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FET Driving

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Suraj143

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I need to drive my FET using a 24V supply.It uses an opto to isolate micro side vs relay side.

Is my resistor values ok?
 
What kind of a relay is that? Most 24V relays have coil current 10-20mA. Using IRF540 is ok but huge overkill, even if you are driving a contactor.
2x 10k will split supply voltage when opto is on so Vgs will be in the 12V range (good). IRF540 has input capacitance of ~1nF so 10K values are ok.

note that output current is 24V/(2*10k)=1.2mA. according to datasheet this opto is meant for 20mA drive current. note that min CTR of 50%. so to reliably drive output, you need at least 1.2mA/50% = 2.4mA. According to Fig5, Vf is about 1V when If=2.4mA or similar.
Assuming that output of your micro is 5V, using 1k resistor gives you If= (5V-1V)/1k=4mA which is good (it is between 2.4mA and 20mA).
even if your micro has 3.3V output, (3.3V-1V)/1k= 2.3mA which should still be ok.
 
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Hi panic mode to get 2.4mA current what do I have to do!!

Yes from this relay output I'll drive a contactor.
 
i already explained, your values should be fine. CTR is current transfer ratio. it tels how much of input current you can expect in output of the optocoupler.
if you consider output of the optocoupler to have low Vce (equivalent to "switch on") then current is only limited by pair of 10k resistors. this happens to be some
Ice=(24V-Vce)/(10k + 10k)= approx 1.2mA since Vce is small (0.1V for example).

but in order for this to work you need some current through input (LED side of the optocoupler). this current is If=Ice/CTR. If CTR is 50% (50/100=0.5) then If=2.4mA.
this current is result of voltage at input of your circuit. it is same as output voltage from your MCU - you need to know this. normally you can assume it is same as voltage powering your MCU (actual voltage at MCU power pin).
if you look at attached picture, using KVL you can write Vin=Vr3 + Vf
as mentioned you need to know Vin, supposedly it is 5V
Vf is found from datasheet and it happens to be 1V.

R3 and LED in optocoupler are in series meaning that current is same through both of them (current flowing through R3 has no other way but to go through LED in optocoupler - therefore current through R3 is the same current as current that goes through LED and here I just call it If).

to calculate If we can use Ohms Law:

If=Ir3=Vr3/R3

Vr3=Vin-Vf=5-1=4V
R3=1000 Ohm
If=4V/1000 Ohm = 0.004A = 4mA

in case you change value of R3 to 470 Ohm for example:

If = 4V/470 Ohm = 0.0085A = 8.5mA.

You can set If to value you want by selecting value of R3.

Why would you cascade so many stages to drive contactor? is the contactor coil AC perhaps? you can get one for 24VDC and in that case you don't need interposing relay. Just put contactor coil in place of relay coil.
 
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