Battery charger - power supply issue affecting ADC

[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

[flotulopex]

Thank you Ingo,

Good observation!

I corrected the schema and the circuit (I was already noticed about this by Acetronis some hours ago - as I say, better two times than never ).

Weird idea this Zener Z1 so far...

As I'm not an electronician but a real "bricoleur du dimanche". Anyway, why is the use of a Zener this way not a good idea?

Ioannis, your math is absolutely correct but when I take my FLUKE87 or my scope, they both show a maximum of 3,45 volts. This has to be, again, one of this famous frontier between theory and real life .

I forgot to ask: what type/model of MOSFET or Transistor would fit my application best please?

[Ioannis]

Your current is up to 1,2A so any transistor that can handle the double to be safe, could be used. TIP31 (3A) or MJE3055 (10A) are a few examples.

The idea is to design a simple classic linear power supply that would be controlled by your PIC about the voltage and current. That would require A/D converters but to be very fast, a good designed Sigma-Delta converter may have to be used instead off the classic ADC's.

That's what is hidden in the Elektor's article I mentioned.

Ioannis

[flotulopex]

I'm searching for this ELEKTOR info - but seems to be available only for members... Still looking for it.

Using PWM, I will generate a voltage variation. If I'm right, a MOSFET can be "voltage" controlled; this is not true for a transistor.

I assume it will be better to use a MOSFET, no?

[Acetronics2]

I'm searching for this ELEKTOR info - but seems to be available only for members... Still looking for it.


I assume it will be better to use a MOSFET, no?

Hi, Roger

Here : http://www.elektor.fr/magazines/2001...1).65895.lynkx

a P mosfet will be easy to use ... a N Mosfet a bit harder ...

But there's some more investigation to do with your circuit before throwing it to the wastebox ...

Alain

[Ioannis]

I repeat again that, your final design might look like a Linear Power Supply.

As such, a NPN pass transistor is the usual choice. The transistor acts like a voltage follower, so whatever voltage is present at the base, that voltage-0,7 is presented at the emitter, with current amplification of course.

Ioannis

[Acetronics2]

Hi, Ioannis

just have a look to the NS Datasheet of the LM 350 page 10 ... there is a very simple Pb batt charger design ... easily usable with the 317 or 338 ( Paper datasheets included it in the 1982 Databook release ...).
fits the I - V charging method ... Roger looks for ...

But do not tell Roger ... he is a friend ...

Alain

[Acetronics2]

Ioannis, your math is absolutely correct but when I take my FLUKE87 or my scope, they both show a maximum of 3,45 volts. This has to be, again, one of this famous frontier between theory and real life .

?

Hi,

IF we consider The 317 has a max ref voltage and a max Iadj current ... those readings are possible ... ( your 317 is "not so good" , Roger !!! )
But that also mean ANOTHER 317 could damage the DS 1804 ...

this design won't be good for series production ...

Now, Roger, there's another point to verify ... your PCB tracks !!! are they sufficient to hold the required current ...
And overall ... did you use a "star" config to link your ground tracks ???
( maximum care for R5 ground return path ...)

Alain