transistor shorting

Thread Starter

cockatoo

Joined Dec 23, 2017
11
hi , this forum is probably a bit technical for my basic/ stupid questions. but i am going to ask one anyway. i have been trying to turn on lights with a 6A transistor. i can get it to work with a small load , but when i try with a bigger load they keep shorting the collecter / emitter leads and staying on when i disconnect base . i came up with the base current [360ma] from data to get 73 ohms r , for the larger load. .for the smaller load , i just pulled a 8k1 resistor out and it worked . what am i doing wrong..i have posted a crude drawing of cirquit which = 1000 words....thanks in advance..bjt 004.jpg bjt 004.jpg
 

ArakelTheDragon

Joined Nov 18, 2016
1,353
Do you have a cooler on the transistor for the high amper load? Also can your transistor bare 8 ampers at least since the spikes will.kill.it?

And if that's 270V that I see on the collector than you should read the datasheet to pick a proper transistor.
 

crutschow

Joined Mar 14, 2008
27,743
To fully turn on (saturate) the transistor, the base current should be around 1/10 the collector current, so for a 6A load the base current should be ≈0.6A, as shown here from the data sheet.
upload_2017-12-25_9-0-19.png
But even then, the maximum saturation voltage is 1.5V so the power dissipated in the transistor at 6A can be up to 9W, thus it needs to be on a heat sink.

You are operating the transistor at its maximum current rating, which is not good practice and why the saturation voltage is so high.
You should use a transistor with at least a 10A rating, such as a 2N3055, or a power MOSFET (one with an ON resistance of ≤25mΩ to avoid a heat sink).
 

Thread Starter

cockatoo

Joined Dec 23, 2017
11
Thanks guys .. Sorry about the poor picture , i must have compressed it too much . It is 27Vdc and a 1.8A [1800Ma] load .
The formula i used was ! [Ic] 1.8A /15 [min gain] = 0.12 A X 3 [ sat factor] = 0.36A [ base current ] .
[ 27 - .8V ] / 0.36 = 72.7 ohm resistor for base ! T he nearest 10W resistor i had was 114R , which i thought would near enough !.
If i used a [sat factor ] of 2 it would have been 109R !. I thought less current through base would be better , i will try again with a lower value resistor !
Thanks for your time , as i am only a learner...
 

MrChips

Joined Oct 2, 2009
24,226
Thanks guys .. Sorry about the poor picture , i must have compressed it too much . It is 27Vdc and a 1.8A [1800Ma] load .
The formula i used was ! [Ic] 1.8A /15 [min gain] = 0.12 A X 3 [ sat factor] = 0.36A [ base current ] .
[ 27 - .8V ] / 0.36 = 72.7 ohm resistor for base ! T he nearest 10W resistor i had was 114R , which i thought would near enough !.
If i used a [sat factor ] of 2 it would have been 109R !. I thought less current through base would be better , i will try again with a lower value resistor !
Thanks for your time , as i am only a learner...
No. You need to increase the base resistor value.
Or use a MOSFET.
 

Thread Starter

cockatoo

Joined Dec 23, 2017
11
thanks mr Chips , i increased value to 160r, and incremently via a rheostat up to about 2kohms. With the last 2 ,tip's , i reduced the load from 1800Ma down to 1200 Ma and then to 890 Ma , and blew these . As shown in first post , these transistors worked with a 500Ma load . to reasure myself sanity , i hooked up a BC338 [800Ma] to a 640Ma load and it worked , but got very hot , due to near maximum current . So to get to the point ! the 338's came from jaycar [electronics shop ] and the tip41c's off ebay . You get what you pay for ! . thanks
 

spinnaker

Joined Oct 29, 2009
7,835
Why do people buy junk from ebay?

Cheap and convenient. Nothing wrong with buying things like switches and connectors. And if you buy other components you just need to be aware you might get a bad part once in a while. But if you deal with reputable sellers that is usually not a problem.
 

BobTPH

Joined Jun 5, 2013
3,679
Because it is 1/3 the price and it works 2/3 of the time?

Actually, I have never received anything that did not work or was way out of specs from EBay, so I must ne charmed.

Bob
 

spinnaker

Joined Oct 29, 2009
7,835
Because it is 1/3 the price and it works 2/3 of the time?

Actually, I have never received anything that did not work or was way out of specs from EBay, so I must ne charmed.

Bob

I never had an issue either. I ordered a constant current regular chip that I could not find anywhere else. Later I got a warning from eBay that the chips were fakes and non functional. For a non functional fake, they worked pretty darn well. ;)

I lot of the components are cheap because they are obsolete. If I am making one or two things form them, I really don't care if they are obsolete.
 
Top