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Convert UPS to DC Power Supply


SmackDown said:
I have an APC model SUA1500 UPS, (uninterruptable power supply). How difficult would it be to convert it to use as a power supply for a DC linear amplifier? The specs are here: APC Smart-UPS 1500VA USB & Serial 120V
Output at AC outlets is 1440VA, whatever that means lol lol lol. I'm hoping it can be done to save some stamps I can put to use on a big FET box.


3's
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You're kidding I hope. I read the specs and it states 120 volts output.
 
psycho said:
You're kidding I hope. I read the specs and it states 120 volts output.
Click to expand...

No, I'm serious. I want to take the part of the UPS that keeps the 12V battery charged and see if "it" can be modded into a DC supply of higher amperage. Maybe my verbage wasn't concise enough. I want to build a variable 5 > 25 V DC supply @ 300 Amps. I feel I have a start with the UPS but need to make some changes to bring what I want into fruition. Thanks for any info.


3's
 
AC or DC?

It can be done depending on what you need as the output.... I have used two CT step-down transformers, 120 in / 12 out, back to back with the use of steering diodes and 400 volt electrolytics to obtain 250 volt plate voltage for a home brew audio amp. This is not DC voltage. The center tap was used to power the filament. :bored:
About the only thing I can see using the UPS for is maybe the case.... The rest would have to be built up. The main transformer would have to be one with 25 volt secondaries.

73 mechanic
 
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Spend about $150 for a supply like a Diamond GZV4000 and get adjustable output 5 to 15 at 35 to 40 amps.
What you have has no capabilty to offer the current you want, in any form.
 
Is it the size of the transformer that will determine the DC amperage output? I understand having a transformer that has 25VDC so I can variable from 5 > 25V but how do I get the amps I want? How does the 12V battery in the UPS make 120VAC @ 1440VA at the output?
And yes it's a nice case. That why I want to utilize it. I feel like I'm getting some good input but don't understand why that xrsmr that keeps the 12V battery charged up has to have an amperage output of about 50 or 60 amps isn't usable. What I think I'm hearing is that isn't usable for my idea ??? ??? ???
Since I also have a second 12V battery I can always craigslist it but first I thought I'd toss it too the engineers here and maybe have a nice project for myself.


3's
 
that xrsmr that keeps the 12V battery charged up has to have an amperage output of about 50 or 60 amps isn't usable.
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That is the thing, I and others doubt that the Supply is capable of anything close to 50 or 60 amps, it takes that supply 3 hours to charge up a ( relativity ) small gel cell.
It is not that big of a supply.

How does the 12V battery in the UPS make 120VAC @ 1440VA at the output?
Click to expand...

It uses a DC to AC inverter.

And if you look at the charts supplied in the link you posted, it can only supply approx 900 watts for roughly 8 min, then the Battery will fall below the voltage needed to run the inverter.
Its not that big of a battery.

These devices are not made to "run" a system, they are more for Allowing you time to save your work in progress and safely shut down your system before your work is lost.
You would be better off with a large lead acid battery and a big commercial Automotive charger than to try to use it for a power supply.

73
Jeff
 
Not easy to put all this down on paper to illustrate all the relationships of power and amps. Several electrical laws come into play.... it comes down to the fact that you need to supply input to obtain wanted output!
The UPS you have is designed to charge a back-up battery and is more or less a trickle charger with an AC voltage sense that switches when there is a AC supply loss and converts the DC battery to AC.
Little of this unit will translate to a high amperage output DC supply with out big circuit design changes with many new parts.
Basic parts of what you need are in this order.... 120 or 240 AC volts in to a step down transformer of ~18 volts AC out, then rectification plus filtering to have 13 to 15 volts DC out, then usually several pass transistor to handle to current load of 10 to 40 amps. This would be for a 13.8 volt DC power supply. The transformer has to be capable of handling 10 to 40 amps or more and that means heavy and expensive!
Powering a linear amp or high current draw load you need lots of amps at needed voltage. This formula is a basic Ohm's law and relates volts and amps to load.... I = V / R
I is amps, V is volts and R is ohms (load)

Read up on this.... 73 mechanic :whistle:
 
okay Mechanic, and Jeff, in the inverse, if using ohms law, then R=V/I. And if the I is equal to 300a and the V is equal to 20vdc the R is equal to .05 ??? Am I looking at this correctly?
Okay if I wanted to build from scratch a power supply with variable dc voltage from 5>20v but delivering 300amps, what kind of transformer are we talking about in nominal value?


3's
 
SmackDown said:
okay Mechanic, and Jeff, in the inverse, if using ohms law, then R=V/I. And if the I is equal to 300a and the V is equal to 20vdc the R is equal to .05 ??? Am I looking at this correctly?
Okay if I wanted to build from scratch a power supply with variable dc voltage from 5>20v but delivering 300amps, what kind of transformer are we talking about in nominal value?


3's
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300 amps is a big draw at 20 volts DC.... .0666 ohms is the resistance! To have a transformer to handle this load it would have to be massive and made with large diameter copper windings on the secondaries! :blink:
This would be a transformer that would huge and expensive, most likely with 240 volt AC input. The electronics to convert this to DC would also be complex and expensive! :scared:
An AC to DC supply like this can be had but it would be heavy in "brute force" configuration and not cheap in "switch mode"!
"Shoot-outs" that are of a high power (watts) out need a big supply for the current draw. Usually high capacity alternators with large or numerous 12 volt batteries with big capacitors to smooth out the draw! Lots of heat is generated so you can see the inefficiency of these systems! :huh:
Vacuum tubes are the best way to go at high power needs, at least above 300 watts.

73 mechanic
 

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what kind of transformer are we talking about in nominal value?
Click to expand...

You are going to have to parallel several Transformers to get 300 Amps DC.
Most of the X/dave/fatboy Transistor Base Supply's are running 75 Amp Transformers.
You need 6 to get 300 amps, and it would be better if you use 7 or 8 to overbuild and give yourself headroom.
You do not have to over build....but I would rather have more than I need than pushing things.
At least 12 or 14 full wave bridge rectifier`s mounted on heat sinks.
A hand full of Big Filter caps to smooth things out.
Regulator circuity, Pass transistors, More heat-sinks, Fans to keep it all cool.
Oh and a 220 volt line in the shack to supply the primary's.

There is a reason that these guys are charging upwards of $1600 dollars for heavy 300 amp power supply's, and most of them are not even regulated!
They "float at about 18/19 volts and then pull down to 13 to 16 volts when loaded....depending on the size of the load.

Fat Boy Sells a 300 Amp Supply with a Big Iron Peter Dahl Transformer in it, runs on 220 Volts, is NOT regulated and draws around 25 Amps AC.
That is what they recommend to run a 16 pill and it cost`s over $1500 Bucks.

The " budget" 300 Amp supply with Stacked 75 Amp Transformers will only run you about $1000 bucks.
This is why you hit a break point with Bi polar transistors were it is much better $$$ wise to go with tubes.....or start looking at devices that run at much higher voltages.
If you are gonna spend $1500 bucks for a 16 pill mobile amp, and another $ $1600 on a supply, you could find a nice amp running a couple of 3-500Z`s or a 3cx1500 sitting on the bench and still have beer money.

73
Jeff
 
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