Another MoFo

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Nick
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#106 Re: Another MoFo

Post by Nick »

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.
Thats just going to make a power supply with a 2 ohm source impedance. (ignoring the extra heat you will have to deal with)
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#107 Re: Another MoFo

Post by Ray P »

For mounting the Mosfet you could use two larger thread holes, one either side, and clamp the Mosfet in place with a shorth length of U channel; with your PCB you would need to have the Mosfet on short leads though, but then maybe leads with a connector would be useful if you plan to kill a few Mosfets?
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pre65
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#108 Re: Another MoFo

Post by pre65 »

Nick wrote: Thu Jul 02, 2020 12:43 pm
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.
Thats just going to make a power supply with a 2 ohm source impedance. (ignoring the extra heat you will have to deal with)
Does the position of the resistor make a difference ?

I was going to put it between 1st cap and 2nd choke.
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pre65
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#109 Re: Another MoFo

Post by pre65 »

Ray P wrote: Thu Jul 02, 2020 12:49 pm For mounting the Mosfet you could use two larger thread holes, one either side, and clamp the Mosfet in place with a shorth length of U channel; with your PCB you would need to have the Mosfet on short leads though, but then maybe leads with a connector would be useful if you plan to kill a few Mosfets?
Could be a plan B. :)

I did a Rapid order yesterday cos I'd run out of red 28/0.2 wire, and I was going to get 2 more mosfets, but Rapid don't stock Irfp 250. :?
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#110 Re: Another MoFo

Post by Nick »

pre65 wrote: Thu Jul 02, 2020 12:52 pm
Nick wrote: Thu Jul 02, 2020 12:43 pm
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.
Thats just going to make a power supply with a 2 ohm source impedance. (ignoring the extra heat you will have to deal with)
Does the position of the resistor make a difference ?

I was going to put it between 1st cap and 2nd choke.
I assume you mean second cap. It will be better. Depends on the size of the caps
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#111 Re: Another MoFo

Post by Nick »

Ray P wrote: Thu Jul 02, 2020 12:49 pm For mounting the Mosfet you could use two larger thread holes, one either side, and clamp the Mosfet in place with a shorth length of U channel; with your PCB you would need to have the Mosfet on short leads though, but then maybe leads with a connector would be useful if you plan to kill a few Mosfets?
If you kill the mosfet, just cut the leads and solder the new ones to the only lead end. I would keep all leads as short as possible.
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#112 Re: Another MoFo

Post by pre65 »

Nick wrote: Thu Jul 02, 2020 1:23 pm
Does the position of the resistor make a difference ?

I was going to put it between 1st cap and 2nd choke.
I assume you mean second cap. It will be better. Depends on the size of the caps
[/quote]

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#113 Re: Another MoFo

Post by Nick »

Well the good news is that that will work as a reservoir, but the bad thing is it will take some time to charge back up through the resistor if you pull a peak from it. But unlikely with your music collection :-)
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#114 Re: Another MoFo

Post by pre65 »

Nick wrote: Thu Jul 02, 2020 1:30 pm But unlikely with your music collection :-)
Nice one. :lol:
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#115 Re: Another MoFo

Post by Ray P »

Nick wrote: Thu Jul 02, 2020 1:25 pm I would keep all leads as short as possible.
Agree, was only suggesting an extension long enough to get the Mosfet from being hidden under the board. As I understand this to be a 'prototype' I thought a bit of flexibility might be handy - can then go an optimal assembly on the proper build.
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#116 Re: Another MoFo

Post by pre65 »

I've decided to stick with the 22V toroids for the MoFo.

The new ones, 28.7V will be saved for a future project.

I know it makes sense. :)
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#117 Re: Another MoFo

Post by pre65 »

According to the IRFP250 data sheet it has "an isolated central mounting hole".

The fixing data says M3, but the hole is bigger, big enough for an insulating top hat washer (for M3) but not big enough for M4.

I thought the first part meant it didn't need an insulating washer between it, and the heatsink but the fixing hole size makes me wonder.

Shall I just use an insulator (and a smidge of paste) and be safe ?
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#118 Re: Another MoFo

Post by Nick »

Still need the insulator and paste if the insulator requires it. The isolated hole means you don't need to countersink the hole as you would with a to220 device to isolate around the top hat washer sides.
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#119 Re: Another MoFo

Post by Nick »

Of course you also don't need the top hat washer. But use as large a flat washer as possible to spread the load.
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#120 Re: Another MoFo

Post by pre65 »

Nick wrote: Thu Jul 02, 2020 4:01 pm Of course you also don't need the top hat washer. But use as large a flat washer as possible to spread the load.
I seem to remember reading about using a coin as the flat washer, a 1p would do.

Mind you, I could be imprisoned for defacing a coin of the realm.
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