Dummy load

[ronv]

Do you have a link to the paste by any chance? Do you make a stencil or just use a needle to put the paste on?

 

[Doctore]

I add in #97 post this link, but I do this again
**broken link removed**
50g for only 4$/free ship
And I just use needle, what you can see in, again, post #97
The violet PCB was fabricated in OSHPark.com for only 5$/pc. from eagle file ;p

EDIT:
Paste is sold out(jocanon? ) so you have to wait...

 

[Doctore]

One more question, I can connect this DL to AC power supply, eg transformer?

 

[()blivion]

Nice work Doctore, you design those boards yourself? Or are they kits?

Man... wish I wasn't occupied with important life stuff, or I would stay and talk. I happened across a bunch of new rail-to-rail op amps for cheap and have been thinking about the P-MOS load ideas I had. Would love to get back on that once again. Maybe some day...

 

[Doctore]

Thanks ()blivion
You asking for these violet PCB's?
I get schematic from web, drawed PCB myself in EAGLE(done pcb was available with schematics but I wanted to make this better myself) and then PCB was fabricated in OSHpark.com(in USA) and then I ordered parts and soldered in oven, So this is not kit
If anyone want, I can send BOM(hex and others) and publicate this PCB in oshpark for easy order(this is easy like in ebay) < this is about uProg(this violet thing - this is pocket programmer for atmega uc).

 

[Doctore]

Hi everyone.
I have bad news
DL is burned...
I connected it to 50V PSU and set 30A for few seconds then magic smoke escaped from my DL.
Almost all 75R resistors are burmed and mosfets shorted, why??!!
SOA says about 8A at 50V, its 80A with 10 of them, I set only 30A for moment, wtf?
Anyone have any ideas what happens?

 

[ronv]

Hi everyone.
I have bad news
DL is burned...
I connected it to 50V PSU and set 30A for few seconds then magic smoke escaped from my DL.
Almost all 75R resistors are burmed and mosfets shorted, why??!!
SOA says about 8A at 50V, its 80A with 10 of them, I set only 30A for moment, wtf?
Anyone have any ideas what happens?

Ahh, That's to bad.
Hard to say what caused it. Maybe the supply you were testing went bad?
Haas it run for the last year ok?

 

[Doctore]

Thanks ronv for reply.
I checked today voltages on this PSU.
As always, you knowed it, yes, its faulty psu, there was 111V ripple on output(readed by multimeter set to AC).
Now I know that I also should check output with AC mode on MM...
<triple facepalm>

But I was thinking about new DL(uc controlled, 3' tft display etc), and I also found that vishay supply samples for everyone(max two orders for fake brands xd) so I can get mosfets for free.
Anyway Im not happy about it....

And of course your design was running all last time w/o any problems!

Also by making this DL, I learned something about cooling, and now im able to put 200W(mosfet case about 60C) per mosfet with standard cooling(heatsink+fan from 10yrs old PC CPU).
The key is soldering mosfet to piece of copper(35x35x4mm, salvaged from CPU thermal contact), and this module connected with heatsink by PC thermal grease.
By this way and 10 mosfets I can get even 2,5kW w/o water cooling
If you cant imagine what I mean, I can make photo of prototype silngle-mosfet, breadboard prototype.

In my dead DL I had soldered mosfets to copper pipe with 1mm wall which is too small, even with flowing 20C water inside pipe :|
So when I was reaching eg. 70C on mosfet case, there was 20C on other side of pipe...

 

[ronv]

Thanks ronv for reply.
I checked today voltages on this PSU.
As always, you knowed it, yes, its faulty psu, there was 111V ripple on output(readed by multimeter set to AC).
Now I know that I also should check output with AC mode on MM...
<triple facepalm>

But I was thinking about new DL(uc controlled, 3' tft display etc), and I also found that vishay supply samples for everyone(max two orders for fake brands xd) so I can get mosfets for free.
Anyway Im not happy about it....

And of course your design was running all last time w/o any problems!

Also by making this DL, I learned something about cooling, and now im able to put 200W(mosfet case about 60C) per mosfet with standard cooling(heatsink+fan from 10yrs old PC CPU).
The key is soldering mosfet to piece of copper(35x35x4mm, salvaged from CPU thermal contact), and this module connected with heatsink by PC thermal grease.
By this way and 10 mosfets I can get even 2,5kW w/o water cooling
If you cant imagine what I mean, I can make photo of prototype silngle-mosfet, breadboard prototype.

In my dead DL I had soldered mosfets to copper pipe with 1mm wall which is too small, even with flowing 20C water inside pipe :|
So when I was reaching eg. 70C on mosfet case, there was 20C on other side of pipe...

Ahh, good. Well not good, but better than not knowing why.
Yes, a picture would be nice. It's hard to cool so much power with air.
Good luck!

 

[Doctore]

Ok, there is it:



Yes I know, it looks messy, but its only for tests.
Tested with 15V/200W.
As u can see I used grey thermal grease its not that white, silicone crap, but good arctic cooling mx-2 grease.
To solder mosfet to this piece of pure copper I used standard alloy 60/40 paste, but it will be better(lower thermal resistance) to use "rohs" 99Sn paste, which I bought from china but that ******** send me dryed one, unusable -,-

 

[ronv]

Ok, there is it:
View attachment 94856

View attachment 94857
Yes I know, it looks messy, but its only for tests.
Tested with 15V/200W.
As u can see I used grey thermal grease its not that white, silicone crap, but good arctic cooling mx-2 grease.
To solder mosfet to this piece of pure copper I used standard alloy 60/40 paste, but it will be better(lower thermal resistance) to use "rohs" 99Sn paste, which I bought from china but that ******** send me dryed one, unusable -,-

So you would have 10 of those? Pretty big - but very dry.

 

[Doctore]

Big, noo, wy?
I could make case in same volume as my WC DL, at least twice bigger.
I can also make wc but with PC system, so heatsink with 3x120mm fans water pump and copper block, it should get 250W/mosfet w/o problem and to copper block we could solder 20 mosfets so 5kW in total.
But before making dl with pc wc I should make some calculations, but I cant find data for that wc heatsinks...

 

[Doctore]

Do you know my friend how to protect that circuits from AC and HV spikes?
Can I use schottky diode and double TVS + fuse on main load rail?
Any other ideas on protecting dl from bad psu's?
What about higher resistance 10W resistors(balancing) to drop more power on them?
For example 1R?

 

[ronv]

Big, noo, wy?
I could make case in same volume as my WC DL, at least twice bigger.
I can also make wc but with PC system, so heatsink with 3x120mm fans water pump and copper block, it should get 250W/mosfet w/o problem and to copper block we could solder 20 mosfets so 5kW in total.
But before making dl with pc wc I should make some calculations, but I cant find data for that wc heatsinks...

I have used 50 divided by the square root of the area in cm to do a rough estimate of a heat sink.
Here is an article that includes air flow.
**broken link removed**
For over-voltage google crowbar circuits and see what you think.

 

[Doctore]

Ok thanks for pdf, I will also google crowabar circuits.
But what about protecting from AC?
Does DL will not be damaged by AC?

 

[Doctore]

Oh, crowabar circuits have a schottky
But now, I need about 600A fuse and about 700A 1kV mosfet :O

 

[ronv]

Ok thanks for pdf, I will also google crowabar circuits.
But what about protecting from AC?
Does DL will not be damaged by AC?

Maybe like this.

https://www.radio-electronics.com/info/circuits/scr_overvoltage_crowbar/scr_over_voltage_crowbar.php

 

[ronv]

Oh, crowabar circuits have a schottky
But now, I need about 600A fuse and about 700A 1kV mosfet :O

Big SCR that turns on when the voltage is to high. Maybe a 50 amp fuse would be enough.

 

[Doctore]

But now, I need about 600A fuse and about 700A 1kV mosfet :O

Correction, not mosfet, thryistor.
I found this:
https://en.wikibooks.org/wiki/Practical_Electronics/Crowbar_circuit
It seems to be ok, and also protect from AC.

Im thinking about using my dl with up to about 500A, of course within SOA.

 

[Doctore]

What about two VS-25TTS12-M3?
Im not sure, but two of them should be able to handle 1000A short and break fuse easy.