DIY power supply issues

Frequently asked questions regarding powering your pedal.
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Benwah
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Post by Benwah »

Hi all,

I'm trying to build a power supply almost identical to this AMZ one:

http://www.freestompboxes.org/viewtopic ... 6&start=20

The only difference to mine is I'm not putting in the -9v supply.

I've made it up on a vero and it seems to work fine, but when I put it into its case (which is metal) it basically self destructs frying the IC's and caps etc.

So my question to the more experienced is, how should I be loading this into the metal case? This is a run down of my approach:

Input: 19V Laptop power supply (centre positive) going through a metal jack that is attached to the metal case - Tip positive, ring negative. Both are wired to separate vero board strips.

Power supply circuits - exactly the same as AMZ power supply. IC's are attached by a screw to the lid of the metal case to act as a heat sink. IC's are fed power from the positive strip attached to the laptop power supply. Ground is attached to the same strip as the negative ring of the power supply.

Outputs: The output from each IC is connected to the tip of each metal output jack. Ground is attached to ring of output jack. The outputs jacks are also metal.

When I insert a plug into any of the output jacks I start to get sparks, bangs, etc and everything breaks. I know it probably has something to do with the metal case but I can't figure it out.

I'm obviously missing a core concept here. Can anyone help me out?

Cheers,

Ben.

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Zokk
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Post by Zokk »

Hi

did you use insulation kits for your regulators and are they correctly wired (check the datasheet of the brand you use)?

Also you should test continuity with your DMM to be sure that the 9V rails are not connected to the ground (of course you should do that before plugging your laptop psu). The case should be connected to the ground, and the sleeves of the jacks as well.

I don't understand why did you connect the ground to the ring ? it should be connected to the sleeve (if we're talking about 3,5mm jacks phone plug type).

Benwah
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Post by Benwah »

Thanks for the reply Zokk.

I didn't use insulation kits on the regulators so maybe that's the problem. They would have been in direct contact with the case which is connected to ground.

Ground is indeed connected to the sleeve, I don't know why I said ring.

Thanks for your help.

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Zokk
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Post by Zokk »

On the LM7809 the pin 3 (middle lug) is connected to ground so if yours are correctly wired insulation should not be really necessary... but maybe you could try to let them unconnected to the chassis just to see what happens, then if that works you'll need insulation.

Does the problem occurs when you try to power a pedal? or just when a plug with nothing at the other end is inserted, maybe there's something wrong with the plugs?

Also I'm thinking about the polarized caps... have you checked they are with the correct orientation?

Benwah
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Post by Benwah »

I didn't think the LM7809 need insulation either but it probably worth a try.

I made up a cable with a plug on each end to test the outputs. I checked there were no shorts on the cable and the plan was to insert one plug and test the voltage by probing the other end. The problem occurs when I insert the plug into the jack. It looks like the sleeve is not grounded until the plug is inserted. When the sleeve is almost completely in, it pushes piece of metal up that makes contact with the body of the jack which is in turn connected to the case. This is when sparks fly etc. I would have thought this should be ok but its obviously not. Maybe I should just buy some isolated jacks instead.

I've double checked the caps and they are positive side to the IC and negative side to ground.

I'm pretty confident the circuit is correct as when I tested it outside the case it was perfect. It when it goes in the case that things go wrong :-(

Thanks for your help Zokk I really appreciate it.

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Zokk
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Post by Zokk »

I think you've pointed where's the problem; the jack socket sleeve should be always connected to ground so maybe you have not soldered the good pin on your socket (are you using a switched type?).

Anyway there is always a small shortcut between V+ and ground when you insert a plug until the plug reach the right position, that's normal with this kind of connector. So you may first try insulation on the LM7809 because, even if they have internal protection, they never like to "see" V+ on a ground pin; then you can try to change your jack sockets with some "normal" mono isolated (that keep hooking simple with only two wires).

This design is pretty rough, you shouldn't have big problems in theory.

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