Transistor limiting current

[GregK]

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.

 

[davenn]

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]

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]

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

https://www.ti.com/lit/ds/symlink/lm2596.pdf

And

TP4056 / TC4056A Lithium Battery Charger and Protection Module

This module uses the TP4056 / TC4056A Li-Ion charge controller IC and a separate protection IC for safely charging and discharging lithium-ion batteries.

www.addicore.com

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]

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.

 

[Bluejets]

I will look into microcontrollers and mosfets.

well, if it's hard on/off, just look at " high side mosfet switching" or if you must " same same in transistors"

 

[GregK]

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]

As you can see below primary determinant of Ic and Ie is base current.




Regards, Dana.

 

[GregK]

As you can see below primary determinant of Ic and Ie is base current.

View attachment 58907


Regards, Dana.

Sorry, I'm new to this. Does that mean the more base current I have going in, the less output current the transistor can put out?

 

[danadak]

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.



Regards, Dana.

 

[Bluejets]

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]

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).