0.03 ohm / 3 watt (two required)
1 μF / 10% ceramic
100 μF 16 V
1 μF 10% ceramic
3. 3 μF 16V
100 μF 25V
1000 μF 25V
5K 10 turn pot
120V-9V / 9V transformer
❑ Variac Output
200 mV DC
RSR LED panel meter #DM3A
from: ELECTRONIX EXPRESS
❑ Variac (mine)
STACO #10-10 ( 10 amp,
120 VAC in, 0-140 VAC out)
This may be whatever brand or
size you prefer.
in general, it is not much worse than the meter’s normal
resolution at these levels, so it’s a moot point.
One word of caution here: In regards to the metering
circuits I chose for this project, these are half wave
rectifiers in which their peak reading is converted to pure
DC for metering. These circuits will lose accuracy for
non-symmetrical AC waveform cycles. Keep this in mind
when making measurements on equipment that would
alter the 60 cycle waveform. The good news is that this
doesn’t happen too often.
A more accurate solution would be to use an
ideal full wave rectifier circuit or an absolute value
circuit in place of the half wave circuitry I chose. I found
that the time constant of the required
filtering slowed the tracking response
down greatly when I used one of
those circuits instead.
This makes voltage adjustment
very annoying as you would have to
creep the voltage control along very
slowly in order for the voltage to
settle to a stable enough reading for
the DPMs to read it. This would not
have presented as big a problem if I
had used analog meters instead of
digital ones as they tend to average
outfluctuations for a more stable
display. If one were to insist on “all
situations accuracy,” those circuits
are easily found in most op-amp
books or even in a focused Google
As I mentioned earlier, nothing is
engraved in stone in this article, so
feel free to upsize, downsize, alter
circuits to better suit your purpose,
or build it exactly the way I did. No matter what you end
up with, I am sure you will enjoy using it as much as I’ve
enjoyed using mine.
As a closing note, I built this unit for just under $65
and some minor parts from my junkbox — a far cry from
the $500 and up for a new commercial unit!
Also, one item I have recently added to the variac
test setup is a 700 VA line isolation transformer (another
steal from eBay). Aside from the safety aspect in some
situations, it really improves oscilloscope presentations by
eliminating a lot of low level noise and garbage since the
scope now has a true common ground to the circuit
under test. NV
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June 2008 41