Underrated power supply.

[thierrywalt]

Hello! I am new here... So, I am a beginner trying to develop a circuit, and I sometimes I get confused about power supplies. If I have a DC power supply rated at, let's say, 12V and 10A, and I connect a device that is supposed to operate at 12V and 5A, everything would be fine, right? The current passing through the device depends only on its resistance, because after all it is like a simple resistor, it does not depend on capacity of the power supply (as long as it is lower), right? Why, then, people say that LED's "draw as much current as they can"? If the voltage is the same, then their resistance varies according to the current? Or is the resistance always the same? If I connected a 3V LED to this power supply, would it draw all the 10A? If so, what other devices display this behavior?

Ok. Now, if I connect a device (thermoelectric cooler) to the same power supply, being that this device is supposed to work at 12V but will draw 15A, what would happen? The current needed is higher than the power supply can supply. Would the device simply operate at a lower potency (the voltage is the same but the current is lower) or would the power supply burn?

Thanks in advance!

 

[albbg]

LED is not a resistor, is a diode that is a non-linear device. The current flowing through it increases exponentially with the voltage across it. To connect it to a power supply you need an external resistor that limits the current at the wanted value.
If you know, from data-sheet, that at a voltage drop (across the LED) Vd the current will be Id and you want to connect it to a power supply Vcc, then you need an external resistor R = (Vcc - Vd)/Id. In this way the current will be Id in both LED and resistor. You will have a voltage Vd across the LED and Vcc-Vd across the resistor.

The answer at the second question depends from many factors, the power supply could also be damaged.

 

[KerimF]

The first question is already (and well) answered.

A professional regulated voltage supply would have also a current limit regulation as well.
So, for a 12V/15A device, the load resistance (like a heater) is 12V/15A = 0.8R.
If this device is connected to a 12V supply with 10A current protection, the supply voltage will drop to 10A*0.8 = 8V (instead of 12V).

Added:
The current protection might be achieved by cutting-off the output voltage till the power supply is reset by turning it off then on for example.
Obviously if the supply has no overload protection, a heavy load would damage it in a way or another.

 

[enjunear]

Beware, a TEC is basically an array of parallel and series diodes. It's comprised of semiconductor P-N junctions and will not act like a resistor (i.e. more like an LED, which is also a P-N junction device).

To be safe, ALWAYS use a power supply with sufficient capacity compared to your desired load. To do otherwise is just foolish, and potentially dangerous.

Some good reading on TEC's: Thermoelectric cooling, coolers, modules, heat sinks, exchangers, Peltier coolers, devices - TE Technology