Is it OK to insulate a heat sink?

[mtl777]

I made a heat sink out of some thick aluminum sheet and used it for a negative voltage regulator (a 7915). The heat sink is a bit big and long, and is attached to the metal plate of the regulator which happens to be connected to the input pin. Because of this, I got worried that the heat sink might act as an antenna and attract EMI/RFI which would introduce noise to the input voltage. So I decided to insulate the heat sink by wrapping it in several layers of vinyl electrical tape. But now I'm wondering if doing this has reduced and badly impacted the heat transfer performance of the heat sink. Was this not good to do? What do you think?

Thanks!

[dnsey]

Wrapping the sink in tape will render it almost useless!
Instead, use a proper insulating washer and bush designed for the device. These are available in a number of materials, but any will do for your application.
i'm still trying to work out why the regulator is connected to the (ungrounded) input terminal...what's the circuit?

[djQUAN]

first of all, any insulating material will not shield from EMI/RFI.

second, you don't need to worry. add a good bypass cap (100nF monolithic ceramic cap) across Vin and gnd will take care of EMI entering the regulator.


dnsey,
the 7915 pinout is pin1 = gnd, pin2 = Vin, pin3 = Vout. pin2 is also connected to the tab in a TO-220 case.

[dnsey]

the 7915 pinout is pin1 = gnd, pin2 = Vin, pin3 = Vout. pin2 is also connected to the tab in a TO-220 case.

I (mis?)understood that the regulator was connected to the input of an amplifier or similar, hence the concern over RF injection.

[richie00boy]

You will also probably find the tape melts after a while and the short appears again. Take it apart and clean the sticky mess off with thinners.

[mtl777]

Originally posted by dnsey
Wrapping the sink in tape will render it almost useless!
Instead, use a proper insulating washer and bush designed for the device. These are available in a number of materials, but any will do for your application.

To be clear, the heat sink is attached naked (metal to metal). Only the non-contacting part is covered with tape. I was just wondering if covering that part with tape reduces the performance of the heat sink in any way. Does it?

Thanks for the washer and bush suggestion! I didn't know such things are used for this, the newb that I am. What kind of material are they usually made of?

[Variac]

to sum up:
EMI RFI won't be stopped at all by wrapping the heatsink with tape so there is no reason to have the tape to start with..

The heatsink needs to transfer heat to the air as efficiently as possible
Tape all over the sink (even though it isn't between the output device and the sink) is not going to help this process..
Get rid of the tape..

[richie00boy]

Look up mica insulator and Silpad for two different solutions.

[mtl777]

Thanks, I get it now. Mica is a good thermal conductor and at the same time it is an electrical insulator. Therefore, it will allow heat transfer from regulator to heat sink but will not conduct electrical noise currents from heat sink to regulator. Beautiful! Just what the doctor ordered.

Now, I have a similar heat sink on the positive voltage regulator. Do I also need a mica washer here? I guess not, since in this case the tab is connected to ground, and any noise picked up will simply go to ground. Nevertheless, I would like to ask this question just in case there's something I don't know.

[Variac]

Mica insulators need to be smeared on each side with a very light coat of thermal grease to move the heat along. This is similar to that used on as used on microprocessors in computers but is a lot cheaper. The generic microprocessor grease will work..

The heat sinks of your project are presumably attached to your case and the case presumably attached to the house ground, so any RFI goes from the heatsinks right to ground.