Maker Pro
Maker Pro

Cons of using large transistors?

BlueJello

Dec 15, 2016
23
Joined
Dec 15, 2016
Messages
23
I'm fairly new to electronics, and I got some transistors from an old CRT T.V. I'm making a circuit, and I wanted to know if there are any drawbacks to using a bigger transistor than necessary. For example, a 24 volt transistor with a 12 volt power supply.
 

Alec_t

Jul 7, 2015
3,587
Joined
Jul 7, 2015
Messages
3,587
Welcome to EP.
For many circuits, particularly those operating at DC or low frequency AC, there would be no drawback.
 

OBW0549

Jul 5, 2016
157
Joined
Jul 5, 2016
Messages
157
I'm fairly new to electronics, and I got some transistors from an old CRT T.V. I'm making a circuit, and I wanted to know if there are any drawbacks to using a bigger transistor than necessary. For example, a 24 volt transistor with a 12 volt power supply.
Using a transistor (or, for that matter, any other electronic component) with a higher maximum voltage rating (or maximum current) than is needed is almost never a problem, provided its other characteristics (such as current gain, frequency response, saturation voltage, leakage currents, and so forth) meet your circuit's requirements.

In fact, it is considered good practice to use components that are rated for voltages and currents quite a bit higher than what will actually be encountered in-circuit, to give an ample margin of safety.
 

BobK

Jan 5, 2010
7,682
Joined
Jan 5, 2010
Messages
7,682
You would have a hard time finding at transistor with a max Vce of less than 24V.

But if you mean using power transistors where a small signal transistor is called for yes, there are disadvantages. The power transistor will have lower gain lower frequency response and higher junction capacitance.

Bob
 

sundy

Aug 5, 2016
64
Joined
Aug 5, 2016
Messages
64
What is your definition of high frequency? The answer could point out all sorts of problems with your transistors depending on what frequency they are designed to operate.
 

Alec_t

Jul 7, 2015
3,587
Joined
Jul 7, 2015
Messages
3,587
What about circuits with high frequency AC, if I'm planning on using a large heatsink?
That's where the frequency response and large gate/junction capacitance that Bob mentioned become a problem, regardless of the heatsink size.
 

sundy

Aug 5, 2016
64
Joined
Aug 5, 2016
Messages
64
Duly noted but since the OP is 'fairly new' I thought a restatement of the challenges would not be redundant. If the OP is working with microwave then TV parts are in no way suitable.
 

cjdelphi

Oct 26, 2011
1,166
Joined
Oct 26, 2011
Messages
1,166
2 people mentioned it here but incase you missed it

1. Larger transistors have a lower gain, eg hfe of 100 vs a small transistor npn might have say a 300 gain

In otherwords, if 1ma is allowed to flow via the base, the collector / emitter a smaller transistor would allow 300ma but a bigger transistor only 100ma

Say you only have 1ma current from a micro controller, you need 1amp of current, it's not going to happen so using a darlington pair you can get a 1000 gain

The other thing is switching speeds and other stuff like capacitance
 
Top