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