DC variable voltage regulator help!

Thread Starter

Skotty319

Joined Oct 8, 2019
7
I am working on a project involving the dimming circuit on an automobile and LED's. The battery voltage feeding the dimming circuit ranges anywhere from 12.9vdc up to as high as 19.2vdc and then is variable all the way down to 0vdc. Is there any basic component or simple circuit I can use that would cap that voltage between 12 and 14 volts but still allow the bottom end of the range to be 0vdc? My problem is that my 12v LED's are burning out easily when the voltage peaks in the 15v and higher range. I tried using a LC7812 voltage regulator but it is all or nothing when the circuit gets dimmed.
Any suggestions are welcomed and appreciated.
 

Thread Starter

Skotty319

Joined Oct 8, 2019
7
What do you mean "all or nothing"?
When the incoming voltage drops below about 11vdc the output shuts off completely. I need something that will follow the input voltage down to at least 5vdc but cap the high end from breaking 14vdc give or take so I don't have the high voltage spikes that ruin the LED's yet still have the variable low end so they are dimmable.
 

Thread Starter

Skotty319

Joined Oct 8, 2019
7
If these are certified for operation in autos, they'll handle voltage variations.
They are design for Automotive applications up to 15v. My problem is that the specialized segement of the auto industry that I am dealing with (custom car audio) has a tendency to run higher incoming voltages for better audio performance and reliability. If there was a way for me to regulate the lighting specifically in the components that I upgrade to LED's it would give increased reliability which is always a bonus
 

dl324

Joined Mar 30, 2015
11,530
They are design for Automotive applications up to 15v.
How are they connected to power? How are these audio devices getting a voltage higher than what the auto electrical system provides? If they have to resort to boosting the voltage, it's a poor design.
 

ci139

Joined Jul 11, 2016
1,696
LC7812 voltage regulator
it will be (all) 12V or nothing unless you pull it's reg pin below the chassis . . . all correct
many portable drills have pwm reg mosfet + controller +"rheostat" inside get one from 12V one or build 1 your own . . . the 555 stuff
 
Last edited:

Ioannis66

Joined Nov 7, 2012
36
If your LEDs are current controled, as they should be you would not have any problem.

But if you rely on voltage control, then get ready to replace regularly the LEDs.

Remember, LEDs are diodes. Not Lamps...
 

Videostar

Joined Feb 4, 2013
13
You could try a simple Zener stabilizer circuit with a control transistor. I have attached a section of circuit which includes that function. What will happen is that the output voltage will remain constant until the input voltage falls below the Zener voltage and at that point the output voltage will start to fall, all the way to zero if so required. In my circuit, the supply voltage passes through D1 (used to stop the capacitors C1 & C2 discharging through the rest of the circuit), through R1 (used to restrict the current flow into the Zener) and the voltage at that point is held constant (in my case by D2, a 6.2V Zener). This voltage is applied to the base of Darlington transistor Q2 (BCV27). The collector is fed from positive supply voltage (which would be your variable supply) and the emitter feeds the stabilized output voltage out to the rest of the circuit (in your case the LEDs). This output would obviously fall once the input supply fell below whatever Zener voltage you had selected. There is a 1.2V drop when using that Darlington transistor so you will need to compensate for that when you choose your Zener voltage. I would suggest much lower values of resistors for your use - I had to reduce the current drain as much as possible for my use. Obviously for automotive circuits that isn't a problem.

Vreg.png
 

robertrau

Joined Jul 23, 2019
1
Are you dimming white LEDs or monochromatic LEDs? If white LEDs and you care about the color temperature, you can't change the current very much from the test current in the data sheet. If you go above the test current more of the blue LED penetrates the phosphor and your LED gets more blue. Less current and it get more yellow-green. To keep the same color temperature you must PWM for dimming. This is less of a issue with single color LEDs. Like Ioannis66 said, LEDs are current devices, device to device you will see a large variation in Vf, but light output will track current. ON Semi and Infineon (NSI45060JD or BCR 320U) have several automotive LED drivers that will protect you from over voltage. Be sure to properly heat sink your driver so it doesn't over heat in an over voltage case.
 

Rasterist

Joined Sep 15, 2020
4
You could try a simple Zener stabilizer circuit with a control transistor. I have attached a section of circuit which includes that function. What will happen is that the output voltage will remain constant until the input voltage falls below the Zener voltage and at that point the output voltage will start to fall, all the way to zero if so required. In my circuit, the supply voltage passes through D1 (used to stop the capacitors C1 & C2 discharging through the rest of the circuit), through R1 (used to restrict the current flow into the Zener) and the voltage at that point is held constant (in my case by D2, a 6.2V Zener). This voltage is applied to the base of Darlington transistor Q2 (BCV27). The collector is fed from positive supply voltage (which would be your variable supply) and the emitter feeds the stabilized output voltage out to the rest of the circuit (in your case the LEDs). This output would obviously fall once the input supply fell below whatever Zener voltage you had selected. There is a 1.2V drop when using that Darlington transistor so you will need to compensate for that when you choose your Zener voltage. I would suggest much lower values of resistors for your use - I had to reduce the current drain as much as possible for my use. Obviously for automotive circuits that isn't a problem.

View attachment 217509
What's wrong with just a 12V zener instead of this circuit?
 

bassbindevil

Joined Jan 23, 2014
100
This might be an application for a cheap little microcontroller. The micro could measure the illumination voltage via a low-pass filter (in case of cars that dim via PWM instead of linearly), then generate a PWM signal to dim the LEDs. Use some kind of switching regulator to limit the LED supply voltage to 12V nominal, or possibly have a resistor that is sized to limit the current at high input voltages, then shorted out via a power MOSFET (controlled by the micro) at low voltages. Something else to consider: the constant current switching LED drivers used in MR11/MR16 LED lamps which handle a wide input voltage range and include a control pin which can accept a PWM control signal or linear control voltage. PT4115 for example. That control voltage input might even be compatible with the automotive dimming signal by adding a voltage divider or something.
 
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