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Old 4th November 2010, 06:16 PM   #411
AR2 is offline AR2  United States
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It is also worth mentioning that Nelson used CL60 in various locations for various purposes. It is used in line to AC feed to transformer, which provides sort of slow start to the transformer, and I found out sometimes eliminates the mechanical broom of transformer. He also use it to separate starpoint ground from chassis ground.
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Old 4th November 2010, 06:19 PM   #412
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by Backbones View Post
........ can cause part of the casing to ride up, away from the heatsink.

The other advantage to the clip is that it rests against the plastic and shouldn't need electrically insulating, whereas a conventional screw would need an insulating collar.
that is partly true
but mostly related to the medium power driver type devices

plastic casing high power devices are cast with built in insulation for the mounting screw
a thick big planed washer will help some on the problem with the offset position of the hole
and a tooth spring/lock washer
I dont know how much the ordinary scew mount will do for heat transfer from device casing to heatsink

but sure, some of the thicker and softie insulators could play tricks that are not so obvious
with the screw hole close to top of device, a softy insulator could be unevenly pressured, theoretically

but there are other ways to place the pressure in the middle
question is what happens to the casing
the device casing is reinforced around hole, and is meant to have the load pressure there
on the other hand, edges are probably equally strong
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Old 5th November 2010, 12:56 PM   #413
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Just received a Mouser delivery containing Thermasil insulators which I think I'll use so I don't need to use any heat transfer compound, rather than the mica insulators.
The Max Clips have also arrived and look fine but at what point on the heatsink to attach them? The heatsinks I'm looking at are 16.5cmX20.0cm so is the ideal position in the middle or doesn't it really matter if they conduct efficiently? I was going to place the Mosfets near the bottom and attach the pcb to the base plate which seemed a logical approach.
I see I've ordered the wrong thermistors which are 5A and 5ohm rather than 10. Twas ever thus for us manufacturers!
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Old 5th November 2010, 03:14 PM   #414
AndrewT is offline AndrewT  Scotland
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try to find heatsink manufacturers' data or info sheet on optimum locations.
For a single row across the width the height from the bottom to the centres of the interface should be ~40% of the heatsink height.
If you have 2devices then locate them at 25% in from each edge.
If you have 3devices then the middle is 50% in from the edge and the outers are at 17% from each edge.
For 4devices the spacing is decided by dividing the heatsink into 4 equal vertical slices and placing each device in the middle of it's slice, i.e. @ 12.5% & 37.5% from the edges.
This rule also fits the earlier examples and for many devices in a single row.

If you have two horizontal rows then the lower row should be ~20% up from the bottom, the upper row ~60% up from the bottom.
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Old 5th November 2010, 05:04 PM   #415
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Andrew,
Thanks. Nothing is quite as simple as it seems!
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Chris
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Old 8th November 2010, 11:28 PM   #416
JimM is offline JimM  United States
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Default BA2 cascode front end

Well the new cacode boards are back and built up (new Layout) but with 55volts +/- the dissipation of R208 and R209 is around 1 watt which is hotter than I like. When the output boards get hooked up and biased the supply will load down to around 50 to 52 volts. My question is can I double the value of R208 and R209 to around 1500 ohm each? This will keep the current approx. the same as original design. I don't quite understand the boot strap function of R208 and R209 so I am not sure what the results of the change will do. Any help would be appreciated.
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Old 10th November 2010, 11:01 PM   #417
JimM is offline JimM  United States
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Default BA2 cascode front end

Changed resistors R208 and R209 as per post 416 and increased the size of heat sink on Q203, everything seems to running ok on bench test.The Amp can swing 80 volts (peak to peak) into 4 ohms load. With out any visiable distortion on scope. Will run bias around 150ma per fet as the heat sink got a little to hot at 250ma, so it looks like 150 to 170 ma is about all the sinks can handle safetly. Now to get the stuff into the chassis will post pictures when get it in the box. By the way the amp will swing 92 volts (peak to peak ) into 10 ohms.
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Old 11th November 2010, 04:19 PM   #418
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Have you increased R208, 209 because you've used a higher voltage from the power supply?

Components from Jack have arrived safely so all I need are the pcbs and a bit of get up and go.

I have some 10mH chokes here (wire wound ferrite). Does anyone have a view on using one of these instead of the four parallel R1 to R4 resistors on the power supply.

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Chris
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Old 11th November 2010, 06:54 PM   #419
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Old 12th November 2010, 12:13 AM   #420
JimM is offline JimM  United States
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Yes with the 750 ohm and +/- 55 volt supply (no load) the resistors were pushing 1.0 watt (calculated) even though they were 3 watt resistors they were getting hotter than I preferred. Also the IRF 610 driver was pushing around 1.5 watts, which was also more than I liked. (Even though this is a burning Amp).
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