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walid

12 volt battery charger

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Hi all
The circuit shown below represent a 12 volt battery charger from "Sam Electronic Circuits" and I have some questions about it:
(1) I noticed in this circuit and all charger circuits that they omit the smoothing cap after the rectifier, why?
(2) Q1= BTY79 or similar 6A SCR and Q2= C106D  SCR, they are thyristors not traiacs, thats means they latch the state even if we disconnect their gates. How they can control the charging process in this manar?
(3) D1= 1N4001 which permit current to fllow to the gate and not from it or I think that. But I see that there is no current may pass from gate, please EXPLAIN?
(4) The GR1= 50V 6A Bridge Rectifier and the transformer is 4A also the Ammeter is 0 to 5A. What making the designer sure that the batt not demand mor current like 10A, what happen if this occur?
(5) Is it necessary to connect the EARTH to the -VE terminal of the supply as shown in the schematic, EXPLAIN?
Thank you.
WALID

post-2833-14279142463598_thumb.gif

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Hi walid,

If you put a smoothing cap after the rectifier you will sabotage the function of this circuit. The circuit uses the half cycles (pulsing DC) to make it possible for the SCR to turn off, if it didn’t turn off each half cycle there would be to much current flowing! 8)

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I guess Q 1 and 2 are answered in my last post.
Q3 Why would there need to be any current flowing from the gate?
Q4 If the circuit is built correctly there are no problems with the current.
Q5 It is always a good idea to ground a project like this; if anything goes wrong with the  transformer it might save your life and the priest doesn’t have to ground you! ;D

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The following answer from Ante is very very very good:
""If you put a smoothing cap after the rectifier you will sabotage the function of this circuit. The circuit uses the half cycles (pulsing DC) to make it possible for the SCR to turn off, if it didn’t turn off each half cycle there would be to much current flowing!""
At first I'm not understand it, but when read it again I discover the GOLD inside it, How this? read on

For long time I know that thyristors when connected in circuits and applying a sufficient voltage to gate it will conduct and remain in conduction (latch) even if we disconnect the gate.
The only two ways to make the thyristor to be OFF are:
1) disconnect its anode from the supply
2) sorry forget it (perhaps to reverse the applied voltage on A and K)

Look at the figure above, Q1 is the main thy. and it controls the charging process, if you suppose wrongly (as I do) that its Anode is connected to purely (constant)DC it is 100% will not be OFF when we disconnect its gate. But if the DC applied is of the form of pulsing DC (ON OFF) the thy. will cuttig the charging current if the gate voltage is zero.
THIS IS THE MAIN RESON TO OMIT THE SMOOTHING CAP AFTER THE RECTIFIER BRIDGE, THIS CAP will sabotage the function of this circuit. 
So we can say that ANTE is a good person, thank you Ante.

Still I have more questions:
(1) I asked before this question:"The GR1= 50V 6A Bridge Rectifier and the transformer is 4A also the Ammeter is 0 to 5A. What making the designer sure that the batt not demand mor current like 10A, what happen if this occur?"
    the answer by tnk2k was:"The battery voltage builds ups to limit the current.  In case the battery shorted out, it will blow the 5A fuse."
    And the answer by Ante was:"If the circuit is built correctly there are no problems with the current."
Both of them don't understand me. What I mean is that: are all batteries demand only 4 to 5 A of current at the beginning of the charging process OR is this current is the upper limit that any car battery want at first. Why not to be 10A, the capacity of my car battery is 60A.h and if it is full discharged it may demand more than 5A at first.
(2) Also I asked:" Is it necessary to connect the EARTH to the -VE terminal of the supply as shown in the schematic, EXPLAIN?"
    The answers from Ante and tnk2k were about risk and safety, and again the don't understad me.
I know the importance of the earth in safty, but I mean: connect the earth to the transformer body and leave the -ve terminal as all circuits do?
I see many many circuits don't use the earth terminal at all and depend on the main earth in the home like the radio/recorder power supply.
(3) For D1 i think it is for maintaining a 0.7V across the gate!!!

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Hi walid,

Thank you for the kind words. I think the M1.s internal resistance (or the shunt resistance) limits to some degree the current through the battery during the pulses. When the battery voltage reaches the preset value (TR1) Q2 will prevent further “pulsing” and LD2 will be continuously lit (charge complete). When Q2 conducts R4, R3 and D1 anode is “grounded” and Q1 gets fewer or no more gate pulses. The internal lag in the battery will probably only allow about 2-3A charging current as a maximum (I am guessing now) with 100Hz switching frequency on Q1 (50Hz mains). I think this circuit will perform very well as a charger when correctly tuned, and the charging pulses will add a sulphate dissolving benefit that a standard type charger do not have. 8)

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Hi tnk2k,

Sorry to hear about your motorcycle battery!
I don’t think it’s a problem if you connect a dead battery to this charger, it will probably never start and not blow the fuse!

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