Matched Transistor Overheating

[Benjamin]

I just read through the post by Lman about Beta Matching and find myself in a similar pickle. I have purchased a set of 7 matched transistors from teslagenx. However, one of my transistors is overheating big time (some smoke happened which I hope was the glue). Here are the symptoms and tests I have made:

*1 transistor overheats even before the wheel is spinning - just by connecting source and charge batteries (I think the charge battery starts the instant heat up)
*I thought maybe a diode failed. I replaced the 4n007 on the base but problem persisted.
*all the 100 ohm resistors to the base are matched
*At first, there was not continuity from all the trigger coils to the collector - I changed this.
*Using the diode setting on my dmm, the "resistance" across the base diode is 63 while the same measurements for the other transistors is about 83
*The transistor is not shorting to the aluminum heat sink.

Thanks for any insight.

[Tom C]

do you have thermal pads between the transistors and the aluminum plate? the back of the transistor needs to be electrically isolated from the heatsink. it is hard to tell where you may have a short in that particular device from the pictures.

if you smoked a transistor, its bad... they don't like getting hot enough to melt inside., and it should not get hot if the rim is not spinning, if it still is getting hot, your c/e junction in the transistor is toast. it could have happened when you were soldering, did you use a clip on heatsink on each leg of the transistor when soldering?

for now unhook that transistor and its wire and run your kit the way it is with six circuits.

Tom C

[Benjamin]

I do have an insulator pad but it has me thinking about the glue I used. I ran out of my official heat sink glue and so I settled with "goop" which appears like a silicon based adhesive. I will test the glue for continuity.

Once (or before) I disconnect that transistor is there a way to test it for sure to detemine it is bad?

Also, if I replace that transistor with an unmatched part, can I modify the base resistor until it becomes matched to the others (like with a pot)?

Thank you.

[Tom C]

silicon is conductive FWIW. you need to remove it from the circuit and... http://www.instructables.com/id/HOW-...R-MULTIMETERS/

as far as unmatched it depends upon if the unmatched one is higher or lower in gain. no way to know unless you take a good one off and beta check it, then match another. most beta matchers do not use enough current, so it will only be so close. we do not keep records of the matched sets, and what the beta is, so I cannot tell you what yours is.


Tom C

[Lman]

Benjamin,

What Tom C said about taking the heat away when soldering is also important. The heat can affect transistors junctions and even though they have been matched they can easily get mismatched when soldering. In the datasheets of some transistors is given sometimes the soldering temperature and time a transistor can withstand. So a clip on the legs is kind of obligatory and this is valid also for resistors and diodes as well.

This is why at Energenx they are doing burn in tests of all the Tracker 5s before they ship them. Bringing them to real working conditions so that components get hot and than check if everything behaves the way it should.

Regards
Lman

[Joster]

Benjamin,

What Tom C said about taking the heat away when soldering is also important. The heat can affect transistors junctions and even though they have been matched they can easily get mismatched when soldering. In the datasheets of some transistors is given sometimes the soldering temperature and time a transistor can withstand. So a clip on the legs is kind of obligatory and this is valid also for resistors and diodes as well.

This is why at Energenx they are doing burn in tests of all the Tracker 5s before they ship them. Bringing them to real working conditions so that components get hot and than check if everything behaves the way it should.

Regards
Lman

Wow! i never new that but it makes perfect sense. does anyone know what these type of clip on heat sinks are called. i mean i take it it clips right on the leg of the transistor right? i definitely need one.

Just a thought...perhaps teslagenx may want to consider putting a warning card with their shipment of matched transistors about the soldering as a courtesy. I get that you folks cant be responsible for how they are used but it could prevent the average Joe from wrecking his $200 plus purchase of 8 matched mlj21194's. Especially since a process that is guaranteed to happen to the product after purchase (soldering) can so easily effect the matching.

[Lman]

Hi Joster,

Look for soldering heat sink. Some people use crocodile clips to do it but you can guess that this is not good as it can't make a good enough contact to dissipate the heat. The proper ones are flat.

About TeslagenX I think they are kind enough to give the option to buy only transistors if one knows how to solder, to buy unpopulated board, or to buy populated board with everything assembled on it in case the person does not have experience with soldering or boards.
I do not expect them to give soldering lessons. The soldering is an art that has to be mastered and there are many different details that have significance on the final results as you can realize.
There are many videos and tutorials on the net about soldering, giving tips and advices and everyone can learn from them. I have learned as well some clever things.
Here is a good one explaining the different soldering iron tips, temperatures, cold connections and other things.

http://www.youtube.com/watch?v=vIT4ra6Mo0s

Regards
Lman

[Tom C]

Wow! i never new that but it makes perfect sense. does anyone know what these type of clip on heat sinks are called. i mean i take it it clips right on the leg of the transistor right? i definitely need one.

Just a thought...perhaps teslagenx may want to consider putting a warning card with their shipment of matched transistors about the soldering as a courtesy. I get that you folks cant be responsible for how they are used but it could prevent the average Joe from wrecking his $200 plus purchase of 8 matched mlj21194's. Especially since a process that is guaranteed to happen to the product after purchase (soldering) can so easily effect the matching.

Joster,

200 bucks gets you everything!! a complete kit including circuit board. mjl's alone are around 90 for 8 matched. I would not guarantee a degrading of the junction every time, only under certain circumstances. you can destroy diodes, small electrolytic caps and hall's the same way. that is why multi pin i.c's are often socketed, to allow easy replacement from heat damage. what is an I.C. but forms of junctions. heat destroys electronics. adding a note about soldering into our full kit instructions is a good idea, we will add it to our products. but also we do expect a bit of knowledge from our customers.

Tom C

[RS_]

I use forceps/hemostats (locking pliers) to heat sink the transistor pins between the transistor and the PCB while soldering..... a 15W iron will get the job done and not over heat things as much as a bigger iron to.....

i also use transistor sockets on some builds, if i think things will blow up and need replacing or trying different part numbers in the circuit to see how they preform...

[Benjamin]

This is helpful information. I will reconsider how I construct the circuit in the future.