Current Draw

Provided the source current availability exceeds the load current consumption, all will be fine. If it is the other way round and the installation has been designed correctly, (a) circuit protective device(s) should operate, disconnecting the load from the source; this situation is called "overload".

Yah, like the others said. Consider a car battery which will power your radio just fine with the ignition off. The battery will also supply power to all accessories powered on driving your merry way down the road.

I am not going to blindly agree like everyone else as somethings do need the current to be limited.

What I do agree with is that most consumer electronics will only draw the current they need.

Some thing such as an LED or charging a NiCad could require the current to be limited or they will burn out. The most common problem here is the heat the devices generate can destroy them.

So while I don't completely disagree with what was said be warned, it is not a blanket statement for all things electronic.

Ah yes...after reading OP again it could be construed that way. Good catch.

Hi Frank, you did raise a good point and a GA was deserved, but the other problem with electronics is the polarity of the device, depending on what device you are using the polariy may change, not sure but not all devices have protection against this problem.

Best Regards

Joe

Thanks Guys,

All comments duly noted. Its just the occasional odd camera but care must be taken with cell phone and GPS chargers as these do have batteries!

The basis is "Ohm's law". If you apply a voltage "V"to an impedance "Z" the current will be "V/Z". The current drawn depends on the "load"=Z. For motors it will also depends on the "inertia"of the load.

Current is like bank balance. You can draw upto what is available and not beyond that . It is safer to leave some balance but no over dawing fecility

If you connect an over-sized load to a generator, the winding will burn if the protections are not in place or engine will stall .

I destroyed my Olympus camera, when I used it with a "big" power supply.

First I checked the polarity of the connection plug, then I selected the correct output voltage and connected my camera. When I switched my camera on, a "psst" was the last sign of live it showed.

The power supply was rated 1.5Amp and output voltage was adjustable 3 - 4.5 - 6 - 9 - 12 - 15 volts DC. The output voltage was adjustable by taps on the transformer secondary. There was no stabilisation circuit like 7805, or LM317, only a rectifier and a capacitor.

Because the current the camera took was so little, the no-load transformer voltage and the maximum voltage over the capacitor, was much more than the selected output voltage.

I don't remember exactly, but I selected the 9 Vdc output, and measured +/- 14 Vdc at the output. This high voltage destroyed the voltage stabilisation circuit in the camera.

Since than, I only use a labo power supply with adjustable voltage and current control, as replace / emergency power supply …

The problem in this case, was that the transformer's (that, in reality, is the the "power supply" was) rated output was supposed to be assumed to be under nominal full load (full current draw). The reverse EMF generated in the transformer windings at full rated current would limit the output VOLTAGE to the rated (or nearly so) voltage. But under virtually no current load, as it was here, an unregulated supply could provide significantly higher voltage, thus driving, at least temporarily, significantly higher input (or at least, inrush, at the moment of turn-on) current through the device.

Hence, "psst" and no more function.

Also, hence, the "labo" supply, with it's attendant regulation, and, no doubt, crowbar safety features.

So, to the OP, it is always a good idea, when using a supply NOT designed specifically for the device under test (DUT, to those of us in the R&D/Test fields), to know what kind of supply you are looking at, and whether regulation/protection is provided onboard the DUT, or assumed to be provided by the Power Supply being used.

In the event that regulation/protection is supplied onboard the DUT, Rudy's original transformer, and a wide range of others, would have worked. But it is definitely misapplied labeling for a common "wall wart" (even, maybe, the switchable variety) to be called a "power supply", with all the latter term implies.

So, as always, Caveat Emptor. Let the buyer beware.