Hi,
Does anyone have any experience of supplying the coil of a 12v relay with 24v? Obviously it will work... for a bit.
I could design in a 5v relay and run it off the 12-24v switch mode I have on the board but wondered whether anyone has, basically, designed a bad board and got away with it.
Why? 24V relays are just as common as 12V ones. Obviously running a 12V relay on 24V is going to burn out if it's on for anything more than a minute or 2.
I want the board to be able to operate from 12-24v and I dont want to design in a reg just for the relay - the relay operates for about 10s max.
From the datasheet:
Max voltage 130% of rated voltage at 85?170% of rated voltage at 23? - that means up to 20v as long as it doesn't heat up.
I've ruled it out now. My board doesn't have a 5v supply either but I can still use the 4.5v supply as the minimum 'must operate voltage' is 75% of the rated voltage. So I could go down to less than 4v using a 5VDC relay. I'll probably do that.
Just like solenoids, the coils in power switching relays have quite a bit of thermal mass and can be overdriven a huge amount as long as you keep the duty cycle low to let it cool off.
Saying "the relay operates for about 10s max" doesn't really tell us the duty cycle. If it's on for 10s how long will it be off before being turned on again? That's the question you need to ask and answer.
See if you can find an 18V relay that that datasheet says will pull in at 11V
That should be ok for a short time at 24v
Or you might look at a solid state relay, some of those have an input range of 3-30V
You could also use a simple current source to drive the relay. The relay has a constant coil resistance, so should work.
add limiting resistor for higher voltage.
add limiting resistor for higher voltage.
That would also limit the lower voltages, he wants it to accept the entire range.
add limiting resistor for higher voltage.
That would also limit the lower voltages, he wants it to accept the entire range.
i dont see it mentioned by OP except he said using 12V on 5V relay which is exactly what i did in my recent project (small relay ie 5V is needed). if so for 12V relay for entire range, then get a proper rated relay without risking shortening life or complicate the board further.
edit: even properly rated 24V relay could not work at lower range. the OP probably can get away driving 12V relay with 24V direct without limiting resistor, but he should be careful of solenoid heat buildup and test for component life and reliability. my 2cnts. YMMV.
It's common to drive, for example, a 12V relay with 24V using PWM. This allows you to adjust the activation and hold voltages to optimize the switching and bounce times. Or, if that's overkill, just PWM at a duty cycle that reliably closes the relay.
Stick a 6 volt incandescent light bulb in series with the relay coil.
Use a bulb with a rated current draw that's in the same ballpark as the relay coil's.
Check one of the 24 volt relays to see if it will pull in on 12 volts many will.
I think introducing a lamp is just a failure waiting to happen.
Some 24V relays *may* pull in at 12V, but how reliable would it be? Would it hold the contact firmly? Would it pull in *every* time?
You could add an emitter or source resistor to make the transistor operate as a current limited driver.
Another solution is to use a PTC with a resistor in parallel. At over 16V or so, the PTC will heat up and let the resistor drop the voltage.
Yet another solution is to have the microcontroller (if that's what you're using) do PWM at above 14V or so.
A Low Drop Out voltage regulator with a heatsink if needed to drop the voltage would work. The voltage drop at 12 volts would bring the output voltage to just under 11 volts, which is fine for a 12 volt coil. Just be sure to add a freewheeling diode across the coil to curb the inductive kick. LDO voltage regulators need a small amount of capacitance for stability, see the datasheet for the example.
It's common to drive, for example, a 12V relay with 24V using PWM. This allows you to adjust the activation and hold voltages to optimize the switching and bounce times. Or, if that's overkill, just PWM at a duty cycle that reliably closes the relay.
This is the answer for a different board of mine. It can take 24v apart from the hardwaired 12v relay therefore it takes the full 24v for a second and then has a low duty PWM (even at 12v).
http://relays.te.com/appnotes/app_pdfs/PWM_application_note.pdf