circuit to extract 7.5v from car battery

I am looking for a circuit that will output 7.5V from the 12V car battery? What is the best solutiosn for this using as little components as possible. TIA

You have not given a very useful specificiation.

What sort of accuracy - +/- 2V, 2mV, 2uV ?

What sort of current - 100A, 1A, 1mA or whatever ?

It could be as simple as 2 resistors if the accuracy requirements are lax, and the current small. It will need some form of complex swith mode design if you want to take 100Aa.

You didn't specify how much current you need, or what the application is.

Here's a standard LM317T circuit with an added TVS diode that might meet your needs. It will provide up to 1.5 amps, if you mount the LM317T on a heatsink:

--------

+12 ---+---+---Vin| LM317T |Vout---+--------+--- +7.5 | | -------- | | | | Adj [R1] 240 | | | | | | + [D1] [C1] .1uf +------------+ [C2] 1 uF | | | | | | [R2] 1.2K | | | | | Gnd ---+---+----------+---------------------+---Gnd

D1 is a 15 volt TVS diode like Digikey part# 1.5KE15CADICT-ND

Ed

The 15V TVS will blow. I would go with something like 27V, followed by an LC filter. If the current is low, he could put a resistor ahead of the TVS.

Tam

Why? It'll handle up to 71 amp transients. Is the steady voltage over 15 volts? Shouldn't the battery hold it lower? I've had one of them across my 2 meter rig for years. Fed from the battery with #12 - say

20 feet total wire - with a 30 amp relay at the rig, energized by the accessory line.

The LC is a good idea, but is contrary to what he wanted - "as little components as possible". I think he might get by with the standard 317 circuit with the added TVS and no LC to fulfill that "little" requirement (which I take to mean parts count).

Ed

I would go with something like 27V, followed by an LC

The minimum breakdown voltage on the 1.5KE15 is 13.5V.That is not good. Depending on how the car is wired up, and where the battery is located, you could have more than 15 VDC at the cigarette lighter socket. My previous car ran about 15.2V. Original battery was still good when I sold the car; so, this was not a problem. Battery was under the rear seat.

The 317 is good for 40V, but noise suppression is poor at high frequencies. That's the reason for the LC filter. Might work OK without the filter, but inductors don't cost anything if you can afford to spend 5 minutes to make one.

Tam

The datasheet for the specified part does not agree. It gives 14.3 V. See:

That is not good.

Ok. That makes your point that a higher voltage TVS is indicated.

Original battery was still good when I sold the car; so,

The reason for the LC filter is not in question. For the OP's requested minimum parts count, it must not be included, unless mandatory. Of course it could be argued that the OP might be able to use a zener and a resistor and nothing more. We're not talking best design, here, we are talking about a minimal circuit that might meet his needs.

Ed

Of course it could be argued that the

That may not be bad at all. A zenar with an emitter follower current booster. (unless he only wants 10 ma)