# Question on my charging circuit - Will the transistor get too hot? There is also more.

#### Sid2001

Joined Dec 7, 2016
28
Hello, for my GCSE Project I am making a solar powered power bank. I found a circuit online and made a few edits to it. Here is my circuit.

The 12v DC power source is representing the solar panel and the 5v battery is the Lithium IPhone battery. The current going into the IPhone battery is 538.93mA as shown. When the circuit is on, the transistor is showing a power dissipation between 2W and 3.5W. What I'm wondering if this will make the transistor too hot and if i'd need a heat sink or something like that. Or maybe if there is a different transistor I can use that will allow a collector current of ~500mA. The transistor I'm using right now is a 2N2222A NPN.
My final question is that I tried making a virtual breadboard of this circuit, and sometimes the transistor would blow up due to an Emitter-Base voltage of 6v exceeding the max limit of 5v. How do I combat this? The voltage at the base is ~7.16v and ~6.26 at the emitter. In case it's needed, the voltage at the collector is ~10.96.

This is the original circuit which i found on the internet (before I changed some things myself).

Sid

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#### WBahn

Joined Mar 31, 2012
25,064
You need to look at the data sheet for your transistor (and this is a good opportunity for you to get started getting familiar with that process).

You will probably find that 500 mA is pushing, but within, the absolute limits for this part, but that your power dissipation is through the roof. The max total power dissipation is in the 0.5 W range.

Also, do you know enough about the proper way to charge lithium batteries to ensure that either the original circuit or your modifications to it adhere to those guidelines?

#### Sid2001

Joined Dec 7, 2016
28
You need to look at the data sheet for your transistor (and this is a good opportunity for you to get started getting familiar with that process).

You will probably find that 500 mA is pushing, but within, the absolute limits for this part, but that your power dissipation is through the roof. The max total power dissipation is in the 0.5 W range.

Also, do you know enough about the proper way to charge lithium batteries to ensure that either the original circuit or your modifications to it adhere to those guidelines?

I thought that IPhones and modern smartphones have built in protection circuits. Would I need a heat sink for this transistor?

#### WBahn

Joined Mar 31, 2012
25,064
I thought that IPhones and modern smartphones have built in protection circuits.
You schematic says nothing about an IPhone or a smartphone, it shows a direct connection to the battery. The battery probably has some kind of protection circuitry, but it may also be designed so that it needs to communicate with the charging device in order to let it charge at all. These are the kind of things you need to look into.

Would I need a heat sink for this transistor?
Go look up the data sheet. I already gave you one big hint. Remember, this is YOUR homework/project.