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High Current DC Switch

Mechanical relays used to be the way to switch high currents; these days, we have a whole class of FETs available to do that job.

I use this small board to gate the power supply current to one of my solid-state amplifiers, but it can be used as a gate for almost anything requiring the switching of DC currents up to 100 amps.

With the FET shown here, this board is set up to switch 28 volts at up to 30 amps, and at that load, will drop only half a volt across the FET. When used to gate the power to one of the 23cm 150w amplifiers (10A or so), the loss across the switch is only about 2 tenths of a volt.

Of course, with minor component changes on the board, and the selection of a different FET, the switching of voltages and currents much higher than that can be achieved. Alternatively, additional FETs can be connected in parallel for higher currents, each one sharing the board connections. Configured like this, the FETs must be identical types, preferably from the same lot number.
To operate the switch, all that is required is grounding the 'on' port. Current at this port is only 5ma. Un-grounding this port turns the switch back off.

When used with a sequencer or an amplifier control board, this port should be connected to event 2 (so that the amplifier is switched on after the antenna relays have been switched at event 1).

I also placed an extra port on the board to allow the switch to be disabled by an emergency signal (the 'disable' port). The use of this port is optional; I added it only to accommodate an older control board design on one of my amplifiers. However, it can be useful in an alternate configuration.

That alternate configuration can operate the switch with a TTL signal at the disable port; by permanently grounding the 'on' port, a TTL high at the disable port (open circuit) will turn on the switch. A TTL low (sinking 5ma to ground) will shut it off.

The table below the schematic lists the correct R5 values for 12v or 28v operation. Values for 2 different FETs are listed. The voltages shown are approximate ranges, and the ranges can overlap a bit. For example, the 12v configuration would be OK for 9 to 20v, and the 28v values would work well from 20 to about 36v.

The kit offered on the parts page (rev 3) is an upgrade to the one shown in the photo above, and can be set up for 12, 28 or 48 volts. The 48v optimization has a range of about 35 to at least 55v. The setup table for this version is shown below the schematic:

The FET can be heat-sinked to a chassis surface as shown in the first photo above. Alternatively, the board and FET can be located on the heat sink of the amplifier it controls. If you are building this project from a kit I supplied, please see the FET mounting instructions below.