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car electronics - rectifier

I'm connecting some electronics to the power in my car. I know that the car electrical system can be a little harsh, so I'm wondering the best way to protect delicate electronics. The device in question is an LCD screen I have installed. The device came with an AC power supply and a cigarette lighter adaptor. The adaptor appears to have some power rectifying circuitry. My question is, if I want to connect to this to the power, what kind of circuitry should I build? I was going to use a 12v rectifier to keep the voltage from going over 12 volts, and a cap between the input voltage and ground and another one between the output voltage and ground. Is this a sufficient design for this application, and what value caps should I use? I think I used small ceramic caps .22uf maybe? last time I made something like this. Also, I don't want to cut up my cigarette lighter cord and use the rectifier, but this is an option too.

let me know if you have any ideas. thanks!

Reply to
Matt Warnock
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You could always hit Radio Shack and buy a female cig lighter adapter. Then you wouldn't have to cut the cord.

Reply to
MZ

A rectifier won't limit output to 12V.

Make sure that it runs off 12V, not 9V. The car adaptor could really contain a voltage regulator.

A simple LC or RC filter works for automotive power spikes. LC (inductor + capacitor) filters are available as "noise eliminator" kits. RC (resistor + capacitor) filters work fine for low power equipment. 1 Ohm series on the input then 2200 microfarads to ground works. It will loose 1 volt per amp drawn. Make sure you use a single ground.

Reply to
Kevin McMurtrie

I don't know what you mean by 12 V rectifier. If you mean 12 V zener, keep in mind that nominal battery voltage with the engine running will probably be over 14 volts, and it could go higher under a trouble condition. To limit high voltage spikes, you could add an automotive transient suppressor. These generally limit at about 20 V.

Tam

Reply to
Tam/WB2TT

I smell dead battery.

A zener-resistor circuit alone is not the answer, short of triggering it with the ignition wire. Even then, the zener will probably get pretty hot. The car voltage and LCD current draw fluctuates too much to be able to accurately choose a resistor that will work well with a small zener.

(when I refer to the LCD current draw fluctuating, I mean that it changes based on the brightness setting on the LCD. I change mine based on whether or not it's dark outside)

Reply to
MZ

That's impractical. an LDO series regulator would work, but be overkill.

Tam

Reply to
Tam/WB2TT

A chip that limits the voltage to whatever its specified. I have one for 6 volts, there are ones for 6 and 9 and 12v. it has 3 pins, 1 in, 1 out, 1 shared ground. I guess I really mean a regulator, not rectifier.

Reply to
Matt Warnock

A zener would require the appropriate resistor in series with it, based on current draw and potential difference, to drop the excess voltage also.

Tom

Reply to
Tom MacIntyre

NO ! A cigarette lighter outlet provides DC. Therefore this adaptor doesn't need to provide any *rectification*.

More likely, it adjusts ( regulates ) the car battery voltage ( which varies somewhat in use ) to the correct voltage for the LCD.

Rectifiers don't do that.

Far too small to do anything. You would need several, tens maybe, of thousands of uF to significantly affect supply disturbances *plus* a series resistor or choke.

Do you know what voltage the LCD requires ? I suspect that the adaptor may contain a voltage regulator that steps down the car's nominal 12 volts to the needed value.

Graham

Reply to
Pooh Bear

Regulator, yes. Those are typically TO220 packages. They should be able to handle just about anything automotive electrically speaking. Even in a worst case scenario.

If the car adaptor has the regulation, why do you need to build anything else? You might want to open up the cigarette adapter and see if there is any regulator in the adaptor.. If there isn't, then just hook it straight to power and don't worry about it.

Reply to
Bruce Chang

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