How much juice can a pedal take?

[navvid]

The dc power jack on a pedal is switched such that when you plug in a power supply, it disconnects the battery from ground, so that you are not running it on both battery and the adapter at the same time. While modding my tube screamer, I ended up shorting those connections with a jumper so the battery is always connected to ground, because I didn't like the crappy jacks I found with the switch, and the only good quality ones were two conductor. I didn't like the contact connection that closed the circuit inside the jacks, and so I figured I'd see if it affected the tone to jump the connections that would normally just make weak contact. All I have to do is remember not to plug in an ac adapter with a battery inside, right?

But what if I did? What would happen? Would I fry some part of the circuit? What could or would be damaged? They say tube screamers can be run on up to 16v, but I don't think the battery and power supply would be in series anyway, so I don't think it would exceed the voltage limit...

[duhvoodooman***]

There's really no need to do what you did to bypass the mechanical disconnect in the adapter jack. I've probably used a hundred or more of those things and have never seen one go bad or fail to make an adequate connection. Analog pedals rarely pull more than a few mA of current, and the contact connection is more than adequate for that....and then some. I've helped "noob" builders fix some of their mistakes, including a couple of pedals that had a dead short between the power rail and ground. They would pull juice from a 9V battery fast enough that the battery would get nearly too hot to hold! SO I really wouldn't worry about it.

As for your question....different pedals will obviously tolerate different max. voltages, based upon the voltage ratings of the components used. A lot of pedals rated for 9V will run on 18V no problem. Others will fail almost immediately due to an opamp or transistor burn-out, etc. Just depends on what componentry was used in them. And the battery and the adapter power are wired in parallel, otherwise the pedal wouldn't run with one or the other absent.

[Eric***]

Analog pedals rarely pull more than a few mA of current, and the contact connection is more than adequate for that....and then some.

Seriously? A few milliamps??? I always assumed it was around 100 mA per pedal, since that's what most of the single-pedal wall warts seem to be.

[navvid]

I didn't expect the mechanical disconnect to fail, mostly I don't like the cheap connectors that have it built in. All the closed switch jacks I have seen are housed in plastic. The two conductor ones (unswitched) have a metal barrel and body, and are just generally beefier, and will take a lot more abuse. As for it's ability to pass current, I suspected it wouldn't make a difference. The reason I jumped the connections in the first place is thatI had pulled the dc jack off the board to make room for my switchcraft 1/4" jacks, and wanted to test the thing. I had to run it on battery power to test it, and it was easier to jump the connections than to reinstall the dc jack. Once I did it though, I figured, why not try it this way?

Also got me thinking, what would happen if it were run on both? It made sense to me that the dc jack and battery should be in parallel, and they appeared to be on examination. If it were two batteries at the same voltage in parallel, they would run at 9v and supply twice the current for a longer battery life. But if you have a constant voltage from the power supply, and the battery is dropping off in voltage with use, then what would happen? Would they even work together in parallel?

[duhvoodooman***]

Seriously? A few milliamps??? I always assumed it was around 100 mA per pedal, since that's what most of the single-pedal wall warts seem to be.

Yep, it's true. The current draw information is available for most of the BYOC kits, and the majority of them draw between 2 and 10 mA, with many under 5 mA. The ones that draw more either have multiple IC's or (especially!) contain digital components. The BYOC Reverb, with the Belton digital spring reverb emulator module, is the highest by far at 77 mA. Only a couple of others are over 20 mA....

If it were two batteries at the same voltage in parallel, they would run at 9v and supply twice the current for a longer battery life. But if you have a constant voltage from the power supply, and the battery is dropping off in voltage with use, then what would happen? Would they even work together in parallel?

To tell you the truth, I'm not 100% sure, but I would guess that the battery wouldn't drain below the incoming voltage from the adapter. But if the wall wart delivers over 9V (most do; 9.5 - 10V seems to be pretty common, from what I've seen, and unregulated adapters can run quite a bit higher than that), it would seem that the adapter would be trying to charge the battery. I understand this is not recommended as standard batteries will overheat and leak after awhile.

[navvid]

So after some research and consultation with a lab technician that I am friendly with, I believe I have an answer. If run on both a supply and a battery simultaneously, the battery would likely overheat, probably leak and maybe even explode a bit. It might also discharge through the power supply, burning it out. It probably wouldn't damage the circuitry of the pedal, aside from the physical damage and acidic mess which would result from the small explosion. In short, it is not a good idea, actually a very bad idea. Also realized that none of the connectors that I wanted to use are going to work because the stock jack has a pin shunt spring, and everything on the market is sleeve shunted. I tried Mouser and even called Switchcraft, but no dice. As of now, my pedal only runs on battery, but I am not ready to concede defeat. I have, (oh god) a plan!

[Ch0jin]

Running a non-rechargeable battery in parallel with an external power supply, regardless of the voltage, will cause your battery to leak/explode. Do not ever do this! Your tech mate is 100% correct in this regard.

You don't have to concede defeat, just use the right jack dude. You don't need to be reinventing the wheel here.

Also regarding current draw, a notable exception are pedals that use a built in voltage multiplier. (MAX1044 or similar). I have a PT80 Delay that steps up 9V to 12V and it'll suck a 9V down in minutes. Older digital multiFX are shocking too, I have a Zoom505 that'll trash a 9V in 15 minutes or so.

[DeanEVO_Dude]

The dc power jack on a pedal is switched such that when you plug in a power supply, it disconnects the battery from ground, ...

I feel the need for my anal-retentive side to rear its annoying head... Just for the record, to correct the info here. On a standard Boss-style plugin (center negative) power jack on an effects pedal, the battery POSITIVE is disconnected from the circuit, not the negative side. Sorry for the butting in here...