Need help to power an external device

**Gobby** · 12-29-2005, 09:53 AM

I'm after powering an audio unit in the car running at 12V but the rating seem very low and I fear I may wreck it if connected directly to the car.

The Main Adapter Output info is:

Output 12V DC: 800mA 9.6VA

Does this mean anything to anyone and can I somehow get the device to work in the car?

Thanks

**raucous** · 12-29-2005, 11:10 AM

The problem is that you need a constant 12v output. Your alternator puts out 14v. If you connected this device (which you could do, by placing an appropriately-sized resistor on the positive input to the audio unit) directly to the positive terminal on the battery, there is a possibility that the unit's life would be shortened due to having a higher voltage than it needs. Not to mention the possible damage when the car is cranking and the battery voltage drops under 10v.

Ideally, I would suggest a power supply designed for a vehicle, that outputs a constant 12v, with an input range of 9v - 15v. The Opus 80 will do this for you, but at $90, it's quite expensive.

Another option is a power inverter, which you could then use to convert your car's 12v/14v DC output into 120v AC, and plug into it the adapter that you have.

This "9.6VA" is bothering me. I feel like I should know what it is, but I can't remember. Anyone?

**RawsonDR** · 12-29-2005, 11:24 AM

Volt-Amps (Watts)
.8 * 12 = 9.6 Watts

**raucous** · 12-29-2005, 11:36 AM

Well now, don't I feel stupid?

**Gobby** · 12-29-2005, 11:37 AM

Well I already have a Carnetix PSU powering the CarPC itself, it was the 800mA I was concerned about, didn't want to mess up the device.

**The unL33T** · 12-29-2005, 11:42 AM

You can get a 12V regulator from an electronics part store. It will output a constant 12V as long as the input voltage is equal or more than that. They are pretty cheap (5 for $5 CAN) and rated at 1 Amp each. The part number is LM7812. Here's a link to the datasheet:
http://alldatasheet.com/datasheet-pd...LD/LM7812.html

**raucous** · 12-29-2005, 11:53 AM

unl33t: But unfortunately, you only need a .37v (usually) drop before your car battery is now BELOW 12v. Plus, how MUCH over 12 does the voltage have to be? .5v? 1v? Car running, you've no problem. Car off, you're coming very close to underpowering whatever audio equipment Gobby has.

Gobby: Which Carnetix PSU? What do you have connected to it? 800 mA at 12V is 9.6 watts. Add that to whatever watts get pulled by everythng else connected to the Carnetix (use the max watt rating of the equipment, to be safe). If that number is within 10% of the PSU's output watt rating, then you shouldn't connect anything further.

**Gobby** · 12-29-2005, 12:26 PM

Ahhh 9.6Watts is more what I understand tbh than the other figures

I can measure how much I'm using at the moment tomorrow.

**raucous** · 12-29-2005, 12:37 PM

1 milliamp is .001 amp. 1,000 milliamps = 1 amp, so 800 milliamps is .8 amps. The output is 12V, and amps times volts equals watts. .8a * 12v = 9.6 watts, as Rawson stated earlier. Hopefully this will help you understand numbers like these in the future. =)

**The unL33T** · 12-30-2005, 06:32 PM

The voltage regulator only needs to be equal or above 12V to output 12V. If it is less than 12V then it will output whatever the input voltage is. If you are concerned about voltage drop while starting the car you might be able to keep it up temporarily by using a capacitor or something.