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DIY Bench power supply oscillating with inductive load

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diy didi

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Hi
I have been woking on a bench power supply for the past two months or so.
I decided on a discrete regulator design with adjustable voltage and and current limit.
Yesterday I boxed most of it, and decided to test using a couple of parallel connected 12V car bulbs. The bulbs were flickering slightly. Scoped the output and revealed a 16- 25HZ almost square wave/ round edges type oscillation or motor boating.
If i turn down the current limiting it gets even worse.
With my DC load all works as it should.
My electronics knowledge doesn't stretch as far as to fix this. The sense leads come together at the croc clips of the output leads. So there are four wires coming out of the box. The negative output terminal connects directly to neg of main filter caps.
Connecting a 4700uF capacitor across the leads at the load doesn't help much.
Any ideas? See schematic attached.
 

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  • Paul Power supp.pdf
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Here are two waveforms measured at output. The first is with some 12V car bulbs connected 12V @3,5A.
The second is with an 8,2ohm resistor also at 12V.
 

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First off, bulbs aren't inductive (not to any degree to affect your problem), can you add another 8.2 ohm to see if the higher current is the problem? - it's possibly shutting down due to over-current?.
 
Have you simulated your voltage/current regulator, especially the response to a step change in load current?
 
Hi
I have been woking on a bench power supply for the past two months or so.
I decided on a discrete regulator design with adjustable voltage and and current limit.
Yesterday I boxed most of it, and decided to test using a couple of parallel connected 12V car bulbs. The bulbs were flickering slightly. Scoped the output and revealed a 16- 25HZ almost square wave/ round edges type oscillation or motor boating.
If i turn down the current limiting it gets even worse.
With my DC load all works as it should.
My electronics knowledge doesn't stretch as far as to fix this. The sense leads come together at the croc clips of the output leads. So there are four wires coming out of the box. The negative output terminal connects directly to neg of main filter caps.
Connecting a 4700uF capacitor across the leads at the load doesn't help much.
Any ideas? See schematic attached.

Hi diy,

It would be difficult to diagnose the problem with your power supply remotely, so all I can do is make a few observations.
(1) It looks like a complicated PSU circuit. Where does it come from. Is it a standard design?
( 2) An incandescent bulb typically only has 10% of its operating resistance when cold and it changes rapidly with current- this may be upsetting your PSU
(3) You have a fairly low frequency medium power transistor feeding the output transistor. That may create a pole at the same frequency as the output transistor and cause problems.
(4) There is no resistor from Q6 emitter to Q7 emitter. This means that Q7 will have a problem with negative excursions and will be very slow, once again introducing an awkward pole in the frequency response.
(5) Low frequency oscillation is more likely to result from layout problems: earth loops etc

These are just some thoughts and may have nothing to do with your PSU problem. Just a word of caution, PSUs are notoriously difficult to stabilize in the frequency domain because, quite simply, you never know what will be connected to the output.

spec
 
I believe you are all correct in saying that it's not an inductive element that is upsetting my power supply but rather the quick changing resistance of the light bulbs.
As a matter of interest, I have been trying a couple of things, I noticed that if I place a 1uF capacitor between D8 Cathode and D15 Anode, this motor boating completely disappears.
Is this an acceptable solution?
SPEC: there is actually a 10K resistor between Q6 emitter and D17 anode. It wasn't drawn in the original schematic.
 
diy,

10K is far to high, I would suggest 470 Ohms

spec
 
As a matter of interest, I have been trying a couple of things, I noticed that if I place a 1uF capacitor between D8 Cathode and D15 Anode, this motor boating completely disappears.
Is this an acceptable solution?
It looks like that effectively parallels C7 so it should do no harm.

Sometimes ground loops can cause the kind of problem you see, but adding a cap is OK.

This looks like a really old design. Using zeners for references will drift voltage. If it was me, I would do something better and use a precision reference.
 
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Hi
I have been woking on a bench power supply for the past two months or so.
I decided on a discrete regulator design with adjustable voltage and and current limit.
Yesterday I boxed most of it, and decided to test using a couple of parallel connected 12V car bulbs. The bulbs were flickering slightly. Scoped the output and revealed a 16- 25HZ almost square wave/ round edges type oscillation or motor boating.
If i turn down the current limiting it gets even worse.
With my DC load all works as it should.
My electronics knowledge doesn't stretch as far as to fix this. The sense leads come together at the croc clips of the output leads. So there are four wires coming out of the box. The negative output terminal connects directly to neg of main filter caps.
Connecting a 4700uF capacitor across the leads at the load doesn't help much.
Any ideas? See schematic attached.


After reviewing the schematic I would issue this WARNING:

This is not a finished design, in fact it won't work as shown. To get the kind of output volatge and current listed, two of the most important factors are for the power transistor(s):

1) SOA (safe operating area) defined by voltage across the device and current through it. In nearly every case, linear supplies with more than 30V input and 3A output will neeed MULTIPLE power transistors in parallel.

2) Power Dissipation. Again, power diss must be distributed between the power devices and handled using a proper heatsink. When Vin is greater than 30V and IL is 3A or more, you are above 100W and have to design for that.

These two aspects are probably the most critical.

If you post exactly what Vout range you need and what current, I might be able to advise.

I would NOT use the design you showed. It will blow up if loaded to higher current levels.

TO ADD:

Showing the SOA curve of the TIP35C used in the schematic, with 40V across it, it can only handle about 1.5A DC current.
 

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You can use the linear regulator design I use in my bench supply (see attached schematic).
 

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After reviewing the schematic I would issue this WARNING:

This is not a finished design, in fact it won't work as shown. To get the kind of output volatge and current listed, two of the most important factors are for the power transistor(s):

1) SOA (safe operating area) defined by voltage across the device and current through it. In nearly every case, linear supplies with more than 30V input and 3A output will neeed MULTIPLE power transistors in parallel.

2) Power Dissipation. Again, power diss must be distributed between the power devices and handled using a proper heatsink. When Vin is greater than 30V and IL is 3A or more, you are above 100W and have to design for that.

These two aspects are probably the most critical.

If you post exactly what Vout range you need and what current, I might be able to advise.

I would NOT use the design you showed. It will blow up if loaded to higher current levels.

TO ADD:

Showing the SOA curve of the TIP35C used in the schematic, with 40V across it, it can only handle about 1.5A DC current.
Hi. Thank you. I'm using 4 in parallel. Also with the tap switching of the transformer I manage to minimize dissapation.
 
Hi Spec. Why too high. What does this resistor do in the circuit?

As I mentioned before, a base emitter resistor speeds up the base drive when the transistor is required to decrease collector current. Don't forget that the base of transistors have actual and virtual capacitances which need to be charged/discharged. For example, if you take the extreme of no base/emitter resistor, the transistor current will increase fast but it will decrease slowly.

spec
 
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Hi diy,

You have no chance of anyone being able to help with your problem if you don't post a full and accurate schematic of your actual PSU.

spec
 
Hi diy,

You have no chance of anyone being able to help with your problem if you don't post a full and accurate schematic of your actual PSU.

spec
Do please notice that there is a 10k resistor missing from my circuit. It is connected between Q6 emitter and D17 Anode.
I have attached the original schematic from the 70's. Apparently it was a very good supply. "Paul" went and simplified the circuit a bit for his needs, but most of the values are identical. The circuit works great except for the problem I mentioned in my first post!
Have managed to get rid of the oscillation by adding a cap between D8 cathode and D15 Anode. I'm not sure how big this cap should be?
So I guess my next step would be to try and better this circuit slightly as I have already made a pcb for it.
Further suggestions welcome.
Definitely gonna try reducing C2 as mentioned by you guys!
 

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I have attached the original schematic from the 70's. Apparently it was a very good supply.
I was going to guess the design had to have been from an era before integrated circuits existed.

I still was wondering what the design objective of the project is: what output voltage and current is it supposed to provide?
 
Hi
I have it upped the spec a bit.
I want 50V @4Amps. Therefore i'm running 4 parallel TIP35C's on a huge heatsink and fan.
 
OK. Just confirmed my cap idea actually doesn’t work!!!:arghh:
Tried the 100uf across sensor wires but no effect.
It’s just strange that if I short the + sense wire to the main + out on the pcb instead of at the croc clip, everything works great, This confirms that the current board is actually working.
Tried making C2 smaller, this too does nothing.
Something is up here....
 
It might be worth a try to temporarily remove the 220 uF cap across the output of the Power Supply to see the effect on the problem.
 
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