Low Power Signal Relay Won't Activate

Thread Starter

dbm55

Joined Aug 18, 2022
2
Hi,

I'm both new here and not well versed in electronics. I have a Niles SPK-1 speaker switcher that can be activated by a 12V (DC) trigger; I am trying to trigger it using a Marantz SR-7000 Amplifier with a 12V output jack that takes a standard 3mm mono mini plug. The SPK-1 requires 12V @ 100mA (minimum) of current to activate the relay and switch the speakers. Unfortunately, the Marantz 12V signal only carries 5mA. Interestingly when I connect the wire to the SPK-1, the voltage drops to about 1.5V. Question 1 is why does this happen? Does it have to do with the coil resistance in the relay inside the SPK-1?

As a work around, I found a low power signal relay with a coil that operates on 12V @ 4.2mA of current. My plan was to have this low power relay sit in between a 12V 500mA DC adapter and the SPK-1; the adapter would be connected to the normally open circuit pins on the relay. The idea was that when the trigger voltage from the Marantz activated the low power relay coil, the DC adapter circuit would close and causes the SPK-1 to switch the speakers (Note, the DC adapter DOES activate the SPK-1 when connected directly to it). Once again, however, the trigger voltage will not activate the low power relay; and again, as soon as I connect the 12V 5mA wire to the coil, the voltage drops - this time to 6.5V - and this is not enough to activate the relay coil.

Again, as to question 1 above, why does the voltage drop simply by connecting it to a relay coil? And question 2 is how can I get this to work? Do I need a different approach - something like a voltage 'amplifier'?

Thanks in advance!
 

Papabravo

Joined Feb 24, 2006
18,981
Yes. When you connect a source with limited current capability to a fixed impedance then Ohm's law says if two quantities are fixed the third one is determined. The fixed quantities are 5 mA current limit from the source and 1.5V measured across the load with 5mA going through it. which says the load has an impedance of 300Ω, and the +12V source is behaving as if it had an internal resistance of 2.1KΩ.
 

Thread Starter

dbm55

Joined Aug 18, 2022
2
Yes. When you connect a source with limited current capability to a fixed impedance then Ohm's law says if two quantities are fixed the third one is determined. The fixed quantities are 5 mA current limit from the source and 1.5V measured across the load with 5mA going through it. which says the load has an impedance of 300Ω, and the +12V source is behaving as if it had an internal resistance of 2.1KΩ.
So is the answer a non-electro-mechanical relay such as a solid state relay? Do they have less impedance?
 

Papabravo

Joined Feb 24, 2006
18,981
So is the answer a non-electro-mechanical relay such as a solid state relay? Do they have less impedance?
I don't know. Nothing jumps out as an obvious solution. An SSR might be optically coupled are require enough current to turn on an LED.
 
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