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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.
For the newer 65v LDMOS amplifiers, I added a
higher voltage version of this switch
to the parts page, capable of handling up to 80v at 50 amps (this switch uses a
100v device).
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). This port is typically handled by the "kill" function of a control board, which can signal an immediate overriding shutdown during a system fault condition. It does this by pulling the port low.
Another application using this extra port is the operation of a remote LNA and it's bypass relay, which are typically energized by default. Connecting the "on" port to ground, and then the "disable" port to event 1 of a sequencer or control board will allow the LNA to remain on during receive, and then bypassed during the transmit cycle.
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
58v. 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.
Note: The first diagram shows hardware for switches supplied with a solder lug (shipped before 4/2015). The second diagram shows hardware for switches supplied with a wire lug; the change in mounting hardware was made after a longer insulating shoulder washer was found to be available, making the switch easier to mount without accidental shorting of the output tab.
The last diagram listed here is the most current one, and the one which is shipping with all FET switches beginning in September (2019). The main change is the insulator, which has been changed to an aluminum oxide ceramic type, providing better heat transfer and scratch resistance than the mica insulator previously used.
Below are the most current mounting instructions:
