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MOSFET switch - sanity check

I have a Milwaukee 12v variable speed tool with a bad control board. Rather than replace the board, I'm planning to just use a switch for single (full) speed. But the switches that I have & will fit are AC only. So, I'm thinking that I'll use the switch to control a MOSFET that will do the heavy lifting. I just want to make sure that I'm not missing anything.

The plan: + ______________ | | | Load | | |__/.____| | | 1k | | | - ________|_____|

The MOSFET is IRF3205 (Rdson of 8 mohm)

I don't think that I need a switch-gate resistor, or do I?

Thanks, Bob

Reply to
Bob Engelhardt
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If it's an inductive load you'll want a back EMF protection diode across it as well; save blowing your MOSFET.

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NO DEAL! NO SURRENDER TO THE EUSSR!
Reply to
Cursitor Doom

If the AC-rated switch can handle the current, it will be fine at 12 volts DC.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

Thanks. I was wondering if I might get away with it. I'll try it ... worse case is a failed switch.

Reply to
Bob Engelhardt

The problem with using AC switches in DC circuits is the hazard of sustained arcing when the switch opens. That's not going to happen if you use a 120 volt AC switch at 12 volts DC.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
John Larkin

That's not necessary; every power MOSFET has a parasitic drain-source diode already.

Reply to
whit3rd

I have experienced the AC-switch-DC=arcing and even though it was years ... decades ago, I am sensitive to the problem. Too much so, I guess. My switch is actually 250v rated for my current, so I'm even safer.

Reply to
Bob Engelhardt

It isn't always that simple. Probably wouldn't hurt to add a snubber network. Then too if it's easy to replace, it isn't mission critical, or you won't be using it much, who cares if it lasts or not?

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Have you ever tried to use a switch in an application and wondered whether the difference between the AC and DC rating on switches really

its current rating exceeds the maximum load requirements in the

Wrong; when it comes to switches, the differences in current-carrying capacity are dramatically different between AC and DC circuits and

some snubber stuff

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Reply to
default

That does not protect the MOSFET from inductive kickback that raises D over VDD. The internal diode only clamps D from going below S.

So, the some means of protection is still necessary, be it either a diode from D to VDD or a TVS (or Zener) from D to S.

The later may be more reliable as the TVS can be placed close to the MOSFET and this avoids the inductance of the wiring to the battery.

Reply to
Dimitrij Klingbeil

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