# zener diode

#### hhsting

Joined Apr 25, 2024
156
In the circuit below Zenior Diode has breakdown voltage of 4.7V. When I take voltage between Zenior Diode I get 4.6V.

When I connect the transistor BSF as shown below I get voltage from the drain to the source of about 4.5V. I want to get 5.2V from drain to source while I want to get from gate to source voltage of zenior diode. How can I do this?

#### dl324

Joined Mar 30, 2015
17,122
Welcome to AAC!

1N750 voltage range at 20mA is 4.47-4.94V.

Didn't try to analyze the circuit because you've drawn it sideways and backwards and it's too much bother.

#### Michal Podmanický

Joined May 11, 2019
112
What is the purpose od V3?
Why not to connect Source to Gnd?

#### crutschow

Joined Mar 14, 2008
34,837
I get voltage from the drain to the source of about 4.5V.
Each MOSFET has a different Vgs to give a desired drain current, so your circuit is not a proper design.
What, exactly, are you trying to do?

#### hhsting

Joined Apr 25, 2024
156
I have load that is not shown that takes 5.2 vdc. The transistor turns load on or off depending upon the vgs. The zenior dido output controls the vgs. The 0.6V dc varies from 0 vdc to 1.8 vdc. I want transistor off when that voltage is 05vdc or higher and on from 0vdc to 0.5vdc
My question is how can I use the 5.2vdc which is constant to drive my load when transistor opens? I cannot connect 5.2vdc from source to drain it gives me some voltage less than 5.2vdc

#### crutschow

Joined Mar 14, 2008
34,837
Show the complete circuit with the load.

#### hhsting

Joined Apr 25, 2024
156
Show the complete circuit with the load.

#### Michal Podmanický

Joined May 11, 2019
112
With Vx you control the Vgs of mosfet according table.
Zener voltage variation (4.6-4.9V) because of current through zener change is neglected there.

#### Attachments

• 91.5 KB Views: 13

#### hhsting

Joined Apr 25, 2024
156
With Vx you control the Vgs of mosfet according table.
Zener voltage variation (4.6-4.9V) because of current through zener change is neglected there.
What is voltage from drain to source be when transistors is open or close? The load requires minimum of 5.2V dc

#### Michal Podmanický

Joined May 11, 2019
112
When transistor is switched On there will be always 5.2V across load.

#### crutschow

Joined Mar 14, 2008
34,837
I want transistor off when that voltage is 05vdc or higher and on from 0vdc to 0.5vdc
For that low and precise a voltage transition you likely need a comparator circuit, such as the LM339/393, along with a Zener or reference voltage IC.
Can you use those?

#### hhsting

Joined Apr 25, 2024
156
For that low and precise a voltage transition you likely need a comparator circuit, such as the LM339/393, along with a Zener or reference voltage IC.
Can you use those?
I am trying to build comparator without using ICs, relays etc. I am confused how I can do it

#### crutschow

Joined Mar 14, 2008
34,837
I am trying to build comparator without using ICs, relays etc. I am confused how I can do it
Why no IC's?
A transistor differential comparator can be built, if necessary.
Example circuit below:
In place of D1 there would be a 0.5V reference voltage.

#### hhsting

Joined Apr 25, 2024
156
Why no IC's?
A transistor differential comparator can be built, if necessary.
Example circuit below:
In place of D1 there would be a 0.5V reference voltage.

View attachment 321761
i dont follow your circuit. I dont have any AC voltages. All voltages are DC.

I want to learn how I can build comparator without Ics or relays and using minimum components as possible.

i am trying to build voltage comparator using only diodes and transistors where if its above 0.5VDC then I get 5.2VDC output and below 0.5VDC I get 0.5VDC output.

know of any such circuits?

#### crutschow

Joined Mar 14, 2008
34,837
i am trying to build voltage comparator using only diodes and transistors where if its above 0.5VDC then I get 5.2VDC output and below 0.5VDC I get 0.5VDC output.
How accurate does the 0.5V trip point need to be?

#### Michal Podmanický

Joined May 11, 2019
112
Bjt itself works like this. It opens at 0.6V across BE.
If you insist to switch at exactly 0.5V input just use resistor divider R1:R2.

However, transistor Vbe changes with indoor/outdoor temperature in range 0.5 - 0.7V, so if temperaure is not stable it won’t be much precise.

#### Attachments

• 89.4 KB Views: 6
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#### hhsting

Joined Apr 25, 2024
156

#### hhsting

Joined Apr 25, 2024
156
Bjt itself works like this. It opens at 0.6V across BE.
If you insist to switch at exactly 0.5V input just use resistor divider R1:R2.

However, transistor Vbe changes with indoor/outdoor temperature in range 0.5 - 0.7V, so if temperaure is not stable it won’t be much precise.
Can you do anything with MOSFET that have positive vgs? Also I want transistor to open between 0V to 0.5V dc and close greater than 0.5vdc.

yours if less than 0.5vdc would close which is not what I want

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#### crutschow

Joined Mar 14, 2008
34,837
very accurate.
"Very accurate" is not a design value, it's an imprecise goal.
Exactly how close to 0.5V trip do you need in percentage value or mV.