# Energizing a relay with a lower coil voltage than my power supply

I'm using a 72 VDC power supply to drive some equipment. I need to drop that voltage down to 48 VDC to drive a relay I'm using.

What are my options to energize the relay coil and what specs do I use from the datasheet?

My initial idea is to do a simple voltage divider. I use the coil resistance from the datasheet (4300 ohm) as my R2 and that gives me an R1 of 2150 ohms (2200 ohm for a common value). As for specifying the power for the resistor it would be 24^2/2200 = 0.3 W ?

Using a resistor will work as relays are fairly tolerant but if someone puts a similar relay in for some reason, the specs may be different. I’d suggest a simple transistor + zener diode regulator. You’ll probably want to use a heatsink as the transistor will probably get to around 30degreesC above ambient (your resistor would get hot as well unless you used a metal clad one and bolted it to a metal panel). The regulator will ensure the relay gets 48V without being dependent on the actual relay specs.

A resistor will work fine. You could also use a 24V 1W zener diode (eg. 1N4749) if the 72V is reasonably well regulated.

That's an odd relay- they appear to use the coil (or maybe a second resistor?) as a voltage divider to derive a voltage for the indicator LED. The reverse voltage waveform across the LED at turn-off might be interesting if they are using part of the coil.

• I'm looking up sample Zener Regulator Circuit Diagrams and I'm confused. Wouldn't I want a 48V Zener instead? Something like the 1N4756A from that same datasheet? RL in this case would be the relay itself for which I know the coil resistance, current, and voltage all from the datasheet? Commented Jan 12, 2022 at 17:52
• The Zener would go in series (like a resistor) and would make the circuit less sensitive to relay coil resistance but more sensitive to variations on the 72V bus compared to using a resistor (+/-5% change in the 72V would represent +/-7.5% change at the relay). It just subtracts 24V from the applied voltage, it's not a regulator. Commented Jan 12, 2022 at 18:09
• If it actually varies that much the resistor would be preferable. But it looks rather well-regulated (0.5%) so if you intend to leave it at 72V then the zener would be a good way to go. Commented Jan 12, 2022 at 18:44
• Just a series resistor will do it. Commented Jan 12, 2022 at 20:04
• I voted for Spehro’s solution - a Zener diode in series - simple and works just that: Voltage Shifter, from 72V to 48V. If @fullyjosh desires to operate at different voltage, select a different Zener. However if the “72V power” is not defined or intends to vary greatly, then Kartman’s solution (48V Zener+resistor biasing an NPN transistor = simple series regulator) might be more flexible.
– EJE
Commented Jan 16, 2022 at 18:55