# Transistor switch circuit not distributing enough current

Discussion in 'Power Electronics' started by programmer6502, Jan 9, 2017.

1. ### programmer6502 Thread Starter Member

Feb 1, 2014
132
6
Hello,

I'm building a NIMH battery charger circuit that will be controlled by a microcontroller featuring current control. This is part of my circuit (without the current limiting portion) that I'm having trouble with. (Note: This is all in a proof of concept/prototype stage.)

So what I have here is 7809 (1 amp) voltage regulator that provides the voltage my particular battery needs, and under that a 7805 (also 1 amp) regulator that represents the 5v a microcontroller would produce. That 5v in turn is fed into a 1k resistor that drops the voltage to about 0.6v, turns on the transistor, and finally starts feeding the load (or battery).

Now the problem I'm having is it won't produce more than about 130-140ma to the load with the 7809 rated at 1 amp! From what I understand the transistor is a current amplifier and the base current governs the collector current. So if it isn't getting enough base current then the output of my circuit isn't going to be very high. But I don't see how the base wouldn't be drawing the current it needs! The transistor I'm using is a E13007F2 (salvaged from a power supply). Datasheet found here.

I'm still fairly new to transistors so it could be that I'm not understanding the specs right and this transistor is not suitable.

Thanks

2. ### ClassOfZero Member

Dec 28, 2016
114
20
What value is the load ?

What value have you calculated the base current to be ?

What sort of gain are you expecting ?

3. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,981
495
hi 6502,
Look at these images of the Ib versus Ic.
Your transistor Ib =(5v -0.6v)/1000 = 4.4mA.
Use the graphs to show the Ic at this Ib.
E

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4. ### #12 Expert

Nov 30, 2010
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Eric showed that 4.4ma isn't enough to drive an amp through the transistor. When you change to a microcontroller, you are stuck with a low current supply. Therefore, you need more current gain. A mosfet could do that, or a second bipolar transistor configured for more current gain.

programmer6502 likes this.
5. ### Dodgydave AAC Fanatic!

Jun 22, 2012
6,198
979
replace the transistor for a Tip141

6. ### dannyf Well-Known Member

Sep 13, 2015
2,196
421
Without knowing BCC, impossible to tell.

I would just short the BJT to see if it is the problem.

7. ### programmer6502 Thread Starter Member

Feb 1, 2014
132
6
Awesome, that's just formula I needed to see! I wondered if maybe the resistor had something to do with it.

Makes perfect sense, thanks for the suggestions on what to use!

Yes, I bypassed the transistor and was able to draw full current to my load so that was indeed the problem.

Thanks all for the help! It's my first time doing something like this and my references didn't mention anything about the limited base current when arranged like so.

(Edit: Rather I learned that my transistor doesn't have the gain I need. My sources probably used something more suitable for their goals.)

Last edited: Jan 9, 2017
8. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,981
495
hi 6502,
I would agree with #12, MOSFET or a Darlington transistor.
E

9. ### dannyf Well-Known Member

Sep 13, 2015
2,196
421
13007 is an old Motorola high voltage switching transistor. It has very low beta.

10. ### #12 Expert

Nov 30, 2010
17,702
8,888
I did one just yesterday for a guy that wants to slam a 30 amp solenoid. Mosfets are amazing at the amount of current you can run through them with a very minimal gate drive current.

11. ### Bordodynov Active Member

May 20, 2015
930
286
See

ericgibbs likes this.
12. ### dannyf Well-Known Member

Sep 13, 2015
2,196
421
replace Q3 with a tl431 and the turn on/off will be much sharper.

U2 really has no reason to exist.

13. ### #12 Expert

Nov 30, 2010
17,702
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1) U2 will be replaced with a micro-controller chip, so it has to exist. The MCU is the heart of the intended circuit.
2) As much as I love Bordodynov, this circuit has missed a few points which seem important.
a) You can't connect R1 to an MCU and get the 40 ma to 50 ma needed by the base of Q1.
b) The current regulation function will be provided by the MCU so Q3 is irrelevant.
c) Replacing Q3 with a TL431 is therefore irrelevant.
d) I don't know where the 592 ma current limit came from because the TS is talking about 1 amp.

14. ### Bordodynov Active Member

May 20, 2015
930
286
See PWM.
PWM signal after divider the filter capacitor . The voltage varies in proportion to the duty cycle. In this example 100% and 30%. The current through the battery ranges from 1A to 300mA.

15. ### programmer6502 Thread Starter Member

Feb 1, 2014
132
6
#12,

I like your reasoning, I'm not opposed to the mosfet idea at all.

And Bordodyno, funny because 300ma to about 1 amp is what I was looking at for my particular battery. I like your utilization of an op amp to control the transistor. In my prototype I was using an additional transistor to divert excess current.

16. ### programmer6502 Thread Starter Member

Feb 1, 2014
132
6
Alright,

I've made two transistor switch circuits here on spice that both utilize a BJT to limit current. One consists of a darlington pair BJT and the other a mosfet; both outputting about 300mA to 1A with some tolerance. As seen on the pics, I've taken two measurements on each. One being the collector/drain current and the other the collector/drain voltage (while R2 is being adjusted in 1 Ohm increments in both circuits). The current measurements look good to me, but shouldn't the voltages be close to the 9v being provided at all times? They're way out there.

(First time on this simulator and the BJT and Mosfet models are somewhat random since I was experimenting)

BJT circuit:

Mosfet circuit:

17. ### programmer6502 Thread Starter Member

Feb 1, 2014
132
6
Never mind, that's right. What's your guys' take on these circuits? Very practical?

18. ### #12 Expert

Nov 30, 2010
17,702
8,888
I can't see the graphs but I can say the mosfet circuit shouldn't work. There is no impedance between the 5V source and Q2.
Q2 should start smoking.

19. ### programmer6502 Thread Starter Member

Feb 1, 2014
132
6
Hah probably so. The idea there is that that Q2 shorts the Mosfet to ground when it exceeds the current limit and repeats an on and off process very quickly to seem as if there is a constant amount of current being supplied to the load. Might not be the best way to go about that.

Edit:
Yeah oops that needs a pull down resistor or something.

Last edited: Jan 11, 2017
20. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,981
495
hi 6502,
It would help if you posted the LTS asc file of your circuits, when you post a plot image.
We could check your work easier and suggest modifications.
E