Using a higher rated DC transformer

Good morning, all.
I have an electronic device that requires a DC Wall transformer rated for 12VDC at 600 mA. Unfortunately, I have lost the power supply and I
cannot find another one with that exact rating. My friendly Radio Shack representative tells me that the device only "pulls" what it needs from the transformer, the transformer does not "push" that much to the device. He tells me I can use a 12vdc at 1A for the device and it will work.
Is this true? I do not want to fry my device by "pushing" 400mA more to it than it needs. If he is right, may I use a transformer rated for 12vdc at 2.5 amps protected by the same principle? You can tell I'm not an electrical engineer. :) I would appreciate any thoughts. Thank you!
Matthew
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Yes its true...You can use a transformer of ANY larger current rating but not a larger voltage rating. Twelve Volts at 2.5Amps is fine for your purposes. However, be sure the polarity is correct. Most wall transformers have the negative terminal on the outside sleeve of the plug and the positive on the inside, But some are reversed. It is critical to get the right polarity.
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Bob Eld wrote:

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| I have an electronic device that requires a DC Wall transformer rated | for 12VDC at 600 mA. Unfortunately, I have lost the power supply and I | cannot find another one with that exact rating. My friendly Radio Shack | representative tells me that the device only "pulls" what it needs from | the transformer, the transformer does not "push" that much to the | device. He tells me I can use a 12vdc at 1A for the device and it will | work. | | Is this true? I do not want to fry my device by "pushing" 400mA more to | it than it needs. If he is right, may I use a transformer rated for | 12vdc at 2.5 amps protected by the same principle? You can tell I'm not | an electrical engineer. :) I would appreciate any thoughts. Thank you!
This is a rare example where a Radio Shack representative happens to be correct.
The voltage is the push. The same voltage on the same load will result in only the same current flowing. The listed amperage is how much current is available without burning out the power supply or the voltage dropping to nil.
Actually, there may be a very slight increase in current because the larger amperage power supply likely has a lower impedance. If this is the case you would actually be seeing a drop in voltage on the older power supply and less of a drop in voltage on the newer one. If the device calls for 12VDC, give it 12VDC and it will take the amps it wants as long as the power supply has the ability to deliver it.
Back when dialup to the internet was the big rage, I know an ISP that took hundreds of modems, tossed aside the wall warts, pulled the boards out of the boxes they came in, stacked them densely on a rack, and fed power to them with a big rack power supply that looked like it could deliver 50 to 100 AMPs. I think this was all at 12 volts. They had a few dozen racks of this setup. They got more modems in the same space, kept them cooler, and saved on some power.
Google has suggested that PC mainboards be re-architectured to accept a straight single voltage (such as 12VDC) instead of the spidery cable it now uses with several voltages. This is actually practical, as these boards want different working voltages these days, anyway. With such boards, you could have the boards stacked up tightly in a rack cabinet and power them all from a single power supply easily. For a company like Google with hundreds of thousands of computers, it would save them a lot on space, maintenance, power, and the cost of the hardware. It would be an example of a big DC power supply in the bottom of the rack, likely with a direct battery backup (no more UPS inverters), and all the boards powered in parallel by that. I'd love to see single voltage PC boards but the next replacement for ATX.
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You have to be careful. Some "DC Wall Transformers" are not regulated. They consist of only a transformer, a rectifier circuit and a filter circuit. The transformer rated at 12 VDC at 400 mA will give a voltage of 12 VDC ONLY at a load of 400 mA. If the load is higher, the voltage will drop and if the load is less the voltage will go higher. Your device MAY be able to handle this.
If the "DC Wall Transformer" has a regulated output, the output will be at 12VDC regardless of the load, unless you excede the maximum load. They you will release the smoke from the "Transformer" and it will no longer work. So a regulated 12VDC "Transformer" rated at a MAXIMUM of 1 amp., will work just fine at 400mA or any other load less than equal to the 1 amp max.
Al
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