Base Resistor Value for PNP Transistor on Solar Light Circuit

Thread Starter

kmarques

Joined Feb 4, 2018
25
Hi Everyone,
I am working on my first circuit design and need help finding the correct base resistor value for the PNP transistor. I've found the following formula, R=(V-Vbe)/Ib but I am not sure which values I should be plugging in. Any help with this would be greatly appreciated. Please see circuit design below.
schematic001-01.jpg
 

spinnaker

Joined Oct 29, 2009
7,835
For starters your LEDs appear to be wired backward. Even if wired correctly they may not light. Your supply is 3.2V and your forward voltage of your LEDs is 3.2v (are you sure that is correct ?). Any variance beyond the rated 3.2 forward voltage and the LED won't light. Even a slight drop in voltage and the LEDs won't light.

Charging Lipos is nothing to play with. You really should purchase a completed controller. Not something that you design. Some of the best engineers have designed charging systems that have failed and caused major damage.
 

Thread Starter

kmarques

Joined Feb 4, 2018
25
For starters your LEDs appear to be wired backward. Even if wired correctly they may not light. Your supply is 3.2V and your forward voltage of your LEDs is 3.2v (are you sure that is correct ?). Any variance beyond the rated 3.2 forward voltage and the LED won't light. Even a slight drop in voltage and the LEDs won't light.

Charging Lipos is nothing to play with. You really should purchase a completed controller. Not something that you design. Some of the best engineers have designed charging systems that have failed and caused major damage.
The LED's forward voltage is 3 - 3.2v. The battery is not a LiPo, but the much safer LiFePO4. Thanks for your input.
 

Alec_t

Joined Sep 17, 2013
13,226
Since you have switch SW1, why do you need the transistor?
The PNP transistor won't turn on with its base connected as shown via the resistor to the +ve rail.
 

dl324

Joined Mar 30, 2015
15,445
What are you trying to accomplish with this circuit? Why do you have so many LEDs? LEDs don't "switch on" at their nominal forward voltage; they start turning on at relatively low currents.

Are you trying to tell us that you want to operate the LEDs at 30mA, or are you telling us their maximum continuous forward current rating? Is 3.2V the maximum forward voltage, or typical or minimum?

With the base pulled up to the emitter, the transistor will never turn on. The LEDs are wired backwards and are drawn upside down.
 

Thread Starter

kmarques

Joined Feb 4, 2018
25
What are you trying to accomplish with this circuit? Why do you have so many LEDs? LEDs don't "switch on" at their nominal forward voltage; they start turning on at relatively low currents.

Are you trying to tell us that you want to operate the LEDs at 30mA, or are you telling us their maximum continuous forward current rating? Is 3.2V the maximum forward voltage, or typical or minimum?

With the base pulled up to the emitter, the transistor will never turn on. The LEDs are wired backwards and are drawn upside down.
I am trying to build a solar powered light to be used in the cockpit of a sailboat. It is very similar to the common solar powered garden light except that I do not need it to light up every night. Here is the LED specs: Forward Voltage / Direct Current : DC 3-3.2 Volts / 30 mA.
 

spinnaker

Joined Oct 29, 2009
7,835
I am trying to build a solar powered light to be used in the cockpit of a sailboat. It is very similar to the common solar powered garden light except that I do not need it to light up every night. Here is the LED specs: Forward Voltage / Direct Current : DC 3-3.2 Volts / 30 mA.

You are trying to DIY for a marine enviroment in a sailboat no less? This is one of the harshest environments there is. Good luck with that. And your 5 LEDs are not going to provide an awful lot of light. Maybe enough to light an instrument panel but that is about it. I don't mean to throw cold water on your project but sometimes it is just better to buy something rather than to build.

Is this a cruising sailboat with existing house battery? If so why not use that? You can supplement charging with a proper solar panel and/or wind turbine.
 

Thread Starter

kmarques

Joined Feb 4, 2018
25
You are trying to DIY for a marine enviroment in a sailboat no less? This is one of the harshest environments there is. Good luck with that. And your 5 LEDs are not going to provide an awful lot of light. Maybe enough to light an instrument panel but that is about it. I don't mean to throw cold water on your project but sometimes it is just better to buy something rather than to build.

Is this a cruising sailboat with existing house battery? If so why not use that? You can supplement charging with a proper solar panel and/or wind turbine.
Spinnaker, I certainly appreciate your input, but I've always believed that building is way more fun than buying... I do have a house battery, but can not wire the lights up to it do to where they will be mounted.
 

dl324

Joined Mar 30, 2015
15,445
I would like to utilize the solar panel as a light sensor as seen here: https://www.evilmadscientist.com/2008/simple-solar-circuits/
Have you tried this circuit that was in the referenced article?
upload_2018-2-4_9-55-4.png
Whether it would work depends on how many solar cells are in series in your array and the base current required by the transistor switch. If the base current is high enough to prevent the transistor from turning on enough, you could use another transistor for current gain.
 

Thread Starter

kmarques

Joined Feb 4, 2018
25
Have you tried this circuit that was in the referenced article?
View attachment 145168
Whether it would work depends on how many solar cells are in series in your array and the base current required by the transistor switch. If the base current is high enough to prevent the transistor from turning on enough, you could use another transistor for current gain.
Yes, that is what I used... I just placed it after the charge controller. I'm just trying to figure out what the base resistor value should be.
 

spinnaker

Joined Oct 29, 2009
7,835
Spinnaker, I certainly appreciate your input, but I've always believed that building is way more fun than buying... I do have a house battery, but can not wire the lights up to it do to where they will be mounted.

More fun yes. Just not always practical. If you are doing this for fun then wonderful. If you want something reliable then that is a whole other matter.

You don't have any existing lighting or instrumentation in the cockpit that the wiring can be utilized? I don't know your boat. Mine was fairly small and hard if not impossible to wire under the cockpit sole but if I got creative I am pretty sure I could have added wiring.
 

spinnaker

Joined Oct 29, 2009
7,835
Updated schematic with LED's direction corrected. Thanks for the call out.
View attachment 145164

I am not so sure you really need the 1n5817. Does the charge controller datasheet call for it? You typically add a schotkey to act as a blocking diode to prevent the battery from "charging" the solar panel when battery voltage exceeds solar panel voltage. But the charge controller should provide this blocking, unless as I mentioned the datasheet calls for it.
 

spinnaker

Joined Oct 29, 2009
7,835
It might be better to put your transistor light switch on the solar panel side of the circuit. There might be plenty of daylight but the panel won't be able to put ut enough voltage to run the controller but still have enough to be sensed by your switch.

Using the solar panel as your sensor is convenient. It is how I did a solar panel project. But they don't make the best sensors. I found that there was plenty of daylight but the light still came on. What I did was to add a timer that started after the panel "thought" it was dark. The light would then turn on after the timer elapsed.

What you might want to do is to add a dedicated sensor that is going to be more sensitive.
 

Thread Starter

kmarques

Joined Feb 4, 2018
25
More fun yes. Just not always practical. If you are doing this for fun then wonderful. If you want something reliable then that is a whole other matter.

You don't have any existing lighting or instrumentation in the cockpit that the wiring can be utilized? I don't know your boat. Mine was fairly small and hard if not impossible to wire under the cockpit sole but if I got creative I am pretty sure I could have added wiring.
I am not so sure you really need the 1n5817. Does the charge controller datasheet call for it? You typically add a schotkey to act as a blocking diode to prevent the battery from "charging" the solar panel when battery voltage exceeds solar panel voltage. But the charge controller should provide this blocking, unless as I mentioned the datasheet calls for it.
I was definitely on the fence about adding the schottky diode... I don't see anything in the data sheet http://ww1.microchip.com/downloads/en/DeviceDoc/22191D.pdf
 

BobaMosfet

Joined Jul 1, 2009
2,053
..fingers and toes...
..maybe your nose..
..all will be gone..
..if your battery blows..

I don't know why, but the song 'silver & gold' by Burle Ives came to mind, and I just couldn't resist changing the lyrics for something appropriate to the TP's level of knowledge and goals...
 
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