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building power supply

galyndavis

Supporting Member
Apr 27, 2005
16
1
11
How much cap per one amp.....seems like 2mf for every amp....
it is a unregulated power supply i am building out of two matching transformers of seventy amps each at 14 volts......so i need how much cap for this project of mine. 140 amps I hope will power eight 2879"s. best i can remember the volts will go up a little when changing to dc.

I have this type of power supply built in my dave made 2x4 base amp. It has worked out great.

new info: I listened to bob and put 12 23000 caps in. When I FIRE it up will post again.

Using twin (my dave made uses three smaller ones)transformers at 75amps ac each. I also have the option of 220 primary that will do even more and taps to dial it in just right. I want 20 volts or so because it being unregulated it will drop 3-5volts at key up.
something else to consider is your house wiring. I ran a 12-2 from my breaker box to my dave made 2x4 and that amp came alive. If you shack is far from the box go to 10 gauge. all I use this outlet is my amp. I will always put an voltage meter on my base amps power supply from now on. I can.t see why they charge about a dollar a watt for most powersupplys. I took two old rack mount units and stripped them .....bolting and rivetting them together. all the dc runs in parallel. the ac on my rectifers is two groups ...each trans has four of them per transformer in parallel. the caps are just parallel at the end of the circuit and guess what......dc.......oh yea i put a 100disk cap and a reisistor across each rectifers two ac connectors(this keeps the ac slightly loaded. the only thing not in parallel is the recitfires are in two groups not letting the seconary ac mix from each transformer before it becomes dc.( bob if you read this you can explain it better than I)....heat paste and bolted to a large alum. shelf in my case.......we may make fire but my son is a fireman.

I found several transformer places......this is where you save your money. I hope the only battery I have to look at is in my car....not under my bench. sorry about my rambling...i am tired and will edit it later. I am going to share everything like this to anyone that wants it. I had to dig and dig and ask questions. out of here.............................davis

once again thanks for helping

davis
 

galyndavis said:
how much cap per one amp.....seems like 2mf for every amp....
it is a unregulated power supply i am building out of two matching transformers of seventy amps each at 14 volts......so i need how much cap for this project of mine. 140 amps I hope will power eight 2879"s. best i can remember the volts will go up a little when changing to dc.

I have this type of power supply built in my dave made 2x4 base amp. It has worked out great.


once again thanks for helping

davis

I don't think 140 amps is going to power (8) 2879's.
 
8x2879=1000 watts out so about 2000 watts INPUT or about 145 amps at 13.8 volts just for the amp alone. Sounds a little light to me without a battery bank and a motor maul.
 
QRN said:
8x2879=1000 watts out so about 2000 watts INPUT or about 145 amps at 13.8 volts just for the amp alone. Sounds a little light to me without a battery bank and a motor maul.

:p
 
linearone said:
are you basing that on like a 50% eff. rating?

Well with 2000 watts INPUT for 1000 watts OUTPUT it sure sounds like it. :p I know, I know,this is CB so I should forget all about operating the transistors class AB and within spec as they should be right? :LOL:
 
not sure if it will help you or not, but my pyramid PS52k 50 amp power supply uses 50,000mfd of capacitance for filtering.
five 10,000uF caps mounted on a PC board in parallel.

i think its a bit chincy and want to add a bit, but havent done it yet.

hope that helps,
loosecannon
 
As bob stated, 2,000uF per amp is a general rule of thumb for power supplies. As to the amperage, If you are putting a maul out there, 140 amps just ain't going to cut it. I have had guys DK 1600+ and drawing over 180 amps on 12.5 volts. I use a general rule of thumb, if you are running (8) 2879's rated @ 100 watts each. Then that would give you 800 watts. DK half of that and see were she goes. But it's your box and you do as you like with it. By the way if those 2 70 amp transformers happen to be the same as the ones that RF parts sells, you are in for a rude awakening. They are rated at 70 amps peak, but only 35 amps continously. Having said that, you would need to double it in order to give those transformers some breath room. Another word, about those 50Amp rectifiers you are probably going to use. Use at least 2 but 3 is better. Especially when you first turn on power supply> Those rectifiers get a heck of a surge. and have been known to blow. RFparts also sells a part, can't remember part #, but it will help to decrease that first input spike when everything first starts up.

Crusher
 
bob85 said:
the old rule of thumb is 2000uf per amp as a good compromise :)

Bob do you think it would be better to check the ac ripple or does the 2000 uf per amp take that into consideration?
 
2000uf per amp is acceptable as a compromise for regulated power supplies but not sure about unregulated,
i think that should give a peak to peak ripple voltage of about 3.5v again not sure its been a long time since i built a power supply based on a design published in radcom in the 80's,
i can tell ya that the rm base amps use less than half that and still work in a fashion though they do have a habit of blowing the 25amp rectifier when they are turned on with our 240v mains,
some sort of inrush limiter /softstart is the way to go and rectifiers rated at least twice or more than max current of the power supply imho.
 

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