Let's say that I have a 37v DC source that I would like to reduce to 12v so it can run a 12v light bar (a string of 6 leds and I assume a resistor to limit the current).
I measure the resistance of the light bar at 35 Ohms, so using Ohm's law I calculate that putting a 70 ohm resistor before the light bar will safely reduce the voltage from 37v to 12v and run through the light bar at the expected 351 mA.
Will this really protect the components of the light bar considering the fact that the inital voltage is 37v until the current starts flowing. If you measure the voltage of the 70 ohm resistor before the current starts flowing: it reads 37v. It's only after the current starts flowing: that the voltage is reduced to 12v. Is it possible for the 37v to damage a component that's not rated or such a high voltage? Initially I tried applying 29v directly to it (no initial resistor) and it instantly smoked and died without heating up first despite being supposedly rated for 12-30v so I'm wondering if some arcing will occur internally with 37v and a resistor.
Best Answer
You can use a resistor to limit current, this is probably the worst way to do that. Why? Because the current will be consumed as power, in this case it will be roughly 9W that's a lot of power, and a large resistor will be needed.
It's not the voltage that damages resistors it's power. (unless arcing occurs, you don't usually worry about arcing until after 60V)
The other problem is the voltage will vary if the current varies, this could have consequences for your circuit.
Use a linear regulator instead, or a few of them or a different power supply.