Hello everyone.

I am new here. I am learning electronics (And English, I am sorry if I make mistakes). I had a BD243CG and I am going to put 2 in parallel to share de current. I know I have to connect 1 resistor in each emitter (Current sharing resistors or sometimes called ballast resistors) I am going to use 0,7 ohms or 1,1 ohms.

But what I don´t know is: In the original diagram the transistor had a 47k resistor in the base. How do I have to connect the bases? I make a diagram with the different options I imagined. I also put the original diagram. Is a solid state tesla coil. I will use the same heatsink for both transistors. The V imput is 30v cc.

I will be grateful for any advice.

Thank you

Best regards

 

C would work well.

 

Thank you very much for your response. I was thinking in B because now the current is split in 2 bases, so we need less resistance.. I have to stydy more!!!

And what do you think is the ideal value to the ballast resistors? I have 2,2 ohms resistors, so I can put 2 or 3 in parallel to get 1,1 or 0,7 ohms.

 

I would suggest that the maximum current on the emitter resistor falls 100 mV.

 

Thank you for your response. The problem is, I have no idea about to calculate that

 

I was thinking in B because now the current is split in 2 bases

The problem with that is, due to likely small differences in their base-emitter voltages, one transistor will tend to hog more of the current.
That's why each transistor needs its own base resistor.

 

The problem with that is, due to likely small differences in their base-emitter voltages, one transistor will tend to hog more of the current.
That's why each transistor needs its own base resistor.

Thank you for your response. I understand. But if I split the current between 2 transistors, shouldn´t the resistance be the half?

 

Yes, the signal source will have to provide twice the current.

The price of using parallel Qs.

 

It depends on why you are putting two transistors in parallel. Usually it is to allow more current to be controlled though sometimes it is to spread the power dissipation around or to increase the Gm (change in collector current vs change in Base-Emitter voltage).

Why are you putting two transistors in parallel? What kind of transistors are these?

 

thank you for your responses. They are BD243CG. I use them to exice a solid state tesla coil. In the first version I use 2 TIP31C in parallel but they got very hot when I increase imput voltage to 30 V.

So, Should I use 2 47k resistors?

 

Variant #1
Consumed current =1350mA


Variant #2
Consumed current = 148mA


Variant #3
Consumed current = 4.7mA@12V
For V1 = 30V, C3 = 10n, HV = 27kV, I(V1) =9.7mA

 

View attachment 205370

Thank you, but I think it will be difficult find more components with the quarantine. I have to work with the bd243 and the 47k and 2,2 resistors

 

With a 30V supply, the 47k base resistor will have 29.3V across it and a current of only 0.6mA. Then for the transistor to turn on well, its collector current will be only 6mA which is close to nothing.

You never said how much current the transistor collectors must pass so how can you calculate the base current needed from the base resistor value?

 

See

 

I have no idea of that parameter. I have only saw the diagram on internet and assembled it.

 

Here are a couple of similar circuits that worked. Perhaps you can get some ideas from their very similar circuits. (Hint: You don't have a diode in the base circuit)

https://forum.allaboutcircuits.com/threads/tesla-coil-slayer-exciter-help.162863/

https://hackaday.com/2014/12/22/micro-tesla-coil-makes-a-perfect-stocking-stuffer/#more-141733

 

Thank you