I want to control a current of about 8A from a car battery for a few milliseconds using output from the arduino. However, my MOSFET is not 'logic level' and therefore, I must use a MOSFET driver circuit.
Would this work?
(see attachment)
When the signal from the arduino goes high, the MOSFET should turn on.
No. When the Arduino output goes high, the base terminal is at 5V. This means the emitter terminal is about 0.7V lower, or 4.3V, and that's not high enough to turn on a non-logic-level MOSFET.
I'd suggest a circuit something like this:
http://ruggedcircuits.com/html/circuit__30.html
Instead of powering an LCD backlight, you can turn on a MOSFET gate. Replace +5V with your +12V from the car battery on the right side, get rid of C1, and it is a reasonable beginning.
However....note that a car is a "hostile" electrical environment. If you just have a car battery in isolation it's OK, but if it's hooked up to a car you might see some nasty stuff, like temporary high-voltage spikes. You will have to condition/protect the circuitry from that (but let's not get ahead of ourselves).
--
The Rugged Motor Driver: two H-bridges, more power than an L298, fully protected
Need this to Source current into the inductor
Connect ground to arduino ground.
I always found it easier to put an N-channel 'below' the inductor to connect to ground, easier to find parts work with.
Hi,
Is the load definitely connected to ground on one end??
You show a "high side switch" which would need to have a P type power FET. Then you could pull the gate down to ground and turn the FET fully on.
Using an NFET as a high-side switch you would need a more complex "charge pump" to get the gate voltage above 12V.
Thanks for the replies. First of all, I realized where my logic when wrong when designing the first circuit. The BJT is configures as an emitter follower and hence the output voltage is always, nearly equal to the input voltage. Maybe someone else will find that useful later on..
Anyway, the other problem I have is with the low-side Vs. High side issue. Can someone please explain why this is of significance when using a MOSFET?
With lowside, you can use an N-channel MOSFET, logic-level parts (can be driven to full-on from arduino output levels) are easy to find and low cost. Plenty of parts with very low Rds.
With highside, you need P-channel MOSFET, to turn them off you need to take gate to the +voltage you are driving the load with - if 12V, then the gate driver must swing from 12V to usually ground for full-on. So you need another transistor or something for sure. Not so many parts to choose from, and more costly. Rds not quite as low, so they can run warmer with high current loads.
Sometimes Highside can't be avoided, sometimes it can.
CrossRoads:
With lowside, you can use an N-channel MOSFET, logic-level parts (can be driven to full-on from arduino output levels) are easy to find and low cost. Plenty of parts with very low Rds.
With highside, you need P-channel MOSFET, to turn them off you need to take gate to the +voltage you are driving the load with
I still don't understand why N-channels are used for low-side and P channels are used for high-side.
You could use an optocoupler to provide the nessesary gate voltage.
Take a look at this:
http://www.bristolwatch.com/ele/opto_isolated.htm
P 4 and 5
Ít has the alt least 2 advantages as far as i can see: 1. it is simple and 2. there is no physical connection between the load and the Arduino, which might be a good thing reg. noise etc.
Won't the MOSFETS insulation ('insulated gate') provide enough isolation?
I still don't understand why N-channels are used for low-side and P channels are used for high-side.
Because in order to turn on a FET (that is make it conduct) you have to have a voltage difference between the gate and the source of 5V for a logic level FET. For an n-channel the gate should be 5V above the source for a p-channle it should be 5V below the source.
So to make an n-channel FET do this with a high side switch you have to generate a voltage that is 5V higher than the voltage you are trying to switch. Where as if you use a p-channel FET as a high side switch and put the source at 5V then putting the gate to 0V, that is 5V below the source, simply turns it on.
Thanks Grumpy_mike..
CrossRoads:
Need this to Source current into the inductor
Connect ground to arduino ground.I always found it easier to put an N-channel 'below' the inductor to connect to ground, easier to find parts work with.
CrossRoads, other than the MOSFET being a low side switch in your schematic, isn't the circuit you have given similar to my circuit? If that is the case, I don't think it will work because emitter voltage will still only have a 0.7 V difference from the base voltage..
I was trying to arrange that so a High from arduino acted the same in both cases.
I myself would wire it like this and just invert the logic of the control line.
I myself was thinking about doing that and after your post, I've made up my mind. That's the way I'll go. Thanks for all your help.
Would anyone else care to comment on this approach?
Yes it is good.
WooHoo!
