Latching transistor problem

Thread Starter

mcgyvr

Joined Oct 15, 2009
5,394
Could use some help here with either improvements or alternate value suggestions or whatever to get this working properly...
I've got this schematic.
I press the switch 1 and the green LED should turn on and then latch the section on the right to have 48V present at RED1VCC..
Seems to work totally fine in simulation but not in breadboard..
I'm having 2 problems with this in breadboard..
1-When the power supply is turned on the LED is dimly glowing (about 10V is present at BAT1) .
2-The reset button is not resetting the circuit.

I'm fairly sure this is all from leakage current but I don't know enough to improve this to prevent that.
Help
Leak1.PNG
 

danadak

Joined Mar 10, 2018
3,770
1-When the power supply is turned on the LED is dimly glowing (about 10V is present at BAT1) .
For 10V to be present means T4 Vbe connection must be wrong or R9 is wrong
valued R, really low, and that would also fry T4. That assumes switch is off.

LED dimly glowing makes sense if 10V is applied means LED has 1/2 mA thru it,
more than enough to excite it.

Regards, Dana.
 

Thread Starter

mcgyvr

Joined Oct 15, 2009
5,394
For 10V to be present means T4 Vbe connection must be wrong or R9 is wrong
valued R, really low, and that would also fry T4. That assumes switch is off.
I have checked wiring/component values numerous times..

What I "thought" I noticed was that upon initial power up the LED did not seem to be glowing.. I then closed switch 1 and opened it again and ever since the LED has been glowing dimly with power supply on and switch 1 open.. Even turning power supply off and back on again the LED is now dimly glowing..
Hence my confusion..
 

ArakelTheDragon

Joined Nov 18, 2016
1,327
The LED is like "Dana" said, you need 10mA to make it glow bright, if it is not an ultra bright LED.

As far as I understand you apply 48VDC on the gate of the bipolar transistor?

Also when the power supply is on, the main transistor will be off as you ground its base until you press the reset button, which actually gives "0VDC" and not the "-5VDC" to make the "pnp" transistor work.
 
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Thread Starter

mcgyvr

Joined Oct 15, 2009
5,394
Also when the power supply is on, the main transistor will be off as you ground its base until you press the reset button, which actually gives "0VDC" and not the "-5VDC" to make the "pnp" transistor work.
The way that the circuit is supposed to work is that when 48V is applied through momentary switch 1 to the base resistor of T4 that this grounds the base of T5 causing voltage to flow through R9 keeping transistor T4 on..
When the reset button is pressed T3 starts conducting and the base of T4 is pulled to ground again causing t5 to also stop conducting..
This is a very common set/reset latching circuit..
https://www.homemadecircuitsprojects.com/2011/12/how-to-make-transistor-latch-testing.html
Its just not working for me.. (I will add C1 too)
 

ArakelTheDragon

Joined Nov 18, 2016
1,327
Maybe I am blind, I have a small monitor(maybe both).

If the voltage needed to drive the "PNP" is maxium base emitter brake down voltage of "-5VDC" wont grounding the base give -48VDC on the base-emitter?
 

MrChips

Joined Oct 2, 2009
19,906
When you close the switch, T4 will conduct and pass excessive current through the base-emitter junction of T5.
You end up blowing T4 and T5.
 

Thread Starter

mcgyvr

Joined Oct 15, 2009
5,394
Doh.. So yes a resistor from the base of T5 to collector of T4 node is needed..
Ok.. Just tried it.. Totally works just fine now.. I didn't even notice I was missing the base resistor for that one.. and sure enough pop on the first time (just not destructive so I didn't notice)..

I would still appreciate a bit of help on determining proper resistor values in the circuit..
Its working find with 10k all around but I just pulled those out of my parts bin for now..
 
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crutschow

Joined Mar 14, 2008
24,035
I would still appreciate a bit of help on determining proper resistor values in the circuit.
Typically, for a BJT to act as a saturated switch, you want the base current to be 5-10% of the maximum collector current when the transistor is on, so you select the base resistors to give that value of base current.
 
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