Laptop Power Supplies WARNING!

Frequently asked questions regarding powering your pedal.

Laptop Power Supplies WARNING!

Postby reubster » 25 Jul 2009, 11:25

So I finished a Carl Martin Plexitone Clone on vero and had it running perfectly on my bench power supply...sounded great!
It was time get it rehearsal ready.

Rather than build this pedal with the transformer based +/- Regulated supply, I opted for the standard R/C volt divider to generate the rails and Vref.......All was good..... :D

I had a few of those (18 volt-ish) laptop supplies hanging around so I thought I'd arc it up with one of those.
Before I hooked it up to the pedal I measured the supplie's DC volts with my DMM and then checked for stray AC....all seemed good.

So I hooked it up, no worries, then I plugged it into my amp.......................FUT! ......The sound of something blowing up, tried it out...nothing but loud hum.... :scratch:
Yup the chip was friedus maximus

I knew it was a supply related issue, so I checked, sure enough the 18 volts actually measured between the positive pin and the earth pin on the AC the DC's negative was tied to AC earth.
Ergo.... With an earthed amp and split supply in the pedal, I had 18 volts between the + rail and Vref....Zap!

So watch out....When I restock my supply of LF347's I may still try a laptop supply but this time without one without the earth pin.

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Re: Laptop Power Supplies WARNING!

Postby HydrozeenElectronics » 25 Jul 2009, 21:09

Thanks for the post, I have thought about using on of them too, I did not think it would be a problem, but now I know.

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