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Transistor limiting current

GregK

May 1, 2023
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I'm building a small solar project and am trying to use a S8550 transistor to work as an on/off switch depending on how much light is hitting the solar panel.

I've attached my schematic to detail what I have done. But I am finding the S8550 transistor is cutting down the current going through so my light is very dim.

I'm very new to this type of work so any advice as to where I am needing up would be super appreciated.

KaxSn.jpg
 

davenn

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Sep 5, 2009
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I'm very new to this type of work so any advice as to where I am needing up would be super appreciated.

Hey Greg,
welcome to EP

1) I am not sure what a S8550 transistor is ? or if it is capable of passing required current from the board
Do you have a link to the datasheet please? :)

2) What is that circuit board ? link to the datasheet please? :)

3) you have the S8550 base being driven directly from the 5V coming from the Solar Panel
this means the transistor will always be on as long as there is 0.7V on the base.

You shouldn't drive a base pin directly, usually a 1k Ohm resistor in series to limit current to the base,
else you are likely to kill the transistor

cheers
Dave
 

Bluejets

Oct 5, 2014
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not sure what a S8550 transistor is
Suspect it's an SS8550......

how much light is hitting the solar panel.
Don't see how that would be adjusted except possibly hard off or on depending on whether sun was out or gone to bed.

Look at microcontroller, light level and also high side switching either by transistor (or easier/better) by mosfet.
 

GregK

May 1, 2023
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1) I am not sure what a S8550 transistor is ? or if it is capable of passing required current from the board
Do you have a link to the datasheet please? :)

2) What is that circuit board ? link to the datasheet please? :)

3) you have the S8550 base being driven directly from the 5V coming from the Solar Panel
this means the transistor will always be on as long as there is 0.7V on the base.

You shouldn't drive a base pin directly, usually a 1k Ohm resistor in series to limit current to the base,
else you are likely to kill the transistor
Hi @davenn that's for the reply.

@Bluejets is correct the transistor is a SS8550. I made a mistake on that one.

1)The data sheet is located here https://www.mouser.com/datasheet/2/149/SS8550-118608.pdf

2) The data sheets for the 2 boards being used are
And

3) I am testing my set up with a 1.2v battery during the build to simulate a low voltage on the base. I will add a 1k Ohm resistor now to ensure I don't forget it in the end as I did not think about burning out the resistor. Thank you for the advice.
 

GregK

May 1, 2023
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Don't see how that would be adjusted except possibly hard off or on depending on whether sun was out or gone to bed.

Look at microcontroller, light level and also high side switching either by transistor (or easier/better) by mosfet.
Hi @Bluejets, then you for replying to my post.

You are correct, I phrased my goal incorrect. I am looking to do a hard on/off. I'm not looking to do any type of dimming.

Thank you for the info. I will look into microcontrollers and mosfets.
 

GregK

May 1, 2023
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Okay after reading a bit I think I am understanding things a bit more. Please anyone let me know if I have this wrong.

I originally thought a transistor would cut flow from the emitter and collector if the voltage dropped below a specific number. But after some reading am I correct in thinking that the negative number stated on the base connector on the data sheets is what the difference has to be for the flow to be cut off?
 

danadak

Feb 19, 2021
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As you can see below primary determinant of Ic and Ie is base current.

1682955222042.png


Regards, Dana.
 

danadak

Feb 19, 2021
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No, more Ib current = more Ic collector current. Look at the graph prior. But as you
keep raising base current at some point you will drive Vce into saturation,
and in sat the Vce no longer changes much, hence the Vload V will no
longer change much, and the current stops changing in the circuit.

1682967724110.png

Regards, Dana.
 

Bluejets

Oct 5, 2014
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Sorry, I'm new to this
Which is why I suggested using a P-channel mosfet high side switch.(simpler)
You will still need to add circuitry/microcontroller whatever to control the switching point.
 

GregK

May 1, 2023
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as you keep raising base current at some point you will drive Vce into saturation, and in sat the Vce no longer changes much, hence the Vload V will nol onger change much, and the current stops changing in the circuit.
I think I ran into this with a previous version I had. I was recommended to remove the SS8550 transistor and use the LM2596S chip as my on/off switch. But I found when I fed 1.2v into the Vin (pin 1) the output would cut off and I unfortunately would have to remove the 18650 battery to reset. As removing the voltage from pin 1 didn't allow output to continue from the LM2596S board (buck converter).dlwsV.jpg
 
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