# Home project- battery vs wall wart

Discussion in 'The Projects Forum' started by LaZyLuke, Jan 24, 2010.

1. ### LaZyLuke Thread Starter Active Member

May 10, 2009
56
0
Hi.
Not so long ago I designed a little project that involves a car stereo and a PC PS and a battery (8x AA battery to give 12v). The battery is there to keep the presets in the radio, however when left over for longer period of time they will drain as there is very lil amp hours on them. I had an idea of either bulding a battery charger that will trigger when the batteries are low, or resigning from the batteries to some old wall wart that gives 12v. The wall wart solution will be more convenient, but I have some doubts about it:

The wall warts are usually rated for amps (mA usually), and I remember blowing some old gadgets when I was lil using a wrong power supply, since I am not sure how much current the radio needs to keep the presets and I dont know how much it can handle (blowing the radio will really make me sad as it costs a lot- nice quality deck). So my idea was to measure the current flowing through the positive terminal of the batteries when the radio is off (radio will drain some small current i assume to keep the settings) using a multimeter and then trying to find a corresponding wall wart- is this a correct approach? Can I accurately measure the current this way? and I want to be 100% sure this wont hurt my radio.

Any help and suggestions will be greatly appreciated. thanks

2. ### ELECTRONERD Senior Member

May 26, 2009
1,146
16
You can't determine how much current your radio consumes while the power is off. If your radio is just a receiver, I doubt it will need more than a couple amps to operate; if even that. Just make sure you have the proper voltage requirements. If you don't have enough current, then your radio will simply not turn on. Hypothetically, let's assume your radio consumes 500mA of current and you have a 1A adapter. You will have an extra 500mA to spare (1A - 0.5A = 0.5A).

Austin

3. ### LaZyLuke Thread Starter Active Member

May 10, 2009
56
0
uhm... dont want to get of the wrong foot here but read the post carefully- I am not trying to power up the radio with the wall wart, i have a pc ps to do that(the radio actually needs about 10 amps to run when cranked - if not more)- the wall wart will only be there to supply the radio with the power to keep the settings... and I remeber few years ago I was connecting some wall wart(1.5amp) to usb steering wheel(apparently required about 750mA) and the gadget simply blew (the voltage was the same but the amps were exceeded so I assume if I have a wall wart giving 1amp and the device needs 500mA it will harm the device)... I am not gonna experiment with a 500 dollar device.... so I will have to use a second opinion about this problem. But thank you for the suggestion.

4. ### Wendy Moderator

Mar 24, 2008
20,766
2,536
The power supply does not dictate how many amps are pulled, this is done at the load (IE, radio). If you have a power supply that has sufficent amps it will work.

USB is a 5 volt system. If you used 12 volts with it then there you go.

5. ### LaZyLuke Thread Starter Active Member

May 10, 2009
56
0
No PS does not, but fuse rating does. For car audio whatever the fuse rating is thats the maximum (with lil overhead) what the device can pull(also on quality equipment with good efficiency the power rating divided by 10 is an approximation of what it will pull, the deck has an external 15amp fuse- assuming the internal a/b class amplifier of 22w rms per channel (22x4=88w- approximately 9amps on the amp itself therefore 10amps plus total current draw by the unit)- the steering wheel i used long ago was a diffewrent case altogether so I am not sure why it was brought up. I am not sure why I am explaining things that are totally irrelevant to the problem. which is pretty straigh forward itself:

I have a 12v device with very lil current draw that needs a corresponding wall wart- how to choose the correct wall wart to be 150% sure it wont harm the device. please and thank you.. (if it was a deck from a pawn shop for \$40 I will just experiment but I cant afford a failure on this)

6. ### Audioguru New Member

Dec 20, 2007
9,411
896
A low current wall-wart is cheap and simple so its voltage is not regulated. A 12V/100mA wall-wart might have a voltage as high as 18V with a light load which will blow up the radio.

When the radio is turned on then the wall-wart will still be connected which might blow up the wall-wart.

7. ### LaZyLuke Thread Starter Active Member

May 10, 2009
56
0
The circuit will be disconnected when the radio is on by a multi circuit toggle switch so wall wart wont be connected when the unit will be on.

Thank you for the advice- what wall wart would you recommend for my application? I was looking online for "voltage regulated" power supplies and wall warts but only the expensive power supplies show up (ac-dc professional power supplies)... would it be a good idea to try low voltage wall wart with an equivalent load resistance (I can try to measure the resistance of the unit when its off)?

8. ### Bernard AAC Fanatic!

Aug 7, 2008
4,176
397
One way is to use a diode OR gate , a 20A shottky diode for main 12 V supply, anode to supply, & 1N4xx for a 9V wall wart, anode to wart, both cathodes to amp. Can use simple zenor regulator for wart. With 150 ohm, 9V zen. 12-15V wart, even with main power missing & radio turned on, wart only tries to supply 100 mA. OK to measure 'off' drain to to allow better selection of series resistor.