Switch current rating - actually current or really wattage rating?

Question

I'm trying to build a dummy load for some power supply testing using some very large power resistors. Basically I will wire up the resistors in parallel and connect more if I want to decrease the resistance and therefore increase the current draw.

I want something to switch on each of these resistors. I thought maybe I could just buy some cheap wall switches but the ones I find are only 15-20A rated @ 125V AC but the testing I want to do could go to as much as 35A at 5V DC. I noticed that the switches would often be rated for 10A @ 240V and 20A @ 120V. Since I'm running such low voltage and only DC, could I run much higher than 20A?

Answer 1

The mechanical design of the switch affects both the voltage and current ratings. The higher the voltage, the greater the chance of arcing as the switch opens. The arcing can reduce the switch reliability over time by inducing oxidation of the surface of the switch contacts and hence increasing the resistance of the contacts. In addition, the voltage rating is provided to offer an insulation guarantee. The switch must be safe to operate at the rated voltage and pose no risk of shock to the user.

Too high a current results in a voltage drop across the switch reducing its functionality (an ideal switch should have 0 resistance and hence 0 voltage drop across it).

In your load circuit, it appears you'd like to use one switch per resistor. Since the resistors are in parallel, the maximum current a switch will see will be 35A/2 = 17.5A when you turn on a second parallel resistor. The 20A switch should suffice. when a third switch is turned on, the current in each switch reduces to 35A/3 = 11.67A.

To summarize, for your application, you need only consider the current rating of the switch. If your application is mass-market, to minimize cost you would perhaps want a lower voltage-rated switch which will be less expensive.