Can a relay coil be a ballast resistor?

[jasonallenh]

Subject is self explanatory... I have an spst illuminated switch that I'm using to turn on a relay, and the LED will need a ballast resistor value that is less than the coil's resistance. I'd simply connect them in series and the spst switch would complete the circuit. I can't think of anything wrong with it, but I'm not an EE either

 

[Audio1Man]

Not enough information. What is AC, DC , Voltage, Current, Switch/lamp data?

 

[jasonallenh]

Not enough information. What is AC, DC , Voltage, Current, Switch/lamp data?

I don't have it in front of me at the moment, but it's literally just the DC voltage, led, and coil all in series with nothing else in the circuit. I know how to calculate ballast resistors and did so with the specs from the datasheet and know the math works out. I'm only asking whether there's any issues with using a relay coil instead of a resistor (assuming the resistance is a safe value)

 

[user 37518]

Any good reason not to buy a ballast resistor instead?

 

[jasonallenh]

Any good reason not to buy a ballast resistor instead?

The LED indicates that the relay is on, so it's not that i don't have a resistor available, it's just a way of simplifying. I guess another way of asking my question is this: should i avoid using a relay's coil as a ballast resistor when its actual residence is the amount i need to protect the led?

 

[Lampie519]

Maybe you need to protect the Led in case you use a coil as ballast as its reactance could destroy it.

 

[Brian Roth]

One thing to consider is the voltage drop across the LED. It might cause enough drop to cause the relay's pull-in to be sporadic.

What DC voltage supply to the assembly? What make/model relay? Relay coil resistance?

Bri

 

[jasonallenh]

Supply to the assembly is 5vdc, relay is a panasonic tq2-2m-5v. Coil resistance is 125 Ohm

LED in my switch is 2v FV, 20ma, and would typically require a 150 ohm resistor with a 5v source voltage, so I'm actually 25 ohms short. Let's say I wasn't, and the coil resistance was exactly 150 ohms- would this (IN THEORY) be acceptable, or would I be breaking some EE rule out there?

 

[JohnRoberts]

This is why we melt solder and bread board circuits.

But... modern LEDs don't require much current so it might be too bright. One other possible concern is inductive flyback so depending on the topology maybe consider a catch diode across the LED.

JR

 

[jasonallenh]

This is why we melt solder and bread board circuits.

But... modern LEDs don't require much current so it might be too bright. One other possible concern is inductive flyback so depending on the topology maybe consider a catch diode across the LED.

JR

Breadboard has been happy, but I don't want to do something that might shorten the life of my super-nice NKK switches. I'm assuming for a 5v relay, a 1n4148 should suffice for the flyback diode, yea?

 

[jasonallenh]

Now that I think of it..... "can a relay coil be a ballast resistor?" has turned into "can an LED be a flyback diode?"

 

[ccaudle]

Minimum guaranteed operating voltage for those 5V coils is 3.75V (75% of rated voltage), so you may not be able to guarantee reliable operation.

 

[gyraf]

coil current min. 40mA, too much for the led

 

[JohnRoberts]

coil current min. 40mA, too much for the led

You could put a resistor in parallel with the LED to reduce the current through the LED.

JR

 

[MagnetoSound]

Now that I think of it..... "can a relay coil be a ballast resistor?" has turned into "can an LED be a flyback diode?"

Flyback diodes go in parallel, not series.

 

[ccaudle]

Flyback diodes go in parallel, not series.

And flyback diodes are reverse biased when the coil is on, so the LED would be off when the coil was on.

 

[jasonallenh]

All good information, thanks everyone!