major aspect of development in broadcast transmitter technology and
power electronics is in the area of power control.
Fig. 1: The
the traditional transformer, we often dealt with the simpler
full-wave and half-wave rectifier circuits; today, the switching
power supply is the norm. Back then, we dealt with heavy 60 Hz
transformers, but now we deal with lightweight but high-energy power
supplies switching at the kHz frequency range.
same is true with power control systems. This article focuses on
repair of a power controller found in many radio and TV transmitters.
your station’s major electronic equipment brings both fun and
anxiety in the process. These days, we more often deal with fixing
modules, be it an RF power module or a power supply module. It may be
easier to send the bad module to the manufacturer, but if the
warranty expires, we probably have to work on it.
where is the fun in just putting the item in a box and calling the
delivery guy to pick it up? The fun is when you learn and make some
Fig. 2: AC voltage
controlled by an SCR. Voltage and current will not pass to the load
until the gate is triggered by a positive voltage.
device used in power control is the silicon-controlled rectifier. The
SCR is a three-terminal device (see Fig. 1). Allow me to refresh you
with the basics. The SCR has three terminals: cathode, anode and
device can be turned on by a trigger voltage at the gate. Once there
is current from anode to cathode, however, it cannot be turned off by
a negative trigger voltage at the gate. It can only be shut off by
bringing the anode to cathode current to zero.
fascinating thing happens when we use the SCR to control AC (see Fig.
2). The positive half cycle of a sine wave goes up and goes down by
itself. If we trigger it at the start of the positive half sine wave,
the SCR automatically will turn off by itself at the end of the
positive half cycle. If the gate trigger voltage is applied at every
beginning of the positive half cycle, then the half sine wave is
delivered to the load.
that if we can set the trigger pulse at any point between 0 and 180
degrees, then we can control the voltage getting into the load.
Notice as well that the load waveforms are pulsating DC (see Fig. 3).
we parallel another SCR but invert its connection in such a way that
the cathode of the second SCR connects to the anode of the first SCR
and vice versa, the full sine wave can then be controlled (see Figs.
4 and 5).
Fig. 3: The effect of
changing the phase of the trigger signal on voltage and current
POWER CONTROLLER FAILURE
power controller we will focus on is a three-phase SCR controller
rated 480 VAC and 50 amps. The control range of this controller is
from 0 to 98 percent of the line voltage. It provides AC power to the
big transformers of the DC power supply. These controllers are
commercially available and are not made by the transmitter
manufacturer. Each controller is loaded at about 12.6 kW, which
translates to about 16 amps at 460 VAC. There are three of these
units for the broadcast transmitter. The controller uses an SCR pack
semiconductor, Semikron SKKT92, which have two SCRs internally
connected in one package. The data sheet is shown below in Fig. 6.
problem arose when we discovered an error in one of our
transmitters. The transmitter indicated “shorted SCR” on its
front-panel alarm. We checked the manual and called the manufacturer
regarding the problem. They then gave us a checklist of static
resistances for different points in the controller (see Fig. 7 for
schematic). Despite the transmitter indicating a short, we found the
three SCR packs to be OK.
controller manufacturer did not offer any more assistance beyond the
Fig. 4: Back-to-back
SCRs result in an AC waveform.
we were left with figuring this out on our own.
Fig. 5: AC waveform
through dual SCRs
YOUR OWN TEST BED
like to check a module outside the transmitter if at all possible. In
this case, I needed three 480 V transformers and a three-phase load.
Fortunately, we had a few 240V/13V transformers that were part of
unused charging equipment that powers the old microwave systems (see
manual specifies that there should at least be 1A of load current in
order to test the functionality of the controller. Assuming the power
factor = 1, the load needed for each phase would be about 277 W. We
do not have resistors this hefty, so the next best thing was to get
some heating elements. I found some cheap heating coils from eBay
that were good enough for the job (see Fig. 9).
connected two of the 240 VAC transformers in series to make up for
one 480 V phase and then connected all the transformers in a delta
configuration. After all the connections were set, sure enough the
fuse blew up when power was applied. It was loud and quick indicating
a dead short. We isolated the problem to be coming from load 2 and 3.
TO THE SCHEMATIC
this point, some analysis was in order. Looking at the waveforms
created above, a single SCR creates a pulsating DC. So, if two SCRs
are connected back to back and one of them fails (or if there is no
firing signals in one of them), there would be a DC current on the
load. DC on the primary winding of the transformers is really bad
because the load for the DC current is a dead short. This is the
crucial point in the troubleshooting process. One of the SCR pair on
either load 2 or load 3 was not getting the firing signals properly.
went on to check the firing circuit. The gate firing circuit is
usually isolated from the main low-voltage circuits because, as in
this case, the cathode-anode of the SCR is at 480 VAC. The power
controller board is powered by these really cute three-phase 480/17 V
step-down transformers (see Fig 10). The isolation devices between
the main board and the firing circuit are HCPL4504 high-speed
optocouplers (see Fig. 11 for complete schematic). I did not bother
much in checking the main board. My main objective was to see the
actual waveforms at the input to the optocouplers. The input of these
optocouplers is connected to a multiplexer. With a dual trace scope,
I checked the pulses at the input of the optocouplers and found them
Fig. 6: Data sheet with
specifications for dual SCR package made by Semikron, the SKKT92
(Click to Enlarge)
then looked at the actual firing voltages getting into the SCR gates.
The scope cannot be used at this point because the firing circuit is
floating at 480 VAC so I used a DC voltmeter instead. These are the
threshold voltage of the SCR gate is 3V and clearly there is problem
with load 3. One of the pair is off while the other one is barely
turning on. The problem then narrowed down to the isolated portion of
the gate firing circuit of SCR 3.
isolated section of the firing circuit is electrically separated from
the main transmitter controller board by the optocouplers. Pictured
in Fig. 12 is the isolated section that controls one of three SCR
pairs. U5 and U6 are the optocouplers for this circuit. The top and
bottom circuits are identical, so I will focus only with one of them.
is provided to the circuit by the 17 VAC coming from the three-phase
step down transformer. Diode D5 (bottom circuit) and capacitor C17
creates a half wave rectified DC supply. The –VCC from
this supply powers the switching transistor Q5 and regulator VR5. VR5
provides the VCC for the optocoupler U5.
from the main board drives the optocoupler, which drives Q5. Q5 fires
up the SCR. Capacitor C17 failed and because this is the DC supply
filter, the DC supply becomes unstable and reduced the voltage to a
much lower DC level because half the time it is at 0 VDC. This
problem made both the optocoupler and switching transistor unstable,
resulting in a very low gate to cathode voltage of half the SCR pair.
This in turn caused DC current to be delivered to the load
Fig. 7: Power control
circuit schematic showing three-phase connection and SCRs, which feed
three-phase wye input AC transformer.
(Click to Enlarge)
Fig. 8: Surplus 240 V
transformers provided a load for our homemade power controller test
Fig. 9: Heating
elements acquired from eBay to dissipate enough load current to test
the power controller properly
|Fig. 10: Three-phase step down transformers to power main board and firing circuit
also noted that the caps are very close to the transistor heat sink,
which is hot in normal operation. This may have caused the caps to
dry up prematurely. A small cooling fan would have prevented this
Fig. 11: Complete
schematic of power control board showing isolated trigger inputs.
(Click to Enlarge)
replacing the caps, the trigger voltages went up to its normal level.
I put all of it together again and connected the load transformers.
There were no more shorts and all the current and voltages were back
our line of work, being resourceful surely helps in troubleshooting
equipment, especially in times of meager financial resources. Basic
troubleshooting procedure, like isolating the problem to a subsystem
or module, is also a good help. There is also a need for some hours
of reading to make sure that we understand the circuit and the
devices that we are working on.
| Fig. 12: Circuit card with optocouplers on power controller board
all of these, safety is paramount in every step; it will not hurt
double-checking each connection before we power up. The fun of
discovery ends here — building your own test bed at high voltage is
a serious business. What a way to have some cardiac exercise, hearing
those loud popping noises from a blown fuse at hundreds of amperes!
Marcon, CBTE, CBRE, is an engineer at Victory TV Network.