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Hello StackExchange,

I'm trying to understand this low side output circuit so that I can interface it with a load.

Lowside Output Circuit

I will have to add another MOSFET/Relay/Driver of some description on the lowside output given that the ECU this is from runs on 12V and the lowside outputs are only rated to 24V. However the load is an 60V 1A Fan.

What is stopping the lowside load just grounding via the 110nF Capacitor when the IRLR3636 MOSFET is off?

Below is another schematic of a lowside driver from the same ECU Series, different model and they still have the Capacitor to Ground (albeit 10nF)

ECU Lowside Driver

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  • \$\begingroup\$ "What is stopping the lowside load just grounding via the 110nF Capacitor when the IRLR3636 MOSFET is off?" - Capacitors block DC. \$\endgroup\$ Commented Dec 27, 2020 at 8:39

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What is stopping the lowside load just grounding via the 110nF Capacitor when the IRLR3636 MOSFET is off?

Nothing. The load will still be 'grounded' - until the capacitor charges up through the load. If the load has low resistance then it won't take long for the capacitor to charge.

If there is no load then the capacitor will only charge up (slowly, through the 300k resistor) to the voltage at the junction of the 430k and 47k resistors, which is ~0.5 V. Therefore you will need to put a resistor in place of the load to pull the output up to the supply voltage.

The 'dummy load' resistance required depends on how much drive current your 'MOSFET/Relay/Driver' needs. A 1k Ω resistor would supply up to 1 mA with voltage drop of 1 V or less.

Remembering that voltage goes down when the ECU output is turned on, you may need an inverter to turn on your MOSFET at the correct time. This could be done with a bipolar transistor like this:-

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Hey Bruce, whilst you were doing that I simmed my own version here: imgur.com/a/pJQwQqf the difference being I didn't use a voltage divder on my BJT Driver, Is that an issue? and what was the purpose of your 12V Zener? \$\endgroup\$ Commented Dec 27, 2020 at 11:53
  • \$\begingroup\$ also the LED's in my simulation are just for visual indication of circuit on/off, not all of them are properly current limited. \$\endgroup\$ Commented Dec 27, 2020 at 12:02

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