Finding circuit fault!

Discussion in 'General Electronics Chat' started by zaman999, Jul 17, 2012.

  1. zaman999

    Thread Starter New Member

    Jan 29, 2011
    26
    0
    Hi,
    I have a problem with an emergency light( that we use during power cut). The battery used is a sealed lead acid rechargeable battery with a stand by voltage regulation of 6.75V - 6.90V. The problem is that the tube does not glow when it is ON (or discharging mode).
    When charging, the multimeter voltage reading on the battery terminals show around 6.7V-6.8V , so i guess the recharging circuit works. When i put it ON, the voltage on the battery terminals read around 5.4V-5.53V only. So is it a problem with the battery??
    I've measured the voltage across the caps in the circuit and they read 50% of the cap voltage rating(fine i think). Only the drop across the wires connected to the tube shows 0V.
    I suspect the battery but i don't know if i missed anything while checking the circuit. I've included the images(pardon the quality).
    I dont know what could be the problem, might have to take it to a repair shop. But how do we generally find fault in a circuit ? do we check diodes/connectivity for each wire apart from checking caps?

    Thanks.
     
  2. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
    230
    It's easy when you have more than one unit to swap parts. There's likely an inverter for bulb start voltage, so I'd check if it was fused. Bulb replacement for test would be in order, as is a replacement battery. Failing those two, then I'd look at the circuit.
     
  3. KJ6EAD

    Senior Member

    Apr 30, 2011
    1,425
    363
    You didn't explicitly state it's a 6V battery or a fluorescent lighting circuit. It sounds like either the battery or the lighting circuit is bad since you have good charger output. If you want to test the lighting circuit, use a 6V supply in place of the battery. Sealed lead acid batteries will have a short life if charged at too high a rate so a good charger will have multiple rates including a very low maintenance mode. This is an example of a good charger.

    http://batterytender.com/battery-tender-junior-6v-at-0-75a.html

    Often, emergency lights are replaced when the batteries go bad since the cost of a new battery rivals the cost of a complete new light and it's an opportunity to upgrade to an LED model.

    http://www.exitlightco.com/category/Emergency-Lights.html
     
  4. zaman999

    Thread Starter New Member

    Jan 29, 2011
    26
    0
    Thank you GetDeviceInfo and KJ6EAD for your reply.
    I'm sorry I should have put it this way, the battery has the following written on it -

    -------Constant voltage charge-------
    voltage regulation
    cycle use : 7.20V - 7.50V
    Stand by use : 6.75V - 6.90V
    max. charging current : 1.35A

    Does that help?
     
  5. KJ6EAD

    Senior Member

    Apr 30, 2011
    1,425
    363
    Not much. There are just 4 functional blocks in the system (charge circuit, battery, changeover circuit, lamp circuit) and you need to isolate the problem to one of them.
     
  6. zaman999

    Thread Starter New Member

    Jan 29, 2011
    26
    0
    Thanks guys, got the battery and the light bulb replaced. Seems like the battery could not supply the required amperes of current. Also the small circuit board had loose connections due to dry solder joints causing problem.
    Now working fine, thanks for your suggestions.

    Regards,
    Zaman
     
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