exploding caps neovibe

[rhdwave]

So, I've been building another neovibe for someone and trying to use the mods so I can use a 10k pot instead of the dual ganged reverse 100k one...this way I could use a 10k for a possible expression pedal as well and won't run into any issues with having to file down part of a chassis...So last night I finally finished all the wiring and this time I decided to use a dc power source instead of ac.  I left out the diode bridge as I thought that was only a crucial component if you were using ac power...I hooked up the power jack directly to the other side where the diode bridge would normally connect and fired her up...           Smoke, pop, pop...both of the huge caps directly next to there blew up...it was actually pretty cool...but I'm not sure why it happened...



Any thoughts?

[rhdwave]

And, yes i checked and rechecked the polarity on the caps

[BubbaKahuna]

Have you checked your PS to see that it's really putting out DC and not AC?
I'm sure it's marked that way, but it might not be truly MilSpec. 

I'd keep the rectifier in the circuit no matter what, especially in a build for someone else.
You'll have no control what they'll plug into it once it's out the door. All it takes is once and phhttt!

Also are the voltage ratings of the caps well over what the PS is actually putting out?
Most wall warts are pretty far out of spec and will put out as much as twice what the label says with no load on them.
25v caps can blow on an 20v PS all day long if the PS is actually putting out 28v when it's first plugged in.

One other thing to look for :: a tiny little accidental solder bridge someplace?

Are you using the GGG NeoVibe board?
If so, have you noticed that every electrolytic cap on it is oriented the same way?
I liked that about mine, very easy to check polarity at a single glance.
Nice attention to detail in the design stage of that one!

Cheers,
- JJ

[rhdwave]

Thanks for the reply JJ!

The board is one i etched myself, but it's taken from the neovibe pdf...all the caps are oriented the same way.  I went back and checked the schematic and compared with the layout and everything is correct.  I'm going to put the diode bridge back in.  It's interesting what you say about the wall wart putting out too much power.  That definitely could be the source of this.  The caps are 1000uf and are rated for 35v...the wall wart is a 24vdc at 800ma...wondering if that's too high ...hadn't noticed the amperage was that much before, not sure if that could effect anything.  But if it's 24vdc and is putting out much more that could account for the problem. 

Will the diode bridge help with this? Probably not right? as i think it's just supposed to orient the positive and negative correctly?

Thanks again...

[sfx1999]

Did you hook the power jack up backwards? That would have screwed things up. Oddly enough, if the rectifier was in it wouldn't have mattered.

Also, is your supply regulated? If it isn't, I can guarantee you that it is actually putting out more voltage than is listed on it because you are drawing much less current than it is rated for.

[rhdwave]

I really don't think i hooked the power jack up backwards...i used a layout from beavisaudio that had really good pics then i checked with multimeter for continuity etc...i just put in the rectifier and was all set to desolder a couple of 1000uf caps from another board (i have no excess now) when i couldn't find my desoldering bulb...i had just used it and now it's missing...nowhere driving me crazy...walked out of the work room and there it is in pieces with one of my dogs chewing on the main piece...well, this'll have to wait i guess...

what concerns me is if i didn't hook it up backwards, what could this be, i'm going to try aagain once i get a replacement bulb with the rectifier in  and report back...

[R.G.]

You either (a) hooked it up backwards or (b) put AC into the caps or (c ) used caps with too-small voltage ratings. Those are the only things in this scenario that could blow the caps like that.

[BubbaKahuna]

The way I understand amperage ratings of a PS is this:
Your PS Ampre/mA rating is the Maximum it can put out safely and consistently.
Your draw is determined by what is pulling (drawing) power from the PS, not what the PS is rated at.
If you have an 800mA PS and hook up a 25mA effect, your effect will draw 25mA from the PS.
You could hook up as many as 32 of those to that PS and not harm it, or any of the effects drawing from it.*

If you have an 800mA PS and you hook up a 1400ma draw to it, you'll likely overheat the PS and you might wreck it.

Basically, you can have as big a PS amperage rating as you like and should always go higher than your total draw.
A big PS won't put too much current to anything running on it.

* However, you shouldn't run too much voltage into anything - ever.
Extra amps available are usually good. Extra voltage is usually not good.

[rhdwave]

Thanks fellas for the responses! How would ac have gotten in there? What i mean is, if the adapter is dc and this is clearly written on the unit, are there any other ways that this may have happened? I've been using this diagram to guide me in the hooking up of the jack:  http://beavisaudio.com/techpages/PedalPower/   i've followed it to the letter.  The caps are rated for 35V, the power supply is 24VDC...is the most likely scenario that the PS is as was suggested putting out over 35V? Is there a way i can measure this w/o hooking it up to the jack again?

Much thanks everybody once more!

[R.G.]

It's possible that the wall wart adapter is defective too. Making DC requires rectifiers that work. If the adapter has been shorted before, could be its rectifiers are shot.

In either case, put in the rectifier bridge on the Neovibe. With that in, it doesn't care whether it gets AC or DC. And make sure the power supply jack does not let any of its terminals connect to the enclosure.

You could tell whether the adapter was putting out DC or AC - or some mixture, which is also possible - if you had an oscilloscope. Failing that, some DMMs will show 0V when reading AC on the DC scales and a voltage on the AC scales, but this is one I use as an indicator, not for measurement.