DC power supply

[Brainfarth]

I have a variac, 3300UF 200VDC cap and a 100v 10a bridge in my possession.  I want a super simple high-voltage DC power supply. I see that people recommend that an isolating transformer should be used... Can I wind a few opposing wraps around a steel rod to comply? Also.. What is a realistic cap and rectifier to use with this variac?

[danadak]

The peak value out of the bridge is 1.414 x RMS value of line. So for 120 VAC the
cap needs some margin, morfe like 300V.


http://www.zen22142.zen.co.uk/Design/dcpsu.htm


Size of the cap in uF and load determine ripple on output. Larger C = Lower Ripple.



For safety sake use an isolation transfer, buy one properly constructed. Or read up
and study transformer design, wire sizes, wire insulation, core size.....



Regards, Dana.

[Brumby]

I see that people recommend that an isolating transformer should be used... Can I wind a few opposing wraps around a steel rod to comply?

To be quite honest, this comment has me rather concerned.  There is a lot more to building an isolation transformer than "going through the motions" - which is what is suggested here.

When working with these voltages with even a half serious current capacity, you need to be a bit better informed than is indicated by the above.  Knowledge leads to safety.

[6PTsocket]

Along with the above mentioned problems, 100 volts is not high enough for a bridge running on 120 VAC. For that voltage, a 200 volt bridge is the minimum. The peak voltage of the AC  will be around 180 volts and you do not even have enough for the RMS or the average value.

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[jeroen79]

The  danger of not using an additional isolating transformer is that you may plug in the variac the wrong way.
Then one output terminal will always be at peak mains voltage even with the wiper turned 'down'.

[6PTsocket]

The  danger of not using an additional isolating transformer is that you may plug in the variac the wrong way.
Then one output terminal will always be at peak mains voltage even with the wiper turned 'down'.

A variac is an autoformer. There is only one winding. If you wired it backwards, you would have more serious problems than that. With the wiper turned down all the way there would be a dead short across the AC line.

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[Zero999]

The  danger of not using an additional isolating transformer is that you may plug in the variac the wrong way.
Then one output terminal will always be at peak mains voltage even with the wiper turned 'down'.

A variac is an autoformer. There is only one winding. If you wired it backwards, you would have more serious problems than that. With the wiper turned down all the way there would be a dead short across the AC line.

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That's not what he meant. He was talking about live and neutral reversal, which is often the case in countries with more lapse regulations, where non-polarised connectors are widely used.

Another issue is that if the winding breaks, then the output equal the full mains voltage, especially under light loads.

[IanMacdonald]

You can NOT use a bridge rectifier without an isolating transformer anyway. Both sides of the input have to be able to swing between the -ve and +ve output rails.

Grounding one AC input of a bridge rectifier can create quite a dangerous situation, because it may cause it to work as a voltage doubler, depending on the supply phasing. This is why those power tool "isolation" transformers which aren't actually isolated can be a serious hazard on the testbench. 

Not trying to dissuade you from learning, but acquiring a wooden overcoat is a very expensive way to get insulated.

[Gregg]

Your Variac is rated for 240 volts and because it is an auto transformer with only one winding, it will not work at 120 volts. 

[Cerebus]

I'm going to be another one that pitches in with please, please stop now and go no further. The basic lack of understanding evidenced by something like

Can I wind a few opposing wraps around a steel rod to comply?

indicates that you don't have a clue what you're going. Working with high-ish energy non-isolated mains voltage circuits when you don't fully understand what you're doing is a recipe for disaster that tends to end in things like fire or death. That's not an overstatement.

Full marks for coming and asking rather than just pressing ahead, but it's time to take a breath and reconsider. Perhaps if you tell us what your end goal is (beyond, obviously, a DC power supply) and what you intend to use if for, then we can offer some helpful advice on a way forward.

[6PTsocket]

You can NOT use a bridge rectifier without an isolating transformer anyway. Both sides of the input have to be able to swing between the -ve and +ve output rails.

Grounding one AC input of a bridge rectifier can create quite a dangerous situation, because it may cause it to work as a voltage doubler, depending on the supply phasing. This is why those power tool "isolation" transformers which aren't actually isolated can be a serious hazard on the testbench. 

Not trying to dissuade you from learning, but acquiring a wooden overcoat is a very expensive way to get insulated.

Cannot safely or cannot electrically? You sure as heck can connect a bridge to the AC line. For many years before flat screens, TVs did just that to save the cost of a power  transformer. Table radios were virtually all wired that way. Without a filament transformer, the tube filaments were all different voltages but the same current and wired in series. The voltages added up to the line voltage and went straight to it. Most of a switching power supply is before the transformer and on the "hot" side. They all depend on good INSULATION not ISOLATION. on an old table radio with an unpolarised plug you could get belted pulling off a knob and touching the shaft. It is the potential difference between two points. It is not necessary for both sides to change relative to earth ground to get AC.

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