# Relay replacement: 150 volt coil down to 12 volt coil

Discussion in 'Electronic Basics' started by [email protected], Mar 7, 2005.

1. ### Guest

I need to replace a broken relay. The form factor is like
the P&B R-10 style.

The coil voltage of the original is 150 volts DC.

The R10's I can find are 12, 24, or 48 volts.

Is there some way to either adapt a 12 or 24 volt relay,
or maybe some simple circuit to lower the voltage just
in this one area of the circuit? Maybe a pwm chip
on a little circuit board or something???

I have found that P&B make a 115V DC version, but it's
going to be pretty expensive and even that isn't as
high as I need.

Suggestions?

Chris

2. ### Robert MonsenGuest

Are you sure the coil of the original is getting 150V across it to
close? Seems excessive, unless the coil is pulling alot of weight.

Digikey has some 110VDC coil relays. That would solve some of your
problem. Then, a power resistor of some kind might be able to do the
rest. I'm not sure if the form factor would be identical.

http://dkc3.digikey.com/PDF/T051/1239.pdf

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.

3. ### John FieldsGuest

---
A month or so ago I was looking for a little buck regulator to run off
of 120V mains, and I didn't have much luck, so if you find yourself in
the same boat, depending on how much current your 150V supply can put
out, you may not _have_ the option of using a resistive voltage
dropper or a linear regulator and going to a lower voltage coil which
requires more current. That is, if the 150 volt relay was running
with the coil dissipating one watt, the current in the coil would be:

P 1W
I = --- = ------ ~ 6.7mA
E 150V

while in a 24V, 1 watt coil it would be 1W/24V ~ 42mA, which would be
about six and a half times more current than the 150W relay needed.
Even if you could get that current out of the supply, the power the
resistor or regulator would have to waste would be:

P = I * (Vsupply - V relay) = 0.042A * (150V - 24V) ~ 5.3W

which is over five times what the relay would be using! It gets
better the higher the relay voltage gets, with a 115V 1 watt relay
needing about 8.7 mA and the dropper wasting only 300mW.

For a 115V 1 watt coil the value of the resistor would be:

Vsupply - Vrelay 150 - 115
R = ------------------ = ----------- ~ 402 ohms
I relay 0.087A

390 ohms and 430 ohms are standard 5% values and either would probably
work, (with the 430 wasting less power, but I wouldn't want to go out
on a limb unless I had the spec's for the particular relay you want to
use in front of me.

4. ### Guest

Yes, it sure is. In fact it may be a little higher than 150VDC.
This is a 70's vintage ham radio transmitter (a Tempo One aka
Yaesu FT-200) which uses mixed solid state and tubes.
Thanks for the link to digikey. I will check it out.

Chris

5. ### Robert MonsenGuest

Is there a lower voltage supply in the box for the solid state
electronics? If so, you could use the 150V signal (divided down with a
voltage divider) to turn on a mosfet, which could then be used to power
the coil of the lower voltage relay. This would be a much better
solution than trying to drop 150V down to 24V, because the divider could
use say 1MEG and 100k, which would give you 14V at the gate with only
20mW required of the 150V supply.

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.