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Building an in-car 12V DC regulator

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JohnnyC

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Hi,

I'm needing to build a regulator, as I simply cannot find anywhere that sells a cigar lighter 12V regulator that provides 5A.

As I also need a 12V 500mA source too, so I may as well kill two birds with one stone...

I have drawn up my circuit diagram, but to be honest I know very little about electronics, and I am completely stuck on what rating & type the capacitors should be - please help...

**broken link removed**

Thank in advance,
John.
 
I don't know why you need 2 regulators. What are you planning on running off the outputs? Some equipment is fairly voltage tolerant and would only need a little LC filtering to remove alternator noise etc. Remember also that you will lose about 1.3V across the LT1085 so you'll need a minimum of 13.3V at the Cig lighter at full load. So the engine will need to be running or the voltage will drop below 12V on the output.
On the top regulator they recommend 10uF, 25V (They don't mention type so an electrolytic should be OK) on the input and a 22uF 16V tantalum (or 150uF aluminum electrolytic) on the output
See datasheet:
http://www.tranzistoare.ro/datasheets/320/57215_DS.pdf
 
Last edited:
The LT1085 has a max output current spec of 3A, not 4A. Its typical input-output voltage differential at dropout when it is no longer regulating is 1.2V. Its max rated dropout voltage is higher.
 
kchriste said:
I don't know why you need 2 regulators. What are you planning on running off the outputs?
I see what you mean about why I'd need 2 regulators when they're both the same voltage - I'm copying someone elses circuit that had both 12V & 5V outputs.
I figured it'd be safer to limit the amps more that go to the camera as it only requires 110mA (although I may had extra cameras in future).
I plan to be running an HDD recorder and a camera.

kchriste said:
Remember also that you will lose about 1.3V across the LT1085 so you'll need a minimum of 13.3V at the Cig lighter at full load. So the engine will need to be running or the voltage will drop below 12V on the output.
Yeah, I'd only be using the recording system when the engine is running. Would this be the case for already-built 12V regulators that you can buy for the cigar lighter?

audioguru said:
The LT1085 has a max output current spec of 3A, not 4A. Its typical input-output voltage differential at dropout when it is no longer regulating is 1.2V. Its max rated dropout voltage is higher.
This is where I got the information on the LT1085:
https://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=4767159
Perhaps I need an LT1084 ? (5A)
 
JohnnyC said:
I figured it'd be safer to limit the amps more that go to the camera as it only requires 110mA (although I may had extra cameras in future).
I plan to be running an HDD recorder and a camera.
Not sure how you plan to control an HDD recorder... but...
Keep in mind that 12v car lines can drop out periodically. Like you first turn the ignition it might go down to 9v. If you turn off the vehicle and run off the battery you might only get 11.5v off that battery after running it awhile, especially if the battery is old and/or cold.

Low voltage will mess with an HDD. It can probably crash (damage) it. What exactly are you trying to do? This sounds odd that you have a lone HDD, which usually needs a complicated device to control, and are only lacking 12v. Some external HDDs run off an unregulated wall wart thus they have their own internal regulators, which are probably only linear regs with no boost.
 
This is the recorder:
**broken link removed**
It's actually a CCTV recorder, but can be used in vehicles as it runs off 12V and has anti-shock mounts etc.
It states 3.5A in the link, but has 4A on the back of the recorder!
 
Do you have the "home standard/advances" or "auto" version?
Just get the auto version.

If you have the home version, you seriously need a buck/boost switching converter. Or more realistically just run it off 110v from an inverter. Though it may seem simple to get 12v +/-0.5v, it's actually somewhat difficult.

It does say 15W- which is little more than an amp.
 
Thanks,
I've only just noticed that in the manual. They only sell the standard or advanced version though.
I had thought of converting to AC with an invertor, then using the power supply that came with it to convert back to DC, but that seems a long way around it. But certainly an option.
I'm confused that it states 15W, as you're right, that's only 1250mA :-S I'll have to look into why it states 4A on the back of the unit, and the power supply is 4.16A. And it does seem quite high for what it actually does.
 
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