Dimmer Risetime Technical Application Sheet

Dimmer Risetime Technical Application Sheet
T E C H N I C A L
A P P L I C A T I O N
Dimmer Risetime
S H E E T
MEASUREMENT TECHNIQUE
INTRODUCTION
Since the introduction of Thyristor (ie Triac or SCR) dimmers in the
70’s, dimmer risetime measurement has been used as a means to
quantify both the amount of noise and buzz that a dimmer will inject
into an adjacent sound system, and the amount of acoustic noise
from lamp filament rattle. This Application Sheet shows the standard
measurement technique as used by Jands for measuring dimmer
risetime. Users can then make their own measurements and/or understand how values from different manufacturers using different
measurement techniques can be compared.
Risetime
end point is
unclear
Phase Controlled Dimmer Operation
Risetime clearly
starts here
Figure 2: Expanded view of the switching event in Figure 1.
A series inductor or choke is used to limit the risetime of the output
and reduce the width and intensity of the interference. In general a
longer risetime results in a reduced amount of interference.
Where does the Risetime Finish?
Figure 2 shows a higher detail trace of the time around
when the triac switches on. The action of the choke can
be clearly seen - the output doesn’t snap to full but slowly
rises from 0 to the full supply voltage.
Figure 1: A typical noise spectrum from a choked thyristor dimmer
source. The top trace shows the source waveform, while the bottom
shows the noise spectra present in this waveform.
Figure 1 shows a noise measurement of a standard triac plus choke
dimmer - in fact it’s a HPX driving a 2.5KW CP91 lamp. This product
is specified as having a 220us risetime. The noise indicated by the
bottom trace can be seen to be approaching zero at around 3KHz.
The plot in Figure 2 shows a risetime as measured using
the 10 - 90% technique of around 220us. However using the 0 - 100% technique the risetime
could be interpreted as anything up to 400us.
Be wary of dimmer specifications that include 0
- 100% values for risetime — although they look
better they are vague and subjective and must
not be directly compared to 10-90% risetime
specifications.
Other Effects
Dimmer risetime is significantly affected by
other parameters - output current / load, supply
voltage and the drive level. Jands dimmers are
always tested at their full rated load, at the
specified supply voltage, at 50% power, using
an incandescent load.
Jands Pty Ltd 40 Kent Road Mascot NSW 2020 Australia
Phone +61 2 9582 0909 Fax +61 2 9582 0999 www.jands.com.au
Specifications subject to change without notice. Manufactured by Jands Pty Ltd ABN 45 001 187 837.
Note: While all due care and attention has been taken in the preparation of this document, Jands Pty Ltd shall not be liable for any inaccuracies or omissions which may occur therein
Dimmer Risetime
Interference
diminishing
to zero
Looking closely at Figure 2, the point at which the risetime
starts can be clearly seen as the instant the voltage starts
to rise. However the point at which risetime completes is
very un-clear - for this reason standard engineering practice takes risetime measurements from 10 to 90% of the
final value. Most CRO’s have 0, 10, 90, and 100%
graduations marked on their display specifically for the
purpose of risetime measurement.
MEASUREMENT TECHNIQUE
Thyristor dimmers use SCRs or Triacs to switch the output load for a
portion of every half cycle—the portion when the load is switched on
defines the output power and therefore the brightness of the connected lamp. The sharp edge created by the Thyristor turning on
generates a large amount of broad spectrum high frequency noise
which, if left unchecked, couples into any nearby sound system. The
result is the classic unwanted dimmer buzz.
A P P L I C A T I O N
Dimmer Risetime
S H E E T
MEASUREMENT TECHNIQUE
The Test
Procedure
The procedure for measuring risetime is fairly quick once
you know what you’re doing. Since mains voltages are
involved, the following procedure must only be attempted by only those competent in the servicing of
mains equipment.
1. Plug in all of the equipment as normal. At this stage
however plug only the small load into the dimmer.
Connect the CRO mains probe on the dimmer channel to be measured.
Mains supply capable of delivering the expected
current.
•
Oscilloscope (CRO) with 0, 10, 90, and 100%
graduations on the graticule
•
Mains probe for the oscilloscope
•
Both small and full power lamp loads
•
Some way to adjust the dimmer level eg console.
•
Dimmer under test
The small lamp is used during setup—the full power lamps
create a lot of heat in the average workshop and are used
only for the final measurement.
Set the equipment up on a clear bench with the lamp
shielded by a non-flammable barrier. The power source
should be protected with a RCD of no more than 30mA trip
current. Test the RCD before switching anything on.
3. The CRO graticule will normally include 0, 10, 90, and
100% graduations on the display. Adjust the 0V reference of the trace so that it is aligned with the 0%
graduation. See Figure 3.
4. Adjust the channel variable gain and the sensitivity
selector on the CRO so that the peak voltage just
touches the 100% graduation.
5. Turn the dimmer power off and swap over the load to
the full power lamp. Put sunglasses on. Turn the
power back on.
6. If necessary re-adjust the timebase on the CRO so
that the complete transition from off to on is displayed. On the CRO display count the number of
major divisions horizontally between where the trace
crosses the 10% graduation and where the trace
crosses the 90% graduation. This is the 10-90%
risetime.
7.
%
Multiply the number of graduations by the timebase
to calculate the actual risetime.
10-90%
Risetime
Voltage
crosses 90%
here
Voltage
crosses 10%
here
Figure 3: HPX choke plot with the voltage scaled to the 0 and
100% graduations. The 10-90 risetime is the time the voltage
takes to rise from the 10% level to the 90% level. Note change of
timebase from previous figures. This choke is rated by Jands at
220us, however using 0-100% technique the same choke would be
rated at around 350- 400us.
Figure 4: Risetime plot of Jands FPX with
standard 120us choke. Using 0-100% technique this choke would be rated at around
175us.
Jands Pty Ltd 40 Kent Road Mascot NSW 2020 Australia
Phone +61 2 9582 0909 Fax +61 2 9582 0999 www.jands.com.au
Specifications subject to change without notice. Manufactured by Jands Pty Ltd ABN 45 001 187 837.
Note: While all due care and attention has been taken in the preparation of this document, Jands Pty Ltd shall not be liable for any inaccuracies or omissions which may occur therein
MEASUREMENT TECHNIQUE
•
Dimmer Risetime
Equipment
2. Apply power to the system, and check that the lamp
can be dimmed up and down. Adjust the drive level
and the CRO triggering so that the output voltage rise
finishes at the peak voltage of the supply ie around
340V for a 240VAC system.
TAS JND-Dimmer Risetime-0902-03
T E C H N I C A L
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