The AM Forum

I checked out DMOD's suggestion for an inrush protector and noted the NTE 4996 was recommended but is one or two used across the HV primary? The diagram looks like there are two or am I mistaken?  I've never used the circuit before so this is something new to me.  There is one spot which I have marked where the leads in the diagram cross and I assume there is no connection at that point.

The symbol in question is LTSpice's symbol for a Bi-Directional Transient Suppressor, a single component. It is essentially two zeners back-to-back.

If you are using this in a 120V circuit you may want to use an NTE4983 or similar. I use them extensively to kill power transients and to extinguish relay contact arcing.

The schematic shows two, 10 Amp relay contacts in parallel.

LTSpice has a problem making connection "dots" in some cases. So the only wires that are not connected are those that cross with dashed lines.

I.E., any line that is solid and intersects another line is a connection.

Phil - AC0OB

I'm using 120 volts.  So the NTE 4983 is a single componant?  Actually Radio Shack has them on line so I can order from the local store.  I imagine it doesn't matter which side of the diode the band is on if it's bi-directional.  Yeah, two contacts in parallel makes a lot more sense.  Thanks for the info!

There are no bands on the Bi-Directional Clamping diodes.

Darned Radio Shack!  That's as bad as taking one photo for several similar products on eBay and not showing you what you really will get if you buy one!  :-\

Noting that R1 and F1 would pass current regardless of whether or not the relay was closed, I assume the actual plate on/off switch would be between the mains and the relay?  I think this is the circuit I need however what is the purpose of the 3 amp slow blow fuse?  I also assume there should be a second fuse between the plate switch and the relay contacts?

The power switch, fuses or circuit breakers are feeding the circuit from the left side.

Initially, the inrush relay contacts are open.

When 120VAC is applied, AC current goes through the resistor and the primary voltage on the HV transformer(s) starts to rise.

When the primary voltage rises to a full 120VAC in this case, the relay closes and applies full voltage to the transformer primaries. All current flows directly to the HV primaries and the 10 ohm resistor is shunted out of the circuit

The fuse is there to limit current on start-up. If a short existed somewhere and the current is sustained above 3 Amps, the fuse would blow and the relay would never close.

Upon turn-off, the inrush relay contacts open. The collapse of the magnetic field from the transformer primarie(s) creates a reverse voltage - HV "kick." Any voltage above 220V is suppressed by the transient suppressor.

Phil - AC0OB

Very good!  Thanks for the info on how the circuit works.  I was a bit vague about that.  Mike - W9ZSL