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  1. crutschow

    Need a new multimeter - any thoughts

    One good thing is to have automatic power off so you don't inadvertently run down the battery. I'm not sure how many of the cheap meters have this feature, but my free little Harbor Freight one doesn't, so the battery goes if I forget to turn it off.
  2. crutschow

    MOSFET 12VDC Relay?

    What issues? What is a "cobbled together" circuit as compared to a regular built circuit?
  3. crutschow

    MOSFET 12VDC Relay?

    The TS wants to use his switches in there original configuration, where a plus voltage from the switch activates the relay, which is why I added the transistor to provide that polarity operation.
  4. crutschow

    Rectified 24VAC to DC value?

    Note that in a real rectifier circuit, you have to subtract the forward drop of the rectifier (one for half-wave and full-wave, two for a bridge) from the calculated AC peak.
  5. crutschow

    MOSFET 12VDC Relay?

    Yes. It's determined by the input current, not the voltage. So you select the input resistor to give the desired input current. The resistance would be the signal voltage minus the diode input voltage (typically about 1.3V) divided by the desired current. But you only need the input current...
  6. crutschow

    MOSFET 12VDC Relay?

    Here's the LTspice simulation of the circuit with an opto:
  7. crutschow

    MOSFET 12VDC Relay?

    With the optocoupler, you likely don't need Q7. Just connect the opto output emitter (pin 3) to ground and the collector (pin 4) to M1's gate (with R1 of course). But depending upon the gain the of opto, you may need to reduce the value of R2 or increase the value of R1, to fully saturate the...
  8. crutschow

    MOSFET 12VDC Relay?

    I forgot to include a protection diode in case the load has some inductance to absorb any negative transient when it is turned off. Revised schematic below:
  9. crutschow

    MOSFET 12VDC Relay?

    That's fine. Just make sure the epoxy is compatible with electronic parts and won't corrode them.
  10. crutschow

    MOSFET 12VDC Relay?

    Yes, R3 limits the base current, which is required for a BJT. R4 is mainly to absorb any base leakage current and insure it stays fully off, especially if the device experiences higher temperatures.
  11. crutschow

    MOSFET 12VDC Relay?

    To turn the P-MOSFET ON when the control signal is high, you can add an NPN driver (example below). The MOSFET has negligible current leakage when off (<1µA).
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