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Old 22nd April 2013, 05:24 PM   #1
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Location: California
Default Variac set to "low" as soft-start for large transformer?

I have the parts to build a variable dual-rail DC power supply. These include:
800VA, dual 60V secondary transformer
100V caps, in sufficient amounts of capacitance
50A monolithic rectifiers
10A variac, 0-100% mains voltage range
fusing
digital DC voltmeter module

My general wiring plan is to connect the variac to the mains, the variac AC output to the transformer primary, then the secondary to the rectifier, and finally a CRC type cap bank (I can use up to 100,000uF total capacitance split between the two rails and CRC) with rail voltage monitoring via the digital meter.

My primary goal with this is to have something to power most any kind of amp board or chip amp that I might want to test and to have the capability to vary the voltage of the rails as needed/desired. With an 800VA transformer and lots of capacitance I should have a nice stiff power supply with low ripple.

So, my question is:
With the variac first in line, can I just dial the voltage ratio down to zero before turning on the mains power and then dial it up to whatever I need? Will this mean I don't need to worry about a soft start as long as I don't forget to start at zero each time?

Also, what's the best place in the circuit for the fusing?

Any other comments on this setup are welcome. It will be primarily testing of DIY amp kits that I build or buy.

Thanks,

-Charlie
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Old 22nd April 2013, 09:48 PM   #2
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Using the Variac this way will work fine, but you need to be sure to turn it down before you energize the circuit. (Actually, the inrush current may not trip a 20 amp breaker.) You should always put the fuse in series with the line before anything else.
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Old 23rd April 2013, 09:08 AM   #3
AndrewT is online now AndrewT  Scotland
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Charlie.
The Variac supplied transformer is great for a settable supply voltage and for a slow power up. I use it. I have a 100VA 12+12Vac wired permanently to the Variac output alongside the mains socket variable output.

But the bulb tester is far better at AUTOMATIC protection if there is a wiring error that tries to draw excessive current during start up.

The Variac does not give this automatic current limiting. You, the operator, has to monitor all the currents to ensure no single item gets overloaded due to a wiring error.
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Old 23rd April 2013, 12:52 PM   #4
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Quote:
Originally Posted by CharlieLaub View Post
I have the parts to build a variable dual-rail DC power supply. These include:
800VA, dual 60V secondary transformer
100V caps, in sufficient amounts of capacitance
50A monolithic rectifiers
10A variac, 0-100% mains voltage range
fusing
digital DC voltmeter module

My general wiring plan is to connect the variac to the mains, the variac AC output to the transformer primary, then the secondary to the rectifier, and finally a CRC type cap bank (I can use up to 100,000uF total capacitance split between the two rails and CRC) with rail voltage monitoring via the digital meter.

My primary goal with this is to have something to power most any kind of amp board or chip amp that I might want to test and to have the capability to vary the voltage of the rails as needed/desired. With an 800VA transformer and lots of capacitance I should have a nice stiff power supply with low ripple.

So, my question is:
With the variac first in line, can I just dial the voltage ratio down to zero before turning on the mains power and then dial it up to whatever I need? Will this mean I don't need to worry about a soft start as long as I don't forget to start at zero each time?

Also, what's the best place in the circuit for the fusing?

Any other comments on this setup are welcome. It will be primarily testing of DIY amp kits that I build or buy.

Thanks,

-Charlie
remember that the transformer will only be a 800VA pc at max voltage (2*60VAC or around +/-80VDC) at say +/-26-27VDC you will only have a 266VA transformer.
you will always be Limited 6.66A pr sec. nomatter what voltage you use.
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Old 23rd April 2013, 08:43 PM   #5
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Quote:
Originally Posted by sawreyrw View Post
Using the Variac this way will work fine, but you need to be sure to turn it down before you energize the circuit. (Actually, the inrush current may not trip a 20 amp breaker.) You should always put the fuse in series with the line before anything else.
I did some more reading, and while fusing first is a good idea the point was emphasized that an autoformer has the same current limit on the output. When the output voltage is only a fraction of the mains voltage, you can draw a LOT of current on the output before the input fuse would trip, meaning it is easy for a novice to overload the output (using more than the rated current).

The article strongly suggested fusing the autoformer's output at the same current as the input. This seems like a sensible idea to me, perhaps using a resettable fuse.

-Charlie
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Old 23rd April 2013, 09:23 PM   #6
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Charlie

I have been working (for quite a while now) on a similar "project" for my bench.

The difference is that my version includes a large isolation transformer that would allow me up to 220V symmetrical DC output and a PIC voltage/current monitoring panel.

So far I collected lots of components and have lots of scribbles... I have not "engaged" any breakers yet...
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Last edited by dudaindc; 23rd April 2013 at 09:32 PM.
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Old 25th April 2013, 10:45 AM   #7
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the variac AC output to the transformer primary, then the secondary to the rectifier, and finally a CRC type cap bank (I can use up to 100,000uF total capacitance split between the two rails and CRC) with rail voltage monitoring via the digital meter.
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