is this simple circuit OK?

[elnaldo]

Hello. I'm fixing an old tape delay machine, and found this simple circuit that doesn't make sense to me. It works, but I don't think it's the best implementation.

It's a 24V solenoid switched with an external foot switch. The switch in my drawing is actually a female connector on the chassis, you need to plug a shorted male plug to activate the solenoid. The solenoid is originally wired to the capacitor and to the chassis.

The capacitor wiring is a hack, a very messy work, so anything in there could be wrong. When the switch is not engaged, the capacitor is floating not connected to chassis ground.

 

[gadget73]

Is the switched side of the cap going to the same AC lead as the diode, or is this the other side of the AC supply?

 

[elnaldo]

Exactly as the drawing. Two AC wires, one to the diode, the other wire to the capacitor "-" and to the switched jack, not connected to chassis. The switch closes the circuit to the chassis, and the solenoid engages. The solenoid wiring is factory original, one end connected to chassis.

Voltage without the solenoid is 35V DC. With the solenoid it drops to 20V DC.

 

[gadget73]

ok that makes more sense. So its just a half wave rectifier setup, and its breaking the path to ground so it opens the load. I would have normally done something like this by breaking the connection between the coil and the power supply positive side but functionally it doesn't really matter.

Maybe consider a flyback diode across the coil just for good practice reasons.

 

[Powertech]

What a nasty little circuit. Whoever designed that needs shooting.

 

[audiotemp]

Does the solenoid have some flyback or snubber built in? Seems like something there would have to give when the current was turned off.

 

[elnaldo]

No flyback diodes, nothing. The solenoid is 40 ohm drawing 0.5A from the 20V supply.

I think the "weird" placement of the switch is related to the switched connector, since that connector is in contact with the chassis.

I could add a flyback diode, and some light load to the DC supply (perhaps 50mA) to reduce the impact (spark) of connecting and disconnecting a 40 ohm (500mA) load.

The solenoid 40 ohm coil:


The device: