Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: hopkinWFG on May 08, 2016, 12:33:03 am
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I am looking to change the voltage regulator on the power section which it has thermal paste between the PCB to the heatsink and thermal paste between the voltage regulator MC7812CT to the another heatsink... I have no idea how hot it runs previously but I am seeing the white clay like substance is crumbling off thus coming to know its a thermal paste....
I am looking to swap the old Motorola MC7812CT to a ON MC7812CT am not sure if the ON semiconductor be reliable enough or quality be better than the Motorola....
I am keeping the old heat sink and do a clean up of the old thermal paste before installing the new voltage regulator... I search thru RS catalog and thermal paste are so much more expensive... I looked thru and found a thermal interface sheet which is more affordable for a one time application...thus I am not sure which is a better choice for a MC7812CT to efficiently dissipate off the heatsink before it gets thermal conduction to the PCB....
I also have no idea for a MC7812CT to be exceptionally hot? Prior to that I also have no idea if what kinda thermal paste or thermal interface sheet to use?
Links to the MC7812CT http://sg.rs-online.com/web/p/products/5164834/ (http://sg.rs-online.com/web/p/products/5164834/)
Links to thermal interface sheet http://sg.rs-online.com/web/p/products/0446493/ (http://sg.rs-online.com/web/p/products/0446493/)
Can anybody with good knowledge or experience advise on this? Am looking more to protect my PCB and PCB tracks
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There is no easy way to answer your question from a distance with precision. It is largely dependent upon how hot the 7812 gets (obviously) which is a function of several things. The idea behind the whole heatsinking notion is to get heat out of the regulator package. This is or can be a challenge because the mounting/dissipating surface is only about 1/2" x 1/2".
1: How much current is being drawn by the circuit being supplied the +12VDC? These regulators (there are newer, more efficient ones) operate in a way where they have to "throw away" at least 2-3 volts from the input supply. You give them (for example) 15 volts, they output 12 volts, the rest has to go away as heat. If you pull 1 amp, then the package has to dissipate volts * amps = 3 watts. No getting around it. If you give them 18 volts, they need to dump 6 watts as heat. That may not sound like a lot of power but the question is always "over how big an area"? I think they can be supplied up to 35 volts as their max rated input volts. Then the package would have to dissipate 23 volts * 1 amp = 23 watts. That would be like a Deluxe Reverb in 1/2" x 1/2". HOTTT.
The most practical approach is to look at what is being supplied the 12 volts. If it is a few op-amps, I would stop your worrying right now and use the silpad. Those use tiny tiny current. Are there LEDs? 2 of them or ten of them? Just like we do when adding up heater currents of tubes to see if a particular transformer with a particular amp rating on the heater winding can light up the tubes....we just add up the requirements and get a feel for whether we are exceeding specs.
2: What's the ambient temperature? Inside an amp chassis, things are probably hotter than they are in a normal room. That makes the heatsink's job harder, because whatever type of heatsink you have works on taking "hot" to "ambient" and can never do better.
3: Let's also not forget that line voltages tend to be higher, which goes back to raising the input voltage level, while the output is still supposed to give only 12 volts. This increases the heat load on the device and thus the heatsink.
Forget about the reliability of the device itself, these things even in commercial grade are phenomenally reliable.
The silpads are pretty good. Personally, I have never quite trusted them as much as the goop + mica insulator but I am old school.
If it were me: The regulator (and if present, its negative voltage brother, a 7912) is supplied DC from the rectifier. If I wanted to reduce the heat load on the thing, I would install series diodes in between the rectifier and the reg input. Either a string of ordinary diodes or a 3-5 volt zener in between the rectifier and regulator input. Every ordinary silicon diode would drop about .65 volt from the input. Of course the great thing about diodes is that they are very cheap. I'd work to knock 2-5 volts from the regulator input with 3-7 diodes and call it a day. Your only consideration is mechanically mounting them but that's not a big deal. Diodes like this mount on their own leads all day long.
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> I am keeping the old heat sink
Re-think. IS it running hot? If so, unless the present sink is huge, that sink IS the bottleneck. Go big, bigger. Many amps look like they were designed for 90-day warranty. If you are keeping the amp, put in the pennies they shaved out. Big metal.
ON Semi _is_ Motorola's operation under different ownership. But dont know why you think it needs replacing?