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### Topic: In car power. (Read 1 time)previous topic - next topic

#### Bert-CWB

##### Mar 24, 2011, 08:39 pm
Hi everyone.

Right I have my project all set up ...but... I need power, I thought it would be easy but after later calculations its harder than I thought.

Soo 12v in a car isn't usually 12v for very long at all. 14.4v or thereabouts whilst the engine is running and about 8v while the car is starting up etc.

I thought about using an in car charger that outputs 5v but i found out the one I have outputs 700ma which means my car battery would only last about 62 hours (about 2 and a half days).

Could I not use a voltage divider to take the 12v and convert it to 5v? Well something like this:

Vin: 14v | R1: 5KOhm | R2: 3KOhm | Vout: 5.25v = Power dissipated by circuit = 24.5mW (Which works out at about 2mA? Correct?)
Vin: 8v | R1: 5KOhm | R2: 3KOhm | Vout: 3v = Power dissipated by circuit = 8mW (Which works out at about 1mA? Correct?)

^ That info was calculated using Electrodroid for Android

So in this case, would it be okay to use a voltage divider or is it not such a good idea?

I have the arduino, an RF receiver board and a bluetooth board to power (RF board is taking power from an arduino pin thats set High and the bluetooth is using the arduino's 3.3v supply.)

Any help is much appreciated.
Thanks.

#### RuggedCircuits

#1
##### Mar 24, 2011, 09:16 pm
A voltage divider is rarely a good voltage converter, only when currents are really really low (like <1mA).

The in-car charger you mentioned outputs 5V and 700mA, but that doesn't mean it always draws 700mA. It only means it can supply UP TO 700mA if required. The amount of current actually flowing depends on what you hook up to that 5V: Arduino, RF receiver board, Bluetooth board. Those could consume quite a few hundreds of milliamps though.

I think the in-car charger is a good solution. As for draining your car battery, that's done by your components (Arduino+RF receiver+Bluetooth) and there's not much to be done about that other than making sure the battery stays charged by running the engine.

--
The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons

#### Bert-CWB

#2
##### Mar 24, 2011, 09:37 pm

The arduino isn't running much code to be honest, probably less than 50 lines? And the RF is used for unlocking and locking the doors, as is the bluetooth board so they will be used minimally to be honest. Unfortunately I don't have a multimeter to test how much it's actually drawing, do you reckon it should be okay and I'd be pretty much good to go? I could maybe borrow a multimeter at the weekend to actually check the current draw.

#### RuggedCircuits

#3
##### Mar 24, 2011, 09:55 pm
Quote
The arduino isn't running much code to be honest, probably less than 50 lines?

Unfortunately that doesn't really matter. It's probably taking 30-40mA of current regardless.

Quote
And the RF is used for unlocking and locking the doors, as is the bluetooth board so they will be used minimally to be honest.

Then you're probably OK to just try it. I don't see anything bad happening.

--
The Rugged Motor Driver: two H-bridges, more power than an L298, fully protected

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