How do you rectify AC to DC right to 0 volts for measurement?

[jolshefsky]

I've had a small variac for a while and wanted to make a variable AC supply with it. I'd like to have a display that reads the output voltage. Once I realized my analog panel meter was too big for the project box, I started to think about how to accomplish this and got lost pretty quickly.

My first thought was to use a small step-down transformer on the output and run it through a full-wave bridge. But then I run into the problem of diode drop—even with a 12V output, 1.2V is 10%. Schottky diodes drop that way down to, say 0.5V or so but still around 5%.

I suppose I could convert the output to DC with a bridge rectifier then use a resistive divider which would knock the minimum-readable voltage down to the <1% range.

For that matter, I could use a microcontroller's ADC to read the transformer waveform and do a proper RMS calculation on it. But that gets into a fair amount of work, what with programming and all that.

So I thought maybe an op-amp circuit that detects zero-crossing and inverts, making a "perfect" rectifier. I think this is probably the best option ... but is there a simpler way? Am I missing something really obvious?

[AlfBaz]

Google "precision full wave rectification"
They're often used for just this purpose. The last time I used one wast to rectify a small AC signal from a sensor

[james_s]

You could use a precision rectifier circuit, or an RMS to DC converter IC. Or you could simply use a diode and calibrate the meter to compensate for it, or you could just buy an AC panel meter, they exist and are already reasonably accurate.

[jolshefsky]

Google "precision full wave rectification"

(First, thanks for not being a jerk with Let Me Google That For You ... I just came here first figuring it had already been asked.)

Definitely the way to go ... Google led me to Wikipedia which cited The Art of Electronics which I now realize I should have checked.

[Ian.M]

There's no real need for a voltmeter on a bench Variac if its line powered.  The knob position can indicate the output voltage with enough accuracy for most purposes.  Occasionally its useful to have a pair of shrouded  jacks so you can directly connect a DMM but you'll rarely use them.   What is essential is a fast-acting ammeter, preferably analog with a redline at the Variac's max load current rating.  It doesn't need to be fancy or particularly large or accurate, you just need to know if the load is drawing excessive current.  A bridge rectifier wrapped around a shunt resistor feeding a small panel meter with a 1mA movement and a series preset to trim it to approximate RMS from the average current is plenty good enough..

[james_s]

I would use a small current transformer rather than a shunt resistor. I do find a voltage meter more useful than trying to read off the dial though.

[Ian.M]

Sure, if you've got a spare current transformer it would be handy.  Its absolutely critical to be able to see the current to avoid burning up the Variac or SMPSUs that don't like being ramped up slowly.

I suppose there's no reason not to have meters for both or to make the meter switchable, but I'd rather monitor the output voltage on a DMM I trust rather than a dubuously calibrated panel meter.

[SeanB]

My variac I wanted a larger scale, so instead of making it from 0V to 360V ( it tops out at around 340VAC or so, depends on the input voltage) I went with a suppressed zero meter, indicating from 60VAc to 360VAC, which gave a nice usable scale on the used meter I had to hand. This also allowed me to use the 6 divisions it had as well, so saving on the remarking after painting out the original lettering, so I could have 50V per tick mark and reading to 5V on there as well.

Used a 723 regulator to make a stabilised 7v rail from a tapping on the transformer that gave 60VAC, using a simple half wave rectifier and some zener diodes to get a 20VDC rail for the 723, and making a simple shunt regulator with it. Then another half wave rectifier and capacitor ( 1uF 400V smoothing) and some resistors to set the current, and a bit of adjustment and it worked.

Variac you rarely need below quarter voltage, most of the time you want from half to full mains voltage. Very important to add some fuses input and output, so that you are protected from shorts or such. I used 10A fuses on the unknown one I had, as it probably would handle this fine, though I did have to add an inrush limiter to tame the turn on surge though.

[james_s]

My variac has a voltage meter but no current meter. In practice this is rarely a problem, I wish it had both and now that you mention it maybe I'll add one, but if the current is exceeded the fuse blows.