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DC Relay pull-in voltage range 3v-12v help

rxrep

Jan 5, 2023
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Hi everyone, thanks in advance for any help. I've searched but couldn't find an answer that is cut and dry.
I'm looking for a DIY multiple relay solution or a single relay that can do the following:
I'm running an aftermarket RGB LED ambient light set up in my car that requires constant 12v 10a input. I want it triggered from the interior lights of the vehicle, so when they turn on, so do the LED lights (via constant 12v pin 30 to 87). The challenge is that the voltage for the interior lights ranges from 3v to 12v depending on the exterior brightness/daylight sensor (interior lights dim to 3v at night and jump to 12v brightness during the day or when car is unlocked at any time prior to starting ignition). The voltage change is not instantaneous. Depending on exterior brightness, the voltage can start at either end and vary to the other extreme over a couple seconds so the change is aesthetically pleasing. The LED require constant 12v and I control brightness for them via an app.
I need to find a relay that engages the coil and stays engaged for any pull-in voltage between 3v and @13v DC.
Is there a relay that does this?
If not, is there a workaround or chained relay setup that does the same thing?
I've seen low voltage relays (the blue colored ones), but they are quite small and don't seem to be able to handle 12v, and the regular 12v relay coils don't immediately engage at the lower voltage. I found a 12V relay that does engage at a lower voltage (out of luck), but it "buzzes" for a second to the engagement point (when the pull-in is on the low end), and eventually stops working over a few months (understandably since it's not designed for this).
If it's a workaround, the easier and more layman the answer, the better. I have only basic understanding of electronics in general. Appreciate it, and Happy New Year!
 

Harald Kapp

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Is there a relay that does this?
no.
If not, is there a workaround or chained relay setup that does the same thing?
yes. In fact a multitude of solutions is possible.

1: use a transistor driver to drive the relay:
1672934403020.png
L is a placeholder for the 12 V relay that drives the additional RGB lights.
To select the right values for the components, you need to supply the data of the relay driving the RGB lights.

2: buy a relay module like this one. It will work from 3.3 V onwards. To limit the input current add a resistor and a zener diode:
1672934771590.png
The ad doesn't state the input current of this module, but typically 5 mA i a good assumption. R needs to be designed to supply 5 mA at 3.3 V with a max. voltage drop of 0.5 V (a ballpark figure, may need some experiments to get a better value). Therefore R = 0.5 V / 5 mA = 100 Ohm.
At 12 V, this resistor will dissipate some power: P = (12V - 3.3V)2/100 Ohm = 1.3 W.

3: One could design a constant current source to drive the relay regardless of the control voltage, but that requires more design effort.


The simplest solution is imho #2.
If you want to go for #1, supply the data for the relay you are going to use (12 V coil voltage).
 

rxrep

Jan 5, 2023
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Thanks a ton for the response and detail. Sadly, much of it is still way over my head - but I'm trying to research how to read electrical diagrams in order to increase my knowledge and see if I can figure it out. #2 does seem logical and I get the math, but understanding the diagram is still just outside my scope. Just my luck that it wasn't an easy fix for an electrical illiterate like me.
Would I have heat challenges with a resistor, and would it be possible that a resistor (or transistor in option 1) might cause an ECU error (meaning, the car's ECU wonders what is happening because of the resistor/transistor and throws a code or shuts the interior lights off completely)? I can't seem to find anything about coil voltage for the box of True Mods EPS131R 12V relays I have. They are the ones I've found that do pseudo-function in that they have lower coil voltage than others I've tried (anecdotal), but they "buzz" until voltage hits @9v before completely engaging and usually burn out completely after a few months. I've designed a multi-relay set up that closes the relay from the interior lights and transfers it to an ignition-based 12v relay when the car is started. It works and extends the initial relay life because that relay off the interior lights is only used for the time between car unlock and ignition on. I may just scrap that and accept that the LEDs will only turn on once the ignition is on and just configure it with a single relay off the ignition 12v. I was just hoping for more sync with the actual interior lights but Volvo went and made them vary in voltage/brightness on me and made it more difficult. Thanks again for everything - don't mean to dumb it down on your end as I'm sure from your perspective it already is dumbed down, lol.
 

Harald Kapp

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Would I have heat challenges with a resistor,
Depends. 1.3 W isn't much, but if the resistor is in a tight enclosed space, it can heat up quite a bit. If it is mounted to the chassis of the car, it will be cooled enough. You need to use a 1.5 W, better a 2 W resistor.
would it be possible that a resistor (or transistor in option 1) might cause an ECU error
Not if correctly wired.
I can't seem to find anything about coil voltage for the box of True Mods EPS131R 12V relays I have.
It's right there in the name: 12V.
I don't seem to be able to find the coil current, though. Without knowing the current it is not possible to select a suitable transistor and the other components. Do you have an ammeter or multimeter to measure the current trough the relay coil when the relay is operated from 12 V?
I may just scrap that and accept that the LEDs will only turn on once the ignition is on and just configure it with a single relay off the ignition
Possibly your simplest option.
 

Harald Kapp

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Right, that's why I added a resistor and zener diode to limit the input voltage to ~ 3V.
What I didn't recognize is that the module needs an additional Vcc input of 3.3 V. You are absolutely right that this will not work on 12 V.
I beg to excuse my oversight.
 
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