LM317T voltage doesn't adjust past 10V

Thread Starter

Pixelview

Joined Nov 2, 2018
9
Hi,

A battery charging circuit below is supposed to give out max charging voltage of around 14.5V and trickle charge voltage of around 13.5V. These values are supposed to be controlled by VR1 and VR9 respectively. However, in real life I'm only able to increase the voltage up to around 10 volts.

I have wired LM317T separately according to the datasheet to verify the that such resistor values would give me the desired output and they did.

I have built the circuit without the 7805, TSR2450,TSR2433 regulators on a breadboard It worked like a charm in the sense that i could control the output voltages with the pots. Then i made both a PCB and stripboard design with full circuit displayed in the image below and in both of them the output can be controlled as far as around 10 V leading me to believe that maybe the regulators introduced some problem but I'm not smart enough to figure it out after many hours of troubleshooting.

Any help would be greatly appreciated :)


*The circuit design was based on a design suggested by forum user with the username crutschow. A link is included for reference to his original design and explantion https://www.electro-tech-online.com/articles/lm317-12v-lead-acid-battery-charger-with-3-step-auto-cc-cv-and-trickle-charge.865/
 

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mvas

Joined Jun 19, 2017
537
Max current flow is 1.5 amps on the LM317.
The OUTPUT Pin is typically 1.25 Volts greater than the Voltage at the ADJ pin.
TR1 should be OFF, unless Max Amps is flowing
TR2 should be OFF, unless in Trickle Charge Mode
So, start by removing TR1, TR2, TRS2450 & TRS2433
Now you have very simple feedback path to diagnose with the LM317 - just like your breadboard.
You essentially have a voltage divider between R1 = 121 and VR1 = 1.3K
Now, can you adjust VR1 to get ... 13.25 Volts at ADJ Pin = ( 14.5 volts - 1.25 volts ) ?

Are your reaching the High End of VR1 before reaching 13.25 Volts at the ADJ Pin ?
If true, then the 1.3 K pot may be to low, try a 2K Pot.

Wire up only the "core" LM317 circuit = LM317 + R2 + VR1 + DUMMY1 + R4
then debug it and get it working first.
Then add and debug each additional circuit ...
1) The Trickle Charge circuitry = TR2 & VR9
2) The Max Amp circuitry = TR1

Divide and Conquer ...

EDIT:
What is DUMMY 1 ?
A wire ?
 
Last edited:

mvas

Joined Jun 19, 2017
537
Analyzing the "Max Amps" circuit ( TR1 + R3 + R4 + DUMMY 1 ) ...

What is "DUMMY 1" ?
A wire or a 1 Ohm resistor ?

If my math and understanding of that circuit is correct then ...
Max Charge Amps is 150 milliamps = ( 0.7 Volts / 4.7 Ohms )
 

crutschow

Joined Mar 14, 2008
25,256
What is the purpose of the added regulators on the output of the circuit?
There should be nothing connected except the battery to be charged, as that will mess up the charging of the battery.
If the circuit worked without the regulators, then something you did when you added the regulators is apparently the problem.
Are you sure they are wired correctly (double check the pinout).

If you need added regulated outputs, the regulator inputs should be connected to the 18V supply.
 
Last edited:

mvas

Joined Jun 19, 2017
537
Some differences that you made ...
a) The original schematic has Diode D3, your schematic does not = Protects TR1
b) The original schematic does not have Diode D2, your schematic does = An additional voltage drop to your Battery
 

Thread Starter

Pixelview

Joined Nov 2, 2018
9
Update:
Thanks for the help :) . The problem was the transistor pin out, in my schematic the legs were reversed (took me obscene amounts of time to figure out). I moved the regulators to the transformer output as advised as well.

However, I think I am observing some unusual behavior in terms of the actual charging.
First of all, I set the output to be 14v but when I connect the battery the voltage drops to current battery voltage(~12.5V). Then the battery started charging to 12.78 V and seems to have stopped. It took maybe an hour to charge to 12.78 from 12.5 but it has been maybe 3 hours and the voltage didn't move up anymore.

Furthermore, I measured the current initially(at 12.5v) and it was drawing around 10mA and now (at 12.78v) when i try to measure current it shows 0mA. Acording my own and @mvas calculations the max charging current should be around 150mA and based on voltage the circuit shoud be in constant current mode.

Full disclosure: I shorted the circuit when I tried to measure voltage with the multi-meter leads still plugged in current measuring configuration. 600mA multimeter fuse broken but I am not sure if some other component could have been damaged as a result causing the circuit not being able to charge past 12.78V.

To be honest I think even in current state (maintaining 12.78 volts ) the circuit would be sufficient to power my project ,but I'm not a big fan of half- assing my final year project so I wish to get it working fully.

Thanks for any help :) @crutschow and @mvas
 

Thread Starter

Pixelview

Joined Nov 2, 2018
9
Sorry, my circuit doesn't make it clear. The power supply is an AC to DC 230V to 18V smps transformer so the input to the circuit is 18 volts.
 

crutschow

Joined Mar 14, 2008
25,256
Sorry, my circuit doesn't make it clear. The power supply is an AC to DC 230V to 18V smps transformer so the input to the circuit is 18 volts.
So you measured 18V when the battery is charging?

If so, post the node input and output node voltages for the circuit when it reaches the maximum charge voltage.
 

Thread Starter

Pixelview

Joined Nov 2, 2018
9
So you measured 18V when the battery is charging?

If so, post the node input and output node voltages for the circuit when it reaches the maximum charge voltage.
Yes I measure 18V at the transformer terminal when charging the battery and 14.5V as the max charge voltage without the battery connected and around 12.8v when battery is connected. should i observe this behavior of the max charge voltage dropping down to the battery voltage?

I just changed the transistors in case one of them was faulty and left the battery charging overnight. The battery voltage got up to 12.9V but after disconnecting the battery its voltage dropped back to 12.8v after 20 mins.
 

Thread Starter

Pixelview

Joined Nov 2, 2018
9
Update: I fully rebuilt the circuit and it seems that is if fully operational now i.e I'm reading ~14.4V while charging the battery. I'll be honest I don't know why it worked now and didn't the previous times. The only thing I changed Is that i used 2 1Kohm resistors for the R7 resistor as opposed to using 1 2.2Kohm resistor. However, I think it should have not mattered because i had a 50k ohm pot for the R6 resistor which i could adjust to counteract the increased resistance.

Thanks for the help, I might post my full project if I have the time to write it up one of these days.
 
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