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  1. #21
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    Re: Extremely simple overnight AAA Ni-MH charger

    Just a series diode from the output of the 78M05 to the battery? (the input to the 78M05 is 13.8V)
    But how the "comparison" of the series discharge diodes and the battery would be done then? (since there is an isolating diode between them)
    The current is the same anywhere in a single series circuit so you can add the diode either in the input to the 78M05 or in the GND pin as long as the load resistor is still connected directly across the 78M05.

    However, couldn't I just use an extra germanium diode in series with the 1n4001 diodes? (I worry about the current that a 1n34a can pass through it, can it be as high as 75mA?)
    The current would always be 75mA, regardless of the state of battery charge. For overnight charging as you requested, that should be OK but an ideal charging circuit would monitor the battery voltage, current and temperature. It would no longer be a "Extremely simple overnight charger" if all of those were monitored. The optimal way of charging is to monitor the temperature and note the increase as full charge is reached but adding a sensor and control circuit would make it quite complicated. Few Ge diodes can handle constant 75mA, the 1N34a is only rated at 50mA. Using two 1N4001 and one 1N5918 might work, they have a Vf of about 0.3V at 75mA.

    A final thing that worries me, is that ok to charge the battery that way, when the preamplifier is being powered at the same time as well?
    It should be completely safe, even if the battery is removed. Bear in mind that if the battery is completely discharged it will short out the pre-amp supply anyway, not dangerous, it just won't work for a few minutes until the voltage has risen.

    Brian.
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    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.


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  2. #22
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    Re: Extremely simple overnight AAA Ni-MH charger

    Quote Originally Posted by betwixt View Post
    The current is the same anywhere in a single series circuit so you can add the diode either in the input to the 78M05 or in the GND pin as long as the load resistor is still connected directly across the 78M05.


    The current would always be 75mA, regardless of the state of battery charge. For overnight charging as you requested, that should be OK but an ideal charging circuit would monitor the battery voltage, current and temperature. It would no longer be a "Extremely simple overnight charger" if all of those were monitored. The optimal way of charging is to monitor the temperature and note the increase as full charge is reached but adding a sensor and control circuit would make it quite complicated. Few Ge diodes can handle constant 75mA, the 1N34a is only rated at 50mA. Using two 1N4001 and one 1N5918 might work, they have a Vf of about 0.3V at 75mA.


    It should be completely safe, even if the battery is removed. Bear in mind that if the battery is completely discharged it will short out the pre-amp supply anyway, not dangerous, it just won't work for a few minutes until the voltage has risen.

    Brian.
    Thanks for all the help all of you!
    It has been really helpful and educational. I like this simple "comparison" technique with the diodes.
    I am going to test it soon to see how well it will perform in practice.

    With 50mA charging current about how many hours should I wait for a 750mAH battery to be charged?
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



    •   Alt15th January 2018, 16:02

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  3. #23
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    Re: Extremely simple overnight AAA Ni-MH charger

    That's very difficult to predict and varies from one battery to another on original discharge state and how much they have been used. 50mA is C/15 rate so a guess would be at least 15 hours. The charge rate levels off though so it may reach 50% in one hour 75% in two hours and take many more hours to reach 90% then 100%.

    Brian.
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    •   Alt15th January 2018, 16:08

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  4. #24
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    Re: Extremely simple overnight AAA Ni-MH charger

    Quote Originally Posted by betwixt View Post
    That's very difficult to predict and varies from one battery to another on original discharge state and how much they have been used. 50mA is C/15 rate so a guess would be at least 15 hours. The charge rate levels off though so it may reach 50% in one hour 75% in two hours and take many more hours to reach 90% then 100%.

    Brian.
    MAny thanks!
    Here is the page. http://qrp.gr/thf7aa/index.htm
    The circuit discussed is at the end of the page.
    Time for some testing!
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



    •   Alt15th January 2018, 16:22

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  5. #25
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    Re: Extremely simple overnight AAA Ni-MH charger

    Quote Originally Posted by betwixt View Post
    The current would always be 75mA, regardless of the state of battery charge. For overnight charging as you requested, that should be OK but an ideal charging circuit would monitor the battery voltage, current and temperature. It would no longer be a "Extremely simple overnight charger" if all of those were monitored. The optimal way of charging is to monitor the temperature and note the increase as full charge is reached but adding a sensor and control circuit would make it quite complicated. Few Ge diodes can handle constant 75mA, the 1N34a is only rated at 50mA. Using two 1N4001 and one 1N5918 might work, they have a Vf of about 0.3V at 75mA.

    Brian.
    Can I put the power switch at this place?
    What happens in that case, where the battery is permanently connected across the diodes?
    Will it discharge to ~1.3v (Vf) and then stay there, or will it continue to discharge through the diodes and get empty?
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



  6. #26
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    Re: Extremely simple overnight AAA Ni-MH charger

    Yes, you can switch it there. The only drainage path is through the series diodes when the switch is open so when the battery voltage drops below 2 x Vf, the diodes will stop conducting (apart from tiny leakage current) and nothing will further drain the battery. Do be careful to heed the earlier advice, a fully charged battery may have a higher terminal voltage than 2 x Vf so current WOULD be drawn until the voltage dropped. Also note that diodes do not have an abrupt conduction voltage, their current gradually tails off below Vf so for a short while some current might flow. This is why it was suggested to increase the clamping voltage to slightly higher than full battery voltage, it is to ensure there is a safety margin against leakage while not letting it go too high that other things might be damaged.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



  7. #27
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    Re: Extremely simple overnight AAA Ni-MH charger

    I wonder what's the reasoning behind using a battery. Why not a permanent supply with bias-T?



    •   Alt17th January 2018, 19:13

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  8. #28
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    Re: Extremely simple overnight AAA Ni-MH charger

    It's an external add on to an existing receiver/transceiver, I guess they don't want to modify it to let DC out.
    I'm puzzled as to why 1.2V is being used though when the device is rated to 6V maximum and characterized at 3V to 5V. It probably doesn't perform well on low voltage.

    Over to you Neazoi....

    Brian.
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  9. #29
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    Re: Extremely simple overnight AAA Ni-MH charger

    Quote Originally Posted by betwixt View Post
    It's an external add on to an existing receiver/transceiver, I guess they don't want to modify it to let DC out.
    I'm puzzled as to why 1.2V is being used though when the device is rated to 6V maximum and characterized at 3V to 5V. It probably doesn't perform well on low voltage.

    Over to you Neazoi....

    Brian.
    Read the info in my page http://qrp.gr/thf7aa/index.htm the device found experimentally to be working great on 1.2v and down to 0.6v! The difference from 3.7v is 2 S-units in the receiver scale. This was an important consideration for minutarizing it. The final active antenna will be a small rod (~10cm in length) of the same diameter as the radio rubber duck. It will fit between the rubber duck and the radio and it could be left onto the transceiver (either switched on or off) even while transmitting, but without using any T/R switching.
    The antenna has been used for over a year now and it performs great out in the garden and even inside the house in "quiet EMI locations".
    The charger is a way to avoid having to remove the battery to charge it, which would be very impractical in such a small enclosure without taking apart everything. Moreover the antenna needs to be splashproof so no holes would be made to it to fit switches and connectors. It is charged by the same SMA connector which is connected to the radio, of course the antenna is removed from the radio first.

    I have built the charger circuit, and now I am trying to empty the battery to 0.6v before trying to charge it again using your proposed schematic to see how it performs. The constant current source works great!

    If you find any good way to make a waterproof on/off switch this would be helpful. The only way I can think of, is an internal reed switch, switched by a small magnet (ring or button) attached to the outside of the antenna.
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



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