Which mosfet?

Captain Haddock

Senior Member
Apologies in advance as I know the usual answer is to read the datasheets, been there and they may as well be in russian at the moment due to MS induced brain fade...
I need to switch the +12V side of a wiper motor that draws about 5Amp with a picaxe, I'm assuming P channel mosfet is the way to go to keep the circuit board small, any recommendations on one that will handle this would be gratefully received as my head is now stuffed with numbers that are not meaning anything to me.
 

SAborn

Senior Member
A P channel mosfet will likely cause you all sorts of problems, as well as the average current rating for a P is often far less than a N channel mosfet.

Look to use a N channel and a lot of problems will be solved.
 

lbenson

Senior Member
I ask out of ignorance and in the hopes of learning something: why not the minus side with something like

RFP12N10L PWR MOSFET 100V/12A/0.200 OHM N-CH LOGIC-LVL TO-220AB Jameco#:787798
IRL520N (BUZ72L), N-CH.MOSFET100V,10A,TO-220 Jameco#:209058
FQP30N06L MOSFET N-CH 60V 32A 27mOhm TO-220
NTD5867NL-1G MOSFET N-CH 60V 18A 43mOhm IPAK
 

Captain Haddock

Senior Member
The 0V side is through the casing and the speeds and self parking are done on the +V side through separate terminals which is the bit I want to control, I'm doing a variable intermittant wiper/washer control for a boat, I have some N channel mosfets that would probably do but can't switch from that side, maybe back to relays then.
Thanks for the advice.
 

lbenson

Senior Member
Here is a fairly new P-CH mosfet:

IRF9540N Power MOSFET P-Channel 23A 100V

I can't vouch for its suitability for your application, but you can drive its Gate from a picaxe through a (for instance) 2n7000 (1K inline picaxe pin to 2n7000), 12V goes to IRF9540N Source, Drain to load to 0V.

Perhaps a schematic would allow the more experienced to make recommendations.
 

fernando_g

Senior Member
Why reinvent the wheel? What you require is an automotive grade high side switch.

You can drive it from ground referred 3 or 5v logic, they internally perform the level shifting and switch multi-amp loads from your 12 or 24 volt battery.
They also have a large suite of protection, diagnostic and monitoring functions.

I've used the STMicro devices and they are almost indestructible.

Check the following page:
http://www.st.com/web/en/catalog/sense_power/FM1965/CL2148/SC1037
 

lbenson

Senior Member
What you require is an automotive grade high side switch.
Fernando_g,

This is the kind of response I meant when I said I was in hopes of learning something, but your link shows 139 products--any further guidance as to what would be suitable?
 

inglewoodpete

Senior Member
Here is a fairly new P-CH mosfet:

IRF9540N Power MOSFET P-Channel 23A 100V
I have been using the IRF9540N for about 3 years as a high-side switch. Very robust when used with an adequate heat sink. I've used them to switch up to 60 volts, using a BC640 PNP + 2 x BC639 NPN to shift the 5v logic voltage from the PICAXE to the higher voltage for the MOSFET.
 

fernando_g

Senior Member
Fernando_g,

This is the kind of response I meant when I said I was in hopes of learning something, but your link shows 139 products--any further guidance as to what would be suitable?
If you scroll down that page, there is a selector guide.

Selecting single channel, a SO16W package (relatively easy to solder) will provide the VN5E025ASO, which should be more than adequate for your application.
http://www.st.com/web/catalog/sense_power/FM1965/CL2148/SC1037/PF252770

But of course, you can play with the selector guide and narrow to a specific device, depending on your own requirements.
 

nekomatic

Member
My go-to P-channel MOSFET is the FQPF47P06, readily available from Farnell, RS, et al. It has a nice low on resistance and a reasonable gate threshold voltage, and it comes in a package with an insulated tab so you don't have to worry about how to mount it. Pull the gate up to the source with a suitable resistor, 10k or so should be fine if you're not trying to do PWM or anything, and use any NPN transistor to pull the gate down to ground to switch it on. If you're switching a motor just remember a suitable flyback diode across the load.
 

nekomatic

Member
...Having said which, those high side drivers do look nifty - thanks for pointing them out. The VN920-E looks like a good choice if you want to stick to through-hole devices.
 
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