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Old 4th May 2013, 06:06 PM   #1
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Default attaching Vbe multiplier to heatsink

Say I have a simple Vbe multiplier in a TO-92 package.

How is this attached to the heatsink, and is there an optimum placement in relation to the output transistor on the sink? I would assume being in close proximity to the metal tab on the output transistor would be good, but I usually assume wrong on these matters! Can it be a few inches away if layout seems to point that way? Also, if the opposite power transistor is on a separate sink, is there a reason to put the Vbe multiplier on one or the other? is it better for the power transistors on bothe halves to share a sink so they are more thermally coupled? So many questions!

Also, even though I would likely use a mica insulator on the power transistor, for sake of argument, lets say the heatsink is otherwise isolated from the circuit and I chose note to insolate the transistor tabe from the heatsink. Can I consider the case on a TO-92 type package to be sufficiently insolated to also be attached to said heatsink? Not something I really plan to do, just trying to expand my undertstanding.
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Old 4th May 2013, 06:12 PM   #2
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I think that it is all irrelevant as long as you have "linear" relationship: Heat sink temperature-Vbe output voltage. You put it somewhere u like when it is cold, measure idle current, than run the amplifier till it become HOT, than you measure I idle again. If it remains the same, it is doing its job very well No need to complicate things.
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Old 4th May 2013, 06:26 PM   #3
Mooly is offline Mooly  United Kingdom
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It helps to understand how it all works. If the output transistors run at a fixed bias voltage to give a given current then as the junction temperatures increase, so the characteristics change. In simple terms it mean that the output stage draws more current, gets hotter, draws still more current and so on. That results in destructive thermal runaway. So the vbe multiplier attemps to compensate by reducing the bias voltage in relation to temperature.

To work properly it has to sense the temperature as quickly as possible and that means mounting it as near as possible to the outputs. Both outputs should share the same heatsink so that both heat and cool the same.

A plastic T092 is just fine, the insulation would be in the high 100's if not low 1000's of volts.

Here's an old thread. Post #18 shows one way I have used in the past and there are examples of other members methods.

Can bias transistor mount on top(body) of output transistor??
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Old 4th May 2013, 07:12 PM   #4
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Thanks guys.

Lets say I have a to-92 Vbe multiplier, and to-220 type output transistors. Any tried and true method for sticking them together that isn't permanent? I am still in the breadboard experimentation stage here, so I would like to avoid gluing parts together for now, although I suppose it wouldn't be a big deal.

Also, for this breadboard setup, I have 2 breadboards, one containing the power supply, and another with the amplifier circuitry. They are both mounted to a piece of 1x4 wood. I also have a separate piece of wood on which I have attached the heatsinks for the output stage, although I have yet to mount transistors on it.

This amp will be ~20 watts into 8 ohms. I was wondering how long I can have the leads from the amp breadboard to the power transistors be and still have decent stability. I will keep the individual wires separate, and will use maybe 22 gauge wire for this. I just want an operating amp that I can measure, scope, and experiment with.

I will be using mje340/50s for drivers. I'm digging up that data sheets right now, but if I bias them at, say, 20mA, will they require heatsinks of their own in this low powered amp? I have Cordell's book, and have reda his section on heatsinks and thermal resistance and what not. I'm not yet clear on how to find out how hot a particular transistor gets at a particular wattage, and how hot it should be allowed to get. I was going to go by touch for now.
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Old 4th May 2013, 07:17 PM   #5
mcd99 is offline mcd99  United Kingdom
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I like the leach amp vbe multiplier where diodes are mounted on the heat sink. You then have the option of having two heatsinks for the output transistors.

This is what I did with my leach amp build. two diodes on each heatsink. Seemed thermally stable but I never did any tests as to how accurate the tracking was.
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Old 4th May 2013, 07:50 PM   #6
AndrewT is offline AndrewT  Scotland
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Glue a sot23 to the collector lead where the lead exists the package.
Possibly as quick reacting as the diode as in the NJL package.
regards Andrew T.
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Old 4th May 2013, 08:25 PM   #7
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The Vbe transistor is usually fixed to the centre of the heatsink.
I use superglue for mine but it might be fun to remove if needed later.
Murton-Pike Systems PCBCAD51 pcb design software.
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Old 5th May 2013, 12:34 AM   #8
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Originally Posted by Dan Moos View Post
Say I have a simple Vbe multiplier in a TO-92 package.
I like Andrew's idea of a SOT223 device, but haven't the dexterity to connect wires to the tiny critters. (Fine when they are lying flat on a PCB).

The problem with any heat sink mounted device is the thermal lag between the time and temperature.

Bob Cordell gave a great presentation for the NJ Audio Society on the beneits of the NJL devices. I have them on deck for a complete remake of a Crown DC-300.
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Old 5th May 2013, 12:49 AM   #9
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Bias current goes through both devices and they tend to heat and cool the same. I drill a hole near the output device, put in some thermal grease and just stick the device in. That seems to hold it well enough for test purposes and probably forever if you have some small clip for the wires. There are about a dozen ways to do this.
I may be barking up the wrong tree, but at least I'm barking!
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Old 5th May 2013, 12:52 AM   #10
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The simplest is to use a to-220 (or somesuch)
and mount it like a power transistor ..
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