Tab Electronics Guide to Understanding Electricity ... - Sciences Club

150 Chapter Five
which can apply to unique situations. Review all of the safety recommendations
presented thus far, and put them to use all of the time.
Set your DVM to measure “DC volts” on the 100-volt range (or higher).
Plug the power supply into the outlet strip. Set SW1 to the “off” position
and turn on the outlet strip. Briefly, turn SW1 to the “on” position, and
then back to the “off” position. Turn the outlet strip off. Measure the DC
voltage across C1 and C2, paying close attention to the polarity (C1 should
be positive, and C2 should be negative, in reference to circuit common). The
actual amplitude of the voltage is not important at this point in the test.
You have simply “pulsed” the power supply on and off, to verify that
the capacitors are charging and in the correct polarity. As you measured the
DC voltages, they should have been decreasing in amplitude as
the charge was draining off. The draining of the charge is caused by the
internal leakage inherent in all electrolytic capacitors (new capacitors can
be very leaky until they have the chance to re-form during circuit operation).
Also, the capacitors will discharge, to some degree, through the internal
input impedance of the DVM while you are measuring the voltage.
If you measured some voltage level across C1 and C2, with the correct
voltage polarities, reapply power to the circuit and measure the DC voltages
across C1 and C2. The calculated voltage across each capacitor
should be about 33 volts (the peak value of the 24-volt secondaries is
about 34 volts, minus an estimated 1-volt drop across the rectifier). In
reality, you will probably measure about 36 to 38 volts across each capacitor.
There are several reasons for this higher level. Transformer manufacturers
typically rate transformers based on minimum worst-case
conditions, so it is common for the secondaries to measure a little
high under normal conditions. Also, you are measuring the voltage
levels under a no-load condition (often abbreviated N.L. in data books). If
you took these same measurements while the power supply was operating
under a full load (abbreviated F.L.), they would be considerably lower.
Leaving power applied to the power supply circuit, set your DVM to
measure “AC volts” beginning on the 100-volt (or higher) range. Measure
the AC voltage across C1 and C2. If you get a zero indication on the 100-
volt range, set the range one setting lower and try again. Continue this
procedure until you find the correct range for the AC voltage being
measured. (When measuring an unknown voltage or current, always
begin with a range setting higher than what you could possibly measure
and work your way down. Obviously, if you are using an “autoranging”
DVM, you won’t have to worry about setting the range.) If the
circuit is functioning properly, you should measure an AC component
(ripple) of about 5 to 20 mV. Turn off the circuit.