Tips for drawing the schematic of the existing device

[arneansper]

Hi!

I would like to recover the schematics of the existing device in order to modify it little bit.

The device is two channel Reflex charger/discharger from Robbe that utilizes the QuickSaver charging controllers. The problem is that the charging current is fixed to 650mA and the controller plays with duty cycle when you select lower capacity battery.

I would like to modify one channel to operate at much lower current for charging small NiMH packs for DLG.

The charger looks quite simple: there are two QuickSaver chips and LM324N, four big transistors on radiator, four big resistors and lots of normal sized resistors, transistors, etc. No SMD components.

I think the LM324N is used as a two channel current generator and some of the surrounding resistors define the 650mA current.

Any tips how to proceed and how to find out what resistors I must change?

Edit: if I try to measure the tracks with ohmmeter to find out what where some tracks go, is there any danger of damaging the circuit?

Arne

[arneansper]

I think I got the analog part of the charger right. This is only one half of the charger.

"Sensor" goes to the input of the charge controller. "Charge" and "Discharge" are TTL level signals that turn on charging and discharging.

Too tired to figure it out now. I suppose R33 or R33 and R34 together determine the charge current.

Arne

PS. The types of the transistors and diodes are wrong. I picked up closest things I found in EAGLE. PNP transistor was TIP32C, FET was IRFZ24 and diodes are quite big and black with silver ring on one end - cannot read the markings

[coro]

I suppose R6 is used to measure discharge current, and R7 to measure charge current. R33,34 are just protecting transistor base. R14,R15 and R3 are used to create desired voltages that are compared with voltage drop over R6 and R7 to limit currents.
You can either modify R6 and R7, increase in value reduces curent and it is most simple path. Either You can reduce reference voltage for example by increasing R14 -15k trim could be added in series with R14 to alow up to 1/10 current - however I am not sure that used op amp is rail toi rail or powered by enough negative voltage, to compare voltages so close to ground.
It seems to me that wires are not corretly connected in Your eagle cheme? (missing joints)

[Bruce Abbott]

The LM324 senses down to ground, but you may get into trouble with offset voltages if the reference voltage is too low. Changing R6 and R7 is the safest way, eg. 2.0 Ohms each should reduce charge/discharge currents to 1/10 (65mA?).

[arneansper]

Thank you very much! Yes, some joints are missing. Eagle sometimes adds joints automatically, sometimes not. I had laptop without mouse and was too lazy to swim around with trackpoint to add manually all the joints.

R14 and R15 are shared between two channels, so I'm going to increase the R6/R7 to leave the other channel untouched.

Thank you very much for your help.

regards,
Arne

[arneansper]

Got the 0.68 ohm resistors today and replaced the R6 an R7. Current dropped ~3 times as expected. Thanks again!

Arne