The stud package devices are supposed to be directly mounted to the heatsink metal (if the heatsink is electrically isolated) or mounted on mica insulators to transfer heat. The plastic insulators I'm familiar with (look like nylon or PE/PP material) don't transfer heat. Maybe you could use some TO-220 or other insulators to cut out some discs. The glass-impregnated rubber insulators would be better than the present plastic ones. Use plastic bushing washers under the nuts if isolation is required.
It seems like a good idea to use parts that one has lying around, but there are 3-terminal regulators that are capable of 5A readily available in most common voltages. Dropping 39V to 13.6V is a large span for a 3-terminal regulator, so a lower output transformer would be more practical.
Transistor regulator circuits have been used for a long time, because they were effective and cheaper, even after 3-terminal regulators became available. Everything from hobbiest PSUs to large industrial equipment commonly had transistor regulators.
-- Cheers, WB .............
"klem kedidelhopper" wrote in message news: snipped-for-privacy@r4g2000vbf.googlegroups.com...
Yes this is related to my post of several months ago. I employed Arfa's suggestion of using two of my 6.8V 10 watt zeners in series to make a 13.6 V diode. And from my previous discussion this was then put in series with the rectified 24VAC, (39VDC) to drop the voltage to the input of my 12V regulator to a safer level. These two diodes were mounted with insulators and silicone grease on a large old black aluminum heat sink that I pulled out of a 1960's Delco automobile radio. With the diodes in the circuit the voltage to the input of the regulator was 19VDC, and under full load it was 16.5VDC, which is just about where I wanted it to be. The only problem is that the diodes as well as the heat sink were getting extremely hot when the regulator was fully loaded, (1.0Amp). In the interest of survivability assessment for my test I ran the unit overnight. So I added more aluminum and beefed up the contact of the heat sink to the mounting surface. Now the heat sink runs cooler however the diodes are still too hot to hold a finger on at full load. This surprises me. If I had good contact with the heat sink shouldn't IT be warmer than the semiconductor mounted to it? The thought now occurs to me that since I had to use plastic insulators from the junk box between the diodes and the heat sink they may not be providing suitable thermal conductivity, and this could also be a problem. I tried to buy a couple of suitable insulating diode mounting kits for DO-4 devices that would provide good thermal contact with the heat sink but I couldn't find a source for them. My distributor couldn't find any and NTE doesn't seem to sell them separately. I remember in my previous post that you had suggested a transistor arrangement last time Jamie, but you're correct in that I didn't fully understand what you were referring to at the time. Now that Phil has provided the schematic it seems quite clear. Thanks Phil. I could certainly try that arrangement too but after putting this all down on paper, (so to speak), now I guess I'm wondering. What if thermal conductivity between my diodes and my now massive heat sink is my problem? My junk box mounting hardware certainly could be suspect. Perhaps I should try to obtain the correct insulators and then reassess this heat issue. Does anyone have a source for them? The diodes are 1N2970B stud mount. Thanks, Lenny
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