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I'm trying to think of a way to switch negative 24v 5amps without using a relay with a big footprint.

All HE* are controlled using negative PWM by the microcontroller and I need to switch it over to HE-OUTPUT that will take over control of the negative PWM that is being controlled by another board that i'm communicating with when required via microcontroller the maximum current will be 5AMP at 24V.

So basically I need to switch the negative wire from my board to another board when required by my microcontroller.

HE-OUTPUT can only accept 1 HE*

HE1 ---------> HE-OUTPUT

HE2 ---------> HE-OUTPUT

HE3 ---------> HE-OUTPUT

HE4 ---------> HE-OUTPUT

Ricky
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1 Answers1

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So basically I need to switch the negative wire from my board to another board when required by my microcontroller.

Use a positive to negative level switch like this: -

enter image description here

When the logic level is high, the load is disengaged from -24 volts. When the logic level is low, the low is connected to -24 volts.

Image stolen and altered from here (a similar question too).

Andy aka
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  • Why do i need to add in a PNP (Q1) to drive the mosfet? can i not just connect it direct to the logic input and pull it LOW? and then I can activate it by send a HIGH from the logic input? – Ricky Sep 23 '22 at 15:29
  • (1) The MOSFET source is at a -24 volts. (2) to activate the MOSFET the gate would nominally be around -22 volts to -12 volts. (3) to deactivate the MOSFET the gate ought to be close to -24 volts. In those scenarios, can what I've said be achieved by your proposed connection given that the logic levels probably range from 0 volts (logic 0) to +5 volts (logic 1) @Ricky – Andy aka Sep 23 '22 at 15:38
  • Thanks for the quick reply @Andy, Yes that correct that will be 0 volts (logic 0) to +5 volts (logic 1) so that will also work if i bypass the PNP? – Ricky Sep 23 '22 at 15:50
  • No, it won't work. The logic levels (0 volts and 5 volts) are totally incompatible with the levels to activate deactivate the MOSFET. That was what my comment was saying. Hence, you need to level shift using the PNP transistor @Ricky – Andy aka Sep 23 '22 at 15:56
  • I was planning on using the following MOSFET IRFZ44N on that data sheet it says Gate Threshold Voltage: Min 2V Max 4V this is why i'm confused maybe it my lack of understanding – Ricky Sep 23 '22 at 16:03
  • I can't explain why you are confused. I thought I'd been as clear as possible; the MOSFET gate/source control range midpoint is centred around -18 volts. Your logic output midpoint is +2.5 volts. Hence total incompatibility without a level shifter. – Andy aka Sep 23 '22 at 16:09