5 Pin Automotive Relay Question

Thread Starter

john2k

Joined Nov 14, 2019
219
I have a 5-pin relay wired up as pictured below which powers a bulb from two different +12v switched sources. Although this works, there is a very brief cut of power between the switch between the sources. Is there a way I can make this smoother? possibly have a dedicated common power from the battery which is then switched using the relay and somehow not let there be any interuption at all on the bulb side?

Any suggestions would be great.

Thanks

Relay.png
 

sghioto

Joined Dec 31, 2017
5,367
A large capacitor across the bulb might work to hold the voltage briefly during the switching if the current is not too high. The relay can be replaced with 2 diodes to isolate the 12 volt sources.
1698937047067.png
Or use the relay and connect the capacitor across the relay coil.
1698938922481.png
 
Last edited:

Thread Starter

john2k

Joined Nov 14, 2019
219
How much current does the bulb require?
The bulb is a HID xenon bulb which i should have mentioned is actually going to the ballast +12v input before it goes to the bulb itself

A large capacitor across the bulb might work to hold the voltage briefly during the switching if the current is not too high. The relay can be replaced with 2 diodes to isolate the 12 volt sources.
View attachment 306486
Or use the relay and connect the capacitor across the relay coil.
View attachment 306487
My worry and concern about using a diode is that it may burn out due to the amount of power the bulb needs and even though the diode doesn't allow current to go back from one circuit to the other, I believe some sort of small power still makes it through which may cause an issue.
 

sghioto

Joined Dec 31, 2017
5,367
Are the two 12 volt sources connected to a single SPDT switch? If so then this arrangement should work.
1698960250679.png
 
Last edited:

eetech00

Joined Jun 8, 2013
3,814
That is true, my bad. Below is the correct arrangement.
View attachment 306505
Um...That still only delays the drop out of the relay, but doesn't span the transfer time of the contacts.
However....

Maybe you can use a second relay with its contacts in parallel with the existing one. Then connect this relay so when energized, its spans the transfer time of the first relay while it transfers. Like this

1698975316700.png

In the graph (zoomed in)

The first trace show the switch state (1V=N.O. closed, 0v=N.O. open).
The last trace shows the current thru automotive BULB.
 

Attachments

Last edited:

sghioto

Joined Dec 31, 2017
5,367
the time it takes for the armature to break one contact and make with the other.
That's what I figured you meant. The TS mentioned:
possibly have a dedicated common power from the battery which is then switched using the relay
To me that indicated a 3rd 12 volt source which is used to power the bulb and the other two 12 volt sources control the relay coil.
If that is true and not wanting to use diodes than 2 relays would suffice or 1 relay coil controlled by two transistor in parallel to isolate the the 12 volt sources.
1699021518569.png
1699021874898.png
 

Tonyr1084

Joined Sep 24, 2015
7,755
you still did not answer the most important question - how much current does the bulb need.
There are two types of HID commercially available; a 35W and a 55W. In nearly all applications a 35W HID kit provides plenty of light. 55W is just too bright.

35W ÷ 13.8V = 2.5A (2536mA)
55W ÷ 13.8V = 4A (3986mA)

How much current the bulb takes is irrelevant because HID is powered via a ballast. Since the kits are rated at 35 and 55 watts, I'd assume that's how much power each kit draws.

The probable reason for wanting to eliminate the switchover time is due to how HID starts. On a cold start they flash bright for an instant of time then drop to a dim level. Over the course of just a few seconds they reach full brightness. Switching between one source and the other might - and I stress "Might" - cause the lamp to extinguish and relight.

I don't think the solution is in managing the switching but rather carrying the ballast through the switchover phase. That means adding a capacitance to the input of the ballast itself. Even if there's a brief dropout the capacitor can hold the ballast powered long enough and at a high enough current to prevent dropout and refiring of the lamp.

What I don't understand is if this is in an automotive application why you're switching sources. If you're using it in a non-conventional situation I can understand the desire to switch without the dropout of the light. So as proposed, two diodes should do. Forget relays. They have just as long an interruption between switching as a toggle switch has. Though I've never tried it, I think there's a chance the ballast will be able to handle the brief switchover without having to refire the lamp. Only way to tell is to try it. Since you're asking I'm assuming you HAVE tried it and don't like the time it takes to refire the lamp. A little more clarity on the subject will help us find you the best solution possible.
 
Top