How to use a timer relay instead of a time delay relay for soft start / inrush

[Opcom]

I got a few new-looking US-made Octal-base timer relays in a nice box of junk. At first I thought they were 1-second delay relays that would be good for soft start, but upon checkout it was found out that they have SPDT relay contacts that switch as soon as power is applied, then switch back after 1 second.

Because the normal state at the beginning can't be avoided, relay logic is used to avoid a state where the inrush resistor is bypassed before the power supply has charged, but no extra relays are needed. The only dependence is that the timer NC contact opens before the plate contactor closes.

This is not a special circuit, just a way to make use of a high quality free part that would otherwise be unsuitable. It does not replace a station sequencer, just a very simple soft start addition to the transmitter.

The circuit uses 120VAC relay coils because that is what is already built into the transmitter on-hand and the timers are 120VAC. A couple of DIN-mount octal sockets should not cost much and there is room on the rail.

[WQ9E]

An interesting setup Patrick and a good way to use an unusual part! 

I would seriously consider putting a fuse in series with your slow start resistive element to prevent problems if something fails and the shorting contactor across it fails to close.  With slow start circuits I use an extra set of contacts on the slow start relay that are in series with the PTT line so that the amplifier cannot be put into transmit condition if the slow start circuit doesn't bypass.  I have seen a commercially built amp with some ugly carnage in the power supply area when the relay failed and heavy current was drawn through the inrush resistor when normal operation was attempted, some flaming occurred a bits of resistor were blown throughout the amp.

[w4bfs]

good snuff .... don't forget the hv off nc pb as well .... first thing in the 'hot' leg

[Opcom]

I'm glad those things were mentioned. Been using those 500-600W conical heater elements for inrush in the TX so there has been no need to protect them. It could benefit from a single "OFF" push button for emergency as there is a convoluted sequence of many switches for turning it on and off. Very manual. I'm using MV rectifiers and the unit has no built in warmup delays so I would not want that switch action to be able to turn the unit back on for just any idiot.

[AB2EZ]

When I added a soft start circuit to the plate supply of my KW-1, I was concerned about a hypothetical failure of the series resistor bypass relay to close... after the 0.25 second soft start delay.

I used a thermal fuse (less than $1.00 each), wired in series with the coil of the main plate relay.

The fuse itself (about the size and shape of a 1/2 watt resistor) was firmly attached (body-to-body) to the body of the soft start, series, power resistor... with a nylon cable tie.

If (hypothetically) the series resistor bypass relay didn't close... then after a few seconds, the resistor would heat up, and cause the thermal fuse to open. This, in turn, would interrupt the current in the coil of the main plate relay... and cause the main plate relay to open. The main plate relay would remain inoperative until the problem was fixed, and the thermal fuse was replaced.

I tested this by slipping a piece of paper between the contacts of the bypass relay. After about 5 seconds, the thermal fuse opened up.

I believe that the thermal fuse I used was rated to open at 100C.

Of note, the thermal fuse's steady state current rating did not have to be very high, because it was in electrically in series with the coil of the main plate relay.... which should be drawing much less than 1A.

Stu