enter image description hereI recently received a variac variable ac transformer. It is a older model so I decided to modify it a bit. I will add analogue amp and volt meters to show what the "current" out put is(pardon the pun) and add a bridge rectifier in parallel to the ac output so I also have a (diode and resistor stabilized) DC output. My question is if it is sufficient to have a automatic fuse(at the rated output amp) on the ac output from the variac before dividing it to go to the ac socket and the rectifier and dc socket (only one to be used at a time)or will the amperage change with the voltage level and requiring a dc fuse after the rectifier and at what amp rating?

I borrowed and modified this picture. Is this a good way to do this or is R1 not needed or even a mistake?

Update. I finished the variac build but did not have room for the dc side, I did run in to a problem however the automatic/resettable fuse I used in series with my output in order to protect the variac will not trip at (or near) the rated 2,5A (250 vac). I tested with a near short through a 1 ohm 100w resistor and the 0-3A meter went of the scale yet the fuse did not trip, any idea why?

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    \$\begingroup\$ My concern would be the "diode and resistor stabilized DC output". The purpose of a variac is to provide an adjustable AC output voltage. You can put that AC voltage through a bridge rectifier to get an adjustable DC output voltage, but just adding a Zener diode circuit is not going to stabilize it. What voltage is it going to stabilize it at? A zener diode has a fixed voltage, but you want an adjustable DC output. (At least I presume that you do). If you explain what you are trying to achieve with the DC output then perhaps we can help. \$\endgroup\$
    – Steve G
    Mar 10, 2016 at 23:46
  • \$\begingroup\$ I never said zener diode, it will be a full bridge rectifier. The dc voltage is not to stabilize at a fixed value but just be smoother with les ripple, but still adjustable. \$\endgroup\$
    – Anders_gbg
    Mar 10, 2016 at 23:50
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    \$\begingroup\$ if you don't want a stabilized output then what do you mean by "a (diode and resistor stabilized) DC output"? If you want less ripple you should add a capacitor. \$\endgroup\$
    – Steve G
    Mar 11, 2016 at 0:06
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    \$\begingroup\$ Ok, no problem. If you add a capacitor across the DC+ and DC- outputs in transistors circuit you will reduce the ripple. You will need to use a capacitor rated for 400V. It's value (capacitance) depends on the current you want to draw, and how much ripple you can accept. 22uF is probably a good starting point, since these are quite commonly used in switch mode power supplies. Be careful, because these are polarized capacitors and can connect only one way. The + connection goes to DC+, - to DC-. Get it wrong and it will explode! \$\endgroup\$
    – Steve G
    Mar 11, 2016 at 0:18
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    \$\begingroup\$ The article linked in the comments below my answer show how to calculate that. Basically, the more capacitance, the less ripple. Using the water analogy: with only the rectifier you have a pulsed water supply. If you add a reservoir capacitor it's like the water tank in your roof that holds charge between pulses - but the more current you draw the more ripple you have in the tank level. \$\endgroup\$
    – Transistor
    Mar 12, 2016 at 9:20

1 Answer 1



simulate this circuit – Schematic created using CircuitLab

Figure 1. Variac with AC and DC output.

You are building a power supply as shown in Figure 1. Everything in your question sounds right except for "resistor stabilized" (so you might want to clarify that).

F2, a fuse or a circuit breaker, but not auto-resetting will give good output protection. The rating should be less than or equal to the max permissible output current of the transformer.

  • Try and connect the "bottom" of the variac to the mains neutral. This ensures that the N output line is always close to ground potential (and eliminating one shock hazard) and when the variac is at a low setting the AC VAR (variable) line is also close to ground.
  • Note that the DC outputs jump around relative to ground. e.g., consider a 50 V peak-to-peak AC output: when AC VAR is at +50 V then DC+ will be at +49 V (through D2) and DC- will be at +1 V (through D3). When AC VAR is at -50 V then DC- will be at -49 V (through D4) and DC+ will be at -1 V (through D1). (For simplicity I've assumed that each diode drops 1 V.) The point is that both positive and negative terminals in this configuration are changing with respect to N and to earth. To learn more about the hazards for the unwary see Help learning from a mistake connecting an oscilloscope.
  • If you want to smooth out the DC then add capacitance of suitable voltage rating between DC+ and DC-, observing polarity. In this case the AC voltmeter will be inaccurate and a DC meter should be added between DC+ and DC-.


If the AC meter is an analogue type then it will be calibrated to give the RMS value of the sinusoidal voltage. The unsmoothed DC voltage will be the same (less the diode voltage drops) and should be good enough for general purpose experiments. You can always hook up a multimeter to get better resolution if you require it.

When you smooth the output with a DC meter the situation changes a bit because the capacitor can store up charge between pulses from the rectifier. The capacitor can charge up to \$ \sqrt 2\$ times the RMS voltage. (Plenty of articles explaining this here and on the web.) That's why you may want an additional DC meter for the DC output.

The AC ammeter will give you a good indication of AC or DC current. The DC is the same current - just rectified.

By the sound of things this is an experimental variable power supply. Fitting even crude meters is a good idea because it gives you some feedback of voltage and current as you wind up the voltage and you'll spot a problem before you smell smoke. Use the multimeter for finer measurements.

By the way, variacs are great for powering up old valve equipment such as radios, amplifiers, etc., as the capacitors may be fragile if they've not been powered up for a long time. There are many articles explaining how to "recondition" the capacitors by powering from a variac and letting the capacitors "soak" at 25%, 50%, 75% for a time (minutes / hours) before applying full voltage.

  • \$\begingroup\$ Will vm1 show correct voltage for the dc to? My variac goes from 0-220v(Sweden) no negative volt. I was planning to have 2 volt meters and 1 amp meter, 1ac 1dc and amp for both(ac amp) will the amp post rectifier be the same as pre? Thank you for the schematic, helps a lot. ps. why not a auto or button resettable fuse? \$\endgroup\$
    – Anders_gbg
    Mar 10, 2016 at 23:59
  • \$\begingroup\$ See the update. ;^) \$\endgroup\$
    – Transistor
    Mar 11, 2016 at 0:15
  • \$\begingroup\$ And the reason a auto(or manual)resettable fuse is undesirable? I was planning on letting it be my overload protection and perhaps have a led light up when it switches of the power(just to know why it stopped) \$\endgroup\$
    – Anders_gbg
    Mar 11, 2016 at 1:55
  • \$\begingroup\$ Would not a high wattage(low ohm i.e. 1-2 ohm) resistor in series with the dc smoothing cap further smooth the dc power? \$\endgroup\$
    – Anders_gbg
    Mar 11, 2016 at 2:02
  • \$\begingroup\$ Can a ac cap on the ac output clean any spikes and/or irregularities from the output sine wave, or what are ac caps for? \$\endgroup\$
    – Anders_gbg
    Mar 11, 2016 at 3:16

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