Another MoFo

[pre65]

But the PCB will be on the heatsink, so all you will need to do is unscrew the heatsink from the mounting bracket to get at the pot. Hardly much of a hardship.

Possibly not, although the heatsink fixing down planning has yet to be done.

Now that this has been discussed, it helps me in how to orientate the pcb and thinking aloud, I could make up a pair of hinges to tilt the heatsink up sideways.

[ed]

But the PCB will be on the heatsink, so all you will need to do is unscrew the heatsink from the mounting bracket to get at the pot. Hardly much of a hardship.

It may be a timely time to point out that the bias needs setting at running temperature....

If I'm reading this correctly you're suggesting Phil manhandle the heatsink whilst hot......cunning! snigger

[pre65]

Ah yes, but.

If I can implement a hinge arrangement, then I can lift up the heatsink when cold, then switch on and let it heat up before checking the bias.

I like a challenge.

Anyway, I'm 8.375 steps ahead of myself at the moment.

HFC bits were posted today.

[pre65]

No post for me today.

After all my efforts to get "better" transformers, it might have been in vain (at least for the time being).

I wired up the breadboard power supply, and using an 11R load (2 X 22R in parallel) i got 23.8V. Which is not bad because (initially) 24V was my target.I only left it on for a few minutes 'cos the heat rating was marginal.

The toroid is 2 secondaries 22V 125VA each in parallel.

[Nick]

Fit the resistors to the heatsink and it will give an idea of how well they will work.

[pre65]

Fit the resistors to the heatsink and it will give an idea of how well they will work.

That is such a good idea.

On test now.

[pre65]

After 30 minutes it's 38 degrees C.

Certainly not hot.

[Ray P]

After 30 minutes it's 38 degrees C.

Certainly not hot.

So you should be able to get an estimate of the heatsinks spec; you know the starting temperature, you know what it got to and you can work out the heat being dissipated from the resistors.

[pre65]

It's warmer now, my thermometer won't measure it, but I can hold my hand on it without discomfort.

[Nick]

you know the starting temperature

That doesn't matter. All you need is the temperature it gets to as a steady state, the room temperature at that time and the power being dissipated by the resistor.

[Ray P]

the room temperature at that time

That's what I meant by the starting temperature.

I'll take 100 lines:

I must be more specific.
I must be more specific.
I must be more specific.
...

[Nick]

That's what I meant by the starting temperature.

I suspected you did, but its likely that the thing will heat the room up as well as itself.

[pre65]

Having drilled a 2.5mm hole, and tapped it M3, I now have a mounting point on the heatsink for the IRFP250.

This has enabled me to think how to mount the PCB. If the heatsink is to be hinged (to access the bias pot) then the PCB has to be mounted on the heatsink, so I measured up and have ordered some M3 x 4mm thick plastic spacers. Once they arrive I can position the PCB accurately and mark out the fixing holes.

As drilling and tapping M3 is a faff I may just use two fixings instead of four.

Then I can populate the PCB as all the parts are here now.

I might try the 28.7v transformer as a test, for I found two 2.2R 100W Arcol resistors in the spares box and at 2.5A they would dissipate 13.75W.

One of the transformers I used on test 1 is powering my chip amp, and as I'm using that in my main system at the moment it's expedient to at least try the new transformers.




*

[Nick]

As drilling and tapping M3 is a faff I may just use two fixings instead of four.

I would use all four holes otherwise you may stress the insulation under the mosfet.

[pre65]

As drilling and tapping M3 is a faff I may just use two fixings instead of four.

I would use all four holes otherwise you may stress the insulation under the mosfet.

Noted.