# help in AUTO TURN-OFF BATTERY CHARGER CIRCUIT

Discussion in 'General Electronics Chat' started by spidermanIIII, Jan 24, 2015.

1. ### spidermanIIII Thread Starter Member

Nov 22, 2013
76
0
hello, my friend give me a link about AUTO TURN-OFF BATTERY CHARGER CIRCUIT my question in this circuit is can i calculate the current that flow R5 mathematically to make transistor T3 cutoff and why 65 mA make transistor T3 on

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2. ### Ramussons Active Member

May 3, 2013
557
92
The transistor T3 action is dependent on the current thru' R4 - not R5.
65 mA x 10 ohms = 650 mV -> cut in voltage for the T3 E-B junction for it to start conducting.

3. ### ScottWang Moderator

Aug 23, 2012
4,855
767
Vdc = 9Vac * 1.414 -(0.7V*2) = 12.726V-1.4V = 11.326V
Ic_T3 = (11.326V-0.2V)/100K = 0.1mA.
The datasheet shown : Vce(sat) , Ic =-10mA, Ib=-0.5mA.
0.65V may not turn on the T3, it should be increase a little bit voltage, because 0.65V only for Vbe, but the R5 still need the current and it will drop some voltage, if only need 0.65V to turn on the T3 then the R5 should be shorted.
So the R3 should be modify to satisfying the conditions for T3.

4. ### spidermanIIII Thread Starter Member

Nov 22, 2013
76
0
sir,can you tell me where the Vdc volt on schematic and how you calculate it

5. ### paulktreg Distinguished Member

Jun 2, 2008
612
120
The 11.326V is the DC voltage across C1.

If you want to calculate the output voltage to the batteries it is controlled by R6 and VR1. Have a look at the LM317 datasheet.

6. ### MikeML AAC Fanatic!

Oct 2, 2009
5,450
1,066
Crap circuit!

First off, if the batteries are really AA Alkalines, then they cannot be recharged. If they are NiCad or NiMh, then this charger's end-of-charge detection method is totally wrong. It would work for Lead-Acid batteries where if charged with constant-voltage, the current into the battery drops after the battery is charged. I does not work for NiCads or NiMh because there is not a detectable current drop at end-of-charge. End of charge detection for NiCads or NiMh is very difficult. Read Battery University.

Last edited: Jan 24, 2015
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