Electronic circuit question

[Ira]

I'm trying to use the contact side of a SPST relay for two purposes. One is to use as a NO or NC (no EOL resistor) input zone on an Elk M1G. The other is to use as input to a data logger. So each terminal of the relay contact would have one wire from the data logger and one wire from the M1G zone attached to it. The data logger can't handle any "outside" power on it's circuit. The diagram below supposedly shows how a diode can be used to keep voltage from getting to the data logger, but I don't fully understand it.

It looks like my setup is most like the "middle" option on the right side. I will effectively be connecting the data logger wires to the zone wires via the relay contact terminals. When the relay closes, the circuit will be complete.

So how do I know what "type" and "size" diode to use? I measure 13.7Vdc and 7.00mA on the M1G zone. I don't know yet if the data logger introduces any power on its wires. If it does, it's got to be a very small amount. The thing runs off a 3-Volt CR-2032 lithium battery that supposedly lasts eight months when used on a continuously closed relay (which is its highest battery draw state).

Thanks,
Ira

 

[fcwilt]

I don't understand what you are trying to do.

Where does the relay fit into the picture? What is the purpose of the relay? What sort of connection does that data logger have?

 

[russban]

A DPST relay would be better to isolate the 2 circuits.

 

[Ira]

I don't understand what you are trying to do.

Where does the relay fit into the picture? What is the purpose of the relay? What sort of connection does that data logger have?

As an example, I have a wire running from one of my standby genset's lugs to one side of the relay coil, and a wire running from the neutral lug to the other side of the relay coil. So when the genset is running, the relay coil is energized and the relay contacts are closed. When the relay contacts close (i.e., genset is running), I want the data logger to log the event, and I want a zone on my M1G to show as "violated" so it can call out, etc.

Referring to the OP picture, the data logger has simple 1/8" stereo jack that plugs into the data logger. One of the three wires from the jack isn't used, so it isn't shown in the picture. The other two wires connect to the two terminals on the contacts side of the relay. The M1G zone hooks up the same way. Everything works fine if I use two SPST relays (one for the logger and one for the M1G) with their coils wired in parallel.

A DPST relay would be better to isolate the 2 circuits.

I know. The SPST relay I am using is a device that is all built on a 1"x4" circuit board (the boards are track mounted), and includes an LED that lights up when the coil is energized, and an onboard switch that allows me to open/close the contacts manually for testing. The manufacturer doesn't offer a DPST or DPDT relay with the switch. I already have everything working the way I want using six SPST relays (three for data loggers and three for M1G zones). If I can get a single relay to do the work of a pair (handle the data logger and the M1G zone), then I can use half the relays in the enclosure for something else I want to do.

Ira

 

[fcwilt]

OK that is all clear now.

What sort of trigger does the data logger take? Do you short the wires or apply a voltage?

Thanks.

 

[fcwilt]

Hmm...

Do you have any relay outputs from the ELK that are free - you need 3 is that correct?

 

[Ira]

OK that is all clear now.

What sort of trigger does the data logger take? Do you short the wires or apply a voltage?

Thanks.

It just needs to have the wires shorted (relay closed). I don't measure any voltage across the relay contacts when the data logger is connected and the relay is closed, unlike when an M1G zone is connected across the relay contacts and I see 13.7Vdc and 7mA.

Hmm...

Do you have relay outouts from the ELK that is free - you need 3 is that correct?

I know where you are going, and it would work. I could connect the M1G zones to three of the relays, connect the data logger wires to three relays on an M1G board, and open/close the M1G relays based on the changes in zone states (i.e., the "external" relays). The main reason for not doing it this way is to keep the two "systems" independent, i.e., the data logger doesn't have to depend on the M1G doing its job correctly.

Ira

 

[shtojo]

A standard 1N4001 diode would be fine (avail at radio shack, etc). The datasheet for that data logger says to not connect anything else but a contact-closure to the input (and you would have the elk connected also), but it would probably be fine with the diode in place. The diode will prevent current from flowing into the data-logger, but will not prevent any current from the data logger from flowing out (probably a pullup resistor in the datalogger) and into the elk zone contact when the your relay is open. And depending on how the elk measures the zone inputs, this could alter the analog zone input value. So you may want a diode on both the data-logger connection and also on the elk connection to the relay. Then, when the relay closes, both the logger and the elk will be grounded (minus the .6v drop across the diode, but the elk will still see this as a closed contact because the analog range would be far below the normal value). You would need your EOL resistor connected before the diode (on the elk side, not on the relay side). Anyway, those are my thoughts, others can chime in with better ideas. Obviously you could add more circuitry, replace the relay, use a transistor, feed off the coil voltage (run a second small SPST relay in parallel with your other relay) etc... but trying to keep it as simple as possible. Good luck.

 

[fcwilt]

Yes a pair of diodes would likely work.