Dual channel 0-15V 0-500mA power supply

[xavier60]

Because the bias current for the CV reference flows though the CS resistors, expect some interaction between unreg voltage and CC set point, Also some output ripple in CC mode. It might not be bad enough to worry about.

[xavier60]

I have just realised the TIP122 is a Darlington. It should have no trouble turning off because of the built in B-E resistors. Leave the circuit as it is.

[David Hess]

Q2 could potentially sink 2 amps of current when pulling down a large capacitive load into the negative supply which is unlikely to be rated for that so the diode is required to avoid damaging C8.

Interesting, so a capacitor can be damage by too large current through it? I had this question is a long time ago, all I could find was that average current through the cap should be within its rating, or it may overheat. But no information that, say, shorting leads of a charged capacitor can damage it. Or charging it too quickly.

In this case it is because the large current could potentially apply reverse voltage across the polarized capacitor.  The operational amplifier might also object to having its supply pin raised significantly above ground.

Limiting the current would solve the problem but then discharging a large output capacitance to ground would take longer.

[kallek]

Some update. I painted the case last Saturday and I was in school this week. So paint was allowed to dry well.
Today I started the most exciting part, putting it all together!

Small toggle switch on the left is momentary switch for setting current and the right small one is normal toggle switch to switch load off and on.

[exe]

Well done!

How did you make the enclosure?

[kallek]

Thank you!

First step was cutting top cover and bottom side with angle grinder. Then I marked and drilled all holes. For big holes I drilled eight small holes around the hole and then filed it to round. It was very hard and time consuming process.  After that I bended pieces with my homemade metal brake. Then I welded small flat-bars to both sides where I drilled and tapped threads to screw in top cover. If one want something similar and do not have required tools, some local school or workshop will do something like this for very little amount of money.

Today I did some wiring. I want to make it nicely, so potential repairs in future will be much easier.

For last picture I just put everything in place to see what finished power supply will look like.

I am not sure how I should do marking to front panel, or let it without markings. 

[ggchab]

Very nice work 

[kallek]

This morning I made some oscilloscope test to this power supply:

[exe]

I think it's not a ripple, the psu is oscillating at freq ~1MHz, imho. So, more compensation is needed.

Edit: and power-on glitch is an inductive kick or something, I have similar on my power supply. So, for my next psu I'll add a soft-start.

[kallek]

I think you are right. But should I be worried about it although it is only 5mV oscillation?

[exe]

Only you can decide if this is a problem or not . Also, I don't know how it was measured. May be it can become worse under different condition. Like, a capacitive or inductive load may make it worse.

I'd say the whole point of a linear *bench* psu is to have a clean and stable output. I treat it as i a piece of test gear that I can trust. So, if I, say, troubleshoot an amplifier, and I see an oscillating output, I want to be sure it's not because of oscillation injected from a power rail. So, yes, I'd call this a problem. And it seems to be easy to fix.

[xavier60]

Try increasing C5 to something like 100pF.

[kallek]

Thank you exe and xavier60. I will try to put 100pf cap parallel with c5 and lets see what happen!

[kallek]

I tested 22pf and 100pf parallel C5 and no effects at all. Problem seems to be only in CC-mode. that picture with 5mV oscillation was taken output shorted. Now I connected 1 ohm resistor and adjusted voltage so that current was just below 500mA. Results:

[xavier60]

Try adding a compensating capacitor to the CC op-amp.

On second thoughts, a compensating capacitor won't do much unless there is a 10K resistor in series with pin 6

[kallek]

I will try it, but at the moment I have no time to do another board. For me the current limit is mostly for protecting things and testing components, so I think it is ok to me.

[xavier60]

It doesn't seem like a  serious problem.

[kallek]

I finally got power supply finished and took some photos. It is now sitting next to my old power supply and soldering station.