# How to design a transistor circuit that controls low-power devices

#### stspringer

May 10, 2019
116
I watched this video and I have an electric motor 12 volt 7.2 watt .06 amps "load amps". I did the math for my motor copying the tutorial example for his "load" is solenoid valve spec .67 watts at 12 volts. His device had really low watts .67

Question 1: did I get it right, the math? I came up with a 220k resistor for the base on the transistor.
Question 2: can this transistor handle 600ma?

Thank you for any help

Tutorial: How to design a transistor circuit that controls low-power devices
pn2222a transistor beta "gain" of 100

load is solenoid valve spec .67 watts at 12 volts

what is current? .67 /12 = .0558 amps x 1000 = 56ma

the transistor must handel 56 ma

transistor beta "gain" of 100
IB the base current Ib = 56ma/100 = .56ma

.7 is an intrinsic value to silicon transistors this is an NPN

Ib = 5v - .7 / rb
.7 - 5 = 4.3

now we are going to increase the ma to be safe so instead of .56ma we will use 1 ma

change 1 ma to amps 1 /1000 = .001

4.3 /.001 = 4300 ohm resistor

they dont make 4300 ohm resistors so use a 4.7k resistor

=============================================================
my electric motor 12 volt 7.2 watt .06amps load amps

12 volt electric motor 7.2 watts

load is motor valve spec 7.2 watts at 12 volts

what is current? 7.2 /12 = .06 amps x 1000 = 600ma

the transistor must handle 600 ma

transistor beta "gain" of 100
IB the base current Ib = 600ma/100 = 6ma

.7 is an intrinsic value to silicon transistors this is an NPN

Ib = 12v - .7 / rb
12 - .7 = 11.3

now we are going to increase the ma to be safe so instead of 6ma we will use 6ma x10 = 60 ma

change 60ma to amps 60/1000 = 0.06

11.3/.06 = 188.333333333 = they don't make 188k resistor so use a 220k resistor

could this be a 220ohm resistor and not a 220k resistor? not clear

Last edited:

#### Audioguru

Sep 24, 2016
3,654
Beta is used when a transistor always has plenty of collector to emitter voltage like in an amplifier.
But you probably want the transistor to fully turn on when it is saturated. Then you should see on the datasheet its "saturation Vce voltage loss" which is always when its base current is 1/10th its collector current.

Please learn about simple numbers. 188.3 (almost two hundred) IS NOT 188,300 (almost two hundred thousand).
200 ohms is a common 5% resistor value.

#### stspringer

May 10, 2019
116
, so
Beta is used when a transistor always has plenty of collector to emitter voltage like in an amplifier.
But you probably want the transistor to fully turn on when it is saturated. Then you should see on the datasheet its "saturation Vce voltage loss" which is always when its base current is 1/10th its collector current.

Please learn about simple numbers. 188.3 (almost two hundred) IS NOT 188,300 (almost two hundred thousand).
200 ohms is a common 5% resistor value.

Yes I caught that as I was writing this post so a 200 watt resistor will work in my calculation for my motor right?

#### majoco

Nov 10, 2019
50
so a 200 watt resistor

One problem with electronics is the need for correct description, especially when dealing with values that range from pico (10 to the -12) right up to Meg (10^6). If I did some sums on a circuit and found I needed a 200 watt resistor, I'd do the sums again!. It's a 200 ohm resistor, 180 ohms is close enough and is a standard value in the E12 range. Almost everything is in the E12 range of values, electronics is not an exact science despite what you might have been led to believe. The E12 range starts at say 1000 ohms with a tolerance of +/- 10% so can go as high as 1100 ohms. The next value is 1200 ohms which may go down to 1080 ohms (-10% of 1200) so if you're really desperate to get an exact value between 1000 and 1200 ohms you'll have to search in your junk box to find one! (everyone has a junk box!) Adding 20% to each value roughly gives us 1000, 1200, 1500, 1800, 2200, 2700, 3300, 3900, 4700, 5600, 6800, 8200 and 10000 is the start of the next decade - and then the sequence goes on again...12 values in each decade. Next thing to learn is the colour code for each number - pretty soon you can pick up a resistor, look at the colour bands and say "47Kohms" just like that! (yellow = 4, violet = 7 and three 0"s = orange!)

#### Audioguru

Sep 24, 2016
3,654
, so a 200 watt resistor will work in my calculation for my motor right?
Please proof-read what you write. 200 Watts is how much POWER (heating) a resistor can survive. A 200W resistor will be huge, heavy and expensive.
Your 200 ohms resistor will have a power of (8.3V squared)/200 ohms= 0.34W. Use an inexpensive little 1/2 Watts resistor.

#### stspringer

May 10, 2019
116
Please proof-read what you write. 200 Watts is how much POWER (heating) a resistor can survive. A 200W resistor will be huge, heavy and expensive.
Your 200 ohms resistor will have a power of (8.3V squared)/200 ohms= 0.34W. Use an inexpensive little 1/2 Watts resistor.

Yes sorry, I meant 200 ohms. I guess no one can answer my original question about if I calculated it correctly.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,101
I guess no one can answer my original question about if I calculated it correctly.
Of course we can. Having correct values and units just makes our life easier.
what is current? .67 /12 = .0558 amps x 1000 = 56ma
That should read 0.67 W / 12 V = 56 mA - always include units in your calculations. 0.67/12 = 0.0558 not 0.0558 A, Where from comes the unit "A"? Iz is the result of W(atts)/V(oltage).
We can let thsi stand as correct.
.7 is an intrinsic value to silicon transistors this is an NPN
Again 0.7 ? horsepower...
You mean the right thing (0.7 V Vbe), you don't give it the correct name.
Ib = 5v - .7 / rb
.7 - 5 = 4.3
Assuming a control voltage of 5 V this should read
Ib = (5 V - 0.7 V) / Rb (note the parenthesis
or Rb = (5 V - 0.7 V) / Ib = 4.3 V / 0.56 mA = 7.7 kΩ
But, as @Audioguru mentioned in post #2 you should not use the beta value of 100 but the lower DC gain of approx. 10, so Rb = 770 Ω. A 1 kΩ should work perfectly in this simple switching application.

And so on.

#### stspringer

May 10, 2019
116
Of course we can. Having correct values and units just makes our life easier.

That should read 0.67 W / 12 V = 56 mA - always include units in your calculations. 0.67/12 = 0.0558 not 0.0558 A, Where from comes the unit "A"? Iz is the result of W(atts)/V(oltage).
We can let thsi stand as correct.

Again 0.7 ? horsepower...
You mean the right thing (0.7 V Vbe), you don't give it the correct name.

Assuming a control voltage of 5 V this should read
Ib = (5 V - 0.7 V) / Rb (note the parenthesis
or Rb = (5 V - 0.7 V) / Ib = 4.3 V / 0.56 mA = 7.7 kΩ
But, as @Audioguru mentioned in post #2 you should not use the beta value of 100 but the lower DC gain of approx. 10, so Rb = 770 Ω. A 1 kΩ should work perfectly in this simple switching application.

And so on.
Thanks for your reply but my motor is on the 2nd half of the original post my motor is 12 volts 7.2 watts I followed the tutorial video and came up with 220 ohm resistor. Can you look at my math and tell me if it is correct. Thanks
=============================================================
my electric motor 12 volt 7.2 watt .06amps load amps

12 volt electric motor 7.2 watts

load is motor valve spec 7.2 watts at 12 volts

what is current? 7.2 /12 = .06 amps x 1000 = 600ma

the transistor must handle 600 ma

transistor beta "gain" of 100
IB the base current Ib = 600ma/100 = 6ma

.7 is an intrinsic value to silicon transistors this is an NPN

Ib = 12v - .7 / rb
12 - .7 = 11.3

now we are going to increase the ma to be safe so instead of 6ma we will use 6ma x10 = 60 ma

change 60ma to amps 60/1000 = 0.06
11.3/.06 = 188.333333333 = they don't make 188 ohm resistor so use a 220 ohm resistor

#### duke37

Jan 9, 2011
5,364
As a nit picker, I would like to see correct display of values.

Amperes are shortened to A, not a.
Kilo is k not K (that is temperature).
Numbers after a decimal point should have a number before the point so that it not lost e.g. .67 should be shown as 0.67
Do not use commas in numbers, different countries use them differently and my display is not good at distinquishing them from a full stop.

#### AnalogKid

Jun 10, 2015
2,701
You are fine so far, but you are flirting with danger. And a misconception.

Transistors are not perfectly repeatable. There is a lot of variation in performance from one piece to another. also, for gbest long-term reliability, you should run a transistor at less than 1/2 of its ratings. So if the rated maximum continuous collector current is 600 mA, do not run it above 300 mA. Same for collector-emitter voltage (Vce). This one isn't an issue with bipolar parts because there are very few small signal transistors rated for less than 24 V. but there are many power MOSFETs fated for 20 V, and these should not be used in a 12 V circuit.

Another issue is the 60 mA base current you calculated. The calculation is correct, but 60 mA is a *lot* of base current for a small-signal, general purpose transistor type. working from memory, it might exceed the max. continuous value on the datasheet.

Conclusion: a 2N2222 is too small a transistor for a 600 mA load. Better to use a small power transistor like a TIP31.

Now about the base resistor. Look into a transistor operating range called saturation. This is an area where the normal base current - collector current relationship falls apart. It also is the are when the collector-emitter voltage is the lowest possible value, maximizing the power delivered to the load and minimizing the power dissipated in the transistor. Your calculations are based on using the transistor as a linear amplifier, but the circuit requires a saturated switch for better efficiency.

The old rule of thumb is that the base current should be 10% of the collector load current for "hard" saturation, but that rule is from the 1950's. Believe it or not, parts are better today. For the TIP31 mentioned above, a reasonable base current for efficient switching is 4-5%, with 3% as a limit where the collector voltage starts to increase enough to matter.

ak

Last edited:

#### stspringer

May 10, 2019
116
As a nit picker, I would like to see correct display of values.

Amperes are shortened to A, not a.
Kilo is k not K (that is temperature).
Numbers after a decimal point should have a number before the point so that it not lost e.g. .67 should be shown as 0.67
Do not use commas in numbers, different countries use them differently and my display is not good at distinquishing them from a full stop.
Thank you for your input I will address what you mentioned in the future. Now back to my question. Can you, will you check my math?
Thanks
stspringer

#### stspringer

May 10, 2019
116
You are fine so far, but you are flirting with danger. And a misconception.

Transistors are not perfectly repeatable. There is a lot of variation in performance from one piece to another. also, for gbest long-term reliability, you should run a transistor at less than 1/2 of its ratings. So if the rated maximum continuous collector current is 600 mA, do not run it above 300 mA. Same for collector-emitter voltage (Vce). This one isn't an issue with bipolar parts because there are very few small signal transistors rated for less than 24 V. but there are many power MOSFETs fated for 20 V, and these should not be used in a 12 V circuit.
Another issue is the 60 mA base current you calculated. The calculation is correct, but 60 mA is a *lot* of base current for a small-shgnal, general purpose transistor type. working from memory, it might exceed the max. continuous value on the datasheet.

Conclusion: a 2N2222 is too small a transistor for a 600 mA load. Better to use a small power transistor like a TIP31.

Now about the base resistor. Look into a transistor operating range called saturation. This is an area where the normal base current - collector current relationship falls apart. It also is the are when the collector-emitter voltage is the lowest possible value, maximizing the power delivered to the load and minimizing the power dissipated in the transistor. Your calculations are based on using the transistor as a linear amplifier, but the circuit requires a saturated switch for better efficiency.

The old rule of thumb is that the base current should be 10% of the collector load current for "hard" saturation, but that rule is from the 1950's. Believe it or not, parts are better today. For the TIP31 mentioned above, a reasonable base current for efficient switching is 4-5%, with 3% as a limit where the collector voltage starts to increase enough to matter.

ak
Thank you for your reply. A little over my head in your explanation but I appreciate it. So if I were to hook up the circuit as shown in the video tutorial using my calculation can you tell me what you think will happen? cooked transistor or what?

So the transistor you mention TIP31 is a Darlington transistor correct? Would you happen to know where to point me to a site with a schematic or diagram of what I am trying to do? I have been googling and can't seem to find one. Seems odd to me no one has tried running a motor with a transistor as a switch on a breadboard. Thanks again

#### Audioguru

Sep 24, 2016
3,654
So the transistor you mention TIP31 is a Darlington transistor correct?
Why do you wrongly guess? Look at the datasheet of a TIP31 medium power transistor. Its datasheet shows the order of its pins and all its spec's including the base current needed when saturated. The starting current of the motor might burn out a 3A TIP31 so maybe a 6A TIP41 is needed. Look at its datasheet.
Seems odd to me no one has tried running a motor with a transistor as a switch on a breadboard.
Because a motor's running current of 600mA and a starting current of maybe 5A is too much for a Mickey Mouse breadboard.

#### AnalogKid

Jun 10, 2015
2,701
Seems odd to me no one has tried running a motor with a transistor as a switch on a breadboard.
Seems odd to me that you would think that. One video does not a universe of experiences make. Solderless breadboards (proto-boards) have been around since at least the 70's, and all manner of things have been done with them. The better ones have a contact rating of 1 A.

ak

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,101
what is current? 7.2 /12 = .06 amps x 1000 = 600ma
7.2 W /12 V = 0.6 A = 600 mA
You really should exercise a bit more care in your posts - errors such as these do not help.
transistor beta "gain" of 100
IB the base current Ib = 600ma/100 = 6ma
Correct from the mathematical standpoint, but you really have to look up the DC gain of the transistor used.
As has been mentioned before, the 2N2222 is not suitable. It has a rated max. collector current of exactly 600 mA. You never use a component at the brink of its parameters..
The TIP31 on the other hand is rated at 3 A continuous collector current and a peak current of 5 A. The TIP31 is not a darlington (see datasheet). DC current gain is very low: gain = [email protected] A collector current. So you need to increase base current to at least 24 mA in your application.

Have you considered using a MOSFET? A selection of suitable ones can be found e.g. here. This website shows how it's done.

#### stspringer

May 10, 2019
116
7.2 W /12 V = 0.6 A = 600 mA
You really should exercise a bit more care in your posts - errors such as these do not help.

Correct from the mathematical standpoint, but you really have to look up the DC gain of the transistor used.
As has been mentioned before, the 2N2222 is not suitable. It has a rated max. collector current of exactly 600 mA. You never use a component at the brink of its parameters..
The TIP31 on the other hand is rated at 3 A continuous collector current and a peak current of 5 A. The TIP31 is not a darlington (see datasheet). DC current gain is very low: gain = [email protected] A collector current. So you need to increase base current to at least 24 mA in your application.

Have you considered using a MOSFET? A selection of suitable ones can be found e.g. here. This website shows how it's done.
Yes, I went on another forum and was explained how a mosfet will do the job on a breadboard. I am going to try that I ordered over kill mosfets and Diodes as suggested by other forum.

Amazon
5 PCS IRFZ44N IRFZ44NPBF N-Channel Field-Effect Power Transitor 55V 49A RoHS Compliant TO-220

and Diodes
McIgIcM 1N4148 Switching Diode 100V 200mA Hole DO-35

Please tell me what you think

#### stspringer

May 10, 2019
116
Why do you wrongly guess? Look at the datasheet of a TIP31 medium power transistor. Its datasheet shows the order of its pins and all its spec's including the base current needed when saturated. The starting current of the motor might burn out a 3A TIP31 so maybe a 6A TIP41 is needed. Look at its datasheet.

Because a motor's running current of 600mA and a starting current of maybe 5A is too much for a Mickey Mouse breadboard.

The reason I stated, "So the transistor you mention TIP31 is a Darlington transistor correct?"
It is because I went on Amazon and looked up TIP31 Transistors and here is what I found in the description

Bridgold 50pcs 10Types TIP Series Transistors Assortment Kit, Including TIP31 TIP32 TIP41 TIP42 TIP120 TIP121 TIP122 TIP125 TIP126 TIP127
by Bridgold
5.0 out of 5 stars 2 ratings
Price: $14.99 ($3.00 / 10 Items) FREE One-Day & FREE Returns
Best price
This package has 50 transistors and 10 TIP series, just a perfect combination to meet your needs.
Professionals and amateurs know Darlington transistors offer extreme efficiency and reliability.
Collector-emitter sustaining voltage - VCEO (sus) = 60 V (minimum) - TIP120, TIP125 80 V (minimum) - TIP121, TIP126 100 V (minimum) - TIP122, TIP127
Complementary NPN - PNP transistors,Includes 5PCS* TIP31C 5PCS*TIP32C 5PCS*TIP41C 5PCS*TIP42C 5PCS* TIP120 5PCS*TIP121 5PCS*TIP122 5PCS*TIP125 5PCS* TIP126 5PCS*TIP127
If you have any suggestions for the product, don't hesitate to contact us immediately.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,101
I went on Amazon and looked up TIP31 Transistors and here is what I found in the description
Amazon, Ebay, Alibaba and co. are not qualified sources for that kind of information. Always look up a datasheet.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,101
Please tell me what you think
The diode is o.k., although a more robust 1N4001 may be more approproate for the current involved here.

The MOSFET is way oversized. The datasheet states:
VGS(th) Gate Threshold Voltage 2.0 ––– 4.0 V VDS = VGS, ID = 250µA

Depending on the Vgs(th) of your specific MOSFET a 5 V drive may or may not be sufficient. Give it a try but do not be dissapointed if the MOSFET doesn't fully switch on. I suggest you use a so called "logic level MOSFET" (Google).

#### Audioguru

Sep 24, 2016
3,654
Amazon sells cheap clothing and shoes. They know nothing about electronic parts.
Bridgold is an unknown Chinese company who might sell fake or defective electronic parts.
Anybody can write a good rating including the owner and friends of the Amazon seller who sells cheap junk.
Why are you using a wrong transistor or a wrong Mosfet to turn on the motor? Why not simply use a piece of wire to turn on the motor?

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