Failed Battery Charger

[Chile72]

This is my first post as a newbie to this site. I want to be in the correct forum. Guidance will be readily accepted. I want to post a schematic of a Traveller Battery Charger that I drew after 3 identical units failed. The aim is to find the weak link and correct it. Though I understand most of the circuit I need help. In what format should schematics be posted in? {Corrected title to reflect subject}

 

[JimB]

Welcome to ETO.

Post your schematic here in General Electronics Chat, it is as good a place as any other.

JimB

 

[Chile72]

Thanks. I understand the use of the shunt regulator and the fan and relay circuit, but I can't figure what produces the gate voltage on the SCR. Does the battery being charged supply the initial voltage? (I read DerStrom's post on drawing schematics and will attempt to better this one.)

 

[Les Jones]

I consider this to be a series regulator rather than a shunt regulator. When transistor P2 is conducting it effectively connects the gate of the SCR to it's anode via R7. This triggers the SCR into the conducting state. Transistor P2 is turned on via transistor N3 and R13. Transistor N3 is biased on by R16. It can be turned off by transistor N2 when it conducts. Transistor N2 is turned on when transistor P4 is conducting, Transistor P4 is turned on by the TL431 when the voltage on it's pin 2 is more positive than +2.5 volts with respect to it's pin 3. (Which is also the ground reference point.) P1, P3 and N1 together with the state of the relay contacts seem to adjust the ratio of the the potential divider that scales down the battery voltage down to the 2.5 volt reference level of the TL431. I have not yet worked out exactly how this part works.

Les.

 

[Chile72]

I am tracing through the circuit now with the explanation you have given. I look forward to clearly understanding the circuit because it has some area that is particularly weak. I bought a second charger after the first failed, and then the company replaced the second when it failed, and finally the one they replaced died also. My plans are to strengthen the weak components since I could repair both units. I drew the schematic by tracing the circuit board. I've doubled checked everything, but it is certainly possible I have missed something. Your help is certainly appreciated.

 

[Les Jones]

What is the function of the switch that switches between taps on the transformer ? What voltage battery and what type of battery is the charger designed to charge ? Which component (or components.) failed in the chargers ? This may be a better rout to find a possible design fault.

Les.

 

[Chile72]

Les , the switch is an external switch that switches between 6 Volts and 12 Volts to distinguish batteries. This charger is a ten amp charger specifically for lead acid or car batteries, but it's controlled output allows for deep charge batteries as well. I've not been able to test all transistors in the circuit because they are surface mount and obviously I get readings from external components. That was to be my final test if I did not figure out the probable components before. I suspect heat as a culprit because the fan is too small. The entire transformer delivers about 15 volts and the tap about 8 volts. The relay of course does not get pulled in except when in the tap position (8 volts) which effectively takes resistor 10 and 15 out of parallel with R12 and I kinda thought that might be to adjust the voltage to the reference pin of the TL431. That's where I was stumped. Somehow it shuts down the SCR when the battery reaches a full charge of somewhere around 12.6 to 12.8 volts. On one, I unsoldered all leads to the board that were not necessary and it worked fine as a brute charger. Therefore all major components are fine. As a side all transistors are 2T and 2X respectively. I did some cleaning on the schematic and here is a revised copy.
Roger

 

[Les Jones]

I have re drawn your schematic to make it easier to understand how it works. I have also shown the potential divider as just two resistors . RL and RH
RH is R11 and R2 in parallel which gives a value of 17.01 K. In 6 volt mode RL is 9.1K (The value of R12) In 12 volt mode RL is R12, R10 and R15 in parallel which gives a value of 3.594 K

In 6 volt mode this means that 7.17 volts is required at the top of RH to give 2.5 volts. (The value that starts the TL431 conducting.)
In 12 volt mode this means that 14.33 volts is required at the top of RH to give 2.5 volts.
When these voltages are reached transistor P3 conducts which puts R4 in parallel with RH. which means that now only 6.49 volts in 6 volt mode or 12.61 volts in 12 volt mode is required to give 2.5 volts at the junction of RH and RL. This means that when the battery is put on charge it's voltage is allowed to 7.17 or 14.33 (Depending on the mode.) before the charging current is reduced. It will only go back to full current charging if the battery voltage drops below 6.49 or 12.61 (Depending on the mode.)
There does not seem to be anything in the design to limit the charging current. (Other than the fuse.)
In what way did the chargers fail and did they all fail in the same way ? If you still have the faulty board you will need to diagnose which component (Or components) have failed. It would also be interesting to monitor the charging current with the working charger to see if the charging current drops to zero or some trickle charge value. after the cutoff voltage.

Les.

 

[Nigel Goodwin]

I have re drawn your schematic to make it easier to understand how it works. I have also shown the potential divider as just two resistors . RL and RH
RH is R11 and R2 in parallel which gives a value of 17.01 K. In 6 volt mode RL is 9.1K (The value of R12) In 12 volt mode RL is R12, R10 and R15 in parallel which gives a value of 3.594 K
View attachment 124790

There does not seem to be anything in the design to limit the charging current. (Other than the fuse.)

In battery chargers the current limiting is normally done by the transformer itself, which is specially designed for that purpose - which makes it more complicated if you're making a home made charger.

BTW, nice redraw - makes a LOT more sense now

 

[Les Jones]

Hi Nigel,
One thing I have not managed to understand is the purpose of transistor N1 shorting the reference voltage to ground. That would prevent the TL431 from having any control of the SCR triggering.

Les.

 

[Diver300]

Hi Nigel,
One thing I have not managed to understand is the purpose of transistor N1 shorting the reference voltage to ground. That would prevent the TL431 from having any control of the SCR triggering.

Les.

I think that it is to stop the charger measuring the voltage at the peak of the half-cycle, when the battery is being charged. The idea is that the charger measures the voltage when the battery is at low current, so that the internal resistance of the battery doesn't make the charger cut off too early.

Thank you for the redraw. I agree that it makes it far more readable.

 

[Les Jones]

Hi Driver, I did wonder if that might be the reason. It will latch the gate drive on to the SCR before the voltage from the bridge is higher than the battery (So the SCR is in a condition were it can latch on itself.) The only possible design problem I can see is the base emitter reverse voltage rating of transistorP1 might be exceeded when the bridge output is close to zero. If that did turn out to be the failure adding a diode across the base emitter junction should solve it.

Les.

 

[Chile72]

I'm grateful for all of the replies. It will take me a bit to assimilate all that has been said. Hopefully I can spend time this evening applying it to the circuit. I will remove P1 to test if necessary. Since I had to exchange the units for the warranty I only have one board in its original condition. Should the transistor turn our to be bad, I will make the recommended modification to the board with a diode. I'm glad I chanced up on this forum.
Roger

 

[Les Jones]

I don't advise just removing the transistor based just on a vague suspicion. I suggest using normal diagnostic methods to trace what is causing the problem. Telling us what the nature of the problem may help us to suggest a diagnostic approach. It is possible that just the nature of the problem could eliminate this transistor as a possible cause.

Les.

 

[Chile72]

I have followed your advice and am methodically testing each section as I have time which is very limited. In a previous post I mentioned all major components appeared to be ok, but it did not occur to me at that time that I had replaced the shortstop fuse on the unit I turned into a manual charger. Les, it appears that your assumption that the fuse is the limiting factor to the current is correct. The battery begins to present resistance and for that reason the amp meter also serves as an indicator of relative charge as it displays gradual drop of the current. I have a fuse ordered but it will be several days before it arrives with the shipping delays we are facing in my area, therefore I will not be posting until then. I have a much better understanding of the circuit due to the comments received on this site, for which I am grateful.
Roger

 

[Chile72]

Thanks Les and others who have contributed to this thread. While inspecting each component on the board I discovered an almost hidden soldier bridge on R7 and my shortstop fuse had what I think is properly described as chatter. The fuse has been replaced and the soldier bridge cleaned up. It is finally evident to me that this charger cannot start charging without first having a battery with some initial voltage that activates N3 through R6 which then causes P2 to latch the SCR. I was able to test this part with a small 9 volt battery and the SCR now latches. I plan to install a larger fan for better cooling and see how the rest of the circuit works.
Roger