# My transistors keep burning

#### hope_it_dont_explode

Joined Jun 3, 2022
7
can someone check my math to make sure this wont burn
so if I have the curcuit below with the led needing .02A and having a forwad V of 2 then the resistor connected in series with it should be = (5v-2v)/.02A = 150ohm then if Ic = B*Ib and im using a P2N2222A transistor data sheet with a B around 100 then .02A = 100*Ib then Ib should be .0002A or 200uA then if the switch is on and the V(sat) is 1.2v then the value of the resistor is (5v-1.2v)/.0002A = 19000ohms. right?

so with the vaules inputed it looks like this?

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#### bertus

Joined Apr 5, 2008
21,939
Hello,

When using a transistor as a switch, the rule of thumb is Ib = Ic/10.

Bertus

#### Alec_t

Joined Sep 17, 2013
12,894
..... so Ib= ....?

#### crutschow

Joined Mar 14, 2008
30,103
im using a P2N2222A transistor data sheet with a B around 100 then Ib should be .0002A or 200uA
Not correct, so it's not surprising your transistor is "burning".

As has been stated here too many times to count, Beta has nothing to do with using a transistor as a switch.
DC Beta is used for bias calculations in AC amplifiers.
Notice the values below in the data sheet you referenced:

The DC current gain is given at a Vce of 1-10Vdc, not when it is fully on as a switch.

The Collector-Emitter Saturation voltage (where it is being used as a switch) shows a base current of 1/10 of the collector current (as Bertus stated), so that's the value you use.

#### hope_it_dont_explode

Joined Jun 3, 2022
7

#### hope_it_dont_explode

Joined Jun 3, 2022
7
Not correct, so it's not surprising your transistor is "burning".

As has been stated here too many times to count, Beta has nothing to do with using a transistor as a switch.
DC Beta is used for bias calculations in AC amplifiers.
Notice the values below in the data sheet you referenced:

The DC current gain is given at a Vce of 1-10Vdc, not when it is fully on as a switch.

The Collector-Emitter Saturation voltage (where it is being used as a switch) shows a base current of 1/10 of the collector current (as Bertus stated), so that's the value you use.

View attachment 268667
ok so then can you explane why the base current is 1/10 of the collector current and is this always the case with npn transistors

#### crutschow

Joined Mar 14, 2008
30,103
ok so then can you explane why the base current is 1/10 of the collector current and is this always the case with npn transistors
That value is used for most NPN and PNP transistors (a few high-gain ones may use a factor of 1/20).
It seems to be a good (but somewhat arbitrary) value to always insure that the transistor is fully on (saturated) when being used as a switch.

#### Papabravo

Joined Feb 24, 2006
18,793
ok so then can you explane why the base current is 1/10 of the collector current and is this always the case with npn transistors
When you "overdrive" the transistor, to use it as a switch, you must rely on the external circuit to limit the current for you, because the transistor is entirely incapable doing that for itself. In this condition of "saturation", further increases in base current CANNOT increase the collector current because the external circuit won't allow it. The external circuit limits the current, the transistor establishes a value for Vce, and the power dissipated is Ic * Vce. So if Ic is 20 mA and Vce is 0.2V that will mean the transistor is dissipating a mere 4 milliwatts: result happiness.

#### Alec_t

Joined Sep 17, 2013
12,894
Nope. Always specify units. Is that Amps? If so that is not 1/10 of the collector current.

Last edited:

#### Ian0

Joined Aug 7, 2020
5,510
If the transistor is burning, then something is connected wrongly.
With a 150Ω series resistance, the maximum power dissipation is 2.5^2/150 = 41mW, which would hardly get it above tepid.
(Maximum power dissipation occurs when the voltages across transistor and resistor are equal)

#### MrAl

Joined Jun 17, 2014
9,158
can someone check my math to make sure this wont burn
so if I have the curcuit below with the led needing .02A and having a forwad V of 2 then the resistor connected in series with it should be = (5v-2v)/.02A = 150ohm then if Ic = B*Ib and im using a P2N2222A transistor data sheet with a B around 100 then .02A = 100*Ib then Ib should be .0002A or 200uA then if the switch is on and the V(sat) is 1.2v then the value of the resistor is (5v-1.2v)/.0002A = 19000ohms. right?
View attachment 268665
so with the vaules inputed it looks like this?
Hello,

I have to agree with Ian in the previous post. That is because you can only get so much power in the transistor with a 150 Ohm collector resistor and a 5 volt power supply, and that max is 41.6666 milliwatts
Now i'd be hard pressed to think that even a tiny 2N2222 would overheat with 42mw unless it was located inside an oven that already had a high temperature inside.

So yes the schematic you are showing can not match the actual real life circuit you are working with because if it was right the transistor would not burn out.

Check the wiring maybe you have the 150 Ohm shorted out or something.

Just for reference, the power in the transistor is related to the power source voltage (Vcc), the collector resistor (R), and what voltage happens to end up appearing at the collector (Vc).
P(watts)=(Vc*(Vcc-Vc))/R
Other units are of course volts and Ohms.

#### hope_it_dont_explode

Joined Jun 3, 2022
7
I should say that the title is a bit misleading no transistor are burning right now.

so to try and refrase every thing said the base resistor needs to be at 1.9Kohms because the max saturation is 1.2V and I need a base current of .002A because that is 1/10th of the collector current so it should look like this?

Am I missing anything else?

#### MrAl

Joined Jun 17, 2014
9,158
I should say that the title is a bit misleading no transistor are burning right now.

so to try and refrase every thing said the base resistor needs to be at 1.9Kohms because the max saturation is 1.2V and I need a base current of .002A because that is 1/10th of the collector current so it should look like this?
View attachment 268869
Am I missing anything else?
That sounds about right although i did not check your resistor values. If you get 0.002 amps into the base at all times then the collector is CAPABLE of sinking 0.2 amps given the voltage levels are not too high like Vc.

#### dl324

Joined Mar 30, 2015
14,689

#### Papabravo

Joined Feb 24, 2006
18,793
1.9k isn't a standard resistor value.
Close enough to 1.91K which is a standard value.

#### bertus

Joined Apr 5, 2008
21,939
Hello,

1.91 K would be the E96 or E192 series.
Here is a sheet with all standard values:

Bertus

#### hope_it_dont_explode

Joined Jun 3, 2022
7
so would a 2kohm resistor work just as well if I don't have 1.91kohm

#### Ian0

Joined Aug 7, 2020
5,510
Yes, and a 1.8k and, more than likely, a 2.2k

#### Papabravo

Joined Feb 24, 2006
18,793
so would a 2kohm resistor work just as well if I don't have 1.91kohm
Yes, the maximum base current will be slightly lower, but you don't care because the transistor is STILL saturated.