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Need a 3VAC Adaptor

Discussion in 'General Electronics Discussion' started by Dan0, Apr 2, 2013.

  1. Dan0

    Dan0

    3
    0
    Apr 2, 2013
    I've got an old Black & Decker "Ergo" can opener and the PS no longer works and i have not been able to find one. It's 3VAC, 100mA. It has a couple of 1.2V AA NiCad batteries packaged together putting out 2.4V. Looks like there is a diode/rectifier inside the wireless, hand held can opener.

    What happens if i try to use a 3VDC adapter??? Any suggestions?

    Thanks
     
  2. shrtrnd

    shrtrnd

    3,674
    453
    Jan 15, 2010
    The diode/rectifier is to convert your 3VAC input to DC. You can't just plug in a DC adapter,
    because you've got the diode/rectifier circuit to contend with.
    If you're sure your NiCads are ok (which may be your problem), you can use a 3VAC adapter with
    a higher amperage than 100ma. The circuit will only draw what it needs.
    You'll have an easier time locating a 3VAC adpater with more than 100mA output.
     
  3. Electrobrains

    Electrobrains

    259
    5
    Jan 2, 2012
    1. Why not repairing the power supply?

    2. I am almost 100% sure, you can use a DC power adapted instead of the AC one (but not the other way around).
    The diodes inside your unit sure are put together as a bridge rectifier, producing DC voltage for charging the batteries. A 3VDC supply could work, even better a 3.5V or 4VDC type. The polarity should not matter (because of the bridge).
     
  4. ModemHead

    ModemHead

    69
    20
    Dec 2, 2010
    Firstly, if the device is old, I agree that the cells are more likely to be the problem. Do you have a multimeter? If a cell measures less than about 0.9V it is probably dead.

    If your can opener is anything like the B&D products I've looked inside of, their "charger circuit" consists of nothing more than a series diode and a "weak" AC transformer. They use the internal resistance of the transformer to limit the charge current. This allows for constant trickle-charging.

    Using a larger transformer may over-charge the batteries, leading to a fire hazard.

    The batteries are probably 1200 or 1300 mAh cells, so the current should be limited to 130 mA. A small DC adapter could be used if you get the polarity right, but only if the current stays limited to the 100mA neighborhood.
     
  5. Dan0

    Dan0

    3
    0
    Apr 2, 2013
    Thanks for the replies

    I replaced the batteries and the unit works fine but then I thought I better check the charger with my multimeter. I got nothing out of the charger.


    That sounds exactly like what's going on.

    That was my concern...

    Thanks... I wonder how I can verify the mA...

    and I guess I can leave the diode in there...

    Thanks again
     
    Last edited by a moderator: Apr 2, 2013
  6. Dan0

    Dan0

    3
    0
    Apr 2, 2013
    Well - I believe I have my Ergo Can Opener fixed!!! Thanks everyone for your help!

    I replaced the batteries with a couple of 1700mAh NiMH cells. The originals had a couple of tabs for the wire connections. We were able to remove those and solder them to the new batteries.

    While I had the can opener opened, I made note of which contact (in the handle) was +positive. The charging cradle that it sits in is "keyed" so you can only put it in one way. When I connected my new charger I made sure I got the polarity right. I left the diode in the can opener and closed it all back up.

    The Adapter that I got is a Universal DC Adapter, 3 to 12VDC, 600mA. Not exactly what I wanted but it seems to work fine for some reason. It's a really super light weight adapter, not like those old heavy things they used to sell. I glued the polarity and voltage selector switches in place so no-one could accidentally change them.

    I'm a little concerned about overcharging the batteries with an adapter rated at 600mA. I'm not exactly sure how that works. I've gotten some conflicting information on that issue. I think of current as something that the circuit draws. If it doesn't need it, it doesn't draw it.

    I've been keeping an eye on this thing. Nothing seems to be getting hot. There may be something in the adapter circuit that prevents it from overcharging.

    Any comments are appreciated!

    Thanks again!

    [​IMG][​IMG]
    [​IMG]
     
    Last edited: Apr 20, 2013
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