I'm having trouble including a switch in a circuit that will be turned on and off via a PIC microcontroller (3.3v). The circuit that it will be switching can go up to 24v and around 20amps. This has been easy to do with a transistor (N-Channel MOSFET) and works very nicely when I put the load before the switch.

The problem is that this switch will control an external circuit where the load could be anywhere. Having the load after the switch causes a voltage drop of around 10%, something I need to get around. Is there a way of getting consistent voltage and current on the second circuit before and after the switch? It doesn't have to be a MOSFET.

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    \$\begingroup\$ Is a MOSFAT a heavy duty MOSFET? You need to supply a schematic so we know what you're talking about. There's a button on the editor toolbar. \$\endgroup\$ – Transistor Sep 26 '16 at 17:47
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    \$\begingroup\$ It has a phat junction... sp. err. corrected \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 26 '16 at 19:03
  • \$\begingroup\$ They're primarily used in phat sounding bass amplifiers. \$\endgroup\$ – Ian Bland Sep 26 '16 at 20:17

I would use a mechanical relay - provides full isolation between the microcontroller and the switched circuit, so you can place the relay contacts anywhere you wish in the switched circuit.

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simulate this circuit – Schematic created using CircuitLab

  • Here we have a regulated "voltage source (low) power supply with a known drop in voltage due to a rise in current due to internal resistance for example, shown as 0.01 Ohm.

We call this the effective series resistance (ESR) or the output impedance Zout.

  • An ideal switch with a 1 Ohm load will drop 1% due to resistor divider.
  • If the RdsOn of the switch is equal to, or higher than the Zout from the supply, then Voltage drop will increase even more.

We call this Load Regulation error and it equal to the ratio of source to (load+source) impedance, like any voltage divider. Therefore it is a good idea to choose the switch according to the % load regulation you need.

One must know the switched and steady currents of the load to determine the effect R values of the load. The FET or relay contact resistance can be selected based on load current rating.

Normally a much higher current rating in FETs is chosen to minimize the resistance if you want good load regulation.

  • A "High Side" switch is shown needs a low Vg to switch on thus a P-ch MOSFET is used.
  • If using "Low-side" switch on Gnd to load then V+ , we use an N-ch switch when there is only a single supply.

  • Distributors carry both for automotive and industrial use in high power switch applications.

  • For 24V,20A you may choose a 30A relay or a 60A MOSFET depending on load inductance, it may need further margin derating.


  • specs: 62A 30V , 10.5 mOhm @ 7A, 10V ($0.91/1pc)
  • SMD only
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