# A potentially daft question Re: sense resistors

Hey guys.
Just curious, on drivers that use sense resistors I’ve seen that people change or piggy back resistors to decrease resistance and increase current and therefore brightness.
What would happen if I were to just bypass said resistors? Would this Fry things or would is be like a pseudo DD driver?

I’ve never done it, but someone said that the driver he did it to, behaved erratic. Someone with actual electronic understanding may have a logical explanation.

When I was an apprentice we got told no such thing as a stupid question just stupid people lol That taught us to think through what we ask lol.

They are doing the same thing in a way because you can lower the sense resistor value till it has no resistance. Some drivers take it and other don’t. I have done it many times with cheap drivers and it worked fine. Lots of people do it with L6 and S70 and run about 9amps.

Boost and buck driver probably won’t like it very much and may fry to much current and heat.

It can also stuff up mode brightness levels.

A sense resistor can be used to measure the current thru the driver by measuring the voltage drop across the resistor. For example if the sense R is 0.01 Ohms, then a 1 Amp current will cause a voltage drop of 10 mV across the resistor, and 10 Amps will result in 100 mV drop. This voltage drop can then be used as a safety set point for shutting off the driver, or as a feedback control signal to regulate the current to a given level.

If the value is changed by piggyback or other means, then the results may be erratic or non-functional depending upon the intended design function.

If the resistor is bypassed with a shorting jumper, then there may be no measureable voltage drop generated by the current. If R ~0, then V = I x R ~0, even for high currents.

http://i65.tinypic.com/4zyyog.jpg

This could explain why some sophisticated drivers will behave erratic , when the sense resistor is bypassed , or the value is too low.

The comparators , in order to do their job , supposed to have something to compare , but in this case , (R sense =0 , or very close to 0 ) . In this case some of the parasitic capacitance of the lay down circuit will trigger the first comparator ,erratic , and in the same time , the second one , that is responsible for the modes .

More complex a circuit is , more chances to have this kind of behave ! that's why , the cheap drivers do not suffer of this , their internal architecture is simple , and the lay down circuit is not "crowded".

Thanks guys, interesting stuff. I will leave everything alone then I think. No point messing up a perfectly good driver.