Hi,

Making a simple tester for automotive.

R3/R4 are 100K and cause TestLead to have 6V. If TestLead is touched to a ground wire, an Arduino picks up the voltage drop at ADC and starts the test. Q1A is turned on and power flows from +12V through R1 load resistor and Q1A to vehicle ground through TestLead. Arduino monitors voltage at ADC and generates a pass/fail for the ground circuit under test.

if TestLead is touched to a 12V wire then Q1B is turned on and power flows from the 12V circuit at TestLead through R2 load resistor, Q1B to ground. Again voltage is monitored by an Arduino to generate pass/fail.

Purpose of this is to allow rapid testing of power & ground circuits in a vehicle simply by touching a probe to a circuit and waiting for a pass/fail indicator.

Now my question. Is there a problem with Q1A having positive voltage on the source pin? Normally 6V low current via R3/R4, but also 12V whenever TestLead is connected to a voltage source in the vehicle. Q1A would never have a gate voltage applied when source has positive voltage, only when TestLead is grounded. I plan on using logic level MOSFETs.

Thanks

 

No, provided that the gate will withstand -12V. (Most will withstand -20V).
Bear in mind that current will flow through the source-drain diode if it gets forward biased.

 

No, provided that the gate will withstand -12V. (Most will withstand -20V).
Bear in mind that current will flow through the source-drain diode if it gets forward biased.

Thanks. My MOSFET has a gate-source voltage of +/- 20V so looks like I’m OK. No worries about the diode getting biased as worse case scenario is the source has the same voltage as the drain (12V).