adding extra lights do i need relay
Printed From: the12volt.com
Forum Name: Lights, Neon, LEDs, HIDs
Forum Discription: Under Car Lighting, Strobe Lights, Fog Lights, Headlights, HIDs, DRL, Tail Lights, Brake Lights, Dashboard Lights, WigWag, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=134639
Printed Date: July 05, 2025 at 4:59 PM
Topic: adding extra lights do i need relay
Posted By: victorious
Subject: adding extra lights do i need relay
Date Posted: August 01, 2013 at 2:27 AM
Hi I've added some extra vanity lights to use as footwell lights to my car interior's (Vw mk4 golf). Its working fine at the moment with the central locking switched ground and power but I have been advised by an experienced electrician that I MUST use a 4/5 pin relay to run this or I will burn out the MOSFET inside the CCM and have no interior lights, as apparently my car is not designed to run extra interior lights or LEDS?
is this true its just I have read on here that you do not need a relay to power some extra LED festoon light bulbs? Its working fine without the relay at the moment and if it's not necessary I would prefer not to use a relay as it buzzes when the lights fade out and i'm not too confident on wiring it up anyhow.
If I need to protect the circuit can I just use an inline fused wire with 10amp fuse inbetween the new footwell lights and 12v power supply from my interior map lighting loom. Or is it fine without the relay or fused wire?
Thanks!
Replies:
Posted By: oldspark
Date Posted: August 01, 2013 at 9:48 AM
If the extra lights are LEDs, it should be fine unless they add significant load - eg, maybe an Amp or more.
If the CCM dimes the lights, then a relay is not the solution.
A few suitable solutions include:
- replacing other bulbs with LEDs to reduce the overall load;
- add a transistor or MOSFET buffer circuit;
- replace the existing CCM MOSFET with a higher rated device.
Posted By: victorious
Date Posted: August 01, 2013 at 11:38 AM
Thanks very much! They are only two extra vanity lights im adding to the interior lighting for footwell use. the two bulbs are rated at about 1.5w consumption each i really think it should be ok on the stock system with no relay? I have replaced all other interior lights with 6 led style festoons or t10's.
I have currently wired a 10amp fused wire inbetween the footwell/vanity lights and 12v + power supply from car is this an ok solution to add some protection to the circuit?
So I don't really need a relay for this then right?
Thanks!
Posted By: oldspark
Date Posted: August 01, 2013 at 7:04 PM
Not if your total load is less than your old.
And not if it dims.
Besides, most relay coils are 3W.
Posted By: victorious
Date Posted: August 02, 2013 at 5:53 AM
Thanks buddy! so what you are saying is that by replacing all my filament bulbs with LEDS I have actually decreased the total load DESPITE adding these two extra 36mm LED footwell festoons?
I was advised the CCM MOSFET is only designed to run/fade out a single festoon bulb and the extra lights im adding should be run via a 5pin 30amp 12v relay to avoid burning out the MOSFET, and also advised my LEDS may not work properly due to resistance issues..was that bad/wrong advice in my case then?
seems to be working fine with no relay and fades out very smoothly with the central locking/ignition as i wanted :)
Last question, I have added an inline 10amp fused wire between my new footwell lights and the 12v+ wire from my car wiring loom, does this offer any additional protection to the circuit or help to cope with the new load at all? or should i just get rid of that if no purpose for it, the whole circuit should be protected by another fuse in the fusebox anyway right?
Thanks!
Posted By: oldspark
Date Posted: August 02, 2013 at 7:24 AM
It's very likely, but you should be able to compare the LED current/power with the bulbs that were removed. Watts = Volts times Amps.
If you are saying that your LEDs dim ok - ie, "linearly" or in proportion to bulbs, or evenly from full to (near) zero/off and NOT "stuff all" from max to maybe ~half way and then suddenly off - that means your dimmer is a PWM (Pulse Width Modulated).
PWM varies the ratio of on to off time - in simple terms for LEDs, flouro tubes etc - keeps the constant voltage they require but turns on the current with a varying duty cycle from say 0% (fully off) to 100% (fully on), and brightness of LEDs etc is proportional to that duty cycle (average current).
Bulbs (tungsten, halogen etc) respond the same way to PWM, but they can also dim merely by using a pot - a variable resistor aka POTentiometer.
Older vehicles use high-wattage pots to dim lights, but that changed to PWM circuits since they ran cooler and could be much smaller (and cheaper). It then didn't matter if you dimmed bulbs or LEDs.
A dimming PWM frequency is usually at least ~50-100Hz (else you see the flicker) but can be 20kHz & higher.
Hence why you can't use a relay - you can't turn a rely on & off 50 times a second.
And of course the relay coil is likely to blow the MOSFET due to its inductance (spike), but I'm sure that experienced electrician told you that (ie, that you must fit at least one spike quenching diode - unless it is already spike protected)?
I assume the electrician did not know your were merely adding 3W of lighting. A typical 30A relay has a coil resistance of ~60 Ohm, hence its current = V/R = 14.4V/60R = 240mA.
14.4V x 240mA = 3.4W. So adding the relay adds as much or even more load than adding your 2 x 1.5W lights. (LOL) (And LEDs don't add spikes. LOL#2)
Your LEDs should be designed for a 12V vehicle and hence tolerate the normal ~14.2V and maybe up to 15V or 16V. Hence there should be no resistance issues (depending on what was meant by "resistance issues").
A single festoon bulb... Is that a 3W, 5W, 10W or a 20W festoon that the CCM MOSFET was designed to handle?
Someone knowledgeable should be able to state what the current rating of the MOSFET is (and maybe if under-heatsinking limits its full current rating).
But MOSFETs with 10A - 100A ratings are cheap (~$3) and might be relatively easy to substitute. (And unlike transistors, little "design" is required for MOSFET input signalling; ie, substitutions are easier - merely N- or P-type, on voltage, and output current handling; no "base" current nor "gain" factors.)
Fusing: Nothing will protect the LEDs so you are merely trying to protect the CCM or the cabling.
The CCM should already have its fuse (+12V input) and that's all that should be required.
Posted By: victorious
Date Posted: August 02, 2013 at 8:47 AM
Phew thanks for that info. I'm really not sure why he said to use a relay then if it would add more load than the two LED Bulbs which i did explain was all i was adding. .had me bit worried about burning out the MOSFET so thanks for clearing that up :)
re dimming i just meant when i turn the ignition on or the car autolocks all the lights fade out very smoothly as i think they should. I dont really need them to be dimmer controlled or i would have wired up to headlight dimmer switch :)
Guess ill remove that inline fuse then!
Thanks again for your help!
Posted By: victorious
Date Posted: August 02, 2013 at 4:09 PM
Soz just realised you had asked me a question.
"A single festoon bulb... Is that a 3W, 5W, 10W or a 20W festoon that the CCM MOSFET was designed to handle?
Had a look at the original bulbs which i replaced...the single festoon was a 10w and it also normally lights up the two adjacent t10 w5w bulbs with the central locking/ doors open + rear alarm sensors (2x 3.8w) and 2 x puddle lights (2x w5w t10 bulbs again) I have replaced all of those with LEDS now.
so old bulbs were 1 x 10w + 4 x 5w + 2 x 3.8 = 33.6W
new bulbs are 4 x t10 LED's rated at 1.5w power usage each and the LED festoon is stated as 1watt power consumption! + the 2 footwell lights ive added which use the same festoon so another 2watts on top. also changed the rear alarm sensors for t5 leds not sure what power usage they are but say they are 3w each.
so new total power usage with LEDS = 4 x 1.5w + 3 x 1w + 2 x3w = 15w (including the 2 extra footwell lights ive added)
So if what you are saying is correct this should be totally fine and I could even maybe add a couple more of those 1w leds without any problems from the mosfet?
Posted By: oldspark
Date Posted: August 02, 2013 at 9:13 PM
You are indeed Victorious! (Or... "you have done well master Luke".)
There you have it - 33W old versus 15W new. You have capacity to spare.
And it seems the MOSFET is capable of ~33W/12V = ~3A. It may well be higher - maybe even 10A or more.
Load wise, LEDs should be friendlier than bulbs. Bulbs have an inrush current which can be 10 to 20 times their normal rated current, though these are very short lived and almost always handled by normal component tolerances (relays, MOSFETs, transistors, fuses). [ Such failure modes are usually thermal - ie, too long an over-current duration. ]
I'm glad you came here for another opinion. IMO it would have been ironic(?) if a well intending electrician caused your CCM to blow.
Whether that electrician was a domestic electrician as opposed to one that works with electronic devices is another issue, but I'm surprised more common sense was not used - ie, the new load versus old load.
Nevertheless, it also shows that any "expert" can be wrong or lacking - and that includes me! But I reckon this is a pretty straight forward situation - ie, the load has reduced, and it's electrically friendlier with no inductive relay-coil spikes etc.
I hope the new LEDs work well. Not that my cabin LED is dimmed, but I love its superior output - a much whiter light that seems to light everywhere. And without the heat that melts my light switch and cover after about 20 years (I drive a 1965 ute).
BTW - I recall some rule of thumb that LEDs consume about 1/20th the current of equivalent bulbs. But that was probably for 20mA LED indicators (dash alarm lights etc) as opposed to newer illumination types that use high power LEDs (50mA, 1A etc).
But since LEDs are well over 50% efficiency compared to bulbs (with ~5% efficient; & flouros maybe 15%), LEDs should always consume less than bulbs.
PS - my festoon size question was directed at the electrician. But that was to find out what the CCM was capable of. (IMO the obvious starting point.)
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