View Full Version : Driving Relays

27-06-2015, 09:53 AM
I have a CSMIO-IP/S and would like to be reassured that the relays on page 138 (the 24v DC ones) http://asp-gb.secure-zone.net/v2/index.jsp?id=1816/2360/9875&lng=en can be driven directly from the outputs of the CSMIO. I am a little wary as some relays can have low coil resistances and the output can only source 250 mA.

I am fairly sure that someone on the forum will have experience of doing this, and would be grateful for the nod.



27-06-2015, 07:03 PM
Hi Rob,

I don't have that control board but I would have thought that you could use the V = IR equation to check. If the relay coil is more than 96 ohms then it shouldn't draw more than 250mA at 24V (R = 24 / 0.250). What is the coil resistance?

27-06-2015, 07:28 PM
Without seeing some proper details for the relays, it's hard to say, however given the physical size and type of relay, I'd say you'll be fine.
I've used similar sized relays from RS, and I'm sure the coil current is well under 100mA. I would check the exact figures, but the RS site is down for maintenance just now :/
Only thing I'd be concerned about, is will you need to add a diode to handle spikes when the relay is tuned off?
(I've never used anything CS-Labs, so you'll need to check the manual)

27-06-2015, 08:15 PM
Ay there's the rub, I can't find the coil resistance, but have since learned that other similar relays have a coil of 750 ohms, so my fears are somewhat allayed. The CSMIO-IP/S has opto isolated VNQ860 as the output stage, and they appear to be diode protected, but I would put a diode local to the relay anyway.

Cheers and thanks for your input on this question of output :)

27-06-2015, 09:14 PM
You'll be fine You'll be lucky if it pulls more than 50ma. I've yet to find 24v Ice cube relay that pulls too much for IP-S outputs.

If your unsure and want to be certain and have Multimeter check the current draw on the bench.!

27-06-2015, 09:44 PM
If you wanted to minimise the current handled by your board you could use solid state relays,

- Nick

28-06-2015, 09:02 AM
Thanks Jazz, I suspected as much.

There is an additional note of caution. I shall use 24v DC relays as the AC versions rely more on the inductance of the coil to limit current. I have recently encountered a relay coil burn out, in a control unit for a furnace, which had been on continuously for at least six months. The coil was energised with 24v DC, but the relay in the socket was an AC version.

Robin Hewitt
28-06-2015, 10:40 AM
The holding current is probably a lot less than the pull in current. To minimise consumption the old trick is to use a capacitor to pass the pull in current with a parallel resistor to pass the holding current.

28-06-2015, 01:30 PM
RS website back online. Omron 24vDC relays take 21.6 mA. So could run 10 on each output.

28-06-2015, 10:53 PM
Just checked, and the Finder relays I used last time (RS part No. 154526), have a 600ohm 24VDC coil , which translates to 40mA.