User Manual

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Experiment 6: Smoothed versus unsmoothed
DC (ripple)
Connection layout:
Connection summary:
GND clip : 4
Probe tip : 6
Purpose:
The purpose of this experiment is to show what smoothed and unsmoothed DC looks like on a scope
screen and how a scope can help you to determine the quality of your DC supply.
A little theory:
In the previous experiments, we have used one or more diodes to convert an AC voltage into a DC volt-
age. The result was OK, but far from perfect.
Why?BecauseitwasstillfarfromtheatlineonewouldexpectwhenmeasuringaperfectDCvoltage.
ItisclearthatourrectiedACvoltageneeds‘smoothing’.Thiscanbedonewithanelectrolyticcapacitor
(see diagram of this board)
How?:
1. Place the probe switch “x1/x10” to ‘x1’.
2. MakesuretoipSW1tothecorrectposition.
3. Turn on the HPS140 Handheld Pocket Scope, It will start-up in auto setup mode as always.
Watch thescreenclosely.Youwillnoticethatthetraceisalmostat,sothecapacitorisdoi ng a
goodjobatsmoothingourrectiedACvoltage.Yet,itstillwobblesabit.
Ifyouiptheswitchfromsinglephasetodualphaserectication,
the wobbling will be reduced.
Why is this?
Basically, the capacitor acts as a temporary storage device.
It provides power to the rest of the circuit during the ‘interruption’ of the waveform (remember, single
phaserectication?).Withdualphaserecticationthisinterruptionisnotpresent,sothecapacitorhas
less work to do. The remaining ‘wobble’ of the waveform is called ‘ripple’. One of the key features of a
good DC supply is low ripple.
NOTES:
Smoothed versus unsmoothed DC (ripple)
Can we measure this amount of ripple?
Yes we can, a scope is the ideal tool for ripple measurement.
FlipSW1backtosinglephaserectication.
Default, your scope starts up with ‘DC-coupling’ selected.
Change that to ‘AC-coupling’
(see manual for how-to instructions).
Now, the scope will only show the AC part of the signal, the DC
part will be blocked. Make sure it is still in ‘auto-setup’ mode.
If you look at the screen below and your scope screen, you
will see a kind of ‘sawtooth’ waveform, this is the ‘ripple’ volt-
age that rides on top of your DC voltage. During the rising
edge the power supply charges the capacitor, during the
falling edge the capacitor supplies current to the circuit.
If more current is drawn from the supply the ripple will be higher, as the capacitor will be drained
more, so it will not be able to keep the output as steady as it would with a small load
.
How can we reduce the ripple?
TryippingSW1forsinglephasetodualphaserectication
and watch the screen. Check the lower right hand corner.
It displays the rms ripple voltage.
Flip the switch back and forward. It is clear that ripple is
reducedwhendualphaserecticationisused.
Smoothed versus unsmoothed DC (ripple)
Good to know: Use1000µFperampère,soe.g.ifyoudesigna2Apowersupply,
you need a smoothing capacitor of at least 2000µF (2200µF is the closest value that
is suited for your purpose).