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# Basic power supply design question

S

#### Steve

Jan 1, 1970
0
Let me start off by clarifying that I'm not a design engineer, so I'm
just seeking some help, and apologize in advance if I'm unclear.

What I want to build is a regulated 0-30v 0-3a DC supply, adjustable
current and voltage. I want to build it for personal gratification, I
know I could just purchase a HY3003, but what's the fun in that?

Looking at the schematic on Bowden's Hobby Circuits,
http://ourworld.compuserve.com/homepages/Bill_Bowden/page4.htm#ps6.gif
it looks like this supply has no line regulation. Am I wrong? I
think it has load regulation, as the input for the voltage portion is
fed directly off the output, but there is no input reference. Please
correct me where I am wrong, I'm sure I am.

Looking at the LM317 datasheet,
http://www.national.com/ds/LM/LM117.pdf
page 17, they show a regulated adjustable voltage and current supply.
But this will not go to 0 V, only to 1.2V. On page 16, they show the
negative supply to allow the output to adjust down to 0v, but how do
you incorporate this into the adjustable current schematic shown?
Just bring the voltage adjust pot down to -1.2V? How will this affect
the rest of the circuit, specifically the current limiter? Am I
making this too complicated?

Thanks for your help, all thoghts appreciated.

Steve

D

#### D from BC

Jan 1, 1970
0
Let me start off by clarifying that I'm not a design engineer, so I'm
just seeking some help, and apologize in advance if I'm unclear.

What I want to build is a regulated 0-30v 0-3a DC supply, adjustable
current and voltage. I want to build it for personal gratification, I
know I could just purchase a HY3003, but what's the fun in that?

Looking at the schematic on Bowden's Hobby Circuits,
http://ourworld.compuserve.com/homepages/Bill_Bowden/page4.htm#ps6.gif
it looks like this supply has no line regulation. Am I wrong? I
think it has load regulation, as the input for the voltage portion is
fed directly off the output, but there is no input reference. Please
correct me where I am wrong, I'm sure I am.

Looking at the LM317 datasheet,
http://www.national.com/ds/LM/LM117.pdf
page 17, they show a regulated adjustable voltage and current supply.
But this will not go to 0 V, only to 1.2V. On page 16, they show the
negative supply to allow the output to adjust down to 0v, but how do
you incorporate this into the adjustable current schematic shown?
Just bring the voltage adjust pot down to -1.2V? How will this affect
the rest of the circuit, specifically the current limiter? Am I
making this too complicated?

Thanks for your help, all thoghts appreciated.

Steve

I always wanted to try turning a 68W amp module into a variable power
supply. Anybody done that?
http://www.national.com/ds/LM/LM3886.pdf
68Watts mono AB amp about $8.00CAD Benefits Neg and Pos output Output down to 0V Low part count Various internal protection such as overload, shorts and thermal runaway. Low noise +/-35V output Output current limited to 7A Sound good? D from BC E #### Eeyore Jan 1, 1970 0 Steve said: Let me start off by clarifying that I'm not a design engineer, so I'm just seeking some help, and apologize in advance if I'm unclear. What I want to build is a regulated 0-30v 0-3a DC supply, adjustable current and voltage. I want to build it for personal gratification, I know I could just purchase a HY3003, but what's the fun in that? Most ppl who make their living from electronics would probably *BUY* a lab supply rather than make their own ! Decision ONE ! Is it going to be linear regulated or not ? And don't forget the centre tap. You'll kick yourself otherwise. I'd want 0-36V CT actually. Graham E #### Eeyore Jan 1, 1970 0 D said: I always wanted to try turning a 68W amp module into a variable power supply. Anybody done that? http://www.national.com/ds/LM/LM3886.pdf 68Watts mono AB amp about$8.00CAD
Benefits
Neg and Pos output
Output down to 0V
Low part count
Various internal protection such as overload, shorts and thermal
runaway.
Low noise
+/-35V output
Output current limited to 7A

Sound good?

It could make a lot of sense actually.

Graham

S

#### Steve

Jan 1, 1970
0
Most ppl who make their living from electronics would probably *BUY* a lab
supply rather than make their own !

Decision ONE !

Is it going to be linear regulated or not ?

And don't forget the centre tap. You'll kick yourself otherwise. I'd want 0-36V
CT actually.

Graham

Sorry I didn't specify. It will be linear. Part of the reason I'm
building it is I have the components. I have a spare 241-7-24, so I
won't get to 30V w/ full load, or get continuous 3a out, but that's
ok. I don't mind overdesigning the regulator and putting in a beffier
xfmr later. I already have enclosure w/ panel meters (30V & 3A),
heatsink, pass transistor (2n3055), caps, rectifier, line filter, etc.

And, as a side note, I already have a HY1802, which is ok, and a Power
Mate Corporation BPA-10E, which I LOVE. I just have the parts around,
and there's something about building power supplies that is just fun.

Also, as another side note on the audio amp subject, one of the
ongoing projects I have is building a 400Hz power supply for powering
avionics. I have a spare P3A pcb from Elliot Sound Products that I'm
going to use as the driver. The transistors I'm using can each
dissipate 200W, and the output I need is max 29VAC RMS, so it'll be
good for a few amps. The only part I still haven't figured out is
some good short circuit protection. If I'm using it as a supply,
sooner or later the leads are going to touch, and I don't know if just
a fast blow fuse will save the transistors. I plan on just using some
molex plugs for sockets and making them easily replaceable. Not the
best solution, but hey..

Steve

S

#### Steve

Jan 1, 1970
0
Also, as another side note on the audio amp subject, one of the
ongoing projects I have is building a 400Hz power supply for powering
avionics. I have a spare P3A pcb from Elliot Sound Products that I'm
going to use as the driver. The transistors I'm using can each
dissipate 200W, and the output I need is max 29VAC RMS, so it'll be
good for a few amps. The only part I still haven't figured out is
some good short circuit protection. If I'm using it as a supply,
sooner or later the leads are going to touch, and I don't know if just
a fast blow fuse will save the transistors. I plan on just using some
molex plugs for sockets and making them easily replaceable. Not the
best solution, but hey..

Steve

Right after I posted, I started thinking about internal short circuit
protection. Why would I build a complicated circuit when they already
have it incorporated in a single chip? Great idea D from BC! I was
on the same track as you, just going about it in a much more
complicated manner. P3A is really a great amp, but overkill for a
simple AC supply where less than .5% distortion is probably fine. Not
to mention, all discrete components. Takes care of the driver and
short circuit secion of the supply. There is a great clipping
indicator circuit on ESP that I plan on using.

I found an old app note about creating amplitude stabilized sine waves
using clipped squares. Simple circuit, single pot frequency
adjustment, less than .2% distortion, no amplitude stabilization time
like with a lamp, really neat.

Now the DC supply I'm working on doesn't seem nearly as fun anymore, I
may push the AC one to the front.

Steve

T

#### Traver

Jan 1, 1970
0
Let me start off by clarifying that I'm not a design engineer, so I'm
just seeking some help, and apologize in advance if I'm unclear.

What I want to build is a regulated 0-30v 0-3a DC supply, adjustable
current and voltage. I want to build it for personal gratification, I
know I could just purchase a HY3003, but what's the fun in that?

Looking at the schematic on Bowden's Hobby Circuits,http://ourworld.compuserve.com/homepages/Bill_Bowden/page4.htm#ps6.gif
it looks like this supply has no line regulation. Am I wrong? I
think it has load regulation, as the input for the voltage portion is
fed directly off the output, but there is no input reference. Please
correct me where I am wrong, I'm sure I am.

It looks like a regulated output with the 2N3055 as a pass element
controlled
by an op amp fed back from the ouput of the supply - a typical linear
regulator.
Looking at the LM317 datasheet,http://www.national.com/ds/LM/LM117.pdf
page 17, they show a regulated adjustable voltage and current supply.
But this will not go to 0 V, only to 1.2V. On page 16, they show the
negative supply to allow the output to adjust down to 0v, but how do
you incorporate this into the adjustable current schematic shown?
Just bring the voltage adjust pot down to -1.2V? How will this affect
the rest of the circuit, specifically the current limiter? Am I
making this too complicated?

Power supplies need a reference voltage to compare the output to and
tell if it is
in regulation or not. The LM317 reference is a set 1.25 volts
developed between the output
and the ADJ pin. Once you drop below the voltage reference it can't
track it to
regulate the ouput and the supply goes "open loop".

All you really need is is an OP amp, a bipolar to level shift the amp
so it can control
the pass element sitting up at 30V, and the pass element itself. This
setup gives you
control over the voltage reference itself and you can bring it right
down to 0V.

The supply you mentioned above works this way with the added feature
of a current
control amplifier. It looks like everything you need.

Traver

D

#### D from BC

Jan 1, 1970
0
Sorry I didn't specify. It will be linear. Part of the reason I'm
building it is I have the components. I have a spare 241-7-24, so I
won't get to 30V w/ full load, or get continuous 3a out, but that's
ok. I don't mind overdesigning the regulator and putting in a beffier
xfmr later. I already have enclosure w/ panel meters (30V & 3A),
heatsink, pass transistor (2n3055), caps, rectifier, line filter, etc.

And, as a side note, I already have a HY1802, which is ok, and a Power
Mate Corporation BPA-10E, which I LOVE. I just have the parts around,
and there's something about building power supplies that is just fun.

Also, as another side note on the audio amp subject, one of the
ongoing projects I have is building a 400Hz power supply for powering
avionics. I have a spare P3A pcb from Elliot Sound Products that I'm
going to use as the driver. The transistors I'm using can each
dissipate 200W, and the output I need is max 29VAC RMS, so it'll be
good for a few amps. The only part I still haven't figured out is
some good short circuit protection. If I'm using it as a supply,
sooner or later the leads are going to touch, and I don't know if just
a fast blow fuse will save the transistors. I plan on just using some
molex plugs for sockets and making them easily replaceable. Not the
best solution, but hey..

Steve

Current feedback is often done with a sensing resistor + some
amplifying components that steer the power electronics to regulate the
current.

D from BC

P

#### Phil Allison

Jan 1, 1970
0
"D from BC"
I always wanted to try turning a 68W amp module into a variable power
supply. Anybody done that?

** No.
http://www.national.com/ds/LM/LM3886.pdf
Benefits
Neg and Pos output

** Total disaster to have the polarity of a DC supply change in use -
destroy nearly any cct.

Two devices could be made to produce dual tracking, one following and one
inverting a reference DC input.

Output down to 0V
Low part count

** Agreed.

Various internal protection such as overload, shorts and thermal
runaway.
Low noise
+/-35V output
Output current limited to 7A

** Not true.

Read the specs properly - that figure is the *minimum* short cct current
for * 10 mS * with a +/- 20 volt supply.

The current into a load can be dangerously higher - until the chip
overheats.

Come up with adjustable output current limiting and you may have a usable
idea.

Stability with capacitive and awkward loads like DC motors cannot be
certain.

........ Phil

D

#### D from BC

Jan 1, 1970
0
"D from BC"

** No.

** Total disaster to have the polarity of a DC supply change in use -
destroy nearly any cct.

Two devices could be made to produce dual tracking, one following and one
inverting a reference DC input.

** Agreed.

** Not true.

Read the specs properly - that figure is the *minimum* short cct current
for * 10 mS * with a +/- 20 volt supply.

The current into a load can be dangerously higher - until the chip
overheats.

Come up with adjustable output current limiting and you may have a usable
idea.

Stability with capacitive and awkward loads like DC motors cannot be
certain.

....... Phil

Oops..Yeah..Those Ispecs can with some fine print..

I'll be easy for me to add a current limiting circuit to the amp
module..
Also..
I have some notes around here concerning feedback networks and
reactive loads..It might help with stability.
I've been tempted to do this project.. Mostly because it's a
"crazyass" use of an audio amp module..
But I'm more into 100khz switchmode.
D from BC

P

#### Phil Allison

Jan 1, 1970
0
"D from BC"
Oops..Yeah..Those Ispecs can with some fine print..

** What the large print giveth - the fine print taketh away.

( Old Biblical quote ...... )

I'll be easy for me to add a current limiting circuit to the amp
module..

** Do tell.

Also..
I have some notes around here concerning feedback networks and
reactive loads..It might help with stability.

** Nothing like a big electro across the output for PSU stability.

I've been tempted to do this project.. Mostly because it's a
"crazyass" use of an audio amp module..

** LOL.

I have long dreamt of using a "spare" Crown DC300A as a classy dual
tracking PSU !!

But I'm more into 100khz switchmode.

** Whatever turns you on - buddy .....

....... Phil

D

#### D from BC

Jan 1, 1970
0
"D from BC"

** What the large print giveth - the fine print taketh away.

( Old Biblical quote ...... )

** Do tell.

** Nothing like a big electro across the output for PSU stability.

** LOL.

I have long dreamt of using a "spare" Crown DC300A as a classy dual
tracking PSU !!

** Whatever turns you on - buddy .....

...... Phil

Ok...I just have many one difficulty when it comes to current
sensing..
Let's say I use a 100mohm sense resistor to monitor the PS current.
I'm not sure if it matters if I do high side current measurement or I
do low side current measurement..??
I have design experience with both but I haven't realized the
difference in benefits from a performance point of view..
D from BC

R

#### Rich Grise

Jan 1, 1970
0
Let me start off by clarifying that I'm not a design engineer, so I'm
just seeking some help, and apologize in advance if I'm unclear.

What I want to build is a regulated 0-30v 0-3a DC supply, adjustable
current and voltage. I want to build it for personal gratification, I
know I could just purchase a HY3003, but what's the fun in that?

Looking at the schematic on Bowden's Hobby Circuits,
http://ourworld.compuserve.com/homepages/Bill_Bowden/page4.htm#ps6.gif
it looks like this supply has no line regulation. Am I wrong? I
think it has load regulation, as the input for the voltage portion is
fed directly off the output, but there is no input reference. Please
correct me where I am wrong, I'm sure I am.

"Line regulation" isn't a component, or separately adjustable input or
anything - it's more like a phenomenon - it's just a measure of the
regulation relative to the input, where "load regulation" is a measure of
the regulation relative to the output. i.e., they're not independently
controllable, unless by circuit architecture or some such.

I have no opinion on the LM317, having never used one - but I think
that they return the adj input to -1.2 relative to the output "ground",
so the circuit itself doesn't really know the difference - it thinks
it's putting out 1.2V, but the user sees 0V.

Cheers!
Rich

J

#### John Popelish

Jan 1, 1970
0
D said:
Ok...I just have many one difficulty when it comes to current
sensing..
Let's say I use a 100mohm sense resistor to monitor the PS current.
I'm not sure if it matters if I do high side current measurement or I
do low side current measurement..??
I have design experience with both but I haven't realized the
difference in benefits from a performance point of view..

The difference is involved with not only what the current
limit is measuring, but with what it is trying to ignore
(the output voltage, in this case). If the pass device
control circuit is referenced to the negative rail, but the
current sensing is taking place on the positive rail, then
the sensing circuit has a lot more to do in ignoring the
large possible voltage swing than it has to do measuring the
small current sense voltage drop, accurately. It is a
signal to noise problem, and a level shifting problem.

By the way, it may not be obvious, but it is not necessary
for the pass element and / or its control circuit to be
referenced to the negative rail. Many lab supplies are
designed to operate completely floating, so the negative
rail has no particular ground or common concept associated
with it.

R

#### Rich Grise

Jan 1, 1970
0
.
Ok...I just have many one difficulty when it comes to current
sensing..
Let's say I use a 100mohm sense resistor to monitor the PS current.
I'm not sure if it matters if I do high side current measurement or I
do low side current measurement..??
I have design experience with both but I haven't realized the
difference in benefits from a performance point of view..

To me, the only time high side sensing makes sense (pun unintended,
but noted) is if the load absolutely, positively has to have its
negative side grounded to system ground.

The other arguments are things like level shifting, CMRR, and all
that stuff - well, you know the drill.

Hope This Helps!
Rich

D

#### D from BC

Jan 1, 1970
0
To me, the only time high side sensing makes sense (pun unintended,
but noted) is if the load absolutely, positively has to have its
negative side grounded to system ground.

The other arguments are things like level shifting, CMRR, and all
that stuff - well, you know the drill.

Hope This Helps!
Rich

Yup... Low side sensing can be as simple as a transistor BE across a
sense resistor..
Whereas..high side sensing ..ugh
My ugliest high side solution was a battery powered op amp that drove
an optocoupler.

I'm wondering if low side current sensing is typically done in high
end lab supplies..
D from BC

J

#### [email protected]

Jan 1, 1970
0
I'm wondering if low side current sensing is typically done in high
end lab supplies..

The sensing (on lab supplies) is generally done on whichever side of
the supply is the signal common for the error amplifier. Integrated
designs more often use the negative rail, but discrete designs use
either, without prejudice. This makes some of their schematics look
strange for people who are used to always seeing the negative rail as
common.

John Popelish

S

#### Spehro Pefhany

Jan 1, 1970
0
"Line regulation" isn't a component, or separately adjustable input or
anything - it's more like a phenomenon - it's just a measure of the
regulation relative to the input, where "load regulation" is a measure of
the regulation relative to the output. i.e., they're not independently
controllable, unless by circuit architecture or some such.

Specifically, the reference is the 4.7V Zener diode. Pin 2 is driven
to 0V, so the output voltage is Vo= 0V ~ 25V. The op-amps are supplied
from unregulated voltage, and the line regulation will be pretty
crappy, maybe 3%, because the current through the zener is not very
constant and 4.7V zeners at a few mA are not great regulators. DC load
regulation should be fine because there is plenty of gain.

This is a pretty cruddy hobby-level 'bench supply'. I'd expect it
would overshoot a fair bit too if you suddenly remove part of the
I have no opinion on the LM317, having never used one - but I think
that they return the adj input to -1.2 relative to the output "ground",
so the circuit itself doesn't really know the difference - it thinks
it's putting out 1.2V, but the user sees 0V.

He could tie the lower end of R8 and C6 to the -1.2V reference (rather
than ground) with 680R to -10V. The current limit should still work
with -10V on the LM301A pin4, but I have not analyzed it in detail.

Best regards,
Spehro Pefhany

S

#### Steve

Jan 1, 1970
0
Specifically, the reference is the 4.7V Zener diode. Pin 2 is driven
to 0V, so the output voltage is Vo= 0V ~ 25V. The op-amps are supplied
from unregulated voltage, and the line regulation will be pretty
crappy, maybe 3%, because the current through the zener is not very
constant and 4.7V zeners at a few mA are not great regulators. DC load
regulation should be fine because there is plenty of gain.

This is a pretty cruddy hobby-level 'bench supply'. I'd expect it
would overshoot a fair bit too if you suddenly remove part of the

He could tie the lower end of R8 and C6 to the -1.2V reference (rather
than ground) with 680R to -10V. The current limit should still work
with -10V on the LM301A pin4, but I have not analyzed it in detail.

Best regards,
Spehro Pefhany

Thanks for the replies. I had already started building the first
circuit, so I may finish and see what sort of regulation I get.
I appreciate the input,
Steve

S

#### Steve

Jan 1, 1970
0
Let me start off by clarifying that I'm not a design engineer, so I'm
just seeking some help, and apologize in advance if I'm unclear.

What I want to build is a regulated 0-30v 0-3a DC supply, adjustable
current and voltage. I want to build it for personal gratification, I
know I could just purchase a HY3003, but what's the fun in that?

Looking at the schematic on Bowden's Hobby Circuits,
http://ourworld.compuserve.com/homepages/Bill_Bowden/page4.htm#ps6.gif
it looks like this supply has no line regulation. Am I wrong? I
think it has load regulation, as the input for the voltage portion is
fed directly off the output, but there is no input reference. Please
correct me where I am wrong, I'm sure I am.

Just curious, how hard would it be to add an LED for CV/CC indication
to the above circuit? Any ideas?

Thanks,
Steve

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