Trouble with newly assembled LM317 based SLA battery charger output

[debasisswan]

Hi all,

I am new to this forum. As an Astronomy hobbyist, now I find reasonable level of electronics knowledge as most important as nothing in this world including astronomy can run without electronics.

Though I have small experience of working with PCBs, I have idea (Very Limited) about the schematic signs of electronics kits and their work pattern and hands on experience of soldering, practically I am near ignorant about the deep inside of electronics world.

So, I need your help. Please enlighten me.

I was looking for a small battery charger to charge my 6V 4.5Ah Sealed Lead Acid batteries those I use in field to run the motor drives of my Telescope. As those mostly run under open sky, fast drainage is a problem and need proper charging.

After doing a little bit of homework, I preferred a circuit diagram (circuitdiagram.org/6v-4.5ah-battery-charger-circuit.html), arranged kits and assembled it. I liked the LM317 based voltage regulation part of the circuit.

So far I could learn, the circuit should give stable 400mA (Around) current at 7.2V DC at its output. The,output voltage is not expected to fluctuate depending upon the load.

But, after assembling the board, I found something different. The output without load is coming around 8V and with a 6V 4.5Ah SLA connected for charging, it is 6.44V.

Moreover, the rotation in VR 2.2K is supposed to change the output voltage. But in my case, I found no change in output voltage at any position of the VR. I used a Preset.

Please enlighten me on what might have gone wrong with the circuit. I have checked it repeatedly and eliminated any chance of my assembling fault or any Vero Board copper foil shortage.

Could you please suggest me how to get the circuit in working condition?

The two minor changes I incorporated in the circuit are,

1. Instead of a 9-0V 500mA secondary transformer with bridge rectifier, I used a 9 - 0 - 9 v 500mA secondary transformer with diode rectifier (Two 1N 4007).

2. I have used an extra 1000-MFD/ 35V filter condenser from Vout of the LM317 and ground.

Thanks in advance-
Debasisswan

 

[betwixt]

It should work.
Try temporarily disconnecting the 1.2K resistor and see what happens. It, the zener diode and the transistor are only there to limit the maximum voltage across the output but they shouldn't start to have effect until it reaches about 7.4V.

With the resistor disconnected the voltage should be adjustable from 1.25V with VR at minimum resistance up to approximately 10V. Get that working first then reconnect the 1.2K resistor and see what happens.

Brian.

 

[debasisswan]

Hi Betwixt,

Thanks for your kind help. I disconnected the Resistor as you advised but even than, it failed to give varying DC voltage output. Naturally that indicated towards some kind of major fault in my assembling the board. I reassembled the entire board and now it seems to be working.

Now please enlighten me on one more thing. I have set the Preset to have 7.2V DC at output without load. Since it's output is regulated, it should give stable voltage output. But when I attach a 330Ohms resistance across the output, the voltage comes down to around 6.5V. After I attach the battery for charging, then also the output goes down to 6.5V. Is it normal? or there might be some more faults in the assembly?

Please advise- in case I want to put a fuse to keep the board and especially the transformer safe, where to put it? At 220V AC input side or at the final DC output side? What should be the Fuse rating? 200mA?

And, is there any way to put a LED full charge indicator in the circuit that can indicate by going either off or on whichever is simpler to assemble?

Thanks again.

Debasisswan.

 

[srizbf]

i think the 16ohms/5w is the problem in your circuit.

may be you have taken the circuit from some source.

it cannot be that high value.
one test is remove and see what happens.(that is short the 16 ohms terminal)