# Relay drop off

Discussion in 'The Projects Forum' started by skinner927, Oct 28, 2008.

1. ### skinner927 Thread Starter Active Member

Dec 31, 2007
36
0
I'm not exactly sure if i'm using the right term, but I'm having a problem with relay voltage drop off.

I have a 12v relay that is powered from a 12v+ lamp line in a car. Problem is, this light doesn't switch off, it dims out. So the voltage drops from 12 to 0 within a few seconds but it gets to the point where the coil in the relay makes the contact "jump" because it's just around the drop off (not sure what it's called) point where the contacts in the relay would become open. Is there something I can do to make the connection quicker, more of an on/off? because right now it flickers and my device that is connected to the relay, which is another light, flickers instead of just on/off.

I hope this makes sense.

thanks.

2. ### alwayslearning Member

Feb 27, 2008
22
0
Yes, it makes sense!

There are two critical voltages for a relay coil.
1) Minimum voltage to cause the electromagnet to “pull in” (operate voltage) the armature operating the relay.
2) Release voltage-voltage relay will release, once operated.
These two voltages are not equal-for example a relay whose coil is rated at 12 V; may have an operate voltage of 10.5 V and a release voltage of 8 V.

You say the light dims-this means you are tied in to the “courtesy lamp" voltage line, which when energized usually stays on for seconds then takes about 2-3 seconds to go dim; just be sure you do not load this line! There are many different ways to accomplish your task. First determine the ways this line is energized; for example, key out after vehicle turned off or upon entering vehicle? Then determine under what condition you want this light to come on? Finally design a circuit to connect between your switching signals/+12 V constant/ignition Voltage, etc and light (load). Two examples for circuits could use either a Op-Amp comparator circuit or digital "And Gate" circuit…. Depending on your conditions and design you may or may not need to use the dimming voltage at all-for example if the light dimming voltage only comes on after you set a switch on then you may be able to tap in to this on/off dimming circuit supply line as your switching signal? Do you have access to a schematic of your electrical system? If not, and you choose to buy one, first try AutoZone.com.

Last edited: Oct 28, 2008
3. ### skinner927 Thread Starter Active Member

Dec 31, 2007
36
0
so an op-amp comparator circuit would be the easiest way to do this?

Is there any such thing that works as a vary relay, like a pot that would adjust with voltage?

Basically, something I could use to get the dimming other than just putting a cap on the end of the relay and having it turn off and have the cap dim the lights. The purpose would be to have all the lights dim at the same time.

4. ### AlexR Well-Known Member

Jan 16, 2008
735
54
The fact that the relay is chattering as the lamp dims suggests to me that the dimming is done by pulse width modulation rather than by dropping the voltage. If this is the case then a comparator won't be of any use to you, instead I would try putting a fairly large capacitor across the relay coil and see if that helps.