Voltage across 100ohm resistor is always 0.6V.

If the temperature is constant.

 

Voltage across 100ohm resistor is always 0.6V. So current thru 100ohms is 0.6/100=6mA. The same current flows thru zener.
If you change 100ohm to 47ohm the current will be 0.6/47=12mA.
So by picking this resistor you can set the current thru zener.

What is purpose of 1k resistor? Also what zener diode has 3V real model and what should the current be? I cant find any Ltspice model breakdown voltage of 3v.

 

1k is for supplying the base of transistor.
Most used is 3v3 zener, but 3v exist also. I don’t use a models.
Pick the current according datasheet VI-chart at what current the zener voltage is most constant.

When you look at 3v3 curve the voltage starts to be out of knee at 10mA and more. Maybe 7mA is usable.

 

1k is for supplying the base of transistor.
Most used is 3v3 zener, but 3v exist also. I don’t use a models.
Pick the current according datasheet VI-chart at what current the zener voltage is most constant.

So what's the purpose of 1k? I dont understand supply base with what and why thats needed?

 

The pnp transistor is conducting (switched on) when the base is pulled to ground thru resistor. That’s it.

 

The pnp transistor is conducting (switched on) when the base is pulled to ground thru resistor. That’s it.

Ok but how did you get value of 1k ohms for the resistor? Could have 10k ohms?

 

1k sets the current flowing thru two 1n4148 also , so it’s good to be at least few mA (strong biased). The base current flows thru 1k also, so another current is added.
It’s better to use lower resistance to base current doesn’t affect the two diode bias.
(To hold 2xVf=1.2V most constant).

The base current is Ib=Ic/Hfe=10mA/100=100uA
Together with 2mA (empirically picked) of two diode bias current the overall current thru R_bias is 2.1mA.
So R_bias=(Vcc_min-2*Vf)/I_bias=(5-1.2)/2.1mA=1809ohm,
so I choose 1k.

Note: Vcc_min=5V is worst case.

 

1k is for supplying the base of transistor.
Most used is 3v3 zener, but 3v exist also. I don’t use a models.
Pick the current according datasheet VI-chart at what current the zener voltage is most constant.

When you look at 3v3 curve the voltage starts to be out of knee at 10mA and more. Maybe 7mA is usable.

3v3 i dont see it in through hole? It comes in other package.

 

1k sets the current flowing thru two 1n4148 also , so it’s good to be at least few mA (strong biased). The base current flows thru 1k also, so another current is added.
It’s better to use lower resistance to base current doesn’t affect the two diode bias.
(To hold 2xVf=1.2V most constant).

The base current is Ib=Ic/Hfe=10mA/100=100uA
Together with 2mA (empirically picked) of two diode bias current the overall current thru R_bias is 2.1mA.
So R_bias=(Vcc_min-2*Vf)/I_bias=(5-1.2)/2.1mA=1809ohm,
so I choose 1k.

Note: Vcc_min=5V is worst case.

I dont follow. Previously you said IC is constant based on diode voltage drop0.65V so Ic= 0.65/100 all the time.

how come then you Ib in the equation is not constant either? How csn resistor pull more or less in ground from transistor base? I dont get all this

 

https://www.digikey.com/en/products/detail/onsemi/1N5226BTR/977562

 

Ib=100uA is current needed to flow 10mA thru collector-emitter junction.
The Ib is almost constant no matter what Vcc is, Vce also (in this circuit).
From equation Ib=Ic/Hfe you can see if Ic=const the Ib=const also.

 

Datasheets show all the ranges of part specs. Simulators show only "typical" specs but not the range of specs from minimum to maximum.

Datasheets of zener diodes show that their voltage drops when heated when their zener voltage is below about 5,6V and their voltage increases when heated when their voltage is above about 5.6V.
The datasheet for a 3.0V at 20mA 1N5225B (through-hole) shows that its voltage drops 0.075%/degrees C.

 

Ib=100uA is current needed to flow 10mA thru collector-emitter junction.
The Ib is almost constant no matter what Vcc is, Vce also (in this circuit).
From equation Ib=Ic/Hfe you can see if Ic=const the Ib=const also.

Ahhh your good . What about the supply voltage drops below 5vdc 1vdc to 5vdc and want to keep 3V constant voltage supply?? I dont think you can keep voltage constant between 0V to 1vdc.

 

Datasheets show all the ranges of part specs. Simulators show only "typical" specs but not the range of specs from minimum to maximum.

Datasheets of zener diodes show that their voltage drops when heated when their zener voltage is below about 5,6V and their voltage increases when heated when their voltage is above about 5.6V.
The datasheet for a 3.0V at 20mA 1N5225B (through-hole) shows that its voltage drops 0.075%/degrees C.

Ahhh diode is Temperature dependent

 

Ahhh diode is Temperature dependent

That's why I suggested the TL431, which acts as a stable Zener, largely independent of current (above 1mA) and temperature.
If you configure it as shown below, you can use it in place of a 2.5V Zener (Vref voltage) for the rest of your circuit:

 

Ahhh your good . What about the supply voltage drops below 5vdc 1vdc to 5vdc and want to keep 3V constant voltage supply?? I dont think you can keep voltage constant between 0V to 1vdc.

When Vcc is below zener voltage the Vout follows the Vcc (approximately).
To circuit be working when Vcc is below zener voltage you need a boost converter or charge pump.

 

That's why I suggested the TL431, which acts as a stable Zener, largely independent of current (above 1mA) and temperature.
If you configure it as shown below, you can use it in place of a 2.5V Zener (Vref voltage) for the rest of your circuit:

View attachment 323270

Its hard to get 3vdc when you have 1v to 12vdc supply varying.

also i dont like the idea of building from ics, built in shunt trip regulators etc

 

Its hard to get 3vdc when you have 1v to 12vdc supply varying.

This constraint is unreasonable for a 3v reference.

also i dont like the idea of building from ics, built in shunt trip regulators etc

Then it will be a challenge.

 

This constraint is unreasonable for a 3v reference.



Then it will be a challenge.

Can you not

This constraint is unreasonable for a 3v reference.



Then it will be a challenge.

can you not have circuit if voltage drops below 3 vdc then it produces 3vdc output using inductors capacitors etc etc?

 

The IC in a Solar Garden Light boosts the 0.9V to 1.5V from a rechargeable Ni-MH battery cell to 3.5V for a colored LED.
You might be able to copy the circuit in the cheap IC with many transistors.