Help with power supply noise., Please.

for too much noise on the

CC from 0 to 4A. I have posted a

under no load on a.b.s.e in PDF

if someone could give me some

a noisey output since it is

to get the ground connections

didn't affect it much either way.

full 4A load, It gets fuzzy under

change with output voltage. I

With C5 at 2200pf the noise is

very little.

to use an OPA2277 for U1. With

a 10ohm load resistor

expected. When I lower the CC

the current being drawn or 1.6A

there down. It did this no

output voltage to excede the CC

quite smoothly.

A couple of thoughts: increase C1 to 470, and add a small cap in parallel; add a small cap directly across the output. Looks similar to 120 hZ ripple mixed with noise. Can't tell what the noise frequency is.

Ed

Have you brought the ground terminal of the 2.500V reference; U3 pin 7; and the earthy ends of R26, C9 and C10 to a common point? Any noise between these nodes would be bad.

for too much noise on the

CC from 0 to 4A. I have posted a

under no load on a.b.s.e in PDF

if someone could give me some

a noisey output since it is

to get the ground connections

didn't affect it much either way.

full 4A load, It gets fuzzy under

change with output voltage. I

With C5 at 2200pf the noise is

very little.

to use an OPA2277 for U1. With

a 10ohm load resistor

expected. When I lower the CC

the current being drawn or 1.6A

there down. It did this no

output voltage to excede the CC

quite smoothly.

Think I'd be schrieking with fear if my own supplies ran that clean :). It's capacitor ripple breaking through. Seems a very non linear control loop from sampling the output to taking corrective action.

lags. I'd be inclined to lose D2 and drop R8 R17 to maybe 1/10 their present values and then add some local linearising with a (say) 220ohm in Q4's emitter. Puzzled by U3, it seem way overkill.

Thanks, I tried the 470uf, but no help, so I tried more bypassing on the +-12v aux supplies and when I put the

470 on the -12v line things cleared up a bit. Being a simple person, I figured I'd try a bigger cap and sure enough with a 2200uf on the neg supply the noise is down below 1mv and stays there even with a full 4A load. :) It's getting hard to measure, so I figure it's probably good enough now. The aux supplies are on a seperate board with the relays, the line input filter, etc. I used plain ole 7812 and 7912 regulators and have 10uf tant caps right at the connector where the supplies enter the regulator board and 10uf tant caps on all opamp and IA power pins. Thanks again.

Mike If there is no absolute truth then nothing can be known.

It is. It started out as a difference amp using an OPA2277, but not having read the data sheet for the AD7706 thoroughly enough I didn't realize that I needed to run it in unbuffered mode in order to read accurately down close to ground. Before I figured that out I had changed to the INA101 and am just too lazy to change it back. It does result in very accurate output readings, so I'll probably leave it in place.

I thought D2 might be a good idea to make sure Q4 was never reversed biased from the output of U1A, but I'll remove it and make the other mods you mentioned and see how it does. Thanks for the suggestions. Not being an engineer tho, I'm not sure I understand all that you said, but I'll work on it.

I thought the noise was pretty high. I see the specs for commercial supplies and a lot of those claim 0.5mv or less noise and ripple on the output.

I sure wish I knew why the OPA2277 doesn't work for U1, it has much better drift specs than the amps I have that do work properly.

Thanks again.

Mike

If there is no absolute truth then nothing can be known.

for too much noise on the

CC from 0 to 4A. I have posted a

under no load on a.b.s.e in PDF

if someone could give me some

a noisey output since it is

to get the ground connections

didn't affect it much either way.

full 4A load, It gets fuzzy under

change with output voltage. I

With C5 at 2200pf the noise is

very little.

use an OPA2277 for U1. With

a 10ohm load resistor

expected. When I lower the CC

the current being drawn or 1.6A

there down. It did this no

output voltage to excede the CC

quite smoothly.

The fact that the noise is largely independant of load suggests that it does not originate in the unregulated input rail, as this source would increase with load-induced ripple.

You may be able to spot it's earliest source - it should not be present in the outputs of U2. The higher frequency stuff might benefit from a little miller capacity around Q4 (picofarads) or an emitter resistor there, (not enough to affect source rejection).

You might try re-orienting your mains transformer, with respect to your circuit board, while monitoring the output noise. You may find that the lower frequency line undamental and harmonics vary favourably under some circumstances. The steel leadframes used in many semiconductor packages (including the reference, tend to be affected by magnetic fields that have a predictable orientation, and which reduce rapidly in strength with increasing distance.

It was once demonstrated feasible to reduce output noise of some models in a commercial series of linear power supplies into the microvolt region, simply by mounting the regulator board assemblies at right angles to convention, on the unchanged industy-defined chassis's metalwork.

All the longer connections to pots, or larger-bodies pots themselves are subject to pick-up. Perhaps R11 should be grounded, and not used as a voltage pick-off point and C5 should have a small resistor in series. C4 or a similar part might be more effective on the actual output terminals.

A more smooth transition between current and voltage regulation might be possible if U3 had reduced influence. It does, after all, only have to over-ride a milliamp from the output of U2B, using it's full output voltage compliance, to zero U1A input pin3.

RL

I have solved the noise problem and I want to thank all who provided input. As best I can tell the noise is down to about 600uV P-P and mostly 120Hz. The

50KW AM broadcast transmitter located close by has a tendency to inject RF into my scope probes which makes for pretty fuzzy traces. The biggest single difference came when I put a very large bypass cap (2200uf) on the -12V line. I thought that was a bit strange because the +12v line has considerable noise riding on it that's coming from a brushless 12V fan and it just barely shows up in the output. The remaining noise does not change under a full 4A load either. At least I can't see it on my scope. The noise may even get lower once I get everything mounted in a metal case and shorten the cables to the front panel pots. I picked up some 2 conductor shielded cable at a hamfest on saturday and using that to connect the pots may help a bit also.

Btw, It concerns me that I can't measure any difference in output voltage between no load and a 4A load. I used an HP 3478A 5 1/2 digit meter and the 40.000V reading did not change when when I connected my 10 Ohm load resistor. I wonder what I have unknowingly traded off for such good regulation. I would have guessed it would be noise and/or instability. I suppose that somewhere down the road I'll find out. It does drift a few millivolts as it warms up tho. That's probably not helped by having to use the LM6132.

Now to make a gizzmo to switch my load on and off so I can measure the time it takes the thing to respond to step change in load.

Thanks again.

Mike

"The scientist is possessed by the sense of universal causation...His religious feeling takes the form of rapturous amazement at the harmony of natural law, which reveals the intelligence of such superiority that, compared with it, systematic thinking and acting of human beings is an utterly insignificant reflection." Albert Einstein (theoretical physicist)

50KW AM broadcast transmitter

for pretty fuzzy traces. The

the -12V line. I thought that

that's coming from a brushless 12V

change under a full 4A load

I get everything mounted in a

conductor shielded cable at a

between no load and a 4A load. I

when I connected my 10 Ohm load

I would have guessed it would

out. It does drift a few

LM6132.

takes the thing to respond to

You should probably also examine the condition where the microprocessor switches the input voltage. A sudden change of this magnitude could have it's own problems. You might need to borrow a storage scope to do this.

It's fairly easy to examine load transient response by simply switching the 10ohm resistor at the load end using a mosfet. With pnp output buffers, you will find that output terminal decoupling is critical. Larger electrolytics may be needed. A measurable load regulation is not neccessarilly a bad thing.

re- bypassing -12V - improving input decoupling to the regulator might be more efficient, and making sure the return reference is optimally located - it's a low current kelvin connection in most commodity regulators - whether it works better on one side of the current sensor or the other will be the question. I'd suggest RC decoupling the fan, in any eventm to keep it's ripple local or give it it's own independant, crude regulation.

I'm pleased not to hear that the input is not from an SMPS ( I assumed you'd mention by now, if so), as this would complicate things somewhat. Suggest you snub the rectifiers and voltage change-over hardware iteratively, while examining output noise.

RL

Oh yeah, I kinda forgot about checking that. I'll check it out for sure. I have an HP 54502A scope that should do nicely to capture the transition.

That's exactly what I did. I used a 555 driving 3 IRF640s in parallel. I figured that would slow down the on/off transitions to avoid inductive ringing and that seems to work nicely. I have it running at 50Hz with a 50% DC. I let it run and have been playing with the output electronlytic as you suggested. I think a 100uf instead of the 39uf might be reasonable. I also found that the output was ringing for 2 cycles when the load turned on and it took 120us for that ringing to die out. I thought that was too long and found that reducing C8 from 5600pf to 2200pf shortened that to 40us.

The aux supplies, relay drivers, and line filter are all on a seperate etched PCB. There is a 1000uf filter cap and a 2" run to the input pin of the 7912. There is also a 2.2uf cap right at the input pin. I just cut the trace between the 2 caps, inserted a 100 ohm resistor, and replaced the 2.2uf with a 10uf tant. I still have 17.5V input to the regulator. I haven't removed the the 2200uf bypasa on the regulator board, but the noise dropped to about 400uv P-P. I think that's plenty good enough. I played with the reference return earlier and found that it is optimum where it is, on the high side of the sense resistor where the neg output lead is grounded on the board.

The case I have has a 120VAC fan already mounted, so I'll just use that and eliminate the 12V fan altogether. It's a little louder but it does moves more air.

Oh no, No switcher! I thought about preregulating with one, but was afraid of the noise and decided to do it the easy way and just select one of 2 input voltages.

I was hesitant to use the micro for the same reason, but it is not a problem at all. I'm glad I did use it because it allows me to easily do some things that wouldn't be too practical otherwise. I have a nice 20X2 LED backlit LCD that shows me the power output, the CV and CC setings, the heatsink temperature, and allows me to select the low or high input voltage and to shutdown in the event of a heatsink overtemp. I also added a standby switch that signals the micro to drop both input relays. I just have to remember to look and see what happens when it comes out of standby.

Sometime I'll need add an overvoltage protection circuit. I spotted a nice simple and accurate crowbar using a triac and a TL431 in a TL431 datasheet.

Will do. At least it only changes the input when I manually change the voltage so it's not quite as critical as it might otherwise be.

Whew! I never thought building a decent power supply was so involved. I'm sure learning a lot doing it tho and it'll be a lot better than the cheapie I've been using!

Mike

"The scientist is possessed by the sense of universal causation...His religious feeling takes the form of rapturous amazement at the harmony of natural law, which reveals the intelligence of such superiority that, compared with it, systematic thinking and acting of human beings is an utterly insignificant reflection." Albert Einstein (theoretical physicist)