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The Projects Forum Working on an electronics project and would like some suggestions, help or critiques? If you would like to comment or assist others with their projects, this is the place to do it.

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  #1  
Old 08-27-2012, 01:04 AM
pastinsain pastinsain is offline
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Default variac + rectifier + caps = dc power supply

Here is what I got

10amp 120 variac + 35amp 800v full wave rectifier + 450v 3500uf caps

Will this work as a 0-120v dc power supply?

will it provide a clean dc current?

if not , what else is needed in addition to the above.?

thank you for your opinion
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Old 08-27-2012, 01:30 AM
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ifixit ifixit is offline
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If it is going to be used as a lab supply you will need to add an isolation transformer to make it safe to use. I've had a variac supply for AC on my bench for decades. It has a 500VA isolation transformer feeding it. I likely owe my live to that isolation transformer many times over.

Discussions of non-isolated power supplies are not allowed on this forum.

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Ifixit
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Old 08-27-2012, 01:39 AM
pastinsain pastinsain is offline
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Default lets isolate it now

wonderful ! so I need an additional isolation transformer?

can you please recommend a forum that will discuss this circuit? "mabe a guitar amp forum"

This forum has an excellent topic on "isolation transformers in the ac section" I just read that.

I have repaired vintage 460v tube guitar amps for the past 10 years and are no stanger to high volt circiuts
however, I have never used an isolation transformers before .

I thank you for the heads up info! it could save my life next time I build a guitar amp.

Last edited by pastinsain; 08-27-2012 at 02:43 AM. Reason: clarity
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Old 08-27-2012, 02:48 AM
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You will need a fuse and then a switch on the input to the isolation transformer. A fuse on the output is wise as well. Make this a fuse that is easy to change from the front panel. The whole supply needs to be enclosed in a metal box with the variac shaft sticking through the front panel. You must have an indicator light to show when the power is on.

Since the variac doesn't have a center-tap (ct) you will need a bridge rectifier to get full wave rectification. Two ct full-wave rectifiers would do if wired as a bridge.

With 3500uF of filtering the DC output ripple will be dependant on how much current you intend to load it with.

What are your load requirements?
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  #5  
Old 08-27-2012, 03:08 AM
pastinsain pastinsain is offline
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Default load requirements

My variac is a 10 amp stacor so 10amp should do.

I just found my 120v trip lite lc 1200 regulator line conditioner.
its got a 10 amp reset breaker. will this work as a " isolated transformer"
it says isolated filter banks.

maybe I could plug the stacor variac into the trip lite conditioner and
have my cake and eat it to!
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Old 08-27-2012, 03:29 AM
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I don't think the tripp lite lc 1200 has isolation.

Regards,
Ifixit
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Old 08-27-2012, 04:28 AM
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If you insist on working on unisolated equipment, make sure you plug it into an outlet with a properly connected ground-fault interrupter. That should save you from getting a lethal shock between the power and earth ground (but you will still receive a good zap, as I've personally experienced after tripping a GFI by accidentally touching my finger to the hot prong of a main's plug).
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  #8  
Old 08-27-2012, 05:07 AM
pastinsain pastinsain is offline
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Default just purchased a huge isolation transformer

I think i'm ready now . sooo

the setup is the following:

Isolation transformer with switch ,on light and fuse. then comes the variac
with fuse , next full wave 35 amp 800v rectifier, then 2-450v 3500 capacitors total 7000uf.

this should make a 0-120v dc up to 10 amp.

a great all purpose ac-dc power supply

thank you all for the help. any more suggestions are welcomed!
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Old 08-27-2012, 05:46 AM
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Hmm !

Not bad. But you missed ballast resistors.
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Old 08-27-2012, 02:25 PM
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Connect as shown in the schematic.




Notes:
  1. F1 fuse, 10 Amp slow-blow or time delay type
  2. S1 switch 15A AC rating
  3. T1 isolation transformer, at least 1200VA rating
  4. T2 Variac, 10 Amp rating, however, 8 Amp continuious, 10 Amp short time only
  5. All parts above must be approved by your local electrical authority. IE: CSA, UL, ETA, etc.
  6. The bridge rectifier diodes, 400V, 15 Amp, (higher rating is better) heatsink may be required for heavy loads.
  7. R1 is the ballast resister that R!@ff suggested. It will limit the inrush current to the 7000uF if the unit should be powered up with the variac set to maximum... try to avoid that. I'm not sure if this is required since there may be enough resistance in the circuit to do the job. What is the surge rating for the diodes and the caps?
  8. Suggestion: Make F2 a cartridge fuse that can be easily changed from the front panel. F2 can be any value that approximately 50% more than the worst case expected DC load, upto 10 Amp maximum. IE if you are powering a circuit that is not going to need more than 0.8 Amp, then use a 1 Amp fuse for F2. That way if there is a problem (short) then the minimum amount of damage will be done
Anything else?

Good Luck,
Ifixit
Attached Images
File Type: jpg Variac DC ps.jpg (52.9 KB, 49 views)
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