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[SOLVED] What does the transistor do in the attached circuit?

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HasHx

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Hello, I have the following power circuit, i was wondering what the transistors do in this circuit? it is an old circuit that is proved to be working though i am thinking of removing the transistors... any thoughts?

download.png
 

I wouldn't remove them, they are voltage followers in apart of a regulator circuit.

Brian.
 
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    HasHx

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So the main reason for adding them is to increase battery life?
 

Hi,

No, they don't increase battery life.
They (try to) stabilize output voltages.

Please check if all the Q6 type an connection is correct. I expect a PNP type with emitter to output.

Btw: VCC is missing a capacitor.

Klaus
 

The circuit (which should have the capacitor mentioned by Klaus) is to produce two stabilized voltages. They make sure those voltages stay constant as long as possible as the battery voltage drops but they don't increase the battery life. Without them the voltages would slowly drop as the battery discharged, it doesn't help when the battery voltage has dropped too far but it keeps the voltages steady up to that time.

U2 is a negative voltage converter. It turns the battery voltage into it's equivalent but in the opposite polarity. So if the battery produced 9V it would produce -9V from it. Q3 stabilizes the positive voltage straight from the battery, Q6 stabilizes the negative voltage from U2.

Both the transistors are drawn incorrectly. The output should come from the emitter pin in both cases and the PNP/NPN are swapped over.

Brian.
 

Hi,

Thanks for this info, truly appreciated!

Actually this was a double rail circuit and I added the negative voltage converter to get rid of 1 battery, now regarding the Cap, I will add a 2.2uF at the VCC pin of the IC, do i need a 10uF on the battery as well?

Will work on the transistors and fix them, will post the new circuit shortly for reference.

Thanks again!

- - - Updated - - -

I have made this, I really want to keep using the same parts and both transistors are PNP, I think there was a mistake while determining transistors pins (it was done by someone with little electronics knowledge) when the circuit was drawn from the original board. However i think this connection will do.

Screenshot_1.png
 

Hi,

negative voltag circuit seems to be correct now and should give -6.0V ... -6.1V output.

Positive side bjt is still wrong. Replace it with an NPN and exchange emitter with collector.

R2, D7 is mysterious....


Klaus
 

The datasheet for the TL7660 shows that the negative output has 10kHz ripple.
The BC212 and BC177 are obsolete and are not made anymore.
The BC177 is a PNP but Q8 is supposed to be an NPN with its emitter at the + output.
 

Hi,

negative voltag circuit seems to be correct now and should give -6.0V ... -6.1V output.

Positive side bjt is still wrong. Replace it with an NPN and exchange emitter with collector.

R2, D7 is mysterious....


Klaus

How come it is just -6v while the source is 9v??

R2 and D7 goes to another part of the circuit.

- - - Updated - - -

Hello,

I confirm that the set up in the attached screen shot is working with no problem. this is a shock sensor circuit from the 80s i think, and we want to modernize it, most of the parts used are obsolete now so we are trying to remake it.

Screenshot_1.png

this was the circuit before modifying, we want to use one battery only and a SPST switch. Now this feeds 4 Op Amps, and a LCD display and driver.

I was asked not to change anything and i thought that the guy who traced back the circuit did some mistakes but it seems not now. i am pretty confused here... thinking of making it the same way leaving the door open to applying the NPN circuitry if this does not work just by replacing the PNP (THT components should be no problem).

if you guys have anything to add please do.

Regards,
HasH
 

Hi,

How come it is just -6v while the source is 9v??

* 9V battery --> bjt + zener combination --> fixed +6V output
* 9V battery --> 7660 circuit --> -9V --> bjt + zener combination --> fixed -6V output

This is what the circuit does. But you should have known this before.. did you use a voltmeter?

Klaus
 

The circuit absolutely will not work with a PNP transistor for Q3. Q3 MUST be an NPN for it to work properly.
Also, the emitter and collector of Q6 are backwards.

- - - Updated - - -

Why not do it correctly like this:
 

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    HasHx

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I confirm that the set up in the attached screen shot is working with no problem.
No, both circuit parts have errors. Q6 has collector and emitter flipped which reduces current gain B to about unity. Respectively the V- output has high output impedance, voltage drops with only small load.

Q3 has forward biased collector-base junction in series with zener diode, causing high input current above 7.5V battery voltage.

Any simulator of your choice will reveal both circuit flaws if you don't believe it.
 

Hi,



* 9V battery --> bjt + zener combination --> fixed +6V output
* 9V battery --> 7660 circuit --> -9V --> bjt + zener combination --> fixed -6V output

This is what the circuit does. But you should have known this before.. did you use a voltmeter?

Klaus

at the first glance thought you were talking about the charge pump, anyway the voltmeter reads as 7.3v

- - - Updated - - -

The circuit absolutely will not work with a PNP transistor for Q3. Q3 MUST be an NPN for it to work properly.
Also, the emitter and collector of Q6 are backwards.

- - - Updated - - -

Why not do it correctly like this:

this is what i am convinced to be true, yet there is something not making sense in here. the customer wants me to keep everything as it is in the old board so will do that and leave the door open for superimposing the right transistors.

- - - Updated - - -

No, both circuit parts have errors. Q6 has collector and emitter flipped which reduces current gain B to about unity. Respectively the V- output has high output impedance, voltage drops with only small load.

Q3 has forward biased collector-base junction in series with zener diode, causing high input current above 7.5V battery voltage.

Any simulator of your choice will reveal both circuit flaws if you don't believe it.

thanks for taking the time to write a comment, I am really convinced with all that, thing is that the customer wants everything to stay as it is, i told him i have questions about the circuit operation and asked him to double check carefully and he did and confirmed that everything i have is correct, he also confirmed that the voltage across +v and -v is 14.5v (mind blown). after all, this is how he wants it, I will just wait for him to manufacture the board and will sure update this thread of what the results be.
 

yet there is something not making sense in here.
True, indeed. The most likely explanation is that the posted schematics are not reflecting the real hardware. Less likely, that the circuit is not working according to state of the art, but nevertheless serving a purpose. In the latter case, we don't have enough information to judge if the circuit shows useful operation under the given conditions, neither load current nor load voltage range have been reported yet.
 

Hi,

at the first glance thought you were talking about the charge pump, anyway the voltmeter reads as 7.3v
Ok 7.3V is a ranfdom voltage. We don´t know whaere you connected the DVM.

I didn´t mean that you need to measure something for us.
I expected you did follow the signals and measured several voltages on your own to find yout the circuit´s function.... before you did your first post.

And: Yes. With "7660 circuit "- I indeed meant the "charge pump" circuit.

Klaus
 

The output voltage of a 7660 IC depends on its input voltage and its load current.
How can the circuit produce 14.5V between its +/- 6.2V outputs? Only when its parts are wrong and backwards. The Zener diodes are 6.8V and each voltage output from the transistor emitter-followers drops it by one base-emitter voltage drop, as I showed.

The BC177B is a PNP but an NPN is supposed to be used in its place. The BC212 is shown connected backwards but is easily fixed by turning it around 180 degrees.
 
Last edited:

Hello all,

I am here to report the readings of the board i made... the circuit i posted did not work, so we simply flipped the PNP and used an NPN transistor for the +ve part... the voltages read after changing transistors were +6.05v and -6.01v.

thank you all for your help, truly appreciated.
 

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