Converting a battery powered device to an AC/DC adapter

Hi there! :slight_smile:

I have a thermal label printer that eats through 4xAAA 1.5v alkaline batteries in 15-30 minutes on average.

I have read of people using rechargeables and getting a low power warning, so my plan is to connect it up to one of the many spare AC adapters I have lying around.

I’m having a lot of trouble working the power needs out though.
I’ve googled for a couple of days and even hit up reddit electronics channel, I still can’t seem to get any decent knowledge.

Also please note, I’m a complete n00b, I only started reading books and watching youtube videos about electronics a couple of days ago. :confused: I’m learning as fast as possible.
This seemed like a good beginners project…

  1. I can’t even actually find what mAh a AAA 1.5v alkaline battery is supposed to rate at, for a ‘heavy duty’.

  2. Even if I do, the time it takes to run down the battery group is not precise, so the calculations would be guess work at best, ( and I can’t even be certain I didn’t get a couple of dud packs, event though they are EveryReady, new from OfficeWorks… )
    How do I measure what mA the device is drawing from the battery? Or “should” draw. There is no rating on the device.

  3. I read on Jan’s great “understanding battery capacity” page, that a circuit will only draw as much as it needs from the batteries. Is this true of an AC/DC adaptor too, or will it quite easily potentially pump in too many Volts/mA and fry the circuit? Would I be better off trying to find a 5v or 5.5v adapter just to be safe?

The only two 6v adapters I have run at 320mA and 500mA respectively, which seems a lot, even if the device is sucking 4 batteries dry in 15 mins…
I’m assuming I can put a resistor or something in there to choke the mA ?
Assuming I can work out what my target is.

Also any tips on making it ‘removable’, so that I can have it plugged in when I’m at home, and running on batteries when I’m out in the garage or somewhere else.

Maybe installing a socket to plug the adapter into would be better than wiring it straight onto the circuit board, so I can just plug in it. I think I have some old electronics with the female for the adapter plug…

If anyone has the patience, can help me out, also give advice and recommendations, that would be awesome. :slight_smile: if not, I’ll keep on trekking…

Thanks peeps!

Hello, Adam.

I am not sure whether modifying your label printer to work with an AC adapter or trying to find better alkaline or rechargeable batteries would be better for you, but I can answer some of your general questions:

  1. I don’t know if there is a standard capacity for alkaline AAA batteries, but if you know the manufacturer of your battery, you might be able to find the battery’s capacity on their website. For example Energizer has a page for finding technical information.

You also mentioned that you have heavy duty batteries, which this article (that I have found helpful in the past) suggests might actually mean that they have less capacity than standard batteries.

  1. You could measure the current that your printer is drawing with a multimeter or ammeter, though it might be hard to convert those instantaneous current readings into a mAh rating, since the current the printer draws will probably change depending on what it is doing.

  2. It is a general rule that a load or circuit will only draw the amount of current that it needs from a supply (this includes AC/DC adapters), so even if your supply is rated to be able to supply a larger current than your load draws, it will not damage the load device. However, it is very important that the voltage your supply outputs is in the operating range that your load device can handle. Supplying too high a voltage is generally an easy way to damage your device.

Also, the amount of current different devices draw varies a lot, so I can’t say what a normal current draw for your device is, but I would not be surprised if it was more than 500mA.

If you want to make the connection to your adapter removable, you might consider using something like our DC Power Adapter Barrel Jack.

I hope that is helpful!