Hi there
I am trying to build an Aleph 3 amp and have all my parts together (finally). when I mount my transistor (Kristijan's PCBs) to the heatsink, should my mounting screw and heatsink be isolated from each other? I know that I have to use some form of insulator(I am using mica insulators with thermal grease) between the transistor and the heatsink. However, once I use a screw to mount the transistor to the heatsink, the screw and the heatsink will have an electrical short. Is this ok?
what about isolation between an aluminium chassis and the heatsinks? Does there need to be electrical isolation between these two?
Finally, do I need insulating bushing between the screw and the transistor? Can the screw end up being in physical contact with the metal part of the transistor through the screw mount hole?
I am a novice and I hope these questions make sense
I am trying to build an Aleph 3 amp and have all my parts together (finally). when I mount my transistor (Kristijan's PCBs) to the heatsink, should my mounting screw and heatsink be isolated from each other? I know that I have to use some form of insulator(I am using mica insulators with thermal grease) between the transistor and the heatsink. However, once I use a screw to mount the transistor to the heatsink, the screw and the heatsink will have an electrical short. Is this ok?
what about isolation between an aluminium chassis and the heatsinks? Does there need to be electrical isolation between these two?
Finally, do I need insulating bushing between the screw and the transistor? Can the screw end up being in physical contact with the metal part of the transistor through the screw mount hole?
I am a novice and I hope these questions make sense
What kind of transistor are you using. Some are plastic and some are metal. If you are using, say, an IRFP240/9240 combination then the bolt will not touch any of the metal of the transistor and that will be fine. If you have any doubt, after you bolt the transistor down, and before you solder it to the card use an ohmeter to check from the screw to each leg of the transistor. You should not get a reading if everything is okay. If you get a reading then you must buy an insulator.
Although I'm a complete novice as well. And I have similar questions. I believe the following will answer one of them:
It appears that the hole in the TO247AC package is isolated. Or insulated? It sounds like you need not worry about a bushing.
Regards,
Dan
An externally hosted image should be here but it was not working when we last tested it.
It appears that the hole in the TO247AC package is isolated. Or insulated? It sounds like you need not worry about a bushing.
Regards,
Dan
Mosfets in use
Hi there
Thanks everyone for replying so promptly. The devices i am using are the IRFP244. I want to say that this is a TO-247 package but I am not very sure. They do however look like this -
http://www.irf-japan.com/package/TO247AC.pdf
Can I do the continuity check on the PCB itself (check for electrical continuity between each of the pin legs and the screw hole ?)
Do I need to keep any leg grounded? Will the continuity checks in anyway ruin the boards?
thanks in advance
VSR
Hi there
Thanks everyone for replying so promptly. The devices i am using are the IRFP244. I want to say that this is a TO-247 package but I am not very sure. They do however look like this -
http://www.irf-japan.com/package/TO247AC.pdf
Can I do the continuity check on the PCB itself (check for electrical continuity between each of the pin legs and the screw hole ?)
Do I need to keep any leg grounded? Will the continuity checks in anyway ruin the boards?
thanks in advance
VSR
Re: Mosfets in use
If you look at the top of the page you linked it clearly states TO247AC
You need not worry about it. On the top where the screw goes through is black plastic and underneath you can see a ring of black plastic around the hole and then metal. Mount this on your choice of insulative pad (mica w/thermal compound) and let her rip. You do not have to do a continuity check if you don.t want and once you have these mounted on insulators your heatsink will not cause a short when you attach it to the rest of the chassis.
The problem with checking a device after it is on a board is, can the meter read what I want it to read...ONLY. That is, will it read through something else in the circuit and make me think I have a problem when I don't, and how long will it take me to figure out I am chasing my tail 1 megaohm to ground is different than 1 ohm to ground. You will be fine but if you want to double check take a device not soldered to the card, say a spare, and bolt it up and look. Just a little peace of mind, but you will see all will be well. You may blow it up from something else but not this Have fun
vsr123 said:
If you look at the top of the page you linked it clearly states TO247AC
You need not worry about it. On the top where the screw goes through is black plastic and underneath you can see a ring of black plastic around the hole and then metal. Mount this on your choice of insulative pad (mica w/thermal compound) and let her rip. You do not have to do a continuity check if you don.t want and once you have these mounted on insulators your heatsink will not cause a short when you attach it to the rest of the chassis.
The problem with checking a device after it is on a board is, can the meter read what I want it to read...ONLY. That is, will it read through something else in the circuit and make me think I have a problem when I don't, and how long will it take me to figure out I am chasing my tail
1 megaohm to ground is different than 1 ohm to ground. You will be fine but if you want to double check take a device not soldered to the card, say a spare, and bolt it up and look. Just a little peace of mind, but you will see all will be well. You may blow it up from something else but not this Have fun
Re: Heat Transfer Abilities
Yes.
Yes.
Actually, I think that the Mica is slightly better thermally, if I remember correctly. Try Bergquist's site, I think that the data is there. Or alternately, Digikey. They have data on everything that they sell. I think that any difference is only a problem if your heatsink is too small to begin with.
I don't quite understand the question "is this an issue...?" Just put it on both sides. It's the right way. That sauce serves to fill any microscopic gaps between the surfaces of the heatsink, mica pad, and transistor. That is its one and only purpose.
Now that I look again, maybe it's not a question. It's worded like one, but it has no question mark (but I notice that they are being done-away-with here )
For Francois, the answer is Yes. Particularly in your case with your extra-mega-huge heatsinks.
I think that mica insulators in that size are somewhat difficult to get. You can get them, but most don't carry them.
Here are two sources you can read to become a real expert:
http://www.chomerics.com/products/documents/PowerSiteThermalPerf.pdf
http://www.irf.com/technical-info/appnotes/an-1012.pdf
Panelhead said:
Yes.
François said:
Yes.
Actually, I think that the Mica is slightly better thermally, if I remember correctly. Try Bergquist's site, I think that the data is there. Or alternately, Digikey. They have data on everything that they sell. I think that any difference is only a problem if your heatsink is too small to begin with.
I don't quite understand the question "is this an issue...?" Just put it on both sides. It's the right way. That sauce serves to fill any microscopic gaps between the surfaces of the heatsink, mica pad, and transistor. That is its one and only purpose.
Now that I look again, maybe it's not a question. It's worded like one, but it has no question mark (but I notice that they are being done-away-with here
)For Francois, the answer is Yes. Particularly in your case with your extra-mega-huge heatsinks.
I think that mica insulators in that size are somewhat difficult to get. You can get them, but most don't carry them.
Here are two sources you can read to become a real expert:
http://www.chomerics.com/products/documents/PowerSiteThermalPerf.pdf
http://www.irf.com/technical-info/appnotes/an-1012.pdf
quote: I don't quite understand the question "is this an issue...?" Just put it on both sides. It's the right way. That sauce serves to fill any microscopic gaps between the surfaces of the heatsink, mica pad, and transistor. That is its one and only purpose.
what he has said is is in his own way is absolutely correct...sil-pad absorbs the compression of the device and fills up any fluctuations in mating surface ...or should I say adhesive properties...mica is a translucent wafer so its "RIGHT THERE"...the compound we use just helps the heat transfer
what he has said is is in his own way is absolutely correct...sil-pad absorbs the compression of the device and fills up any fluctuations in mating surface ...or should I say adhesive properties...mica is a translucent wafer so its "RIGHT THERE"...the compound we use just helps the heat transfer
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