How to determine the correct potentiometer to control the intensity of the photo Transistor?

Thread Starter

Paresh Chauhan

Joined May 20, 2021
3
Can any one help to as to how to select the potentiometer of correct rating.
I have 22,3 V (max) of supply from solar panel which is connected to zener diode circuit with parallel resistor of 1 k ohm to zener diode (3.0V) and parallel to zener diode is another resistor of 820 ohm which is connected to the photo transistor (BPX38-3 OSO) with Vce = 50 V, Vbe= 7 V maximum values. Between the ground and the emmitter of the photo transistor i have to connect the potentiometer which control the sensitivity of my photo Transistor to supply voltage to the gate terminal of MOSFET with Vgs= 16 V and Vds = 100 V which is connected to the actuator which is to be controlled with the phototransistor.
Files attached: Ckt diagram, Mosfet datasheet, Phototransistor data sheet
 

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dl324

Joined Mar 30, 2015
12,927
Welcome to AAC!

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Do you really have the base of the photo transistor grounded?

What is the pot value value? What voltage is PV? Is it a solar cell?
 

michael8

Joined Jan 11, 2015
147
Can any one help to as to how to select the potentiometer of correct rating.

Unlikely since the zener limits the mosfet gate voltage to 3.3 volts which isn't likely to turn on the mosfet.
I'm assuming that since the mosfet is in the circut, at some point it is to be turned on.

Also my understanding of a phototransistor is that the photons create charges in the base-collector junction
and this via transistor action causes collector/emitter current. With the base grounded this charge will
drain off and any transitor gain will be minimal (likely zero). The emitter current is thus likely to be
zero.

I think you need to write a description of what the circuit (or you) are trying to do. What would it do if it worked?
What does it actually do? (or where do you see problems or parts you are uncertain about).
 

Alec_t

Joined Sep 17, 2013
12,153
Welcome to AAC!
A problem with your simple circuit is that because of component tolerances and temperature effects there are no sharply-defined consistent points at which the MOSFET turns on and off. Also, the slightest variation in light intensity will cause the actuator to 'stutter' around the turn-on/off points. I suggest you research 'hysteresis'.
 

Ian0

Joined Aug 7, 2020
2,652
What does it do? It appears that the intention would be to switch on the actuator when there is solar power and off when there isn't. But wouldn't that be achieved simply by connecting the actuator directly to the panel without any other circuitry?
 

dl324

Joined Mar 30, 2015
12,927
Max volatage of Pv panel is 22.3 V
  1. What is the panel current capability?
  2. What voltage and current does the motor require?
  3. What phototransistor current corresponds to the light level you want the actuator to be on?
  4. Is the actuator inductive?
  5. What is the actuator resistance?
 

Thread Starter

Paresh Chauhan

Joined May 20, 2021
3
Can any one help to as to how to select the potentiometer of correct rating.

Unlikely since the zener limits the mosfet gate voltage to 3.3 volts which isn't likely to turn on the mosfet.
I'm assuming that since the mosfet is in the circut, at some point it is to be turned on.

Also my understanding of a phototransistor is that the photons create charges in the base-collector junction
and this via transistor action causes collector/emitter current. With the base grounded this charge will
drain off and any transitor gain will be minimal (likely zero). The emitter current is thus likely to be
zero.

I think you need to write a description of what the circuit (or you) are trying to do. What would it do if it worked?
What does it actually do? (or where do you see problems or parts you are uncertain about).
The base is not connected to ground. the circuit description is as follows:-
through zener diode 3.3 V is supplied to the photoTransistor which is meant to supplied to the gate approximately above 2.7 V which would then run the actuator. The Phototransitor will turn on when the light falls on it and the potentiometer is used to control the intensity of light at which the the phototransistor is turned ON/Off
 
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