Ebay bargains ?

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pre65
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Ebay bargains ?

Post by pre65 » Tue Jun 30, 2020 11:40 am

I had been looking on Ebay for toroidal transformers for the MoFo. The ones I have were purchased for a stillborn project years ago and are not ideal, but would have done to get the prototype up and running.

So, whilst searching for 24v or 30v secondarys I came across this gem.

300VA - single 230V primary - single 28.7V secondary and £16 + post.

I've ordered two, and when they get delivered I will have the sellers contact details, which I can pass on if not wanting to use Ebay. The 30VA in the advert is a typo.

https://www.ebay.co.uk/itm/Toroidal-Tra ... 1438.l2649
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Ray P
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Re: Ebay bargains ?

Post by Ray P » Tue Jun 30, 2020 12:19 pm

You're planning to be running a pretty 'hot' MoFo then Phil?

The prototye used 19VDC from laptop bricks?

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pre65
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Re: Ebay bargains ?

Post by pre65 » Tue Jun 30, 2020 12:25 pm

Ray P wrote:
Tue Jun 30, 2020 12:19 pm
You're planning to be running a pretty 'hot' MoFo then Phil?

The prototype used 19VDC from laptop bricks?
Some like it hot. :lol:

But, using the MOT with it's higher DCR means more volts are needed.
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Re: Ebay bargains ?

Post by Nick » Tue Jun 30, 2020 12:55 pm

28.7v = 40v.

Say 1.5A, thats 3v across the choke, so 37 * 1.5 or 55W

0.55 C/W heatsink (guess), 25C ambiant = 55C,

Assuming IRFP250, junction temp = 25 + ( 0.55 + 0.24 + 0.65 ) * 55 = 100C so less than the max 150C

1.5 W/C derated so derate 45W from 190W leaves 145W

So all should be ok.

However, I am not sure what higher voltage is going to give you. say 18W peak out into 8R so a voltage of 12v, so maybe 20v from the supply that's never going to be used other than heating the room. To make use of the higher voltage you will need higher current so then the power dissipation starts to get silly.
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Re: Ebay bargains ?

Post by pre65 » Tue Jun 30, 2020 1:42 pm

Nick wrote:
Tue Jun 30, 2020 12:55 pm
28.7v = 40v.

Say 1.5A, thats 3v across the choke, so 37 * 1.5 or 55W

0.55 C/W heatsink (guess), 25C ambiant = 55C,

Assuming IRFP250, junction temp = 25 + ( 0.55 + 0.24 + 0.65 ) * 55 = 100C so less than the max 150C

1.5 W/C derated so derate 45W from 190W leaves 145W

So all should be ok.

However, I am not sure what higher voltage is going to give you. say 18W peak out into 8R so a voltage of 12v, so maybe 20v from the supply that's never going to be used other than heating the room. To make use of the higher voltage you will need higher current so then the power dissipation starts to get silly.
Thanks Nick, I was thinking of starting at around 2.5A ish, so 5.5V across the choke.

I'll wire up the prototype power supply and see what I get with a dummy load using the transformer I have now.
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Re: Ebay bargains ?

Post by Paul Barker » Tue Jun 30, 2020 1:57 pm

Ray P wrote:
Tue Jun 30, 2020 12:19 pm
You're planning to be running a pretty 'hot' MoFo then Phil?

The prototye used 19VDC from laptop bricks?

99% of torroids are unwindsble regards secondaries low voltage are open not core filled with resin. I do it all the time to get exact voltage.
Depends on core size how many tpv start assuming 30 tpv and adjust on test.
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Re: Ebay bargains ?

Post by Nick » Tue Jun 30, 2020 2:21 pm

Thanks Nick, I was thinking of starting at around 2.5A ish, so 5.5V across the choke.
Ok...

So assume 40v from the power supply as before. 5V across the choke, thats 35v across the mosfet. So 35 * 2.5 = 88W

So assume the 0.55 C/W heatsink as before, and 25C room temperature, thats a heatsink temp of 73C.

Junction temp = 25 + ( 0.55 + 0.24 + 0.65 ) * 88 = 152C so you have a dead device as you have exceeded the chip temp.

See how the junction temp needs to come into it ed? No so pedantic now :-)

You may get away with it with a bigger heatsink than I have used, but its still a bit close. 2.5A into 8R gives you 20v, add 5v for the choke and 5 across the device. You still would only want 30v rails. The other 10v is just creating heat.
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Re: Ebay bargains ?

Post by steve s » Tue Jun 30, 2020 2:44 pm

I've just bought 4 s/h power supplies from ebay for £11 each, 24 v 6.5 amp, adjustable between 19 and 29 volts. They are made by mean well, and recommended on the DIY audio thread. I'll see if they better my laptop supplies..be interesting to compare the different types of supply Phil once yours is running
Looking forward to out next owston...
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Re: Ebay bargains ?

Post by Ray P » Tue Jun 30, 2020 3:00 pm

I've got a pair of single rail 'Smooth Like Butta' (SLB) cap multiplier power supply boards but aside with my own MoFo boards.

I will run it at around the same supply volts and curent as the original so something around 15VAC should be about right for my toroid. I thought I had a suitable one in the garage but when I took a break earlier I popped down to have a look. What I thought was a 250VA with two 0-15VAC secondary windings turns out to be two 15-0-15VAC windings plus another at 7VAC. Not only that but the label doesn't specify the current of the windings! Its a Toroidy so maybe if there's a reference I could contact them and see if they have it on record.

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Re: Ebay bargains ?

Post by Ray P » Tue Jun 30, 2020 3:06 pm

steve s wrote:
Tue Jun 30, 2020 2:44 pm
I've just bought 4 s/h power supplies from ebay for £11 each, 24 v 6.5 amp, adjustable between 19 and 29 volts. They are made by mean well, and recommended on the DIY audio thread.
I've got one like that too Steve, kept it back for testing when I finally try a MoFo. Will be interesting to see how you get on.

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Re: Ebay bargains ?

Post by Paul Barker » Tue Jun 30, 2020 3:16 pm

pre65 wrote:
Tue Jun 30, 2020 11:40 am
I had been looking on Ebay ....
300VA - single 230V primary - single 28.7V secondary and £16 + post.

I've ordered two, and when they get delivered I will have the sellers contact details, which I can pass on if not wanting to use Ebay. The 30VA in the advert is a typo.

https://www.ebay.co.uk/itm/Toroidal-Tra ... 1438.l2649
Thanks Phil I’ve ordered two for my GK 71 filaments.
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Re: Ebay bargains ?

Post by ed » Tue Jun 30, 2020 3:31 pm

Nick wrote:
Tue Jun 30, 2020 2:21 pm

See how the junction temp needs to come into it ed? No so pedantic now :-)
ner ner ne ner ner....

snigger

you're still a pedant though. more snigger

I do beleive you ..it would seem I've been misled in the past by empirical naivity.....
I'm intrigued by your figures though....
I'm looking at the sp400 datasheet and it states resistance at 0.45 degrees C per watt per square inch. Using that I was reckoning on something like 25 + (0.55 + 0.45) * 88..... = 113 degrees on the dye....

but the 0.45 is for 1 square inch and I always work at about 1/2 square inch...does that count?

I guess the question is where do you get the 0.24 and 0.65 from ?
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Re: Ebay bargains ?

Post by steve s » Tue Jun 30, 2020 3:38 pm

Ray P wrote:
Tue Jun 30, 2020 3:06 pm
steve s wrote:
Tue Jun 30, 2020 2:44 pm
I've just bought 4 s/h power supplies from ebay for £11 each, 24 v 6.5 amp, adjustable between 19 and 29 volts. They are made by mean well, and recommended on the DIY audio thread.
I've got one like that too Steve, kept it back for testing when I finally try a MoFo. Will be interesting to see how you get on.
Mines running on the laptop supplies at the moment Ray, I cant imagine how it could sound better than it does..
i'm going to build a 2nd amp once I get a few jobs sorted (decorating and a proper ish hi fi in the motorhome fitted, amongst other things)
I like have two to compare, should give me a good reference.

Last week I built an amp for the van, based on the tda2030, it sounded as good as any I've built before ( I've built about 8 or more with that chip so far this year) I've been on a mission to build a decent solid state job.

I compared it to the mofo, and got quite a surprise at the difference. Some may call them subtle differences, but the way overtones where reproduced was obviously superior on the mofo. Sweeter.. very much worth playing with, better in the bass too.
You set me off, selling me those boards ray..
The tube manual is quite like a telephone book. The number of it perfect. It is useful to make it possible to speak with a girl. But we can't see her beautiful face from the telephone number

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Re: Ebay bargains ?

Post by Nick » Tue Jun 30, 2020 3:53 pm

I guess the question is where do you get the 0.24 and 0.65 from ?
I wasn't sure what output device was in use, but I had seen mention of irfp250. From that datasheet

https://www.vishay.com/docs/91212/91212.pdf

Case to sink, Flat, Greased Surface : 0.24
Junction to case: 0.65
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Re: Ebay bargains ?

Post by Ray P » Tue Jun 30, 2020 4:36 pm

Nick (Jack) mentioned in the buying and selling section that he has a surplus of IRFP450 devices that he's selling for a pound or so each; I wonder if they might be suitable instead of the IRFP250s in the schematic.

Must have a look into it later...

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