[Amps] AC-DC Relay Coils

[Will Matney]

Since what come on TV at 9:00 tonight, I have better time to spend than listen to that BS, so I figured I'd do the post I promised about the voltage interchangability of AC and DC relay coils.

From; Struthers & Dunn Relay Engineering, 1950

For midget type 1 relay assemblies (Like antenna relays, P&B KUP Series, 10 amp contacts, 1, 2, pole);

AC Volts = 115
Armature open current = 0.09 amps
Armature closed current = 0.057 amps
60 Hz impedance = 2037 ohms, P&B is 2250 ohms for 120 VAC, P&B 3PDT for 120 VAC is 1700 ohms
Coil mark = 50A
Number of turns = 5000, 38 ga.
DC Volts = 35
DC current = 0.058 amps
DC resistance = 605 ohms
Core dia. = 3/8"

For For industrial type 1 relay assemblies (Large open frame clapper type, 25-30 amp contacts, like Magnacraft 300 and 389 series, 1, 2, pole);

AC Volts = 115
Armature open current = 0.14 amps
Armature closed current = 0.08 amps
60 Hz impedance = 1437 ohms
Coil mark = D
Number of turns = 4200, 32 ga.
DC Volts = 24
DC current = 0.13 amps
DC resistance = 180 ohms
Core dia. = 1/2"

Acceptable variance in resistance (tolerance) is 7-1/2%

For me, I would start at say 15 to 20 VDC and go up until the relay closes normally. The hold in voltage is not near as great as the pull in voltage. I remember someone saying they was operating 115 VAC relays on a higher DC voltage around 24 or more volts DC I think on here. Between the two styles above, we have a range between 24 to 35 VDC for normal operating voltage. Anyhow, once it looks to be pulling in at the right speed, check the coil to see if it's getting hot. Without the core having the shading coil, I would want the DC supply to have a good filter cap so a steady DC current would be fed to it. Without a capacitor, it will probably buzz. Also, make sure to put a diode across the relay coil to kill the spike.

Best,

Will

Amateur Transformer