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Interfacing a PIC w/ high-side mosfet

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OK, this is what I have been working on. I haven't had time, in the not yet together lab, to breadboard this up, and my power supply wont get here 'til friday evening. My heathkit trainer doesn't supply enough current to test this out.

The fets modeled are the through hole parts that I plan to test/prototype with. So far, I know more about fets than I have ever known, after about two hours with the simulator, and thanks to Daves videos and the help here on the the forum, thanks guys!

Any questions or comments are welcome...


On edit: I thought I'd give a rundown on the circiut for those who need it, so here it goes....

V1 is the high current 12v supply
V2 is the output from the MCU
J1 is the switch that lets me simulate the MCU high and open output
U2 and U4 are voltage and current to the gate of Q2
R3 is the pull-down resistor for the gate of Q2
R1 is the pull-up resistor for the gate of Q1
Q2 is 2N7000 N chan mosfet used as inverter/driver for Q1
Q1 is MTP20P06 P chan power mosfet 20a 60v
R2 is the simulated load that is high side powered from Q1
U1 and U3 are voltage and current across the load R2

I hope that will help someone on here.


I'm working on a similar project to give some control to some older car lighting.  I ended up getting this p channel mosfet  AOD4185.  People have mentioned to me to also watch out for the high initial current draw of incandescent lighting.  Until it warms up the filament has very little resistance.  This is the main reason I over sized the mosfet so much.  I'm planning on only 3Amps steady state.  Also, you can use PWM on your micro to slowly power on the light.

Your circuit looks good to me though.


--- Quote ---Your circuit looks good to me though.

--- End quote ---

Thanks, I have learned a lot just being on here.
I hope to have this little project complete by the weedend and start another.

I am using LEDs for the flasher I'm building, they flash faster and are brighter. Now they are cheap enough too @ $16usd each.

Incandecents are really high current until warm, that is why most new stage lighting dimmer packs keep the filament warm for faster flash rate and to keep the lamps from burning out due to inrush current.

Is your project something that you can use LED lighting? Is your project something that could be done with automotive relays that are controlled by the MCU? I am a huge fan of relays in the accessories that are required on the trucks at work, switches last longer and I can shut off everything via relay when the Ign key is removed.It saves a lot of dead batteries.

Those relays are cheap and can switch quite fast, I considered using the relays, but switching is not quite fast enough for me. 
30-40 Amp contacts and I have never had to change out a bad one that I have installed. Factory ones on the other hand go bad all the time, if I even suspect a stock relay it gets replaced, been burnt by them a couple times, never again.

Good luck...


For automotive lighting, I am all about using the smart fet's.  Very low on resistance, 50mOhms about. The pins in the connectors get hotter than the device.  High current, 10amp cont. with 40amp max.  They are pretty simple to operate, just send a command to them.  They can detect and report short circuits/faults in the system.  Here are a few links

Infineon -http://www.infineon.com/cms/en/product/automotive-ics/smart-high-and-low-side-switches/profet%E2%84%A2-smart-high-side-switches/channel.html?channel=ff80808112ab681d0112ab69e2d40357
Freescale -http://www.freescale.com/webapp/sps/site/taxonomy.jsp?nodeId=01435979968459

There are others these are just the ones I have used.  Go to any manufacture and click on their automitive solutions page somewhere and you can see if they offer such a thing. 


--- Quote from: andersendr on February 23, 2012, 08:43:04 am ---For automotive lighting, I am all about using the smart fet's.   

--- End quote ---

Yes, thanks! Rufus pointed those devices out to me also. They're great but expensive, and if I would have went that route, I wouldn't have learned how to use fets in the real world. If I were using incandescents, that would have been the chip I would use, or just a simple relay.

The newer cars and trucks use these to report lamp out and if there is more current draw than there should be. A real hassle for me at work adding the aux lights for plows and such. I end up isolating everything with relays because the computers can't detect the extra load of the relay, but will shut down the circuit if I try to add just one turn signal, marker lamp. What a pain.

Perhaps Sile may find them useful for his project...




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