reduce AC voltage to match relay nominal voltage

[alonjr]

Hello,

I am trying to solve an AC voltage misalignment between a transformer and a relay.

The transformer steps down from 230V AC to 16V AC (15VA) while the relay bobbin nominal voltage is 12V AC, 90mA and has a resistance of 80Ω.
Given that this relay will not be "activated" frequently for lengthy periods of time (perhaps 3 a week and few seconds each time), is there a simple solution even if it is not efficient and wasteful?
(I cannot change the transformer’s secondary voltage because it drives a device that was designed for 16V AC)

Thank you
Alon

 

[che12345]

Dear Mr. alonjr
"...The transformer steps down from 230V AC to 16V AC (15VA) while the relay bobbin nominal voltage is 12V AC, 90mA and has a resistance of 80Ω. Given that this relay will not be "activated" frequently...".
There are two possible alternatives:
a) use [another transformer] primary 16V and secondary 12V or a little higher to ensure that the voltage maintained at 12V when the relay pulls in.
b) use [resistor bridge] method using a 15W wire-wound resistor total resistance of 17.77 Ohm; with an adjustable tap at 13.32 Ohm. Use an ohm meter to determine the location of 13.32 Ohm. Connect 16V to the 17.77 Ohm and than check the voltage at the 13.32 Ohm tapping. It should be around 12V. Move the tapping to say slightly higher than 12V in order to ensure that the voltage drop is OK during relay pull in. Attention! the resistor is HOT! this is NOT an economical solution unless this circuit is activated for a very short duration and hardly used, as you had stated.
c) refrain from using a [series resistor] method to drop 4V . The [voltage drop would be very high] which would prevent the relay operation during pull in.
These are small toys but nevertheless it is part of electrical engineering.
Che Kuan Yau (Singapore)

 

[RRaghunath]

Dear Mr. Alon,
I don't think anything needs to be done.
You said the relay is actuated infrequently that too, only for a few seconds each time. I don't think the excess voltage can do any harm to the Relay. You can verify the relay literature for its withstand capability.
Moreover, 16V at the transformer secondary is no-load voltage and when the relay draws pickup current (that is typically many times the holding current of 90mA mentioned), the voltage at the relay terminals is likely closer to 12V than 16V. You can verify in the field.

 

[LionelHutz]

I would tend to agree with the above. It's close enough that a couple of seconds maximum should be fine.

Trying it as is would be far easier than sourcing a 17.77ohm resistor.

 

[cranky108]

Check if the transformer has different taps. If not, just let it be, you are fine.

 

[alonjr]

... wow ... what an active forum!

I'll try the "don't do a thing approach" and hope that in my circumstances, it will be good enough.
Thanks so much for all of you who responded.
@che12345, thanks for introducing me to a few new elec. eng. concepts, I had fun researching them.

 

[waross]

First check the current/voltage curve of the relay.
Increase the applied voltage while monitoring the current. The current will be mostly linear with the voltage until the onset of saturation. That is the limiting voltage. Running into saturation will guarantee a rapid failure of the relay coil.
I would leave a 5% or 10% safety margin on the applied voltage. Don't forget that on some days and times, the grid voltage may run high.
Alternatively:
calculate a series resistor. Short the series resistor with a set of normally closed contacts on the relay.
Forget the resistance on AC. Use 12 Volts/90 ma = 133 Ohms impedance. 4 Volts/90 ma = 44.4 Ohms series resistance.
That should give 16 Volts pull-in, 12 Volts hold-in. Check your voltage IRL and trim the resistor if needed to get 12 Volts hold-in.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!