Back to Dr. Greg Latta's
Electrical Engineering and Amateur Radio Pages Safety Note: Working on or testing the QSK-5 and associated circuitry may involve operating the amplifier containing the QSK-5 with the cover off. This is extremely dangerous since very high voltages are present when the amplifier is turned on, sometimes in close proximity to the QSK-5. If at all possible, do all work with the amplifier off and unplugged. The operator assumes all risk and liability in such matters! Never operate the amplifier with the cover off unless you are experienced with working around very high voltages!
What Is Hot Switching?
Hot switching is a term that refers to turning a switch off or on while the
circuit is live. In most cases, hot switching is normal. For instance, the
light switches you use in your house turn the lights off and on while the
circuit is live. And the circuit breakers in your house turn the branch circuit
off and on while the power grid is live.
What Problems Are Associated With Hot
Switching?
As everyone knows, when a switch is turned off while the current is on a spark
usually occurs at the switch. This spark can burn and pit the switch unless the
switch is designed for it. Most switches are designed for hot switching and it
causes no problems. However, with a PIN diode switch hot switching will
usually ruin the switch by burning out the diodes, as will be discussed
below.
Why must hot switching must be avoided with a
PIN diode switch?
When any switch, mechanical or otherwise, is turned off, the resistance of the
switch changes rapidly from a very low resistance to a very high resistance.
During the transition, the resistance takes on all values of resistance in
between. (Imagine turning the volume control on your transceiver very
quickly from one end to the other. No matter how fast you turn the knob, it
must be at all points in between.) In a mechanical switch or relay the
resistance changes because the contact area decreases as the switch opens. Once
the contacts separate, a small arc forms and then the arc extinguishes as the
contacts open further.
In a PIN diode switch there are no
mechanical contacts. The resistance of the diode changes because the bias
across the diode is changed from positive to negative. As in the case of the
mechanical switch, during the transition the resistance takes on all values
of resistance in between a very low resistance and a very high resistance.
Now for the crucial point: At some point in the transition,
the resistance of the PIN diode switch will equal the impedance of the load
(typically 50 ohms), and at this moment half of the power being delivered by
the transmitter will be absorbed by the diode switch! For a typical 100
watt transceiver this means that at some point while it is shutting off the
diode switch will be asked to dissipate 50 watts! Whether the diodes can handle
this depends on the diodes and on how fast the transition occurs. In the case
of the QSK-5, shutting off the receive PIN switch
while more than about 20 watts is flowing through it will burn out the diodes.
It is safe to summarize by saying that hot switching of PIN diode switches
must be avoided at all costs.
What causes hot switching?
One major cause of hot switching is a transceiver that, when switching to
transmit, emits RF before, at the same time, or too quickly after it brings the
T/R line low. This is, unfortunately, very common. It doesn't cause a problem
with mechanical T/R relays, but can burn out an electronic T/R system.
(Fortunately, the QSK-5 is fused to protect against this). Another common cause
of hot switching is forgetting to connect the T/R line between the transceiver
and the amplifier T/R switch. If this happens with a mechanical T/R
switch/relay, the signal simply goes out unamplified. But with an electronic
T/R switch, the result is hot switching and blown diodes (or fuse lamps, in the
case of the QSK-5).
How do I detect hot switching?
Safety Note: Testing for hot switching may involve operating the
amplifier containing the QSK-5 with the cover off. This is extremely dangerous
since very high voltages are present, sometimes in close proximity to the
QSK-5. The operator assumes all risk and liability in such matters! Don't do
this unless you are experienced with working around very high
voltages!
As you might guess, hot switching is common. The designers of the QSK-5
anticipated the occurrence of hot switching and designed the QSK-5 with a
pair of fuse lamps in series with the
receive PIN diode switch. These lamps light/flash if hot switching is
occurring. If the problem becomes serious enough, the lamps burn out,
protecting the expensive receive diodes. When the lamps start to light, their
resistance increases, further protecting the receive diodes. Under no
circumstances should you try to replace the lamps with normal fuses, which do
not have this feature!
To test the system, hook up the amplifier and transceiver and be sure to
connect the T/R line. (Connect the system to a dummy load so you don't cause
interference while testing the system.) Turn the power level of the transceiver
down to a very low level and begin to transmit. Allow the transceiver to drop
in and out of receive as you do this. Keep an eye on the QSK-5
fuse lamps, particularly when the
transceiver goes into transmit mode. Gradually increase the transceiver power
while keeping an eye on the lamps. If the lamps light/flash it indicates that
hot switching is occurring. Mild hot switching can be tolerated, but if the
lamps start to get bright enough to burn out you have a problem that must be
dealt with.
Test the system at both low and high keying speeds. Hot switching may occur at
one speed but not the other.
What Do I Do If I Detect Hot Switching?
If the hot switching is mild enough, you may be able to live with it.
In other words, if the fuse lamps don't light too much and don't blow, you
might be able to get away with it. But that is your choice. If you have hot
switching in any form, I recommend that you talk to
customer support at
Ameritron. You should also consult with your transceiver manufacturer's
technical department. They may have some fixes that might help you out. Some
transceivers may have a software update that can cure the problem.
In an extreme case, it might be that you simply can't use that particular
transceiver with an electronic T/R system such as the QSK-5. Electronic
switching has many advantages and disadvantages. This is one of the
disadvantages: it may not be compatible with all transceivers.
Back to Dr. Greg Latta's
Electrical Engineering and Amateur Radio Pages
If you have any questions or
comments, you can send E-Mail to Dr. Greg Latta at
glatta@frostburg.edu