MOSFET switch - sanity check

B

Bob Engelhardt

Guest
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
 
On Mon, 13 Aug 2018 13:40:56 -0400, Bob Engelhardt wrote:

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
Click to expand...

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



--
NO DEAL! NO SURRENDER TO THE EUSSR!
 
On Mon, 13 Aug 2018 13:40:56 -0400, Bob Engelhardt
<BobEngelhardt@comcast.net> wrote:

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
Click to expand...

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


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
 
On 8/15/2018 7:35 PM, John Larkin wrote:
If the AC-rated switch can handle the current, it will be fine at 12
volts DC.
Click to expand...

Thanks. I was wondering if I might get away with it. I'll try it ...
worse case is a failed switch.
 
On Fri, 17 Aug 2018 13:32:33 -0400, Bob Engelhardt
<BobEngelhardt@comcast.net> wrote:

On 8/15/2018 7:35 PM, John Larkin wrote:
If the AC-rated switch can handle the current, it will be fine at 12
volts DC.

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

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
 
On Wednesday, August 15, 2018 at 7:41:03 AM UTC-7, Cursitor Doom wrote:
On Mon, 13 Aug 2018 13:40:56 -0400, Bob Engelhardt wrote:

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 MOSFET

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

That's not necessary; every power MOSFET has a parasitic drain-source diode
already.
 
On 8/18/2018 11:40 PM, John Larkin wrote:
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
Click to expand...

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.
 
On Sun, 19 Aug 2018 17:50:55 -0400, Bob Engelhardt
<BobEngelhardt@comcast.net> wrote:

On 8/18/2018 11:40 PM, John Larkin wrote:
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

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.
Click to expand...

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?

https://www.mouser.com/blog/which-switch-who-cares-if-its-ac-or-dc

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
matters? ­­There is a fallacy that any switch can be used so long as
its current rating exceeds the maximum load requirements in the
circuit. Who cares if it’s AC or DC, it’s only a 12VDC system right?
Wrong; when it comes to switches, the differences in current-carrying
capacity are dramatically different between AC and DC circuits and
this is typically reflected in the switch’s AC and DC ratings.

some snubber stuff
https://www.allaboutcircuits.com/textbook/digital/chpt-4/switch-contact-design/
 
On 2018-08-19 22:13, whit3rd wrote:
On Wednesday, August 15, 2018 at 7:41:03 AM UTC-7, Cursitor Doom
wrote:
On Mon, 13 Aug 2018 13:40:56 -0400, Bob Engelhardt wrote:

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 MOSFET

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

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

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.
 
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