Fried DIY Power Supply

[Macavity]

Having had a NICE multiple secondary 12VA transformer lying around, since the mid-80's, I thought I'd add a few diodes, capactitors, regulators etc., and make myself a dual (12V / 0 / -12V) bench supply. Finished the electronics yesterday (it worked first time!), so I decided to put all the components on a chassis in a box etc.

What could go wrong...

Sadly, quit a lot! At switch-on 50Hz hum, crackling, sparks, smoke etc.  

Then I had my suspicions... No... surely NOT, I thought! But actually yes:

Not merely are the pins of NEGATIVE regulators transposed (I had sussed

that one!), but the metal tag is also connected to input, rather than ground. 

Up until now, I had only needed positive regulators and used the box as a

heat sink. Clearly, grounding negative regulator TABs creates a short!!!

Doubtless there is some sound manufacturing reason for the above, but...

  

[Kropster]

I remember 'in the old days', power transistors etc always came with mica washers for the body or heatsink tab.

I guess you need to buy as an extra now.

[Mick J]

Ouch,  has that fried the whole thing Chris or just taken the regulators out.

[Macavity]

Ah yes, Kropster, I remember those mica washers.

Oddly enough, much (all?) of it seems to have survived!

Once unscrewed from the chassis, voltages look good.

The earth line vero board strip got rather fried though. lol

But there are a couple of unused strips left either side...

The circuit is quite conventional:

http://www.circuitstoday.com/regulated-dual-power-supply-circuit

But a rather interesting little GOTCHA for the unitiated...

(Well me anyway) Buying stuff is probably cheaper now!

[Macavity]

Interestingly, they DO still make TO-220 insulators...

http://cpc.farnell.com/bergquist/k10-54/thermal-pad-to-220/dp/SC10730

So real thanks for that idea.

Aside: Despite huge advances in electronic chip technology, the world

of discrete components seems to have changed surprisingly little?

[Kropster]

Try making the next version with surface mount components!