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How can I get 4VDC from 12VDC?

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This is for a school bus and I'm trying to fix one of the flashing yellow lights. The buses used to use a incandescent light bulb, but has been converted to LEDs. The 12VDC power flashes (by design) at about 0.5 seconds on 0.5 seconds off.

I have a an LED array that had a potted built in power supply. A new costs $150. The potting cracked and made it work intermittently. I took it apart and while it was working I saw it was delivering 4VDC to the LED board. I chopped the old power supply off, and ran 5 VDC to the LEDs and they worked. I decided to solder a USB connector on the leads and plug in to a USB cigarette lighter. That worked fine, but here's the problem. The USB charger doesn't get the 5VDC started before the 12VDC shuts off again. The LEDs never light up with the short interval.

So how can I build a 4VDC power supply that runs off 12VDC that is instantly (or close to) on. The current draw is probably less than 1 amp. I can build whatever board and get whatever components needed. I have a full electronics workbench available including scope if needed.

I really appreciate any help.

An LM317 regulator plus two resistors, value to be determined by reading the datasheet.


I think 100 and 220 ohms might be the correct values.

Or, if 5 volts will work, you can use the LM7805 type with no external resistors needed.

Great, Thank you. I will look into this. Will there be a delay in the output voltage?


--- Quote from: mikef187 on July 26, 2021, 05:20:38 am ---...Will there be a delay in the output voltage?

--- End quote ---

If using a linear regulator as @bdunham7 suggests, the delay will be minimal.
For an indicator flasher you are unlikely to perceive the delay with the human eye.
Don't use too big a filter capacitor on the output of the regulator...

If you want something instant, another option would to simply connect multiple diodes in series.

A basic diode like 1n400x (1n4001, 1n4002... 1n4007 - all the same just maximum voltage they can handle changes) will have a voltage drop of around 0.7v so if you want to drop 12v - 4..5v =7..8v, then around 10 diodes should do it.

You can put them on a small stripboard alternating cathode and anode ends to make it easy to solder the leads and keep it small and they'll use about as much space as a to-220 regulator and you won't need input and output capacitors and feedback resistors (if you use an adjustable linear regulator)

Example : 1n4004 (20 cents each if you buy 10, 10 cents if you get 100) : https://www.digikey.com/en/products/detail/micro-commercial-co/1N4004GP-TP/2233923


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