PSU: Is this right?!

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Rarkov

Rarkov

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Joined 2002
garethbradley.co.uk
#1
Hi,
I have just modelled up a power supply in Duncans Power Supply Designer II. The circuit uses a choke directly after the rectifier. The amp is it providing has 5A (I think) rail fuses. I assume I need a choke that is rated at 5H and 5A?! These must be huge!

Can anyone tell me if this is right - and Where I can get them?
I can give more information / graphs on request...

Thanks,
Gaz

BTW - If these chokes don't work out - I will be using a high power potential divider!
 

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#4
Hi Gaz,

There are a couple things you need to check. As you found, a 5H choke is huge. 5mH sound more like it. The other item is that when I’ve used Duncans’ program in the past, I would set the load as a constant current draw. Your PSU is only providing 10mA to a 5kohm load.

Try it again with 5mH and 5A constant current.

Rodd Yamas***a
 
#5
Hi,
The amplifier is a Randy Slone designed MOSFET 140W-200W class B amp. It needs +-55V to operate at those ratings.

When I replace the resistive load with the 5A constant current load - the rectification goes out of the window! I can't make it look like a normal graph at all! Even when I change the choke to a more reasonable value!

Edit:
If I only pull 1 Amp and leave the choke at 5H - I get a response alot like a very underdamped impulse response with huge overshoot.

Gaz
 
#6
I don´t know about this PSU design programm but I agree that 5H is a little too big. 5mH is I think more logical as stated before.
Also I would split the capacitance in half and use 2 caps on either side of the coil in a pi configuration. It works better and is easier to find smaller caps.
 
#7
Hi,

The 13.6mF cap represents 2 6.8mF in parallel. Obviously 5H is a normal kind of value for Choke Input Supplies - but as it has been pointed out to me - That is for hundreds of voltages and mA!

I got quite good results from the Choke-Cap-Choke-Cap-Load configuration - when drawing a resitive load!

Gaz
 
#8
You might also want to change the diodes from 1 A types (which indeed would blow up used here) to something more suitable like a 35 A bridge or similar. If using the supply for Class B amp I would also not use a choke supply but normal C supply or possibly CRC.

/UrSv
 
#11
HBarske said:
Rarkov,
your schematic shows 30 Ohms for the 5H coil. You are going to lose 30 volts on your voltage rails for every ampere of current you draw.I can't imagine this is meant to be serious...
Click to expand...

It's my PSU skills that are the joke! Let me put this another way.

I have a 35A rectifier. The output of my Xformer-Recifier-Cap-Cap PSU is 70V. I want it lower than that (55V ish). I suggested to a friend that I would be using a potential divider and he suggested a choke. SS regulators have also been suggested.

What is peoples preferred method of changing voltages (without buying a new xformer)

Thanks,
Gaz
 
#12
Rarkov,

I am afraid the preferred method is to buy a new
transformer. They are expensive, but it will save you a lot of
trouble. Second to that, you may try to rewind the one you've
got, but that ís usually quite a lot of work.

Otherwise,
1) Linear voltage regulator. Will waste a lot of power and
dissipate a lot of heat.
2) Switching regulator. Much better efficiency, but difficult
to design and higher risk of failure.

My suggestion:
Get a new transformer. Keep the other one for future projects
or try to sell it to somebody building a bigger beast than you.
 
#13
OK, OK, You win!!!

Maplins have been kind enough to order in some 40V varients so the output voltage will be 56V. Because I have not soldered them - They will let me swap them, even though it is outside the 14day limit! Thanx guys!!!

I was getting worried anyway - because the caps are rated at 63V...One wrong wiring...!

Thanks alot lads! Great help! 😉

Gaz
 
#15
It's just as well that they did exhange them...they had to redeem themselves. When they delivered the xformers, caps, bridge rectifiers and switches etc. they put the box down somewhere. When I picked it up - the box looked like it had been thrown from a 10 floor high building! It crumpled in my hands!

I had a look at the capacitors and they were bugg**ed! They was dents and holes in the sides! I took them back and one lad said - they should be alright!!! 😉 Like I was going to build my first major PSU with dud / dangourous caps!!!

Anyway - they did change them in the end! 😉

Cya,
Gaz
 
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