****12v battery charger to 24v****

Discussion in 'General Electronics Chat' started by giannishalkida, Jan 29, 2014.

  1. giannishalkida

    Thread Starter New Member

    Jan 29, 2014
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    Hi,
    I would like the opinion of people who have more knowledge
    about electronics from me.
    I found a simple and nice circuit (picture attached) of a12v lead acid battery charger with thyristor.
    Does anyone know what part i must replace in order to convert it in 24v charger?

    I know the easy answer.... the transfoirmer..... hahahaa

    Anything else needs to be changed?????

    Thanks !!!!!!!!
     
  2. tcmtech

    Well-Known Member

    Nov 4, 2013
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    If it's a full wave rectifier power source to start you will need to change the transformer.

    After that the voltage reference will need to be changed as well.
     
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  3. giannishalkida

    Thread Starter New Member

    Jan 29, 2014
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    voltage reference?????? the zener?????

    A friend told me that i must change the bottom right resistor
    from 5.6 to 2.2 Kohm. how that sound???
     
  4. giannishalkida

    Thread Starter New Member

    Jan 29, 2014
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    It seems tricky question.........
     
  5. tcmtech

    Well-Known Member

    Nov 4, 2013
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    That would work too.
     
  6. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
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    Not being willing to do the math.

    I'd start with doubling all the values including zener.

    That should start a discussion.:D
     
  7. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
    1,190
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    I recommend people to buy a charger instead of playing with something that can cause alot of damage if it can not be watched at all times ..I wish you good luck on the build ...
     
  8. #12

    Expert

    Nov 30, 2010
    16,261
    6,774
    I calculate 1400 ohms.

    All the other parts will do just fine with no changes.
    Still, you must understand that the proper setting is the "float" voltage for that battery.
    There is no shut off feature, so this must be a float charger.
     
    inwo likes this.
  9. giannishalkida

    Thread Starter New Member

    Jan 29, 2014
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    From my basic electronics i assume that there are two ''stages'' in this circuit.
    First stage: the battery voltage monitor
    Second stage: The thyristor controller.

    So (i think....) i must replace something which monitors the battery voltage?

    :confused::confused::eek::rolleyes::D

    electronics is my hobby.....i only build ready circuits... i wish i could understand how this simple circuit ''thinks''.
     
  10. inwo

    Well-Known Member

    Nov 7, 2013
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    Thanks, as a practice exercise, I came up with similar. 1600 from my scribblings last night. :)

    It depends on the ultimate battery voltage and the rheostat I guess. I hate doing the math.

    Seems my first guess might also be close? Doubling the voltage divider resistors and the zener value.

    Have you found a circuit explanation? Other than the bottom (switch), I don't understand the circuit.

    I could design a similar circuit, but have difficulty understanding other's designs.:eek:
    Like our "dryer" discussion.:p

    Is the top of the voltage divider at the battery pure dc? (for design purposes)
    Is the scr and base of top transistor a phase control circuit timed from the output (pulses) of the bridge?

    That's how most scr chargers I've run across work.
    With low battery, scr triggers ~90 deg., triggering later as battery charges.
     
    Last edited: Jan 30, 2014
  11. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
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    That's what #12 and your friend suggested.;)
    The resistors on the right form a voltage divider. A lower value of the bottom resistor (or higher value zener) means it takes a higher battery voltage to reach the trigger point of the bottom transistor.

    IMO
    I'm hoping for a complete circuit explanation from an expert or simulator.
     
  12. #12

    Expert

    Nov 30, 2010
    16,261
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    1400 ohms, 1655 ohms...depends on where the pot is adjusted to. I tried the full 2500 ohms first and saw that the float voltage would be way too high. Then I tried zero for the pot and got answers that seem about right. Just change one resistor.
     
  13. #12

    Expert

    Nov 30, 2010
    16,261
    6,774
    Now for the explanation.

    The transformer is providing pulses that peak at about 30 volts.
    The top transistor is biased to always fire the SCR whenever there is enough voltage on the rising edge to exceed the battery voltage by about 3 silicon junctions.
    The voltage divider string is attached to the battery and thus measures DC.
    If the DC is high enough to start up the lower transistor, it sucks the bias out of the top transistor so it can't fire the SCR.
    The 2k2 on the left vertical line is what I call a warming supply for the zener. Without that, the zener would start allowing current through the bottom transistor at random voltages that depend on temperature.

    Any questions, boys and girls?
     
    inwo likes this.
  14. ian field

    Distinguished Member

    Oct 27, 2012
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    You can make the bridge rectifier into a voltage doubling rectifier by adding a series pair of capacitors from positive to negative and the tap between the two caps to *ONE* AC arm of the bridge - just one problem though, it'll no longer be rough DC, so no commutation of the control thyristor (once triggered it'll stay on!).

    If you go that route, you'll have to use a linear regulator instead. The capacitors have to be quite hefty to get decent output current.
     
  15. #12

    Expert

    Nov 30, 2010
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    Yikes. You caught me. I was thinking in terms of a capacitive input filter when I said 30 volts. This transformer will not do that. It will not charge a 24 volt battery.
     
  16. ian field

    Distinguished Member

    Oct 27, 2012
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    Not quickly anyway - unless you use very large capacitors and forced air cool the transformer.
     
  17. giannishalkida

    Thread Starter New Member

    Jan 29, 2014
    5
    0
    thank you all for your answers.!!!
    i will try to build it this weekend by replacing the bottom
    right resistor with a smaller one (around 2kohm) and see what is going to be happen, i hope i will not fry the transistors
    :D:D:D:D:D:D
     
  18. inwo

    Well-Known Member

    Nov 7, 2013
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    Looked thru the thread.

    You may be wrong, thinking you were wrong.
    The transformer OP is going to use was never specified, was it?

    It can be assumed that a transformer double the voltage of the 12v version is needed.

    Does someone know the answer before OP builds this?
    If not, I'll measure on here.
    I'm thinking 12vrms + diode/scr drops? ~14vrms? 13.8 rings a bell.
    Doubled for 24v of course!
     
  19. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,520
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    I built a 12v version about 25yrs ago, it just used TL431c to regulate the SCR.
    A couple of resistors and a diode.
    Max.
     
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