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Problems with my school project Automatic ON OFF lamp using LDR and MOSFET

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kylie1023

May 26, 2023
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my transformerless power supply on the left side of the volt meter works just fine until i connect it to my load circuit on the right. the volt coming out just stays 1.5V at most and i cannot figure out why, please help thank you1685099514899.png
 

Harald Kapp

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School project? This type of "power supply" is absolutely not suitable for a school project.

Please do not use this so called "power supply" design. Although the voltage on the secondary is low, the whole circut is connected to mains and thus potentially lethal.

Throw away the power supply section and use a well insulated safe power supply, e.g. a USB wall wart. Or an adjustable wall wart for higher output voltage and power.
 

Alec_t

Jul 7, 2015
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Welcome to maker.pro, but I agree with Harald.:eek:
i cannot figure out why
Because R10(?) prevents any significant current flow in the circuit.

What is the lamp rating?
Which MOSFET?
What are the LDR specs?
 
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bidrohini

Feb 1, 2023
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I agree with the first answer. I also suggest you check all the parts separately with DC voltage. See if all of them are functional.
 

charly2choo

Apr 25, 2023
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Uhm, an interesting case.

The most likely reason why your transformerless power supply is only giving out 1.5V when you connect it to your load circuit is that the load circuit is drawing too much current. You can fix this by increasing the value of the resistor in series with the load.

So for troubleshooting your project, I would make sure that:

the load circuit is connected correctly,
the zener diode for damage is alright,
and also what about connections between the components.

I hope you'll be able to figure out the problem.
 

danadak

Feb 19, 2021
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What is your Vgas of the MOSFET when you think it should be off ?

I am in agreemen with all the safety comments. I would bet if I take a poll
of the group we would all like to be placed on your end of life will.
Tell your lab mates to stay a few meters away when you are working on it.

:)


You have no part numbers, like that of the MOSFET showing.....hard to help
you w/o adequate info. Tell your lab mates to stay a few meters away when
you are working on it.


Regards, Dana.
 

AnalogKid

Jun 10, 2015
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The most likely reason why your transformerless power supply is only giving out 1.5V when you connect it to your load circuit is that the load circuit is drawing too much current. You can fix this by increasing the value of the resistor in series with the load.

I don't see a resistor in series with the load. Reference designator - ?

Unless you mean R15 or R16. Increasing these would make the problem worse. Other than the FET, the most significant impedance in series with the load is C9.

Also, a 1N4749 is a 24 V zener. Most power MOSFETs have a maximum Vgs of 20 V, so that's a separate problem.

ak
 

bertus

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Hello,

I have inverted the colours of the picture for better readability:
ldr lamp mains inverted colours.png
R16 will reduce the current to much.
I hope this is in simulation only.
Also it not recomended to use the circuit as it is connected to the mains and can be lethal.

Only one post and no reaction from the TS.

Bertus
 
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