DIAGRAM or SCHEMATIC needed 12 volt 172 ampere NPN MOSFET using an opto coupler

Will switch NEGATIVE power wire of 13-ampere automotive radiator fan. Long strings of words are confusing. Does a diagram exist to do this? I don't need heatsink advice merely a diagram or schematic to wire a circuit board with correct resistor values and correct capacitor values. I assume a single 470Ω would be enough to bias input of 12 volt signal for the opto coupler.

Thank You From Mexico

If you mean an N-channel MOSFET in the low voltage (ground side) of the fan, connect is source to ground, drain to the fan and gate to source through a 10K resistor. The connect the optocoupler emitter to the gate and it''s collector throgh a 1K resistor to +12V. No capacitor is needed. It might be useful to connect a diode across the fan, cathode to +12V side to trap back EMF but that depends on the fan type which you haven't specified.

For the input (LED) side of the optocoupler, choose a resistor that lets the manufacturers recommended current flow though it, again with no type or specification it's difficult to advise what value to use.

Why do you need a 12V 172A MOSFET?. This must be a huge fan! In a normal automotive system, a fan drawing 172A would flatten the battery in minutes.

Brian.

The fan draws 13A when it is running but draws 130A to 172A when it starts and when he sticks his foot in the blades.

Ah!, a special version for prospective amputees......

What they really want is a MOSFET rated at say 13A+50% = ~20A and maybe 24V for safety. Most would handle the stall/start-up current for a short period and after that, if their foot is still stuck, the fuse should blow.

Brian.

betwixt said:

Ah!, a special version for prospective amputees......

What they really want is a MOSFET rated at say 13A+50% = ~20A and maybe 24V for safety. Most would handle the stall/start-up current for a short period and after that, if their foot is still stuck, the fuse should blow.

Brian.

Click to expand...

My life with that fan, aluminum cylinder heads and an un-upgradable single-row radiator, is ruled by

Terror

Cylinder heads for this engine or parts down here are unavailable. I have had three Bosch type relays fail on me. I opened all three and found a mound of green powder and no parts inside. By sheer luck of the draw I had managed to gaze at the temperature gauge before it pegged-out.

The circuit is going to be entombed in a salvaged Ford ECU computer case. Large finned aluminum. I am going to use 18-volt MOVS + the MR2535 avalanche rectifiers plus a hyperfast free-wheeling diode across the circuit input to chassis negative. I will bond the NPN to the case with Arctic silver adhesive.

An easy-to-build 210F audio temperature alarm circuit would also be nice. Ding-Dong chime so alarm doesn't cause heart failure.

Many thanks. I have adequate experience with auto electrical (built and re-built ten thousand plus alternators with my own hands). But I have been retired long before MOSFETS came out.

Understood.

The basic rule when considering MOSFETs as simple switches is to treat them as resistors that change from a very high resistance to very low resistance as you lift the gate to source voltage high enough. In general, you can lift the gate up to and beyond the 12V supply and it will draw negligible current but check the actual maximum voltage on the data sheet. The heat you dissipate will be almost entirely due to the (I^2)*R where I is the fan current and R is the resistance between the drain and source pins with the MOSFET fully conducting (Rds on the data sheet).

Many modern MOSFETs have an Rds of 0.1 Ohm or lower so the power loss can be quite small and certainly much lower than you would see with a bipolar transistor. For example, if the fan draws 13A, which is quite high by the way, a MOSFET with 0.1 Ohm Rds would dissipate only about 17W. The fan start-up and stall current (when your foot is stuck in it!) will obviously be much higher but if you look at the specifications for MOSFETS you will see they usually have quite high peak current ratings so they should survive brief surges. It would still be a wise move to fit a fuse in the system though. A prolonged stall period would probably damage the fan itself, regardless of how you control it.

Brian.