I am using this module for wireless charging, as a receiver, and i want to switch it on and off, because firstly it consumes about 5mA when idle and secondly i want to power it off when the battery is fully charged. So i decide to use this NPN transistor. I build the following schematic

enter image description here

in order to control the wireless charger receiver. The problem i am facing now is that the charging procedure takes much longer.

Any clue why is this happening? I dont think the the circuit is wrong, maybe a diode in Vbat in parallel with the collector pin, but i am not sure that this has anything to do with charging procedure.


I am using a lipo battery and i am starting charging at 3v and i should stop it around 4.1v. The capacity of the battery is 400mAh and the charger receives 300mA maximum. Therefore, when i am not using the transistor the charging procedure takes less than 50 minutes, but when i am using the transistor the charging procedure is about 6 hours(and i am guesing the battery is broken after that).

To control the transistor i am using and arduino uno gpio, therefore 5V/40mA maximum. During the charging procedure usinng the transistor the current floating from collector to emiter is only about 32.5, measured with a multimeter.

So, am i "opening" the transistor enough? I removed the base resistor and the I_CE current increased to 118.2mA and the charging procedure is more faster now. It's much better know, but still there is about 100mA kind of "lost".

Moreover, when base is set to low, there is a 15mA current floating from collector to emmiter.

  • \$\begingroup\$ Perhaps if you included a few specifics it might help get an answer. What is "much slower"? V_BT1? I_CE compared to I_GND without the NPN, what is the GPIO (V_high, I_max)? \$\endgroup\$ – P2000 Jul 2 '20 at 6:58
  • \$\begingroup\$ I added more information. Many thanks for your comment. \$\endgroup\$ – alexisicte Jul 2 '20 at 7:57
  • \$\begingroup\$ OK good start. So what is the V BT1, the WC+ supply? And I think you have mA (current) mixed up with mAh (capacity)? You should also measure V across the charger with and without transistor, since you'll have a V_CE drop of perhaps 0.2V. \$\endgroup\$ – P2000 Jul 2 '20 at 8:11
  • \$\begingroup\$ Yes i mixed up mA and mAh. When the charger is applied, i measure 4.28v on the wireless charger receiver without the transistor and 4.70v with the transistor. V_CE 0.55 while charging. The WC+ supply is 5.05v coming from the wireless charger transmitter. \$\endgroup\$ – alexisicte Jul 2 '20 at 8:36

The wireless charger module you referenced (https://www.dfrobot.com/product-1283.html) is based on a switched capacitor voltage regulator on the receiver side.

This regulator has a ""skip mode" which is an automatic shut down if the current drops below 4mA. https://datasheet4u.com/datasheet-pdf/WillSEMI/WD3168/pdf.php?id=1112865

It is quite possible that the V-CE voltage drop across the transistor reduces the charging current to below 4mA when the battery is charged somewhat but not fully; at which point the regulator is shut down.

If your design is married to this regulator/charger, one work-around might be to artificially increase the current drawn from WC+ by shunting a resistor that guarantees a current draw of at least 4mA. This would slow down the charging a bit during the charge phase but keep the charging going for longer, and on balance provide a faster charge.

For a supply of 4V and a current of 4mA, a 1KOhm across WC+ and GND might be just what you need.

  • \$\begingroup\$ The receiver module uses this IC, SOP-8 package. The datasheet you shared is 6-pin IC. Nevertheless, should i remove the resistor from the module that enables skip mode, and attach 1k resistor across WC+ and GND? Is that what you are proposing? \$\endgroup\$ – alexisicte Jul 3 '20 at 7:48
  • \$\begingroup\$ Following your link, I only see the T3168 receiver, and there is no skip mode resistor. In any case, yes, you could try to cause a current draw that prevents the unit from going into "skip mode" (shut down). \$\endgroup\$ – P2000 Jul 3 '20 at 23:34
  • \$\begingroup\$ Yes now it's much better. The charging procedure continues. But still I_CE is less than expected. \$\endgroup\$ – alexisicte Jul 8 '20 at 10:20
  • \$\begingroup\$ @alexisicte It will be less because of the added resistor (current split) and the V_CE drop. If that does not account for all the differences, maybe something else inside the regulator is going on, but I think that will be hard to pin-point, let alone resolve. Anyway, I am glad the trick worked. At least you have it working, and you can choose to optimize later. Good luck! \$\endgroup\$ – P2000 Jul 8 '20 at 16:46

You have very strange NPN part of circuit. You connect ground to collector permanently and ground to emitter permanently. Something wrong there. Try removing ground connection from collector, you don't need it, although I would rebuild the circuit with opposite polarity transistor on the battery side, kinda more intuitive.

  • \$\begingroup\$ Sorry my bad, i draw the circuit incorrectly. Just edited the schematic. The collector is not connected to groung permantly. The collector is connected to wireless charger negative pin. \$\endgroup\$ – alexisicte Jul 2 '20 at 6:26
  • \$\begingroup\$ still try to flip it around and connect a transistor between positive supply and charger. See what happens \$\endgroup\$ – Ilya Jul 2 '20 at 6:56
  • \$\begingroup\$ i did it, the same.. \$\endgroup\$ – alexisicte Jul 2 '20 at 7:34

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