Low Voltage Across Current Sense Resistor

[Suraj143]

Hi in this circuit the voltage across the 1 ohm current sense resistor is 0.4V not 0.6V why is this?

**broken link removed**

Let say i need 450 mA. so the resistor value must be 0.6/450 = 1.3 ohms.

but when I measured its passing 350 mA because it is automatically adjusted to 0.4V not to 0.6V even to a dead battery.

Can somebody tell me how to keep 0.6V across the current sense resistor.I need to pass 450mA.

Thanks a lot.

 

[kchriste]

Hi in this circuit the voltage across the 1 ohm current sense resistor is 0.4V not 0.6V why is this?

The Vbe junction voltage of the 2n2222 will vary with temperature; the hotter it is the lower the voltage. Also, when passing low collector currents transistors will tend to "turn on" at a slightly lower Vbe voltage.
Another thing you must consider is that Vin must be greater than BatteryVoltage + 3V + 0.6V where 3V is the dropout voltage of the LM317 and 0.6V is the Vbe drop across the current sensing resistor.

Can somebody tell me how to keep 0.6V across the current sense resistor.I need to pass 450mA.

Change the resistor to 1hm: and you'll get 400ma if Vbe is at 0.4V

 

[Suraj143]

Thanks Kchritie

I changed it to 1 ohm now it’s showing 380mA.

But the original circuit diagram shows for 1 ohm it’s sending 600 mA. But I get 380mA.

I need 450 mA. I cannot find below 1 ohm resistors in my place.

Also my 2N2222 it’s not getting hot it’s very cool. Shall I change the transistor to BC547 & see?
Also my input is 12V.

 

[kchriste]

Also my input is 12V.

What is the voltage rating of the battery you are trying to charge? Is it a NICAD, NIMH, or Gel Cell?
If you replace the battery with a 5hm: 5watt resistor, how does the circuit behave?

Shall I change the transistor to BC547 & see?

Can't hurt to try, but it'll probably make no difference.

 

[Roff]

If your LM317 is getting hot, it will limit the current to protect itself. If your LM317 has 4.6 volts across it, it will dissipate 1.75 watts at 380mA, enough to make it very hot. You may need a big heatsink.

 

[Suraj143]

I’m charging 6V SLA 4.5AH battery.

I’ll put a bigger heat sink & check whether it’s sending 450mA current.

Thanks.

 

[ericgibbs]

I’m charging 6V SLA 4.5AH battery.

I’ll put a bigger heat sink & check whether it’s sending 450mA current.

Thanks.

Hi Suraj,
When you measure the 'current' in the Isense resistor, whats the internal resistance of your test meter?

 

[Suraj143]

Hi I don't know whats internal resistor value in the DMM.I just make series with the resistor and measuring.

 

[ericgibbs]

Hi I don't know whats internal resistor value in the DMM.I just make series with the resistor and measuring.

hu Suraj,
Consider that your meter and test leads have say, a total resistance of 0.5hm: [this example is a high value, but its to demonstate the point]

Then the Rsense = 1hm: + 0.5hm: = 1.5hm:

So if the transistor has a Vbe ON of 0.6V, a current of 0.4A would switch ON the current limit transistor.

 

[Suraj143]

Oh i see.....but in the data sheet they have put 1 ohm resistor for 600mA.

In my case its passing only 380mA for 1 ohm resistor.because its 0.4V.

What do I have to do to get 450 mA.

thanks

 

[ericgibbs]

Oh i see.....but in the data sheet they have put 1 ohm resistor for 600mA.

In my case its passing only 380mA for 1 ohm resistor.because its 0.4V.

What do I have to do to get 450 mA.

thanks

Hi,
You will only get Vbe= 0.6V when the circuit is current limiting.
If your battery is almost fully charged then the charging current will be low, so the voltage across the 1R will not be 0.6V.
Remember: its a current limiting cct. Not a constant current charger.

Measure the voltage across the battery while it is charging and tell me what it is.

Try discharging the battery down to its lowest recommended 'discharged' voltage, then connect it to the charger.

 

[Suraj143]

Hi Eric thanks for your input

I measured the battery voltage its 6.5V.

Now I get the charging current 150mA.I adjusted the charger voltage to 6.8V.
So still there 0.3 V voltage gap.

My battery there’s no discharge voltage or current given.
Any how I’m going to discharge & see what’s the progress.

Thanks a lot

 

[ericgibbs]

Hi Eric thanks for your input

I measured the battery voltage its 6.5V.

Now I get the charging current 150mA.I adjusted the charger voltage to 6.8V.
So still there 0.3 V voltage gap.

My battery there’s no discharge voltage or current given.

A 6V SLA discharged voltage is 5.4Vmin.
A 6R load resistor [10Watt] should discharge the battery in 3 to 4 hours.
[a 6V 6W car lamp is a good way to discharge the battery]

DONT allow the SLA to go below 5.4V!

Any how I’m going to discharge & see what’s the progress.

Thanks a lot

If you want a fast cyclic charge set the charger voltage to 7V or a float charge 6.8/9V.