built a variable dual 3 amp supply (LM317 iirc), with an additional 5V supply built in, in a rack mount chassis. I put in a digital panel meter and a dial to read V or A on any of the 3 supplies. It is big, heavy, and was not cheap to built. The panel meter is non operational now because you need a separate supply to power it (I may buy a DC-DC converter to power the meter, that will isolate the supply voltage). I recently bought an RC car and all the associated gear. The battery charger I bought came with an AC/DC switching power supply much like the typical "wall wart" only this one is switching, has a fan, and puts out 12V at 7A! It is VERY lightweight, I almost can't believe it. The supply and smart charger were only ~$60. This
is not what you are looking for, but just an example of what power supply

--Dan

J

Jan 1, 1970
0
J

James Harris

Jan 1, 1970
0
Robert Monsen said:
Watch EBAY for power supplies. You will occasionally find an old bench
supply for a reasonable price.

Building linear power supplies is pretty easy, at least the fixed voltage
ones. Just get some transformers of the right size, and use the appropriate
78xx regulator (with bridge, caps, etc.)

The variable supply is also fairly easy using something like an LM317.
However, you waste lots of power (and thus generate lots of heat) when using
something like this at 3VDC and 2A.

EBAY occasionally has variacs on sale, which is a good solution to the
waste/heat issue; I've seen them for as low as $16 US (which I lost to a sniper, drat!) One thing is that unless you are a craftsman, your enclosure won't look as nice. Also, the displays, current limiting, control knobs, etc that you get with a typical bench supply are features you'll eventually want. Regards, Bob Monsen Variacs sound good to cut down on power and heat. I would buy from a retailer but the variacs I can see seem designed to work up to mains voltage. I guess they could just be used at the lower end of the range. I'll give it some thought. I can see myself ending up making more than one PSU. I wonder why variacs don't isolate from the mains. I can't think of a reason the secondaries need to be linked to the primary. Yes, am very very far from a craftsman. Cheers, James D Dimitrij Klingbeil Jan 1, 1970 0 James Harris said: Hi. Am looking for a bench power supply that has positive and negative supplies. Ideal would be a device that supplies +12, +5, -5, -12 and a variable 0-30V DC, preferably with current limiting. <snip> A computer psu might be a good choice, it has most protections already built-in. But it does not provide voltage regulation facilities. I'll describe an idea how to construct an (a little advanced) linear psu, if you have further questions about linear regulators, please ask. The choice whether to build or buy a psu is yours, but be warned that building one may take more time and be more expensive than one may want. However you won't learn anything (except hating the prices) by buying one while constructing your own psu is likely to be a lot more interesting. If you have some spare time left, my advice would be: Try it. If you have old components salvaged from some no-more-use equipment, constructing a psu might even be cheap (I built mine entirely from this sort of things, it's working reliably for 3 years now, params: 1 to 30V, 8A). The enclosure will be one of the hardest parts to find, so a poor man's solution may be a speaker box with transformer and heatsink on top unless you have the means and skills to create a more suitable one. In your post you only gave the voltages, no maximal current values, so I can hardly assume anything about the power throughput you need. So for the simple reason of not disappointing you with a 'weak' psu let's assume 7A approx current at 30V. There are 3 classical approaches to such a thing: 1: Linear, 2: Switching, 3: Triac-controlled transformer with linear fine-regulation electronics. The first solution is the easiest, but the most inefficient. The second one is the most complicated and exceptionally hard to build at home (do not try unless you are really experienced), but the most efficient. And finally the third one might be a viable medium-complicated solution with acceptable efficiency. The concept of this device is to roughly pre-control the voltage on the primary of a transformer by using a triac (rated at two times the maximal transformer power), then transform the voltage down, rectify it full-bridge, filter it, and finally use a linear regulator to fine-tune the voltage, limit the current (if necessary), implement a short circuit protection and this like. The disadvantage of this design is that there is a need to operate 3 (if you are skilled, maybe 2) transformers independently of each other while only one of them delivers useful power. Since I have only built similar, but not exactly the same sort of power supplies I cannot give you a very exact description, nevertheless I'll try to outline a likely possible way of handling this. Everyone who is more experienced is encouraged to correct me. First, the easiest-to-test structure of any device is a modular one, so I'll try to make use of it. The psu shall thus consist of 3 parts (apart from the xformers): 1: the triac circuit, 2: the linear regulation circuit and 3: the control circuit. First the triac circuit. Note that it is not isolated and operates at mains voltage. To control the primary of a transformer, a triac is placed in series with it. To prevent the triac from tripping accidentally, a resistor (around 500k) should be connected to the cathode and control pins of it. The voltage to 'fire' the triac can be taken off a capacitor that is charged from the mains through a resistor (resistor value depending on the capacitor used, the more capacitance, the less resistance, but not less than 30k, you'll need to do some testing). To make the thing work more reliable, 2 zeners in series, but with opposite directions and a voltage of about 6 V can be used in series with the triac gate. Controlling is done by shunting the cap with a transistor (bridge rectifier needed), this will delay the time until the triac triggers, if the transistor is open completely, the triac will shut down. The transistor base voltage / current is provided by an additional power source (very small trafo, maybe half a watt) and controlled via an optocoupler (Isolation required!). Schematics in ASCII: -------x------------------------------ from | R__ to trafo mains '--|__|-. C | from .--||---x--------. mains | | to trafo -------x-------|><|------------------- | / Triac | | R__ | | x-|__|-x-|<|-|>|-x | 2 Zeners| | .-------------' | | --------- |rectifier| .---. R__ --------- | | .------|__|---- - -| |+ | C \ B | R__ opto | '-----' |---x-|__|-. .---- + | E</NPN | C | | | | '-||---x '---- + | | | 6V DC '------------x------------x------- - When the LED part of the optocoupler is on, the transistor will be on, it will shunt the triac-controlling cap and the triac will stay off, the less current will pass through the opto LED, the more power will reach the main transformer. The next part would be a linear regulator. with its control circuitry. I'll leave it up to you to choose one appropriate for your needs and draw it as a simple square in the schematics. Note that if you use a custom regulation system (a couple of 2N3055 with appropriate control or this like), an additional transformer will be needed to power the control circuit. .-------. | regu | ----------|-----|-----x---x--| lator |--x---x----- from | rec | + C_|_ | | (lin) | | _|_ + trafo | tif | ___ | '-------' | ___ OUT secondary | ier | - | | | | | - ----------|-----|-----x----------x----------x----- | | | R__ |Zener 4V '-|__|-. '-|>|----- 1K | - | To opto LED | + '---------------- Depending on the voltage difference between the connections before and after the regulator the triac circuit will be auto-adjusted using the optocoupler. When the voltage rises, the LED will light stronger and the optocoupler will lower the triac throughput thus limiting the voltage. That's all. The whole thing can be repeated with 2 transformers if a positive and a negative voltage are needed simultaneously. It is also possible to use one power transformer and connect the 2 optocoupler sensors in series to control one triac. This may be less efficient though. Please note that this design is certainly a waste of parts and time if less than some 5A at 30V output are needed. For a less powerful psu just use linear regulation with no triacs. Don't forget a BIG heatsink in either case. More questions -> please ask here, you can also email me (address valid). Dimitrij J James Harris Jan 1, 1970 0 Chris Oates said: Years ago I built all my own equipment it looked like crap & barely functioned but it was so much fun. You won't learn anything buying 'off the shelf' Agreed. This is the one I saw about using an old computer PSU, http://academic1.bellevue.edu/robots/powerSupply/. What do you think about the use of just 2Ohms between +5 and ground to preload the PSU. This seems an incredibly small value to me, passing a large 2.5A just to get the PSU working. Another point. Would it be naiive to add zeners on all except the +5 to ensure the outputs don't exceed the rated voltages by too much? - James J Joel Kolstad Jan 1, 1970 0 James Harris said: I wonder why variacs don't isolate from the mains. I can't think of a reason the secondaries need to be linked to the primary. Just economics -- you don't need two windings if your VARIAC isn't isolated! You certainly can buy isolated VARIACs, though -- I own one, and I believe they're still readily available. Other than cost, I can't think of a good reason you wouldn't buy one! ---Joel Kolstad D Dimitrij Klingbeil Jan 1, 1970 0 James Harris said: Agreed. This is the one I saw about using an old computer PSU, http://academic1.bellevue.edu/robots/powerSupply/. What do you think about the use of just 2Ohms between +5 and ground to preload the PSU. This seems an incredibly small value to me, passing a large 2.5A just to get the PSU working. Try a higher resistance. A lot of PC PSUs will work with a small load or sometimes even no load at all. Don't waste 2.5A Another point. Would it be naiive to add zeners on all except the +5 to ensure the outputs don't exceed the rated voltages by too much? Usually the don't exceed the specs much anyway. If they do, use some other protection. Do not add zeners, a current of up to 10A (+ 12V line) is likely to kill them. Instead try to find out where the 5V loopback controls the PSU power and add there a 12V-controlled protection circuit. Dimitrij D Dimitrij Klingbeil Jan 1, 1970 0 James Harris said: Variacs sound good to cut down on power and heat. I would buy from a retailer but the variacs I can see seem designed to work up to mains voltage. I guess they could just be used at the lower end of the range. I'll give it some thought. I can see myself ending up making more than one PSU. I wonder why variacs don't isolate from the mains. I can't think of a reason the secondaries need to be linked to the primary. Use a variac that goes up to mains voltage w/o isolation. Connect a stepdown transformer to it. This will provide isolation and a better efficiency for low voltages and high currents. Note that it would be less efficient than a variac only for high voltages and low currents. As to why a lot of variacs don't isolate: They have no secondary, the (only existing) primary is used in auto-transformer mode. There are exceptions with (most likely isolated) secondaries though. See also my reply about triac control. You may want to substitute the triac with a variac and adjust it manually instead of by optocoupler. In this case a voltage meter in the place of the opto LED will show whether variac adjustments are necessary. It does not have to be exact, just use the cheapest one available. Dimitrij D Dimitrij Klingbeil Jan 1, 1970 0 Sorry, there was a mistake in my post. Not a real one, but it could end up with a less efficient device. Use a 2V Zener instead of a 4V one. | R__ |Zener 2V '-|__|-. '-|>|----- 1K | - | To opto LED | + '---------------- Part of original message: The next part would be a linear regulator. with its control circuitry. I'll leave it up to you to choose one appropriate for your needs and draw it as a simple square in the schematics. Note that if you use a custom regulation system (a couple of 2N3055 with appropriate control or this like), an additional transformer will be needed to power the control circuit. .-------. | regu | ----------|-----|-----x---x--| lator |--x---x----- from | rec | + C_|_ | | (lin) | | _|_ + trafo | tif | ___ | '-------' | ___ OUT secondary | ier | - | | | | | - ----------|-----|-----x----------x----------x----- | | | R__ |Zener 4V '-|__|-. '-|>|----- 1K | - | To opto LED | + '---------------- Depending on the voltage difference between the connections before and after the regulator the triac circuit will be auto-adjusted using the optocoupler. When the voltage rises, the LED will light stronger and the optocoupler will lower the triac throughput thus limiting the voltage. Dimitrij R Robert Monsen Jan 1, 1970 0 James Harris said: If you want to build it yourself, go ahead and do it, but be warned that you will probably be better off in the long run with buying a pre-built supply. I built a variable dual 3 amp supply (LM317 iirc), with an additional 5V supply built in, in a rack mount chassis. I put in a digital panel meter and a dial to read V or A on any of the 3 supplies. It is big, heavy, and was not cheap to built. The panel meter is non operational now because you need a separate supply to power it (I may buy a DC-DC converter to power the meter, that will isolate the supply voltage). I recently bought an RC car and all the associated gear. The battery charger I bought came with an AC/DC switching power supply much like the typical "wall wart" only this one is switching, has a fan, and puts out 12V at 7A! It is VERY lightweight, I almost can't believe it. The supply and smart charger were only ~$60. This
is not what you are looking for, but just an example of what power supply

--Dan

The thing about modern supplies is that they are usually switching supplies.
That is how they get away with such small transformers (they use higher
frequency transformers, which require much less metal.)

One problem with these, however, is that they are really noisy. If you are
doing work with an oscilloscope, you can end up with lots of noise in the
signal of the device under test from the supply. I was using a switch mode
PSU from an old cable modem for a bit, and it was great in terms of
generating lots of different output voltages (35V, 12V, 5V, -5V, -12V, all
at different amperages.) However, I couldn't get away from the 250k 10mV
signal it was adding into everything. It didn't even have to be connected,
it was radiating the signal so intensely that my probes would pick it up.

Switching <g> to a linear power supply did the trick. Much cleaner signals.

Regards,
Bob Monsen

J

James Harris

Jan 1, 1970
0
Dimitrij Klingbeil said:
A computer psu might be a good choice, it has most protections already
built-in. But it does not provide voltage regulation facilities. I'll
describe an idea how to construct an (a little advanced) linear psu, if you

The choice whether to build or buy a psu is yours, but be warned that
building one may take more time and be more expensive than one may want.
However you won't learn anything (except hating the prices) by buying one
while constructing your own psu is likely to be a lot more interesting. If
you have some spare time left, my advice would be: Try it. If you have old
components salvaged from some no-more-use equipment, constructing a psu
might even be cheap (I built mine entirely from this sort of things, it's
working reliably for 3 years now, params: 1 to 30V, 8A). The enclosure will
be one of the hardest parts to find, so a poor man's solution may be a
speaker box with transformer and heatsink on top unless you have the means
and skills to create a more suitable one. In your post you only gave the
voltages, no maximal current values, so I can hardly assume anything about
the power throughput you need. So for the simple reason of not disappointing
you with a 'weak' psu let's assume 7A approx current at 30V. There are 3
classical approaches to such a thing: 1: Linear, 2: Switching, 3:
Triac-controlled transformer with linear fine-regulation electronics. The
first solution is the easiest, but the most inefficient. The second one is
the most complicated and exceptionally hard to build at home (do not try
unless you are really experienced), but the most efficient. And finally the
third one might be a viable medium-complicated solution with acceptable
efficiency. The concept of this device is to roughly pre-control the voltage
on the primary of a transformer by using a triac (rated at two times the
maximal transformer power), then transform the voltage down, rectify it
full-bridge, filter it, and finally use a linear regulator to fine-tune the
voltage, limit the current (if necessary), implement a short circuit
protection and this like. The disadvantage of this design is that there is a
need to operate 3 (if you are skilled, maybe 2) transformers independently
of each other while only one of them delivers useful power. Since I have
only built similar, but not exactly the same sort of power supplies I cannot
give you a very exact description, nevertheless I'll try to outline a likely
possible way of handling this. Everyone who is more experienced is
encouraged to correct me. First, the easiest-to-test structure of any device
is a modular one, so I'll try to make use of it. The psu shall thus consist
of 3 parts (apart from the xformers): 1: the triac circuit, 2: the linear
regulation circuit and 3: the control circuit. First the triac circuit. Note
that it is not isolated and operates at mains voltage. To control the
primary of a transformer, a triac is placed in series with it. To prevent
the triac from tripping accidentally, a resistor (around 500k) should be
connected to the cathode and control pins of it. The voltage to 'fire' the
triac can be taken off a capacitor that is charged from the mains through a
resistor (resistor value depending on the capacitor used, the more
capacitance, the less resistance, but not less than 30k, you'll need to do
some testing). To make the thing work more reliable, 2 zeners in series, but
with opposite directions and a voltage of about 6 V can be used in series
with the triac gate. Controlling is done by shunting the cap with a
transistor (bridge rectifier needed), this will delay the time until the
triac triggers, if the transistor is open completely, the triac will shut
down. The transistor base voltage / current is provided by an additional
power source (very small trafo, maybe half a watt) and controlled via an
optocoupler (Isolation required!).
Schematics in ASCII:

-------x------------------------------
from | R__ to trafo
mains '--|__|-.
C |
from .--||---x--------.
mains | | to trafo
-------x-------|><|-------------------
| / Triac |
| R__ | |
x-|__|-x-|<|-|>|-x
| 2 Zeners|
| .-------------'
| |
---------
|rectifier| .---. R__
--------- | | .------|__|---- -
-| |+ | C \ B | R__ opto
| '-----' |---x-|__|-. .---- +
| E</NPN | C | |
| | '-||---x '---- +
| | | 6V DC
'------------x------------x------- -

When the LED part of the optocoupler is on, the transistor will be on, it
will shunt the triac-controlling cap and the triac will stay off, the less
current will pass through the opto LED, the more power will reach the main
transformer.

The next part would be a linear regulator. with its control circuitry. I'll
leave it up to you to choose one appropriate for your needs and draw it as a
simple square in the schematics. Note that if you use a custom regulation
system (a couple of 2N3055 with appropriate control or this like), an
additional transformer will be needed to power the control circuit.

.-------.
| regu |
----------|-----|-----x---x--| lator |--x---x-----
from | rec | + C_|_ | | (lin) | | _|_ +
trafo | tif | ___ | '-------' | ___ OUT
secondary | ier | - | | | | | -
----------|-----|-----x----------x----------x-----
| |
| R__ |Zener 4V
'-|__|-. '-|>|-----
1K | -
| To opto LED
| +
'----------------

Depending on the voltage difference between the connections before and after
the regulator the triac circuit will be auto-adjusted using the optocoupler.
When the voltage rises, the LED will light stronger and the optocoupler will
lower the triac throughput thus limiting the voltage.

That's all. The whole thing can be repeated with 2 transformers if a
positive and a negative voltage are needed simultaneously. It is also
possible to use one power transformer and connect the 2 optocoupler sensors
in series to control one triac. This may be less efficient though.

Please note that this design is certainly a waste of parts and time if less
than some 5A at 30V output are needed. For a less powerful psu just use
linear regulation with no triacs. Don't forget a BIG heatsink in either
case.

you can also email me (address valid).

Dimitrij

Dimitrij,

Thanks for all of the info. I get at least some of the
principles but I think this is a bit above me for now. I'll keep
it, though, for future reference - maybe a second project! I do
have a question, though, about displaying the voltage and
current. It would be essential to avoid dotting my meter between
circuit and PSU. Can you recommend a cost-effective but
reasonably accurate solution? If possible I would like to
display V and A at the same time.

Thanks,
James

D

Dimitrij Klingbeil

Jan 1, 1970
0
James Harris said:
<snip>
Thanks for all of the info. I get at least some of the
principles but I think this is a bit above me for now. I'll keep
it, though, for future reference - maybe a second project! I do
have a question, though, about displaying the voltage and
current. It would be essential to avoid dotting my meter between
circuit and PSU. Can you recommend a cost-effective but
reasonably accurate solution? If possible I would like to
display V and A at the same time.

Thanks,
James

Sorry, I don't have much experience with meters. I actually have only one
and using it for all sorts of testing (V, A, Ohm, hfe, F, H, Hz, diode test,
duty cycle, etc) I find myself disconnecting and reconnecting it in another
mode over and over . It was not an especially cheap one either.

W

Watson A.Name - Watt Sun, Dark Remover

Jan 1, 1970
0
"James said:
Hi. Am looking for a bench power supply that has positive and
negative supplies. Ideal would be a device that supplies +12,
+5, -5, -12 and a variable 0-30V DC, preferably with current
limiting.

I have been thinking about making such from a computer power
supply (to get the +/-12 and +/-5) and a torroidal with 2x23V
RMS to get circa 30V with full-wave rectification. I could then
use a semiconductor regulator (eg. L200x on the positive and
maybe something on the negative side also) to adjust the output.

Frankly the above is a bit daunting. I am OK with the safety but
unsure of the details of the electronics. It seems to be
expensive to get any kind of panel meter, let alone the two or
four required (V and A on both variable outputs). And a decent
case will be costly also.

Would I be better buying? I have seen this one,
http://www.cybermarket.co.uk/ishop/923/shopscr1565.html, which
is the Skytronic unit here,
http://www.skytronic.com/uk/prod/search.php?s=650.685. It has
one +5V and two variable 0-30V for UKP200. It has -, 0, and +
outputs so does that means it will supply up to (-15 to +15) on
each output? I've read the manual on the web site which is,
frankly, dismal. This is a little offputting but other devices
with similar specs are at least twice the price.....

Should I buy or build? Any guidance much appreciated.
- James

I think it's more economical to buy than to build. I bought four HP
66312A 0-20V, 2A power supplies for \$200 each, I could never come
close to getting the capabilities on anything I build. Here in the
U.S., inexpensive PSes are available, for an example see URL
http://www.mpja.com/productview.asp?product=14600+PS

--
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W

Watson A.Name - Watt Sun, Dark Remover

Jan 1, 1970
0
Years ago I built all my own equipment
it looked like crap & barely functioned
but it was so much fun.
'off the shelf'

There's not a whole lot to learn when you assemble a PS made out of a
Xfmr, a FW bridge, a filter cap, a LM317, two resistors and a meter.

--
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###Got a Question about ELECTRONICS? Check HERE First:###
http://users.pandora.be/educypedia/electronics/databank.htm
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W

Watson A.Name - Watt Sun, Dark Remover

Jan 1, 1970
0
A computer psu might be a good choice, it has most protections already
built-in. But it does not provide voltage regulation facilities. I'll
describe an idea how to construct an (a little advanced) linear psu, if you

The choice whether to build or buy a psu is yours, but be warned that
building one may take more time and be more expensive than one may want.
However you won't learn anything (except hating the prices) by buying one
while constructing your own psu is likely to be a lot more interesting. If
you have some spare time left, my advice would be: Try it. If you have old
components salvaged from some no-more-use equipment, constructing a psu
might even be cheap (I built mine entirely from this sort of things, it's
working reliably for 3 years now, params: 1 to 30V, 8A). The enclosure will
be one of the hardest parts to find, so a poor man's solution may be a
speaker box with transformer and heatsink on top unless you have the means
and skills to create a more suitable one. In your post you only gave the
voltages, no maximal current values, so I can hardly assume anything about
the power throughput you need. So for the simple reason of not disappointing
you with a 'weak' psu let's assume 7A approx current at 30V. There are 3
classical approaches to such a thing: 1: Linear, 2: Switching, 3:
Triac-controlled transformer with linear fine-regulation electronics. The
first solution is the easiest, but the most inefficient. The second one is
the most complicated and exceptionally hard to build at home (do not try
unless you are really experienced), but the most efficient. And finally the
third one might be a viable medium-complicated solution with acceptable
efficiency. The concept of this device is to roughly pre-control the voltage
on the primary of a transformer by using a triac (rated at two times the
maximal transformer power), then transform the voltage down, rectify it
full-bridge, filter it, and finally use a linear regulator to fine-tune the
voltage, limit the current (if necessary), implement a short circuit
protection and this like. The disadvantage of this design is that there is a
need to operate 3 (if you are skilled, maybe 2) transformers independently
of each other while only one of them delivers useful power.

Another serious disadvantage of a PS with a TRIAC or switch mode PS is
that they make a lot of EMI and RFI, so if you are using these kinds
of PS to power a low level amplifier, you will probably see some
interference in the circuit. Linear PSes don't have this kind of
problem. But Dimitrij has some good ideas if you don't plan on using
the PS for low level amplifiers.

Since I have
only built similar, but not exactly the same sort of power supplies I cannot
give you a very exact description, nevertheless I'll try to outline a likely
possible way of handling this. Everyone who is more experienced is
encouraged to correct me. First, the easiest-to-test structure of any device
is a modular one, so I'll try to make use of it. The psu shall thus consist
of 3 parts (apart from the xformers): 1: the triac circuit, 2: the linear
regulation circuit and 3: the control circuit. First the triac circuit. Note
that it is not isolated and operates at mains voltage. To control the
primary of a transformer, a triac is placed in series with it. To prevent
the triac from tripping accidentally, a resistor (around 500k) should be
connected to the cathode and control pins of it. The voltage to 'fire' the
triac can be taken off a capacitor that is charged from the mains through a
resistor (resistor value depending on the capacitor used, the more
capacitance, the less resistance, but not less than 30k, you'll need to do
some testing). To make the thing work more reliable, 2 zeners in series, but
with opposite directions and a voltage of about 6 V can be used in series
with the triac gate. Controlling is done by shunting the cap with a
transistor (bridge rectifier needed), this will delay the time until the
triac triggers, if the transistor is open completely, the triac will shut
down. The transistor base voltage / current is provided by an additional
power source (very small trafo, maybe half a watt) and controlled via an
optocoupler (Isolation required!).
Schematics in ASCII:

-------x------------------------------
from | R__ to trafo
mains '--|__|-.
C |
from .--||---x--------.
mains | | to trafo
-------x-------|><|-------------------
| / Triac |
| R__ | |
x-|__|-x-|<|-|>|-x
| 2 Zeners|
| .-------------'
| |
---------
|rectifier| .---. R__
--------- | | .------|__|---- -
-| |+ | C \ B | R__ opto
| '-----' |---x-|__|-. .---- +
| E</NPN | C | |
| | '-||---x '---- +
| | | 6V DC
'------------x------------x------- -

When the LED part of the optocoupler is on, the transistor will be on, it
will shunt the triac-controlling cap and the triac will stay off, the less
current will pass through the opto LED, the more power will reach the main
transformer.

The next part would be a linear regulator. with its control circuitry. I'll
leave it up to you to choose one appropriate for your needs and draw it as a
simple square in the schematics. Note that if you use a custom regulation
system (a couple of 2N3055 with appropriate control or this like), an
additional transformer will be needed to power the control circuit.

.-------.
| regu |
----------|-----|-----x---x--| lator |--x---x-----
from | rec | + C_|_ | | (lin) | | _|_ +
trafo | tif | ___ | '-------' | ___ OUT
secondary | ier | - | | | | | -
----------|-----|-----x----------x----------x-----
| |
| R__ |Zener 4V
'-|__|-. '-|>|-----
1K | -
| To opto LED
| +
'----------------

Depending on the voltage difference between the connections before and after
the regulator the triac circuit will be auto-adjusted using the optocoupler.
When the voltage rises, the LED will light stronger and the optocoupler will
lower the triac throughput thus limiting the voltage.

That's all. The whole thing can be repeated with 2 transformers if a
positive and a negative voltage are needed simultaneously. It is also
possible to use one power transformer and connect the 2 optocoupler sensors
in series to control one triac. This may be less efficient though.

Please note that this design is certainly a waste of parts and time if less
than some 5A at 30V output are needed. For a less powerful psu just use
linear regulation with no triacs. Don't forget a BIG heatsink in either
case.

you can also email me (address valid).

Dimitrij

--
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###Got a Question about ELECTRONICS? Check HERE First:###
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My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 <at> hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@

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