# I don't understand how I could optimally use this transistor(2N5550)

Discussion in 'Analog & Mixed-Signal Design' started by 64C113M Abe., Sep 23, 2016.

1. ### 64C113M Abe. Thread Starter New Member

Sep 13, 2016
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https://www.fairchildsemi.com/datasheets/2N/2N5550.pdf

I have in my inventory the 2N5550 BJT, and what confuses me about it is the 600mA collector current.

Reason why I am confused is that as I understand the TO-92 package could only max around 625mW, and that would only be for about a few seconds at best.

So if I where to build a circuit that utilized 120 volts and 300mA I would have a maximum power of 36 watts which is well beyond the capabilities of the TO-92 package

2. ### crutschow Expert

Mar 14, 2008
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3,359
It certainly is.
A maximum current rating does not imply a power rating. That's the maximum current it can momentarily carry when saturated.

Maximum ratings are just that, maximum values you should never exceed.
You can't look at just the device's maximum parameter values and extrapolate its operation from that.
Note in the data sheet curves, the transistor gain starts to drop off above about 10mA of collector current and the saturation voltage rises rapidly above about 100mA, so you should normally limit that transistor's operation to not exceed those currents, depending upon how you are using it.

What do you mean by "a circuit that utilized 120 volts and 300mA"?
For a transistor to carry 300mA with 120V applied (transistor not saturated) you would need a power transistor on a heat sink that can dissipate 36W.

Last edited: Sep 23, 2016
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3. ### 64C113M Abe. Thread Starter New Member

Sep 13, 2016
14
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What I meant was a basic transistor switch circuit that had Vcc of 120 volts and an Ic of 300mA.

4. ### OBW0549 Well-Known Member

Mar 2, 2015
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I don't think you'd be wise to try switching that much current with a 2N5550; above about 10 mA its current gain drops drastically, and beyond 100 mA its Vce(sat) rises very rapidly. In fact, the curve of Vce(sat) vs. Ic on the data sheet (see Fig. 2) doesn't even go all with way to 300 mA.

If you need to switch that much current and voltage, you'd be better off using something like a KSC2383; at 300 mA Ic, it has plenty of current gain and very low Vce(sat).

5. ### crutschow Expert

Mar 14, 2008
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For a switch, the transistor power is not the supply voltage times the current, it's the ON saturation voltage times the current.
Thus if the transistor saturation voltage (with a base current of at least 30ma) is 0.2V with 300mA of collector current, then the transistor power dissipation is 0.2V * 300mA = 60mW.

But as OBW0549 noted, the 2n5550 won't operate properly at that current level.
The maximum for using it as a switch is about 100mA.

6. ### 64C113M Abe. Thread Starter New Member

Sep 13, 2016
14
0
crutschow, thanks so much for pointing that out to me about the ON saturation voltage, just one thing is it the same case if I were using it as an amplifier?

I should point out I had brought these transistors for my attempt at a high powered slayer exciter, my idea of high power was to use 120volts DC.

7. ### ian field Distinguished Member

Oct 27, 2012
4,447
791
Any scrap CRT displays gathering dust?

They often have TO126 package transistors on the CRT PCB that can handle that sort of voltage.

A high definition monitor could have transistors with fT as high as 1GHz - usually not a problem, but something to bear in mind if parasitics crop up.

8. ### crutschow Expert

Mar 14, 2008
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3,359
No, it will be decidedly different for a linear amplifier.
The power at is always equal to the voltage across the transistor times the current through it any instant in time.
For a linear amp the power thus varies over time with the frequency and amplitude of the waveform signal.

The average dissipation of a linear amp varies with the type.
A Class A type has a maximum efficiency of 50% so 1/2 of the output power is wasted as heat in the transistor.
Class B (push-pull) types have a higher efficiency of up to 78% so the transistors dissipate less.

9. ### ian field Distinguished Member

Oct 27, 2012
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791
I think the TS may need to search for some datasheets that include a SOAR curve.

It doesn't necessarily have to be for the particular transistor - it illustrates the basic principle of what you must not do.