12-24v sensing circuit to operate a relay

[pigman]

Hi,

I'm building a circuit that runs on 12VDC however needs to have the ability to sense when a higher voltage occurs (say 15v or so) and turn on a relay to divert power via a 12v regulator IC. I was considering a 12v zener to opperate the relay, which works, but I'm concerned about how long the zener would last in a full time 24v situation. Any help would be much appreciated!

Cheers
Josh

 

[MikeMl]

You mean "subtracting" 12V from a nominal 12VDC relay's pull-in voltage by putting the Zener in-series with the coil?

The Zener will work just fine, provided that it is operated within it's power-dissipation limits.

Only you know how much current your relay coil draws at 12V? With an input of 24V, 12V is across the Zener, and 12V is across the coil. The coil current flows in both the Zener and coil. The Power in the Zener is Icoil*12V. Is that substantially less then Pd max for the Zener? Only you can know that, since you didn't tell us the coil current, nor the Zener number.

Most people just put a resistor in-series with the coil....

 

[Reloadron]

Hi,

I'm building a circuit that runs on 12VDC however needs to have the ability to sense when a higher voltage occurs (say 15v or so) and turn on a relay to divert power via a 12v regulator IC. I was considering a 12v zener to opperate the relay, which works, but I'm concerned about how long the zener would last in a full time 24v situation. Any help would be much appreciated!

Cheers
Josh

Hi Josh

You may want to consider the use of a comparator based circuit. Give this link a read.

As to the use of a zener diode in any application the zener will run along indefinitely as long as it is working within its power ratings. I am not saying to use a 0.5 watt rated zener to drive a 0.5 watt load indefinitely but for example a 1 watt zener would drive a .5 watt load indefinitely. Based on your post I am not sure exactly what you are after.

<EDIT> and what Mike said who was quicker than I </EDIT>

Ron

 

[pigman]

Wow, you guys rock. so fast!

Ok the Relay is a 12VDC unit and the coil is 285 Ohms. The Zener I have tested is the 1W 1N4742 12V zener and I have just now tested the 5Watt 1N5349B Zener which does the same job but seems to obviously operate at a lower temperature.

At 24v the output of the zener to the relay coil is roughly 12.5v

what Zener should I use based on that Ohm rating? Thanks again in advance.

 

[Reloadron]

12 Volt DC coil resistance = 285 Ohms so the coil will draw 12 / 285 = .0421 amp (42 mA) so you have .042 * 12 = .5 watt coil. As I see it and I may have that wrong. If I have it right a 1 watt zener or greater would be fine.

Ron

 

[pigman]

Thank you so much! I had tested this on the bread board and proved my very basic concept, but wasn't sure until now about its ability to last the test of time when in use. thanks for that!

 

[Blars]

Relays are not acurate voltage comparors, a 12v relay may "chatter" if it has 8v on the coil. I'd recomend using some amplification if not an actual comparor.

 

[pigman]

Hi, thanks for your help. In my testing of the circuit I've found that during the rising voltage when the input gets to about 17.5v on the Zener the out put is about 6-7vdc which is enough to activate thewithout coil without chatter and when the input voltage to the Zener is at 24v the output to the relay coil is 13v, as the circuit only runs in either 12 or 24v mode, nothing in it's environment in between, would it be ok to leave this circuit in place?

Also the rise in voltage is never slow, as it is battery operated it is always eith 12 or 24.

I am a little concerned about damage to components during this initial spike before the relay operates to divert power via the regulator tho, any ideas the would be appreciated. The caps and whatnot used are all rated to handle 24v however there is a 499 ic out putting a 5v rail to operate a pic, in my testing is holds true during the voltage changes but any suggestions would be appreciated.

Cheers
Josh