2n3055 transistor question

[TheOne]

From the picture attached, my transistor work, but I would like instead connect it like the right section (of the image) but it does not work.


Can I do it like the right section?


Because I want to put 2 transistors on the same heat sink, I want to put the 35V on the collector on both transistors (direct on heat sink) but
just want the other part with coil separated from each others, I don't have 2n3055 termal insulator and put the 35v on each transistor and without touching the heatsink...


Anyway any suggestion/help would be appreciated!

[MarkE]

Your circuit presents three problems:

1) The emitter cannot rise any higher than 1 Vbe below the base.  Consequently the transistor dissipation will at least double from a low-side configuration and your heat sink sharing will be self-defeating.
2) In order to turn the transistor on, you will have to pull up to the 35V rail.  This means you will have to add a predriver stage.
3) The predriver will inject extra current into the load.

There are several alternatives that make more sense:

Use an N channel MOSFET in a low-side switch configuration that has a low enough on resistance that you can use a small heat sink or no heat sink at all.  How many amperes does your load draw?  This MOSFET is under $1. each from Digikey.  It's on resistance is nominally 12.5mOhms.  Even at 4A the power dissipation will be about 200mW.  No heat sink is needed. NTP5864NG  http://www.digikey.com/product-detail/en/NTP5864NG/NTP5864NGOS-ND/2748420  If you need a logic level gate then this is a few cents more and will dissipate about 300mW.  It is also fully insulated.     AOT2618L http://www.digikey.com/product-detail/en/AOT2618L/785-1438-5-ND/3603378

Use an integrated high or low side switch again with a low enough on resistance as to obviate the need ofr any shared heat sink.

Oh, and by the way:  Connect the catch diode around the inductor:  cathode to the 35V, anode to the switched side.

And you could always just buy one of these kits from SparkFun:  https://www.sparkfun.com/products/10618

[TheOne]

Your circuit presents three problems:

1) The emitter cannot rise any higher than 1 Vbe below the base.  Consequently the transistor dissipation will at least double from a low-side configuration and your heat sink sharing will be self-defeating.
2) In order to turn the transistor on, you will have to pull up to the 35V rail.  This means you will have to add a predriver stage.
3) The predriver will inject extra current into the load.

There are several alternatives that make more sense:

Use an N channel MOSFET in a low-side switch configuration that has a low enough on resistance that you can use a small heat sink or no heat sink at all.  How many amperes does your load draw?  This MOSFET is under $1. each from Digikey.  It's on resistance is nominally 12.5mOhms.  Even at 4A the power dissipation will be about 200mW.  No heat sink is needed. NTP5864NG  http://www.digikey.com/product-detail/en/NTP5864NG/NTP5864NGOS-ND/2748420  If you need a logic level gate then this is a few cents more and will dissipate about 300mW.  It is also fully insulated.     AOT2618L http://www.digikey.com/product-detail/en/AOT2618L/785-1438-5-ND/3603378

Use an integrated high or low side switch again with a low enough on resistance as to obviate the need ofr any shared heat sink.

Oh, and by the way:  Connect the catch diode around the inductor:  cathode to the 35V, anode to the switched side.

thank for your reply!


More explanation: actually I test with 5v + led and resistor, the coil stuffs is not done yet, but I have the problem with the LED so I guess the coil version would not work


I don't need a lot of amp since it is used for MEG input coil, I want higher voltage and low amp and all my circuit is powered by a raspberry pi2 GPIO 5v.


But about this transistor, I dont want to buy mofset and wait 2-3 weeks to continue to work on my circuit.


Can I use my transistor, if yes, they need to be insulated from the heat sink and each collector are managed like this new circuit diagram? I dont quite understand your pre-driver  stage

[Void]

From the picture attached, my transistor work, but I would like instead connect it like the right section (of the image) but it does not work.
Can I do it like the right section?
Because I want to put 2 transistors on the same heat sink, I want to put the 35V on the collector on both transistors (direct on heat sink) but
just want the other part with coil separated from each others, I don't have 2n3055 termal insulator and put the 35v on each transistor and without touching the heatsink...
Anyway any suggestion/help would be appreciated!

You are basically asking if you can use an NPN transistor as a high side switch.
Normally a PNP transistor is used in a high side switch configuration when using bipolar transistors.
However, even if you were to use a PNP transistor such as a MJ2955 instead of the NPN 2N3055,
you would stil have the same heatsink problem, as the emitter of the PNP transistor goes to V+ when
it is used as a high side switch. See the attached diagram.
Maybe someone who is very familiar with these sort of switching arrangements can offer a better solution
while still using your 2N3055 transistors, but it looks like you may well have to go with two separate 
electrically isolated heat sinks, or order some thermal heat sink insulators for the transistors if you can find some.
All the best...

[TinselKoala]

But Mark... the 2n3055 is a "magic" transistor. If you get a genuine one (as opposed to the fake Chinese ones with tiny dies inside probably from a 2n2222) they can do wonderful things that seem to be way beyond their published specs. Perhaps the OP wants to see these miraculous effects, and perhaps his aesthetic sense requires that they be on the common heatsink. I have a JT sitting here on my desk that uses a 2n3055 and lights up a NE-2 from a "dead" AA battery. (Of course it works with a 13003 and some other types also.)

It's too bad he's not my neighbor, I just checked my stash and I have more TO-3 mica insulators and mounting kits than I'll ever use in my lifetime.

http://www.ebay.com/itm/4-sets-TO-3-Transistor-Mounting-Hardware-Kits-Mica-Insulator-Screws-Bushings-/251202298797