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# UC3906 LED Indicator Problem

Greetings,

I am using UC3906 to charging my SLA battery 6V 4.5Ah, the following are the specification:

Voc=7.45V    V12=7.08V    Vf=6.9V     V31=6.21V    Vin=12V  Imax=500mA

R2=0.51Ω   R7=82KΩ   R12=10KΩ   R13= 39kΩ   R8=220Ω   R6=390KΩ

other the resistor value are same as below circuit.

My input is connected with an adapter 12V 1A.

The problem i facing is that when i on the power, my led float(green) and bulk(orange) are on without connect to the battery.May i know it that my resistor calculation are wrong or anyway i can check the problem that i facing.

Thank You .

Hope can get the reply as soon as poosible.

### 13 Replies

• Hi,

Your voltages looks right to me, but for the resistor values, I can not see the attached circuit, can you please send it again? You can use the design procedure on page 5 in the datasheet.

I deleted the second post, it is the same question.

-Tahar

• In reply to Tahar Allag:

This is the circuit i use but the problem i facing is the bulk and float led is on at the same time when power is on.

• In reply to dum me:

Hi,

The resistor values are correct, the only thing I can see may cause the issue is on the comparators (U1) and the values of R3 and R5.

-Tahar

• In reply to Tahar Allag:

Hello,

Did you got any way that i can indicate the float and the bulk  or calculate the value of R3 and R5???

thank you

• In reply to dum me:

measure the voltage on the comparator pins. From the schematic, the reference is the portion of the input voltage (I can not read the values of R3 and R5). Then compare that to the pin 10 of the charger, you will have a better understanding on what is going on in the circuit.

Send me some scope plots to look at.

• In reply to Tahar Allag:

Hi,

I had measure the input voltage for between R3 and R5 is 0.84V.

I found that when float led is on the voltage at pin 10 of the charger is 2.2V.

when the bulk led is on the voltage at pin 10 of the charger will drop until 11.8mV.

For the scope plot,which scope should i need to take???

Thank You

• In reply to dum me:

This makes sense, when you have 2.2V at pin 10, which means the V- of the comparator is higher than V+, thus the output (7) drives low and the LED turn on. Can you tell me what the battery voltage is at this time? Why do you use pin 10 instead of pin 13?

Bulk LED, do you mean D1 or D2?

-Tahar

• In reply to Tahar Allag:

The voltage from the battery is around 7.04 when connect to with the charger, if i unplug without the chager and measurethe battery voltage is around 6.83V. I though the pin 10 is to indicate the battery whether is float charge or not.

For the circuit,

D1- bulk charge( taken from pin 7)

D2-overcharge( taken from pin 9)

D3-float charge(taken from pin 10)

Thank You

• In reply to dum me:

Please see figure 2 and figure 3 in page 5 in the datasheet to understand the functionality of the pins and typical charge cycle.

• In reply to Tahar Allag:

I had see the datasheet for few times already.I found out that  the pin 7 is power indicate(prevent the divider resistor from discharching the battery when the input is removed), so i can't using the pin7 as my bulk charge indicator.Correct me if i wrong. Got any other way i can indicate my bulk charge.

Which pin of UC3906,scope can  take to make sure that my battery charger is working correctly???

Thank You