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Basic AC -> DC Converter

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cabe

New Member
I've been trying to find information on building an AC to DC converter for quite some time now, as I need to power a vapor phase refrigeration unit.

I'm trying to build a converter that would provide me with up to 30 amps of current at 12v.

Any help would be greatly appritiated, just point me in the right direction! :)
 

Scubasteve

New Member
Hey,

It sounds like you need a lot of power! You will need a tranformer to bring your voltage down to ~14-15Vac. This transformer will of course need to supply 30+ amps. You will need to find a very beefy transformer (420VA!).. It makes me feel like a wimp because I am using 20VA transformers in a design I am working on right now.

Then, you simply get a full-wave bridge rectifier that can take at least 30A, which are easily available. You need a few 0.1 and 0.01uF caps across the output with the addition of several large paralleled caps to keep your ripple down. I forget the formula for ripple in this application, I am sure someone will help.

This sort of supply is going to cost around 300$-500$ to build, just because of its high power nature. You may want to check out surplus, but you might want to avoid it if you want to make it reliable..

Steve
 

daviddoria

New Member
all you need is 4 beefy diodes in a square shape to take the sine wave from AC and chop it up into a negative and a positive (DC)

if you've never seen that i'll draw you a picture.

i'm not sure what the output of a wall socket is, but shouldn't it multiply the amperage by 4 in the transformer because we are taking the voltage down so much?

i would just try a 4:1 transformer followed by a normal DC rectifier (its even easier to just buy one of these instead of doing the 4 diode thing, they sell them already together in a nice little ceramic cube :))
 

cabe

New Member
The thing is i think the compressor only needs 30 amps to start. I have looked here:

http://www.ecokitchen.com/shop/refrigerator/specs.html

and the section on current draw for DC compressors makes me think that my compressor needs a lot less current while operating compared to starting. I think i need 30 amps to start it because of an air compressor (albeit not the same thing, but closest i could find) I have seen here:

http://www.streetbeatcustoms.com/aircom29100d.html

Now I have pretty much figured that I need a peak of 30 amps to start this thing, and 20 amps to run it continuously (it's a 1/3HP compressor, so 20 amps at 12v?).


Hey Scubasteve, were you ever at the UHLE forums? I seem to remember someone with your handle. ..maybe it's just a popular name.
 

cabe

New Member
daviddoria said:
all you need is 4 beefy diodes in a square shape to take the sine wave from AC and chop it up into a negative and a positive (DC)

if you've never seen that i'll draw you a picture.

i'm not sure what the output of a wall socket is, but shouldn't it multiply the amperage by 4 in the transformer because we are taking the voltage down so much?

i would just try a 4:1 transformer followed by a normal DC rectifier (its even easier to just buy one of these instead of doing the 4 diode thing, they sell them already together in a nice little ceramic cube :))
I'd really appritiate it if you'd draw me a picture. Really, thanks for all the help! :)

So you say I just need to get a DC rectifier and that will convert the AC signal to negative and positive?
 

Gene

New Member
Maybe we can be a little clearer.

1. First you need to convert your house voltage (120 volts in the US) down to the voltage your project requires. This means a transformer. The transformer will not only reduce your house voltage to 12 volts but it must be capable of supplying the current (20 - 30 Amps) your project needs. Transformers of this current level are usually very expensive.

2. Next you will need to convert the transformer output from AC to DC voltage. This will require 1-4 rectifiers or a single bridge rectifier. These are cheap and easy to find.

3. The rest depends on your project.
a. A voltage regulator can keep the voltage right at 12 volts.
b. A filter circuit will provide smooth voltage. It's easy to build and parts should be no problem. Maybe you don't need smooth voltage - don't know from your description.

For solid state electronics, we use all of these elements. For your project, the first two may sufice - hard to tell from what you have written. I recommend you find a source for the transformer first.

As you may have guessed, the 30 Amp requirement is the kicker.
 

Scubasteve

New Member
Whoa, big mistake!

You didn't read the chart correctly, it said "Daily Useage" and gave a value of ampere hours for the particular appliance. It looks like a continuous draw of 1.5A, which is a heck of a lot cheaper to produce!

Daily useage of 30Ah.. If it said hourly useage of 30Ah, then you would need to produce 30A..

Steve
 

Phasor

Member
appliance. It looks like a continuous draw of 1.5A, which is a heck of a lot cheaper to produce! Daily useage of 30Ah.. If it said hourly useage of 30Ah, then you would need to produce 30A..
You are partly right, but bear in mind though, that the compressor will not be running continuously. If it runs at (for example) 25% duty cycle, the current draw would be (30Ah / 24h) / 0.25 = 5A.

Perhaps a switching power supply would be less expensive? Not sure how well they handle inductive loads, though. Can anyone comment on that?
 

Phasor

Member
I would like to correct myself... after looking at the electrical specs...


Current Draw for DC Compressors

12 volt DC 24 volt DC
Running current for single compressor 4-5 amps 2-2.5 amps
Starting current for single compressor 15 amps 7.5 amps
Maximum current draw, one compressor starting and one already running 20 amps 10 amps
Note: The wire gauge should be sufficiently heavy to carry 20 amps with no more than a 1/2 volt drop.
This info was taken from http://www.ecokitchen.com/shop/refrigerator/specs.html#electrical
The PSU needs to supply 15A. 8)
 

cabe

New Member
Well I can tell you that I have connected the vapor phase refrigeration unit to a spare computer PSU (Antec TruePower 430, 20 amps at 12v) and the compressor wouldn't start up. That, along with what I found that the 1/3 HP air compressor mentioned 30amps and 12 volts, makes me think this thing is going to need 30 amps to start up, but significantly less to run.

And thanks for clearing it up for me Gene, that helps. :)

I'm just wondering what kind of circuitry was in the original refrigerator. That ecokitchen site also mentions 24v power, which would cut my amperage requirements in half. I may look into that, would be a heck of alot cheaper for a transformer.
 

Mosfet

New Member
My fridge don't run continuously,
it seldom runs more than once an hour.
When it does, the cycle is 5 minutes or so.
Can we assume your need is not continuous?
Buy a battery and charger.
 

Phasor

Member
Buy a battery and charger
A 300VA, 12V-0-12V toroidal transformer costs (retail) AUD$83.95. An average car battery also costs about $80.

Six of one, half a dozen of the other...

Of course, you could use a solar cell to charge the battery, and cut your electricity costs :) Then again, at 12c/kWh (I don't know how much it is in the USA), and 0.38kWh/day, trying to save 4.6c/day isn't really worth it.
 

Mosfet

New Member
I worry about a relative newcomer building a 30 amp supply.
I would include a soft start circuit.
If you just 'bang' turn it on the diodes won't last very long.
I imagine the lights will dim a bit also.

Those Sun Frost fridges were made for the 'outback' types.
Those people beyond the power grid.
You could sell it to one of them and buy a normal fridge with a nice profit :!:
 

cabe

New Member
See the thing is I don't have one of those fridges. What I've got is a vapor phase refrigeration unit pulled from an old '70s refrigerator. It says:

WR7X
138
1/3HP
3ARR20
A300

on the side.

Unfortunately the circuitry from the original refrigerator is unavaliable. :(

The closest thing I've been able to find as to what kind of power to supply to the compressor is 12v 30amps to get it to start. I guess I could also try 24v and 15 amps.

And actually, in my application I'll more than likely be needing to have the compressor running for several hours at a time. ..You'll think I'm crazy if I tell you what I'm doing..
 
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