Battery charger - power supply issue affecting ADC

[flotulopex]

Hello,

I'm finishing my 24V lead-acid battery charger and get stuck on a power supply issue.<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=2167&stc=1&d=119651200 3">
The circuit works well but there is a strange (to me) voltage drop on the PIC's side while the current raises on the load and this impacts the ADC measurements.

If no load is connected to the circuit, the controlled output voltage range is from 25V to 30,5Volts and there is a clean constant 5V at the VR1's output (VDD) through the full range.
R2 drops the transformer's voltage from 44VDC to around 40VDC at the LM317K's input (to stay within specs).

When the load is connected, as the charging current raises from 200mA up to 1,2Amps (maximum I need), the VDD drops from 5V to 4,8Volts while the load's voltage is then raising from 25V up to 29Volts.

There is a slowly blinking LED (currently I have one, should be two finally) indicating the loading process goes on (timer toggles the LED every 2 seconds).

I check the differents measured values by the PIC via a serial connection. This is actually how I noticed the voltage drop.

Each time the LED is on, the values differ from the trend.

I naturally thought about an overload on VR1. I can measure between 5,54mA and 14,51mA at the VR1'a output wich is far under it's limit (100mA). I the doubt, I replaced it with à 7805 (1A). Same problem.

For a try, I replaced the actual 22V Zener by a 8,3V one; no change.

I then built up a totally appart 5VDC power supply with a second transformator (230:9VAC). Again, the same voltage drop happens as the load raises (this is more than strange!!!).

Does anyone have an idea what I'm doing wrong here?

[Acetronics2]

Bonjour Roger,

Just one question ... Temp of the LP2950 case ???

see the " LP2950 Maximum Rated Output Current " current and remember the input voltage is close to the maximum of the LP ...

the current you draw is ALSO very close to the max ...

also the reason from DS DATASHEET:

''
H - High-Terminal Potentiometer. This is the high terminal of the potentiometer. It is not required that
this terminal be connected to a potential greater than the L-terminal. Voltage applied to the H-terminal
can not exceed the power-supply voltage, VCC, or go below ground.

"

It seems I already had cut and pasted this section .....

We already had this discussion too ... about frying the DS 1804 ...


Amitiés
Alain

[flotulopex]

Bonjour Alain,

Merci de m'aider encore une fois
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=2168&stc=1&d=119651738 4">
When there is no load on the transformator, I have around 13Volts at the VR1. This is maybe just a little to much... At this time, the PIC has nothing to do.

But when on full load, the transformator's voltage drops down the VR1 input's voltage to 7Volts wich is now okay. Now the PIC has to "go to work".

About the DS, I don't understand what you mean; there is nothing connected to the "H" terminal. The DS is 5VDC powered and the maximum voltage I can measure between GND and "W" is 3,45Volts. Do I miss something basic?

How do I "manage" the huge voltage difference from the transformator (with or without load)?

[Acetronics2]

Sorry, Roger

it was :

" W - Wiper of the Potentiometer. This pin is the wiper of the potentiometer. Its position on the resistor
array is controlled by the 3-terminal control port. Voltage applied to the wiper cannot exceed the powersupply
voltage, VCC, or go below ground."

that means voltage on the wiper must not Exceed 5 Volts ... so, your output can't safely exceed 24 + 5 - .8 ( By 500 ! ) = 28 v

you tell 25 to 30v out ... that's 2 v too much !!! I'm oK, only two "little volts" ... but too much ...

I told you on another thread the DS couldn't bear the voltage you want !!!

Alain

[Ioannis]

Hi boys. If I did the math correctly, at max output 30.5 volts the drop on the 10K DS pot would be around 6.8 Volts. As Alain stated this cannot be tolerated by the DS chip.

I believe that it would be more practicle to use PWM on an output of the PIC, filter it and then feed a Pass transistor instead of the 317. As an example please refer to Elektor December 2001 issue (Digital controlled power supply).

Ioannis

[mister_e]

Weird idea this Zener Z1 so far...

But yeah i'll second the PWM idea.. maybe just remove the LM317 as well and use a Mosfet/Transistor instead and do everything in software. a 12F683 would be perfect for that.

1 current senses
1 voltage sense
1 PWM and
remaining pins for leds and maybe other safety inputs.

[inse]

Hello Roger,

one thing I have noticed in your schematic: do you have a ground connection on the OPAMP?
It's ground pin (4) seems to be floating...

Regards,

Ingo

[Acetronics2]

Weird idea this Zener Z1 so far...

But yeah i'll second the PWM idea.. maybe just remove the LM317 as well and use a Mosfet/Transistor instead and do everything in software. a 12F683 would be perfect for that.

1 current senses
1 voltage sense
1 PWM and
remaining pins for leds and maybe other safety inputs.

Hi,

I didn't remember it at first ... but the 317 can be used as a "switching transistor" for PWM !!! ... just do not use too high a PWM frequency and it will work rather pretty ...

YES ... the "off" state voltage will be 1.25 v ... but, here, no issue with that !!!

do not laugh ! ... I have the proof ... it was ... 18 years ago, for a model train controller !!!

Alain

[wbjunkie]

BMW have actually made a charger using the PIC16F series PIC
Have posted a pic of the charger in the schematics section (sorry if I got the forum wrong)