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Making a stable 8V power supply from a car outlet?
tegwin - 27/3/07 at 10:23 AM

I want to be able to plug my video camera into my cars power outlet so I dont have to worry about the batteries in the camera going flat whilst im filming...

The cars power outlet ranges from 12-14V...

The camera needs 7.9v (8v)

Is there a simple circuit that I can nock up that will give a smooth stable 8V supply?

The current drain is going to be tiny!

Any thoughts?

nitram38 - 27/3/07 at 10:25 AM

Buy a multi voltage unit from maplins. They are only about £8 to buy.

iank - 27/3/07 at 10:33 AM

quote:
Originally posted by tegwin
I want to be able to plug my video camera into my cars power outlet so I dont have to worry about the batteries in the camera going flat whilst im filming...

The cars power outlet ranges from 12-14V...

The camera needs 7.9v (8v)

Is there a simple circuit that I can nock up that will give a smooth stable 8V supply?

The current drain is going to be tiny!

Any thoughts?


Yes, a LM317T and a couple of resistors and caps. All parts available from maplins, would cost £1.50 tops built on veroboard.

Let me know if you need a circuit drawn up.

RazMan - 27/3/07 at 10:34 AM

If you want to hard wire something then a small DC-DC converter should do the trick. Maplins to the rescue

tegwin - 27/3/07 at 10:42 AM

Chears IANK. A schematic would be handy.

I dont supose I can use an LM1458 or an OPA2134PA instead can I?....I already have boxes of those, so would be handy to use them up!

Chears.

iank - 27/3/07 at 10:52 AM

quote:
Originally posted by tegwin
Chears IANK. A schematic would be handy.

I dont supose I can use an LM1458 or an OPA2134PA instead can I?....I already have boxes of those, so would be handy to use them up!

Chears.


OK, will put one up tonight. Those chips you mention are op-amps, the LM317T is three leg adjustable voltage supply (69p from Maplin )

Winston Todge - 27/3/07 at 11:42 AM

An LM7808 reduces component count and would only require a couple of capacitors for bypass and response. But doesn't strictly need these either... Rescued attachment 8VReg.png
Rescued attachment 8VReg.png

MikeRJ - 27/3/07 at 01:36 PM

quote:
Originally posted by iank

Yes, a LM317T and a couple of resistors and caps. All parts available from maplins, would cost £1.50 tops built on veroboard.

Let me know if you need a circuit drawn up.


Don't forget a diode connected between the input and output pins (reversed biased with regulator powered).

This saves the regulator in the event of the regulators input going to 0v (i.e. ignition off with ignition controll cig lighter) if there is any capacitance on the output (e.g. inside the camera). Often missed out, but considerably improves the reliability for the few pence that a 1N400x rectifier costs.

tks - 27/3/07 at 03:34 PM

Just sample a 7808 from TI
and add the diode on the input in forward bias

add a couple of caps for a smooth voltage and you are done..

Tks

iank - 27/3/07 at 03:57 PM

quote:
Originally posted by tks
Just sample a 7808 from TI
and add the diode on the input in forward bias

add a couple of caps for a smooth voltage and you are done..

Tks


Well yes, but try getting one if you aren't a company that looks like a prospect to some sales droid

I've always found I had to sit through 20mins of nonsense to get any freebies even when I worked for major electronics companies who bought thousands of components.


While a 7808 means 2 fewer resistors the lack of availability over the counter at an national electronics shop makes it much more awkward to make.

iank - 27/3/07 at 08:27 PM

OK, I think this is correct, but obviously measure the voltage before blowing up any expensive electronics kit. Using a multi-turn trim pot (or E96 range resistors) would allow fine tuning of the output voltage.

In theory you don't need any fuses as the power reg is short circuit protected, but I'd get the input from a convenient fuse anyway.

8v power reg
8v power reg

tks - 27/3/07 at 08:37 PM

cant see any reason wy you would put that diode in it..

infact the car will pull from the caps from your device..

i would add a input diode for minimum
atleast it disables the car from pulling from the device and from the input cap..

Tks

iank - 27/3/07 at 08:54 PM

quote:
Originally posted by tks
cant see any reason wy you would put that diode in it..

infact the car will pull from the caps from your device..

i would add a input diode for minimum
atleast it disables the car from pulling from the device and from the input cap..

Tks


Read MikeRJ's post for the diode. If you do an internet search you will see it's common practice to prevent the regulator being damaged by the output staying up longer than the input.

What function would in input diode have?
The caps wouldn't power a car for very long so I'm not sure what you mean by pulling power from them.