Heat sink problem

[DaveP]

I normally work with tubes but I need to make a transistor bass amp and could do with some help.

I need to find a way to attach a heat compensating transistor to a heatsink without electrical contact.  The transistor is a BC107 with a metal body which is also the collector.  The heat sink has two 2N3055's and the BC107 is supposed to be in "contact" with their heatsink to adjust the current to the driver as they warm up.

I have searched the internet to no avail so I'm asking the forum so I don't have to reinvent the wheel so to speak.  I have thought of wrapping some silicone pad around it and resin gluing in place, I guess others have solved this problem before me?

DaveP

 

[gridcurrent]

I need to find a way to attach a heat compensating transistor to a heatsink without electrical contact.  The transistor is a BC107 with a metal body which is also the collector.

transistor with insulated case such as a TO-220-3 FP.

 

[zephyrmic]

isn't a BC547 the equivalent but in a plastic TO92 package? You could use that and clamp it to the heatsink with a small metal clamp with the flat face on the heatsink.
Kindest regards,
zephyrmic

 

[DaveP]

isn't a BC547 the equivalent but in a plastic TO92 package? You could use that and clamp it to the heatsink with a small metal clamp with the flat face on the heatsink.

Yes, this would have been a better idea, but the  circuit called for a BC107 and that's what I have now.

After my first post, a list of similar topics came up underneath, and one mentioned heat shrink tubing which is a very good idea so I'll trying  insulating the device with that before gluing it between the fins.

Best
DaveP

 

[Tubetec]

I have seen these heat compensation components  mounted with silicone on heat sinks from time to time ,always plastic cased transistors though .
Maybe drill a hole in the heatsink just big enough for the transistor to fit ,apply a small amount of heak sink paste ,push the component into its new home clean up excess thermal paste , use a blob of silicone to hold it in place .

 

[JohnRoberts]

Yup metal case is no bueno, but common way is to drill a hole in the heatsink larger than the transistor and sit it inside the hole...make sure it isn't touching metal, then maybe add some heat sink grease. Heat shrink is not much of a thermal conductor but for bias  temperature tracking it may be fast enough.

JR

 

[Audio1Man]

Hi Dave
The BIAS TRANSISTOR is to be mounted on the heatsink between the output devices. I would use a plastic case device that is mounted to the heatsink via a screw or clamp. The BC107 is a metal case part and drilling a hole in the heatsink and trying to keep it insulated is a hand grenade with the pin removed.

The bias tracking needs to keep the idle current constant through the output transistors as the temperature changes over the operating range.  You can use a NPN or PNP to change the BIAS VOLTAGES on the output transistors.  Use a TO225 case77 plastic flat pack MJExxx or other type of part with a mica or thermal pad and screw for good mounting.  Even a flat pack fully insulated type could be a solution. The TO92 with C clamp or drilled hole and pressed into the heatsink are some of the options.  I have used several types of mounting in many of my power amplifier designs (Acoustic, BGW, JBL & others). To test just use a hot plate or thermal heat gun to change the temperature and watch for min change in output current over the temperature range.
I hope this helps
Duke

 

[DaveP]

Thanks for all your very good ideas.

In the end I fitted some heat shrink tubing and drilled a hole just large enough to fit.  Then I used epoxy resin on both sides to hold it in place.  Plastic covered transistors are made of something similar.

The heatsink is massive (recycled from old equipment) so I am not expecting problems there.  The only reason I am making a 50W bass amp is because the output resistance with transistors is so low and that is just what is required to control the speaker cone.

Best
DaveP

 

[Newmarket]

Am I missing something. Why use Heat Shrink rather than one of the many insulating 'thermal interface' sheet / tube materials ?

 

[JohnRoberts]

Thanks for all your very good ideas.

In the end I fitted some heat shrink tubing and drilled a hole just large enough to fit.  Then I used epoxy resin on both sides to hold it in place.  Plastic covered transistors are made of something similar.

The heatsink is massive (recycled from old equipment) so I am not expecting problems there.  The only reason I am making a 50W bass amp is because the output resistance with transistors is so low and that is just what is required to control the speaker cone.

Best
DaveP

That should work, since the bias transistor is not dissipating any power to speak of, the thermal resistance between it and the heat sink should  not cause a significant temperature difference. It may affect the time it takes to stabilize.  Not much mass involved, but maybe give it some extra settling time when setting up the amplifier class A current.

FWIW they used bias transistors before they had plastic packages, but that does make it easier to insulate.

JR

 

[DaveP]

Am I missing something. Why use Heat Shrink rather than one of the many insulating 'thermal interface' sheet / tube materials ?

The BC107 is a very old design with a tubular metal collector case, there are no tubes for it that I know of, hence shrink tubing.  Transistors are so old school now that you have to get them on Ebay.

DaveP

 

[JohnRoberts]

The BC107 is a very old design with a tubular metal collector case, there are no tubes for it that I know of, hence shrink tubing.  Transistors are so old school now that you have to get them on Ebay.

DaveP

Yup the TO-18 metal case was very common for small transistors back in the day. I even burned cute little red circles into my finger tips from touching them to see if they were hot and they were. 

You learn to put a little spit on your finger before touching them. If the spit boils away, they are more than 100'C.

JR

 

[Newmarket]

The BC107 is a very old design with a tubular metal collector case, there are no tubes for it that I know of, hence shrink tubing.  Transistors are so old school now that you have to get them on Ebay.

DaveP

Okay. I see. I was thinking you could sort of make a tube out of sheet material but maybe that doesn't work out.