Author Topic: BJT properties  (Read 523 times)

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Offline sureshot

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BJT properties
« on: July 14, 2018, 11:08:52 am »
I've had a good look round for an answer to this question, but can not find anything specific. I have some heatsinks i would like to mount T03 packages to (transistors) the nature of the heatsink is close packed aluminium fins. Gap is about 2.5 mm between them. I've routed holes through heatsinks like this before, but had greater space between the heatsink fins. The maximum conductor wire i can route through the said heatsink can't exceed 2.5 mm including heatsrink tube. I normally do a 180° through the heatsink via two close holes.

My question is, do the emitter and base conductor wires have to be the same gauge as the collector conductor wire ? This is for a pnp transistor. The collector conductor gauge wire is not a problem as it comes off the case of the T03 package. For a base and emitter conductors my wire gauge is limited by a 2.5 mm hole size looped forward and back through the heatsink. I've sized up 4 Amp stranded wire, the heatshrink it fits fine. I could probably get it to 5 or 6 Amp conductor wire, just about with heatshrink tube. So what I'm asking is does the base and emitter conductors have to be the same gauge as the collector conductor wire. The maximum current i would draw under full load would not exeed about 8 Amps. Thanks for reading any help appreciated.
 

Offline kg4arn

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Re: BJT properties
« Reply #1 on: July 14, 2018, 11:27:19 am »
The emitter carries the collector current plus the base current.
Since base current is a factor of beta smaller than Ic, the collector and emitter leads would be the size (for practicality). The base lead could be smaller.
 

Online Ian.M

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Re: BJT properties
« Reply #2 on: July 14, 2018, 11:44:53 am »
The base wire can be thin - even if you are using the transistor as a saturated switch, Ib wont be greater than Ic/10, and if you are using it in its linear region, Ib=Ic/hFE

However the emitter wire has to carry Ic + Ib.   For high currents, you need wire with really thin tough insulation, but at only 8A, 18AWG (1mm dia) wire would be fine as long as its insulation is rated for a temperature of 35 deg C above the max possible heatsink temperature.   That may be problematic for heatshrink, and you may need to use enamelled magnet wire so the heatshrink isn't the only insulation layer.   If you can get stranded high temperature wire with an o.d. of 2.5mm or less and 0.8mm2 or better CSA, use it.
 

Offline sureshot

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Re: BJT properties
« Reply #3 on: July 14, 2018, 05:06:24 pm »
Thank you for the replys.  I had a feeling the emitter conductor must carry a large percentage of the current, and the base some what less. The wire I use is in the image below. I normally use twisted pairs doubling it up for 5 Amps or more. The single will go through the 2.5 mms heatsink hole with shrink wrap tube on it for extra insulation. The heatsink won't get much above 65°C or so I think, and I can keep wire lengths short. I know it seems pointless, just go buy another heatsink. But they have a really good footprint for the power heat they can dissipate. If I had access to a milling machine I'd mill out one of the fins to accommodate twisted pairs of the wire below. Trying to cut a fin out by hand would probably look terrible. I'm tempted to try with a coping saw though. Wire and heatsink below. It's out of a Dell dimension pc.
 

Online Ian.M

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Re: BJT properties
« Reply #4 on: July 14, 2018, 05:23:40 pm »
7/0.19mm stranded wire has a CSA of fractionally under 0.2mm2.   Assuming PVC insulation with a maximum working temperature of about 90 deg C, you need at least four times that to safely carry 8A with a 60 deg C ambient temperature. 

==> get better wire.   
 

Offline sureshot

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Re: BJT properties
« Reply #5 on: July 14, 2018, 09:12:11 pm »
Yes better wire might be a better idea. Anyway I managed to fit the heatsink out with 2 x T03 packages, and a KBPC bridge rectifier package. The surface area is 80 mm x 64 mm, with a cordless drill  :D and only snapped one drill bit which I had to drift out. The conductor holes are 3.5 mm and a twisted pair of that wire is a very snug fit. The mounting holes are tapped in M3. I'm only looking for 8 Amps total, and typically 4.5 Amps is the intended load. A small HF radio and 35 watt linear amplifier. I will look around for some better quality wire though. Thank you for the help. Needs a clean up with a bit of scotch bright, and get the W40 off it that was tapping lubricant.
 


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