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Post by vonFrenchie on Jul 9, 2006 17:46:24 GMT -5
Im building a pedal that calls for a 9v battery. I also have a 9v 300mA AC adapter from radio shack that I would like to use. How could I wire a switch that would allow me to choose between the battery or adapter? Should I even use a switch?
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Post by fobits on Jul 9, 2006 20:58:14 GMT -5
A switch isn't necessary. If both are connected at the same time, the current will come from whichever one has the highest voltage.
The figure of 9 volts is nominal, and shouldn't be taken too literally. A fresh battery has a bit more than that, and it gradually declines as it runs down.
I just checked a 9-volt adapter, and that particular one puts out 11.4 volts, more than even a new battery.
So when the adapter is plugged in the current will come from that, and when it's unplugged the battery will take over.
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Post by UnklMickey on Jul 9, 2006 21:06:28 GMT -5
Frank,
most pedals that are designed for dual power use (battery or ac adapter) have diodes in series with each source.
that allows either one to power the circuit, without the possiblity of allowing the ac adapter to recharge the battery.
recharging a battery like an alkaline or zinc-carbon can lead to leakage, a real mess.
unk
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Post by vonFrenchie on Jul 9, 2006 21:25:35 GMT -5
Well the thing is.... Im building the pedal. Its not designed to have two power supplies. Its designed to be powered by a battery. I wouldnt know what diodes to use and where. Also I understand the power usage thing. I have some 9 volt batteries that have a charge of 8.5 volts. The thing is I would prefer, PREFER, to have it from supply A or supply B. I dont want the pedal to chose.... I want to. Unless it would be easier with the diode thing.
And yes.... leakage is REALLY messy..... muy.
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Post by UnklMickey on Jul 9, 2006 22:37:04 GMT -5
vF,
since most ac adapters run a little higher than their rated voltage,
the AC adapter will, as Frank suggested, be supplying the current to the circuit when it is plugged in.
the other benefit to diodes, rather than a switch:
if the power cable gets accidentally unplugged when you are on-stage, the battery takes over, rater than the pedal going dead.
voltage rating on the diodes wont be an issue, i don't think any diodes are marketed that have a piv of less than 50v.
i'd just use whatever you can find in rectifier diodes, that has a forward current rating that is greater than what is required by the pedal.
you'll probably end up with 100V, 1A diodes, way more than adequate and still very cheap.
unk
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Post by sumgai on Jul 9, 2006 23:29:20 GMT -5
vF, As unk says, if something goes haywire in the middle of a stage performance, who ya gonna call? So, which would you rather put into a box, a switch that might be hard to fumble in the dark, while under pressure to get the pedal back up and running, or this: That's a pair of simple diodes, as unk describes above. The pair of them together are smaller than a switch, will probably need about the same amount of soldering, and will never do you dirty. Hey, that's my last contribution for the nonce, tally ho! sumgai
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Post by vonFrenchie on Jul 9, 2006 23:35:46 GMT -5
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Post by fobits on Jul 10, 2006 10:19:11 GMT -5
Unklmickey and Sumgai - Thanks for correcting my mistake. You guys are absolutely correct. You don't want the current to go backward through either component. vonFrenchie - The full URL doesn't work (something about ProBoards, I guess) but either the package of 25 rectifier diodes or the package of 50 switching diodes should do fine. Personally I would go with the rectifiers and choose medium-sized ones, just to be on the safe side. Even medium-sized ones are small and cheap. I hope that this makes up for my previous bad advise
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Post by vonFrenchie on Jul 10, 2006 12:12:25 GMT -5
I'm not sure what would qualify as a "medium-sized" rectifier. Im more of a cap and resistor kind of guy.
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Post by fobits on Jul 10, 2006 14:54:45 GMT -5
All right... that can make a point of reference. A diode which is about the size of a standard 1/4 W resistor would qualify as medium size. So would one the size of a 1/2 W resistor, although there's no need to go that big here. Switching diodes, which are intended mainly for digital circuits, are much smaller. Instead of being wrapped in a case that looks like cardboard, they are sealed in a little glass bead. If they are described as "rectifier diodes", they should be larger than "switching diodes". On the other hand, if the price is $3 for 25, they can't be the size of a breadbox. I haven't seen that package, but from the description any of them should work fine.
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Post by vonFrenchie on Jul 10, 2006 15:02:29 GMT -5
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Post by UnklMickey on Jul 10, 2006 17:45:32 GMT -5
...I hope that this makes up for my previous bad advise you're already way ahead of the game Frank. (for previous good work) overlooking one detail, is a very small step backward. vF, keep it simple. if you have a radio shack nearby, either get the 25 pack of rectifiers so you have extras to put in your parts bin for future use. or buy 2 of the cheapest rectifier diodes you can find. do not use germanium diodes. do not use zener diodes. do not use small signal diodes. do not pass Go. do not collect $200 connect them as in the drawing Sumgai provided. that connector looks as though it might disconnect the battery when the adapter is plugged into it. but i can't be certain. also it looks kinda cheap, so i would be concerned that the contacts that disconnect the battery, would not reliably connect the battery, when the adapter is unplugged. it would probably be okay at first, but i wouldn't count on it lasting very long. unk
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Post by vonFrenchie on Jul 10, 2006 18:26:04 GMT -5
Dang... I really wanted that 200 bucks too.
Yea I understand that that looks cheap. But that was just an example.
The plugs at radioshack say that they have a built in switch. What do you think thats all about?
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Post by ChrisK on Jul 10, 2006 22:46:52 GMT -5
If I were doing this, and the load was less than 50 mA, I'd use the glass switching diodes such as 1N4148 or 1N914 since they have substantially less reverse leakage current than the garden variety of 1N400X power diodes.
Diode reverse leakage current will result in inadvertent charging of the battery when powered by the adapter.
Most adapter jacks have a switching contact built in. The battery can go thru the jack switching contact, and is disconnected when the adapter is plugged in.
Q.E.D.
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Post by vonFrenchie on Jul 10, 2006 22:59:14 GMT -5
So basically when using the radioshack jacks that have switches I do not need diodes?
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Post by UnklMickey on Jul 11, 2006 10:10:59 GMT -5
yes
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Post by vonFrenchie on Jul 11, 2006 12:00:41 GMT -5
Thanks
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Post by ChrisK on Jul 11, 2006 19:23:33 GMT -5
Yes, if the jack has the switching function built in. On such a jack, one would see at least three solder terminals.
Can you post the RadioShack part number?
Did you check the RadioShack web site for technical info?
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Post by vonFrenchie on Jul 11, 2006 20:11:51 GMT -5
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