Look for better transistor by reading datasheet

Thread Starter

Kelvin Lee

Joined Oct 22, 2018
105
Dear Sir/Madam,

I connect a PN2222A NPN transistor and a variable resistor at 1K, the motor on but the transistor is very hot. I think I need to replace another model transistor but I have no idea which model should be chosen when I reading the datasheet, which information I should focus? Can anyone suggest me the model of the transistor with an explanation by the figure from datasheet?

Should I need to replace 1K resistor with other resistance if I change the transistor, how much I need? How do I calculate the resistance need?

Best regards,

Kelvin.
 

Attachments

bertus

Joined Apr 5, 2008
19,939
Hello,

How much current does the motor draw?
For good switching, the base current of the transistor should be about 1/10 of that current.

Bertus
 

Thread Starter

Kelvin Lee

Joined Oct 22, 2018
105
Hello,

How much current does the motor draw?
For good switching, the base current of the transistor should be about 1/10 of that current.

Bertus
Dear Bertus,

If 3V power directly connects to the motor, it requires 600mA to start and then around 290mA to run. There is no other load to the motor, later will add a fan blade.
 

bertus

Joined Apr 5, 2008
19,939
Hello,

At startup, the motor is hitting the absolute maximum current allowed for the PN2222:

PN2222_ratings.png

Better look for a transistor that can handle a bit more current.

Bertus
 

Attachments

Thread Starter

Kelvin Lee

Joined Oct 22, 2018
105
Hello,

At startup, the motor is hitting the absolute maximum current allowed for the PN2222:

View attachment 163466

Better look for a transistor that can handle a bit more current.

Bertus
Dear Bertus,

Thanks for your reply. I tested with a fan blade installed to the motor, it requires around 800mA to start. According to your suggestion, I should look for a transistor with Ic more than 800mA, right? If 800mA is required, then the base current should be around 80mA, so the resistance should be around 39 Ohm, am I correct?

May I know the base current is normally at 1/10 of collector current for every transistor? Is it the rule?

Best regards,

Kelvin.
 

bertus

Joined Apr 5, 2008
19,939
Hello,

For hard switching the 1/10 is a rule of thumb.
It will force the transistor in saturation, so there is less voltage accross the transistor to avoid heating.

Also a kind of rule of thumb is that the switched current should be about half the maximum allowed for the transistor.

Bertus
 

Thread Starter

Kelvin Lee

Joined Oct 22, 2018
105
Hello,

For hard switching the 1/10 is a rule of thumb.
It will force the transistor in saturation, so there is less voltage accross the transistor to avoid heating.

Also a kind of rule of thumb is that the switched current should be about half the maximum allowed for the transistor.

Bertus
Dear Bertus,

Many thanks for your patient to explain to my question, because of my unclear concept, do you mind quoting some example about half the maximum allowed for the transistor? At what situation I should consider 1/10 or half the maximum allowed?

Best regards,

Kelvin.
 

bertus

Joined Apr 5, 2008
19,939
Hello,

When you want to switch 600 mA, the base current should be about 60 mA.
The transistor should have a maximum current of at least 1200 mA.

Bertus
 

Thread Starter

Kelvin Lee

Joined Oct 22, 2018
105
Hello,

When you want to switch 600 mA, the base current should be about 60 mA.
The transistor should have a maximum current of at least 1200 mA.

Bertus
Dear Bertus,

Then how do I read the datasheet to know I find the correct transistor?

Best regards,

Kelvin.
 

Thread Starter

Kelvin Lee

Joined Oct 22, 2018
105
He already showed you. Look at the maximum ratings.
Dear dl324,

Sorry I don't understand the datasheet, do you mean 600mA is the maximum rating for PN2222A transistor, so the maximum current to the load is better less than 600mA but the transistor maximum allow current will be 1.2A but it will be very hot.

What will happen if the load is over 600mA or 1.2A to the PN2222A?

Best regards,

Kelvin.
 

dl324

Joined Mar 30, 2015
8,761
do you mean 600mA is the maximum rating for PN2222A transistor, so the maximum current to the load is better less than 600mA but the transistor maximum allow current will be 1.2A but it will be very hot.
It depends on duty cycle. At 100% duty cycle, the 2N2222A is rated for 600mA. Conservative designers would derate that to improve reliability. For hobby circuits, you can operate at 100% ratings if you want.

The datasheet I have doesn't give data for peak currents above 600mA, so you need to do power dissipation calculations and insure that junction temperature doesn't exceed the maximum allowed. You also have to worry that the bond wires can carry the current you desire; if it's over the maximum continuous rating.
 

dl324

Joined Mar 30, 2015
8,761
This is curious. I checked another datasheet (FSC) and it gives a higher maximum current:
upload_2018-11-9_11-7-6.png
upload_2018-11-9_11-7-32.png

EDIT: I see that FSC states that 1A is absolute max; which the device isn't guaranteed to survive. And OnSemi lists 600mA as a maximum rating.
upload_2018-11-9_11-11-31.png
 

crutschow

Joined Mar 14, 2008
23,141
The common 2N3055/TIP3055 power transistor would be more than adequate for your requirements.
It has a 15A maximum current rating with maximum gain at around 0.3A.
For the maximum startup current, a gain of 20 (base current = Icmax/20) should be sufficient.

upload_2018-11-9_11-20-49.png
 

Thread Starter

Kelvin Lee

Joined Oct 22, 2018
105
This is curious. I checked another datasheet (FSC) and it gives a higher maximum current:
View attachment 163469
View attachment 163470

EDIT: I see that FSC states that 1A is absolute max; which the device isn't guaranteed to survive. And OnSemi lists 600mA as a maximum rating.
View attachment 163471
Dear dl324,

Many thanks for your information, yes, sometimes we find like this, so it is very confused to our beginners to understand.

Best regards,

Kelvin
 

Thread Starter

Kelvin Lee

Joined Oct 22, 2018
105
The common 2N3055/TIP3055 power transistor would be more than adequate for your requirements.
It has a 15A maximum current rating with maximum gain at around 0.3A.
For the maximum startup current, a gain of 20 (base current = Icmax/20) should be sufficient.

View attachment 163472
Dear crutschow,

Very informative, may I know how I should interpret the statement DC Current Gain − hFE = 20−70 @ IC = 4 Adc from the datasheet of 2N3055?

-hFE is 20 - 70, your equation let me understand how I can find how much of current I need to produce on base, at what situation we will do base current = Icmax/70 ?

Best regards,

Kelvin.
 

Thread Starter

Kelvin Lee

Joined Oct 22, 2018
105
It depends on duty cycle. At 100% duty cycle, the 2N2222A is rated for 600mA. Conservative designers would derate that to improve reliability. For hobby circuits, you can operate at 100% ratings if you want.

The datasheet I have doesn't give data for peak currents above 600mA, so you need to do power dissipation calculations and insure that junction temperature doesn't exceed the maximum allowed. You also have to worry that the bond wires can carry the current you desire; if it's over the maximum continuous rating.
Dear dl324,

What is the reference 1200mA from post https://forum.allaboutcircuits.com/threads/look-for-better-transistor-by-reading-datasheet.153844/#post-1322662 ?

Best regards,

Kelvin
 

crutschow

Joined Mar 14, 2008
23,141
-hFE is 20 - 70, your equation let me understand how I can find how much of current I need to produce on base, at what situation we will do base current = Icmax/70 ?
Those values of hFE are when you are using the transistor as a linear amplifier, not as a switch.
When using it as a switch, an hFE value of 10-20 is typically used to insure the transistor is fully saturated.
Notice that, for the saturation voltage measurement @4A, they use an hFE of 10.
upload_2018-11-10_9-27-41.png

I would use a value of about 20 for the maximum startup current, since that's well below 4A and its gain is significantly higher at lower currents.
That will insure good saturation at the running current.
Bordodynov, do you have an LTspice model for that transistor?

Edit: The ZTX690B that Bordodynov posted looks like a good option.
They use an hFE of 200 for the saturation tests.
Bordodynov, do you have an LTspice model for that transistor?
 
Last edited:

Thread Starter

Kelvin Lee

Joined Oct 22, 2018
105
Those values of hFE are when you are using the transistor as a linear amplifier, not as a switch.
When using it as a switch, an hFE value of 10-20 is typically used to insure the transistor is fully saturated.
Notice that, for the saturation voltage measurement @4A, they use an hFE of 10.
View attachment 163528

I would use a value of about 20 for the maximum startup current, since that's well below 4A and its gain is significantly higher at lower currents.
That will insure good saturation at the running current.
Bordodynov, do you have an LTspice model for that transistor?

Edit: The ZTX690B that Bordodynov posted looks like a good option.
They use an hFE of 200 for the saturation tests.
Bordodynov, do you have an LTspice model for that transistor?
Dear crutschow,

I can now understand why most experts tell me to use 10 to divide the Ic and estimate the current need for the Ib, and then calculate the resistance value on Ib. It is because I use the transistor as a switch.

Because of rule of thumb, that's why I cannot find the information from datasheet. Many thanks for your teaching.

Best regards,

Kelvin
 
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