Wirewound as sense resistor

T Chap

I have been using this one specific .1ohm 2w resistor and 1.21ohm I found at All electronics for several years now on some motor drivers. The current sensing IC with the .1 works incredibly well for what I need it to do. I remember reading somewhere a while back to avoid wire wound resistors, and to be honest I never knew these were wire wound until recently I had a driver IC go crazy and some hi and low mosfets were on at the same time, the resistor lit up and the coil was glowing red inside. I just ran out of resistors and need some more right away, and All Electronics is out (I live close by there so they are convenient). I need to order some online but figured I'd look into this warning I read about not using wire wounds. My theory is if it has worked great for 4 years now on various drivers, why bother changing, but I thought I'd see if there was any legitimate reason behind it.

kwinn

Todd, as far as I recall the reason for avoiding wire wound resistors is due to their inductance. Not a problem for DC and low frequency circuits, but it may be an issue for high frequency PWM control. Never run into problems myself, but I have always avoided wire wound sense resistors for any but DC or 60HZ circuits.

davejames

Todd,

If you still wish to use wirewound resistors for your application and are concerned about the related inductance, you may consider the "non-inductive wirewound" type. Check out the Vishay NS series (http://www.vishay.com/docs/30204/rsns.pdf). We used something similar in the automotive test group at Phillips Semi and had no problems with inductive effects.

Regards,

DJ

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Instead of:

"Those who can, do.· Those who can't, teach." (Shaw)
I prefer:
"Those who know, do.· Those who understand, teach." (Aristotle)
·

erco

I have used 3 silicon diodes as a simple on/off current sensor. Schematic attached shows 3 series diodes (1N4001 or equivalent, current rating needs to match your application) in motor circuit. Obviously the diodes·slow your motor down slightly, you'll lose about 2 volts from your motor battery supply. But you can feed the top diode connection·into a stamp input pin through a 1K protection resistor, and you'll read logic 0 when the motor is off and 1 when the motor is on.

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·"If you build it, they will come."

T Chap

Thanks guys for that info. I have been looking at substitutions since I ran out of the surplus 1.21ohm wirewound I had a lot of. Digikey doesn't carry anything at 1ohm current sense in 3w, only goes to 2w. I am thinking now that this wirewound I had must be a 2 watter. I can't find anything on it online marks: ULT or ULI CS-4 1.21ohm wirewound. Maybe get one of the 2 watters from digi, non-inductive wirewound and test it.

Post Edited (Todd Chapman) : 10/29/2009 9:03:00 PM GMT

kwinn

In theory a 1ohm 2W resistor would be good for up to 2A, although in practice I would not use it for much more than 1A if the current draw is continuous for long periods.

davejames

kwinn - how do you figure that 2 amps?

DJ

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Instead of:

"Those who can, do.· Those who can't, teach." (Shaw)
I prefer:
"Those who know, do.· Those who understand, teach." (Aristotle)
·

erco

Power dissipated through a resistor is resistance times current squared, so I make max current for that 2W 1 ohm resistor to be 1.414 amps.

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·"If you build it, they will come."

kwinn

davejames, erco, thanks for catching my error. I should know better than to send late night answers without double checking them.

erco

kwinn:

These days I'm making all kinds of late-night mistakes, too. According to Child Services, diapering the wrong end of a baby at 3 am is far more serious than your minor miscalculation...

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·"If you build it, they will come."

kwinn

Erco, that is so true.