# Driving a 24V isolated load from a 3.3V MCU

I am looking to drive a 12-24V purely resistive load from a 3.3V MCU. I would like to use an opto-isolator to separate the circuits electrically. I've drawn up the following circuit, but I'm not sure about exactly how to simulate it in CircuitLab. Can someone please tell me if the following circuit will work for my needs or what changes I need to consider?

My concern with driving the MOSFET is that its Vgs max voltage is 20V, so I've put a voltage divider to limit it to ~10V when the opto-isolator turns on. I know I can use a MOSFET with a higher Vgs voltage, but I couldn't find any with the package size of the FDMC0310AS.

EDITED:

Original Image:

Modified Circuit:

• You should omit R13, in parallel with the opto's LED - it just adds an extra load on the MCU output. – Peter Bennett Oct 4 '15 at 18:26
• Re: "I'm not sure about exactly how to simulate it in circuitlab" I'd say forget about that websim and try any of the free SPICE-based simulators that run on your own computer. Almost all big IC manufacturers give a SPICE-based simulator for free now. – Fizz Oct 4 '15 at 21:52
• FYI, here is a fairly introductory (but not just utterly basic) note to using MOSFETs. There is actually no small number of more advanced documents on the web, but they usually are overwhelming for the beginner. A good 2nd read is radio-sensors.se/download/gate-driver2.pdf (That one was a TI note/seminar SLUP169 at one point, but it seems it's not on their site anymore for some reason). Hope this helps. – Fizz Oct 7 '15 at 8:10

This is mostly like your circuit, but the center of the voltage divider is being driven instead of the end, which will drive the load (R4) with nice snappy edges.

Be aware that when the MCU's output is low the load will be hot, so in order to start with the load cold the MCU's output must be high on power-up.

and here's the LTspice circuit list if you want to play with the circuit.

Version 4
SHEET 1 1576 736
WIRE 656 0 560 0
WIRE 784 0 656 0
WIRE 656 32 656 0
WIRE 784 32 784 0
WIRE 784 144 784 112
WIRE 128 208 96 208
WIRE 256 208 208 208
WIRE 656 208 656 112
WIRE 656 208 448 208
WIRE 656 224 656 208
WIRE 736 224 656 224
WIRE 96 304 96 208
WIRE 256 304 192 304
WIRE 480 304 448 304
WIRE 560 304 560 0
WIRE 656 304 656 224
WIRE 96 416 96 384
WIRE 192 416 192 304
WIRE 192 416 96 416
WIRE 288 416 192 416
WIRE 480 416 480 304
WIRE 480 416 368 416
WIRE 496 416 480 416
WIRE 560 416 560 384
WIRE 560 416 496 416
WIRE 656 416 656 384
WIRE 656 416 560 416
WIRE 784 416 784 240
WIRE 784 416 656 416
WIRE 496 464 496 416
FLAG 496 464 0
SYMBOL nmos 736 144 R0
SYMATTR InstName Q1
SYMATTR Value FDMC8462
SYMBOL voltage 560 288 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V2
SYMATTR Value 24
SYMBOL voltage 96 288 R0
WINDOW 3 24 96 Invisible 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value PULSE(3.3 0 100u0 1u 1u 100u 500u)
SYMBOL res 768 16 R0
SYMATTR InstName R4
SYMATTR Value 2.4
SYMBOL res 224 192 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R1
SYMATTR Value 75
SYMBOL res 640 288 R0
SYMATTR InstName R3
SYMATTR Value 1000
SYMBOL res 640 16 R0
SYMATTR InstName R2
SYMATTR Value 1000
SYMBOL Optos\\PC817A 352 256 R0
SYMATTR InstName U1
SYMBOL res 384 400 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName Rsim
SYMATTR Value 1G
TEXT 502 440 Left 2 !.tran 1000u uic

• And again, charging the gate with high resistance. And discharging unknown cpapacitance with unknown current through optocoupler. Right, burn both of them, why only risk one component? – Gregory Kornblum Oct 5 '15 at 15:21
• I see. Playing fast and loose is better than perusing the data sheets and running the simulation? – EM Fields Oct 5 '15 at 15:40
• Whatever (15 characters) – Gregory Kornblum Oct 5 '15 at 15:57
• Kudos for putting this into a sim (although really that's the OP's job for a circuit of this level). I'm curious if it's you who -1'd me or someone else (Gregory). I'd be glad to take on board any criticism regarding what I said; especially from someone more experienced, as you are. – Fizz Oct 6 '15 at 2:33
• @RespawnedFluff: You can determine the opto's dynamic CE resistance by looking at the slope of voltage across the CE versus the current through it and then doing $R = \frac{E}{I}$. I agree that the opto seems to be the main damping element, and the best way to minimize the damping - I believe - is to use an opto with a high CTR and a low CE ON resistance at the desired working current. – EM Fields Oct 6 '15 at 4:39

You have to use isolated gate driver. It will drive your gate with high current, it's healthier for the MOSFET. And in addition i recommend isolated power supply, 3.3v to 15v. Google recom or traco for power supply and isodrivers are really good at silabs or avago, vishay and fairchld are nice too.

• Why does he have to use an isolated base driver and why can't he use the 24 volt supply for both the load and gate supply? – EM Fields Oct 4 '15 at 19:34
• He can do anything. But if he wants to avoid putting MOSFET to linear operating, he has to drive the gate without serial resistance (or with low resistance which cannot be voltage divider on 24V). Of course, he could split isodriver to optocoupler and non-isolated driver. It's just a little more complicated. 24V he can't use because most MOSFETs absolute maximum rating is 20V on Vgs. – Gregory Kornblum Oct 5 '15 at 4:38
• In general, there are clean universal ways for such circuits and there are amateur patches. Most differences are discovered during debugging and second version design. – Gregory Kornblum Oct 5 '15 at 4:43
• Wow, somebody votes from his heart. Should better design several devices. – Gregory Kornblum Oct 5 '15 at 9:46
• In fact, i could even design, produce and ship a board. But i respect the guy enough, and i do believe he can find anything he needs online, and one hour research will only contribute his knowledge and experience. – Gregory Kornblum Oct 5 '15 at 14:11