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Transistor Query


tom80smith

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Hi, I am building a hand held controller where the actual enclosure is made of aluminium, I have fashioned a heatsink for the transistor (a BDV64C PNP Darlington) to be bolted to, which, in turn is bolted to the underside of the enclosure, as per photo...

 

post-21029-0-95635900-1388743738_thumb.jpg

 

 

I have stopped in my tracks as I am thinking it may not be good to have the collector in contact with the main enclosure. Has anyone any thoughts, recommendations before I proceed one way or another, thanks.

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I concur with smokebox. Insulate it using a TO218 insulating washer and bush together with a small amount of heatsink paste. If you do this then you could mount the transistor straight to the aluminium enclosure thus using the whole of it as a heatsink with much better heat transfer.

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You really need to use an an insulating washer under the transistor and a insulating bush where the mounting bolt holds the device down.

AS thats a fairly beefy device there are two ways you can insulate the device,

the simplest is to buy a TO-3 mounting kit containing the mica washer and a plastic bush, be careful some suppliers like Farnell separate the screws

and insulating bushes and washers and sell as separate kits.

http://www.rapidonline.com/FFSearchResults.aspx?qFS=&query=TO-3+mounting+kits&x=0&y=0

You could use either the TO-3 kit or the MI3055 kit

there are pictures so you can see what you need if you use another source.

 

The alternative way is to insulated the smaller heatsink you have made from the enclosure, you could use Kapton tape or thin PTFE sheet for this.

You can use nylon screws to mount the heatsink, and countersink the transistor mounting screw, so there is nothing to short the heatsink to the enclosure.

 

Using a insulating mount kit is easier, but if you have something to hand which you can insulate the separate heatsink/spreader from the enclosure you can save the p&p .

 

HTH

Richard

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Update... I had a look in a box of 'bits' that I have been given and found a suitable insulating kit from a large TIP141...

post-21029-0-80745100-1388750448_thumb.jpg

 

I have decided to use two of the redundant bolts for mounting the circuit board (will be bolted down, and the copper strips have been cut underneath), The transistor is a fairly hefty device as the shop didnt have a TIP147 in and I was assured that this was an equivalent, looking at the datasheet it has higher current and voltage ratings, and double the gain. I am also tempted to cut some vent holes in the enclosure top.

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Don't forget you'll need to isolate the bolt that goes through the tab of the transistor as well - or you could use a nylon bolt.

(BTW that is not a TO3 case transistor, of course)

I prefer to use silicon rubber insulating kits rather than (brittle) mica as you don't need to use grease.

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The insulating kit should have included 'top hat' bushes, thereby providing the means to isolate the mounting screw. The package shown appears to be TO218, similar to TO3P.

I wouldn't use a nylon mounting screw on any device that is likely to get hot, though I concur with the preference to use 'Sil Pad' type insulating kits rather than silicone grease - which tends to spread everywhere you don't want it, and you cannot get rid of from clothing.

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Well, yes, but if it gets hot enough to melt nylon, then a) there's a serious problem and b) you won't be able to pick up the enclosure anyway!

[in normal, continuous, use none of my own (design) controllers get more than slightly warm... but then I run 009]

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If there is no other connection to the case and from what I can see there is none . So there is no need to insulate the transistor from the case. There is no need to fit ventilation holes in the case as the heat will be dissipated around the metal case.

The problem is if the outside of the case comes into contact with a stray conductor. The transistor should be insulated.

 

Andrew

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  • RMweb Gold

If there is no other connection to the case and from what I can see there is none . So there is no need to insulate the transistor from the case. There is no need to fit ventilation holes in the case as the heat will be dissipated around the metal case.

So if the case just gets rested touching the track it would be quite ok? No case should be anything other than earth.

Don

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So if the case just gets rested touching the track it would be quite ok? No case should be anything other than earth.

Don

Ok , that would be a problem. So why not solve both the heat and insulation problems and move the power transistor out of the handheld unit and fit it on a decent heatsink under the baseboard. The handheld unit only needs the controls. There appears to be a multicore cable going into the unit, so that is fairly simple to do. 

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  • RMweb Gold

Ok , that would be a problem. So why not solve both the heat and insulation problems and move the power transistor out of the handheld unit and fit it on a decent heatsink under the baseboard. The handheld unit only needs the controls. There appears to be a multicore cable going into the unit, so that is fairly simple to do. 

That works well I have done this myself with a pot a few resistors and a capacitor that feeds into an emitter folwer circuit. Does get hot in your hand

Don

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  • 2 months later...

Update... I had a look in a box of 'bits' that I have been given and found a suitable insulating kit from a large TIP141...

attachicon.gifTransistor Update Photo.jpg

 

I have decided to use two of the redundant bolts for mounting the circuit board (will be bolted down, and the copper strips have been cut underneath), The transistor is a fairly hefty device as the shop didnt have a TIP147 in and I was assured that this was an equivalent, looking at the datasheet it has higher current and voltage ratings, and double the gain. I am also tempted to cut some vent holes in the enclosure top.

I know I've probably come to this a bit late in the day but you really would have made life easier for yourself with the original TIP147 (CPC,Maplin, Farnell etc) idea -it's a third of the price, more than man-for-the-job and all insulated so could be mounted straight on the box using a  metal bolt and a little heatsink compound  (or even vaseline), making sure the mounting hole is adequately deburred.

 

 

Trevor

 

An (old) engineer  who does for a tanner what anyone else does for a bob.

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  • 3 weeks later...

Interesting thread, coming to it rather late I have misgivings about the whole concept.   I have been re wiring OnTrack hand held remote controls to have stereo jack plug connectors instead of 4 pin din plugs because I keep stepping on the damn wires and wrenching the controls out of my hand to slam onto the floor or concrete path. My outside leads stretch to well  over 15 feet long but  the standard 2 metre length seems to be the one which I step on most.  OnTrack and I think Morley controllers have remote units with three wire connections and only a single "centre Off" Potentiometer in the hand held,   If i had to use a hand held I would use a plastic outer shell, a multi pin connector between lead and hand held, and a small fan if there is any likelihood of over heating.

But surely the transistors are better located with the transformer on the baseboard while the hand unit just holds the potentiometer, or pot and reversing switch if you like that sort of thing.

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But surely the transistors are better located with the transformer on the baseboard while the hand unit just holds the potentiometer, or pot and reversing switch if you like that sort of thing.

 

In principle maybe, but in reality this would complicate the reversing switch wiring, needing more ways in your umbilical cord. You would actually need a 6 core cable (two for power supply, one control pot wiper, one power transistor drive output, and two reverse switch outputs back to the track. The only way round this would be to use a remote DPDT relay to do the reversing and control that from an on/off switch on the handset, bringing the ways back to four. However, that method would not allow the usual 'centre-off' option which most controllers give on their reversing switches. You would also have more voltage drop due to the increased length of wire through which the current passes.

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  • RMweb Gold

But surely the transistors are better located with the transformer on the baseboard while the hand unit just holds the potentiometer..

 

In theory...

It all depends on what you wanted to do with the induced 'error' and where you wanted the power 'not to go'.

 

But if I were you, I would worry about such things.   :)

 

Kev.

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The OnTrack and I believe the Morley have three wires to the hand held slave, that is why I use a single Stereo Jack plug, the OnTrack has separate forward and reverse transistors and has only a centre off pot on the handset, no reversing switch, clockwise for one way anticlockwise for 'tother.

Current drop, voltage drop, is not a problem when you are only talking a few milliamps, I have detected no difference in controller output voltage between a 25 foot curly lead plus 40 feet of iffy "Hard wiring with a 4 pin Din and two Stereo Jack plugs and the alternative pot on the PCB on the Transformer/ controller.

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  • RMweb Gold

The On track (or at least my version 15 years old!) uses two windings to give + and -12v for the centre 0ff to work. The  power output does not go through the handheld so there would not be much power loss in the wander lead.

Either you have a single way pot and need a reversing switch or you go for a centre off control.

Don

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The On track (or at least my version 15 years old!) uses two windings to give + and -12v for the centre 0ff to work.

 

We are unlikely to see any new commercial designs like this, since EU energy efficiency regulations virtually mandate the use of switch mode supplies. Try getting a centre-off switch mode power supply for anything like the cost of a normal 12 or 15V one  :no:

 

You could use a 24V supply and create a virtual ground at 12V to achieve the same effect.

 

Andrew

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  • RMweb Gold

We are unlikely to see any new commercial designs like this, since EU energy efficiency regulations virtually mandate the use of switch mode supplies. Try getting a centre-off switch mode power supply for anything like the cost of a normal 12 or 15V one  :no:

 

You could use a 24V supply and create a virtual ground at 12V to achieve the same effect.

 

Andrew

Ah well I dont think they can stop you building your own

Don

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