Turn off transistor at some voltage

[nmbg011]

I need help!

How to turn off transistor on some voltage level, should be simple circuit as possible ?

I know transistor as shunt with zener on transistor base, shunt will turn On on some voltage, but I need reverse situation, to turn Off transistor at some voltage level.

 

[allenf]

instead of using an NPN (voltage to effectivly turn ON the EMITTER use a PNP (effectivly use voltage to turn OFF the EMMITTER) see pic below


so with an NPN source goes in at C (collector) nothing comes out of E (emmiter) until G (gate) is given signal (gate is open until closed ) Diag shows NPN BASE to emmiter thus NO signal until you close it
with a PNP source goes in at C (collector) and comes out at E (emmitter) until G(gate) is given signal (gate is closed until opened) diag shows PNP Emmiter points to COLLECTOR (personally i always think the arrow should be on the Collecter to gate in a pnp buts thats just me)

To remember the symbols
NPN = Not Pointing iN PNP Points iN Perfectly

Hope that helps you a bit

 

[nmbg011]

I didnt understand how to turn off transistor at 4V voltage for example, transistor have led which is turned on and on 4V led should turned off ?



Circuit represent NiMh constant current battery charger, and I whant to charge 3xAA to 4V, and at 4V-4,3V to cut off charging.

 

[allenf]

K you can only really turn them one way NPN to ON pnp to OFF so to turn an led OFF when a transistor is ON use an NPN to turn OFF an led when the transitor is on use a PNP

 

[nmbg011]

Can you help me on my circuit, please?

 

[allenf]

K try this



Does that help any more

 

[nmbg011]

You didnt understand, circuit in my post #3 should turned off transistor at some voltage level automatically!!! not with finger.

Example at 6V to turn off.

 

[allenf]

Sorry didnt see your diagram.
If you move R2 to the GATE (middle connection) and changeit for a 10K variable resistor (for sake of simulation) then you will get the result you want all you have to do is set the variable resistore find the value and subsitute the correct resistor for it
Hope that makes sense

 

[nmbg011]

But R2 must be on emitter its current limiter.

Its 0,65V / R2 = current

Thise two diodes are always on and give 1,3V to base, that give constant 0,65V at emitter.

R1 just protect diodes from exploding can be with value greater then 500 ohm.


This working great like constant current charger but I whant to stop charging when batteries is charged at some level example 6V.

 

[allenf]

so basically you are after this battery charger??


You would need t set the VR to the right voltage then subsitute a resistor of that value for it

 

[nmbg011]

That circuit use relay I dont need that and is not constant current charger. I need constant current charger from post #3

I just whant to switch off transistor in post #3 circuit at some voltage level!!!

 

[nmbg011]

Can I turn off transistor like this, can anyone help ?

Transistor voltage Off = zener 5,1V + 0,7V = ~5,8V



Charging will go with constant current and when voltage goes to 5,8V level then turn off transistor, am I right ?

 

[mister_rf]

@nmbg011
One idea for the circuit:

 

[nmbg011]

@nmbg011
One idea for the circuit:

yes but pot p1 will discharge battery?

 

[nmbg011]

@nmbg011
One idea for the circuit:

Dear Mister_rf I try your sugested circuit and i must say circuit dont work!

Acording parts marking on your circuit I put 10K pot for P1, R3 is 4,7K, T3 is BC556 PNP, T2 is 2N3904 NPN, T1 is BD139 NPN, R1 is 1K, D1 and D2 are 1N4007.
Input voltage is 12V DC and batteries are 4xAAA NiMh

I dont know what to do, I dont know how to stop T1 at some voltage level. I whant to stop T1 when batteires reach 5,8V.

 

[mister_rf]

Try this way, simulation OK...

 

[mister_rf]

Resistors values not critical.
R1 = 470-680 ohms
R3 = 1k-2.2 k
R5 = 3.9k – 5.6k
R4 = 3.3k-3.9K for P1 = 1k
R4 = 3.9k – 5.6k for P1= 2.2k
I batteries = 0.6V/R2

 

[nmbg011]

Dear MisterRF I try circuit from post 16 but no success. Circuit works but cant establish threshold of voltage top level, voltage goes over 6,2V, and in other hand when try to adjust pot again, canot stop completely charging (10-30mA stay).

When manualy adjust pot small area is se of treshold, and in one case charging start with high current 200mA (I put 4R), and in another case stops but minimum is 10-30mA.

 

[Ogu Reginald]

They have answered well just use a PNP instead of an NPN. But I have a further innovation "just put an inverter ie cd4069 at the output of your existing design and its ON will be OFF and vice versa.

 

[dselec]

U will ned a comparator that can change output + to zero, and comp can compare voltages as low as 1micro volts.
i mean u can tune in a ref for 1.000000v ans as soon as the voltage goes up by 0.000001 v... that's a change as 1 micro volts , u can assume it changes on off at almost same voltage.