OT - Help with LM317 voltage regulating circuit
Turbo Mini Forums

I'm trying to create a stable power supply for use inside my camper van, I've bought some LM317, and I've wired them up like this:


I''m trying to get 13.75Vso I'm using R1 = 330, R2 = 3300

I expected to see a constant output of 13.75, but I find I always have an output aprrox 1.5v lower than the input. Have I missed something, or have I put some duff IC's ?

Team Racing

This is the info I used : http://www.reuk.co.uk/LM317-Voltage-Calculator.htm

Team Racing

That information is a little confusing and perhaps missleading?

It sugests that you can take any voltage between 3 and 40 volts, which then can output any voltage of 1.2 or 37 volts.

What it does not tell you is that your output voltage is dependant on input voltage. For instance, how can you achieve a 20v output with a 12v input with such a simple circuit? as that is what it suggests.

Reading between the lines, looking at the highest values, an input voltage of 40 will regulate to a max of 37. Thats a difference of 7.5%

Basicaly a regulator limits an input lower. so if your input is 15.2v you might ahieve your 13.7, but if your input is 14.7v you will only achieve 13.2v

What is your input voltage?

Edited by Sprocket on 21st Feb, 2010.

http://www.national.com/ds/LM/LM117.pdf

What Colin is getting at is with a reg such as the 317 the highest voltage you can get out is about 1.5v lower than the input voltage,

I do have some 12v "low dropout" regs at work that only drop about .5 of a volt, we use them in a circuit where we need 12v but only have about 12.6 available.

you could posibly use one of those and stand the ground pin up on a couple of diodes (each diode will raise the output by .6v)

If they're any use i can pop some in the post

That would be really handy, this is mainly for my diesel heater, (which so far I've blown up two ECU's due to crappy voltage spikes) so it will need to able to supply 10amp peak. How many would I need?

Team Racing

If money isnt an issue, you can buy proper voltage regulators/boosters/whatever.

Some people use them in race cars so that important items always see a specific voltage, regardless of input voltage ( within reason of course )

Its basically like a big audi amp, except for power circuits. No doubt costs a fortune too.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

if its power spikes surely a capacitor, Metal oxide varistor, avalance, zener doide sircuit would be more suitable? couple that with a regulator and ought to be nigh on bullet proof.

if you want a cheap version of what stevie is referring too, your average 200W car audio amp has a -30,0,+30v power supply regulated and filtered if you have an old one lying around you could take it apart and regulate teh output down to 13.8v with your circuit above.

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

Jim, what ecu are you killing with spikes?


you cant parrallel regs up, IIRC the current will just flow through the one with least resistance, kill that, kill the next one etc....

what i would suggest is putting in a fuse and a transorb, these are designed to short voltages to ground if they go to high, then if there is a spike the tranzorb conducts it to ground, if its a big one it may take the fuse out.

it is for my eberspacher diesel night heater, just trying to make sure it gets clean power really

Team Racing

The eberspacher is designed for cars boats etc so shouldn't require any additional filtering.

Sounds like some other issue, unless your using some shite powersupply rather than a car battery to power it.

The LM317T current rating is no where near enough for a eberspacher as they munch the power when they first fire up.

Give us some more details of the install or check your power feed and earth connections are good first.

The heater is in my camper /race transporter.
I do have a bank of 4 batteries with a split charge relay, I've also got a mains charger to charge the batteries when there is mains available.

So far I've killed 2 ecu's so trying to be carefull. I suspect that it the mains hookup that is the problem.

My thought is that the 240v mains earth is common with the van chassis, which is also ground on the 12v. This is how it was in the caravan, but I'm thinking of removing the mains earch from the van, any thoughts?

Team Racing

Schrödinger's cat - so which one am I ???

Yeah don't remove the earth!

Probable cause is the 12V charger ripple voltage.
Try adding some capacitance to smooth it.
I would recommend 2200µF 35V rating.

Also connect the eberspacher +12V and GND back to the leisure battery directly. (Using appropriate fuse also!)
This should ensure a clean feed.

At least its only the heater not working and not any thing critical like the fridge!

When I killled the eberspacher ECU's did have the power feed for the heater going directly to the batteries (with fuses).

How do I connect these capaciters, accross live and gound? (on the 12v side) and how many should I connect

Edited by Jimster on 24th Feb, 2010.

Team Racing

yep, from live to ground.

PM me your addr and i'll sort you some out.