Output loading at relay

[crash123]

When I measure the unloaded output, it measures 13.9V. When I connect it (and the -) to a ELK912B relay, the voltage measures 9.75V across the + and - terminal of the relay.
It switches the relay OK, but I wasn't expecting the Output10 voltage to drop so much when connected to the relay. The outputs are rated at 50mA and the relay at 30mA.

Does 9.75V indicate I have a problem with something?

 

[icellama21]

When I measure the unloaded output, it measures 13.9V. When I connect it (and the -) to a ELK912B relay, the voltage measures 9.75V across the + and - terminal of the relay.
It switches the relay OK, but I wasn't expecting the Output10 voltage to drop so much when connected to the relay. The outputs are rated at 50mA and the relay at 30mA.

Does 9.75V indicate I have a problem with something?

A voltage drop across a relay is normal. The coil of the relay acts like a resistor and "consumes" energy, or in this case turning it into a magnetic field and heat. The voltage is a measure of potential energy in a circuit, it roughly tells you how much energy is being consumed between two different points.

If your wiring was perfect (the power source, wiring, connections, etc), then the voltage drop at the relay coil would be the same as the unloaded drop(13.9V). Essentially your relay is dropping 9.75V and the rest( 13.9 - 9.75 = 4.15V) is being dropped by the resistance of the wire, connections, power supply(voltage output is definitely not constant with respect to current, the 30mA draw changes the output voltage), etc.

Is this normal for the relay? The voltage dropped across the relay terminals is related to the ratio of relay resistance to wire/power-supply resistance. You can try using larger wires, better connections and a larger power supply(the ELK outputs are only designed for minimal current), and see if the voltage drop increases across the relay terminals. But I'm not sure what use this would be. At 30mA a 9.75V drop at the relay equates to 325 ohms for the relay coil and 138 ohms for the wire/supply. That doesn't sound unusual.

 

[crash123]

So I guess you are saying the outputs are current sources? The manual seems to say they are voltage sources, but it's not entirely clear.

I have a variable power supply (voltage source) and when I set it at 12V, then hook it to the relay, the relay terminals read 12V.

 

[icellama21]

So I guess you are saying the outputs are current sources? The manual seems to say they are voltage sources, but it's not entirely clear.

I have a variable power supply (voltage source) and when I set it at 12V, then hook it to the relay, the relay terminals read 12V.

Yes, most likely, although nothing is really a voltage or a current source. The voltage on any source is only constant(or semi-constant) for a certain current range. That is just the nature of the components used. To guarantee a constant voltage essentially entail tripping a circuit breaker while still in the constant part of the voltage/current curve. Any kind of current limited device is going to have a voltage drop(that is how it limits current) that gets larger as the current increases.

If you want to find the resistance, just load the output with a resistor at the output and check. Otherwise it is your wiring between the m1 and the relay that is causing the voltage drop.

BTW: If you are using an output extender you can try giving it a separate power supply. Sometimes the data bus power/wiring is not enough.

 

[crash123]

After searching a bit, I found an old reply that says connecting an output (output 7-16 anyway) directly to the 912B relays is potentially not reliable.

The replay said: "Strange as it may seem, I don't think the 912 relays are recommended for direct use with the M1 series. The 912 series require a 30mA trigger for the coil. According to some chats with David, he said this could drop
the voltage too close to the minimum 9VDC pull in voltage and may not be reliable. He recommended the 924
sensitive relays, which only need 1.2 mA to for the coil. The M1RB appears to have the same (or similar)
transistor trigger as the 924 sensitive relay. The 924 relays are about $9 each, which makes the M1RB a
better value."

and the link to the thread from 2005: http://www.cocoontech.com/index.php?showtopic=2936

(I assume the David mentioned is Spanky, so there's no better source than that)

Wish I had known this before buying the 912B six pack. I guess there aren't any SPDT ELK relays that are recommended for connecting directly to M1G outputs.

Spanky: do you guys sell a retrofit kit (something like the 924 relay board, minus the relays) that sits between on output (Out7-16) and a 912B relay? (Or is it possible you could sell such a thing?) Sign me up for 6 kits if you do ;o)