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[ts4z]

time to assemble hobby power supply: 2.5 hours

time to blow up capacitor in hobby power supply: 90 seconds

Since I forgot to study electrical engineering, electronics, ham radio, or anything that would otherwise answer this question, where did I screw up?

I have one of these: Jameco Adjustable Power Supply kit. My soldering is decent enough,

Comments

[bk2w]

Which cap blew? I see four in the design.

Have you rechecked that all the components are connected in the right direction?

The fact that the AC fuse didn't blow is not all that helpful. The voltage on the mains side of the transformer is so large that really all the fuse is protecting against is transformer failure. At full capacity (two supplies running at 750mA), you're only pulling ~200mA of AC mains.

You should be able to turn the adjustment knob all the way up with no load without causing any problems.

[ts4z]

The cap that blew was C1, one of the large filter caps.

I am reasonably sure the cap was installed correctly. I checked it several times, including when I removed it. (I knew that hooking up a cap backwards will blow it.)

I replaced the cap with a 35V one I got from Fry's. Unlike the original, it has nice radial leads that don't involve looping back over the cap. Also unlike the original, it has not blown. I have not tried turning either side of the power supply up all the way, but I might have to.

At this point, I'm wondering if this power supply is made of parts Jameco was trying to get rid of. I mean, what worries me is that the voltage regulator's data sheet says it can do more than 15V, but the cap can only do 16. If the transformer was running a bit high, and the cap got a bit more than 16 volts...

[bk2w]

Replacing the C1 (and C2) caps with 25V or 35V is the right thing to do. Fortunately, C3 and C4 are already rated to 35V, so they should be fine. CR3 and CR4 are good to 35V as well. R2, R3, R4 and R5 should be good to 25V output.

Interestingly, the resistors they chose tell the regulator to run as high as 18.6V. Now, the transformer should be putting out a lower voltage (12.6VAC), which would act as a ceiling to what the regulator can do, but it might be putting out more voltage than expected.

Check what the voltage is coming out of the transformer.

[ts4z]

I re-checked the discarded cap. I had it in right.

Transformer, no load, is showing about 14V. Wall current in is a bit over 120V.

I haven't tried turning the negative supply all the way up, but perhaps that's the last thing to do, once I've gotten another nice radial cap ready...