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..

 

Are you connecting the base to 0V to turn it off or leaving it open?

The first has 270V/114R = almost 2.4Amps of base current.
Also, the max volts of a TIP41C is 100V and you are running it on 270V.
No wonder you killed the transistor.

Have a look at the data sheets.
http://www.st.com/content/ccc/resou...df/jcr:content/translations/en.CD00142950.pdf

 

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.

 

If you look carefully you might see 27.0V.

@cockatoo Your base resistor value is too low and you are destroying the transistor.

Try using a power MOSFET instead.

 

I am blind, don't take it personal! Plus I said "if". Yep you are right. The collector current is about 80 times the base one.

 

TS is trying to switch 6 amps @ 270V. That is well over the limit to cause death.

 

I would like to comment but I can't make out the drawings.

 

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.

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).

 

From what I “can” make out from the drawing, there is only a 1.8 amp load on the Q.

 

From what I “can” make out from the drawing, there is only a 1.8 amp load on the Q.

TS mentions 6 A in the post.

 

OP states they are using a 6 amp transistor, not a 6 amp load.

 

OP states they are using a 6 amp transistor, not a 6 amp load.

I need to learn to read.

 

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...

 

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.

 

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

 

Why do people buy junk from ebay?

 

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.

 

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

 

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.