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rv-9a-online(at)telus.net
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 Posted: Sat Oct 07, 2006 3:21 pm Post subject: Burned out switch |
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I just experience my second switch failure in 100 hours. The first time
is was the master switch (loose switch terminals), this time it was my
Strobe power supply switch.
Symptoms: tripping breaker at shut down, and burned fast-on terminal
insulation at the switch.
An inspection of the switch showed that the terminals were loose and
discolored, indicating high internal temperatures.
Bench testing with a constant current power supply indicated flaky
terminals and overheating.
I would expect the Strobe switch would be more susceptible to failure
due to the constant power characteristics of the strobe power supply.
As switch resistance increases, the current in the circuit would tend to
increase as the strobe power supply tries to keep putting out the same
power. This would quickly lead to thermal runaway in the switch.
For example, if the switch resistance was high enough to cause a 4 volt
drop across the switch, the strobe current would rise from 7 amps
(nominal) to more than 10 amps, tripping my breaker. The switch would
also overheat.
This would also tend to cause a breaker trip at shut-down, because the
voltage would drop from 14 to 12 as the engine stopped, increasing the
strobe power supply draw.
So... lesson learned. It also confirmed my suspicions that the breaker
was tripping for a reason!
Vern Little
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planepubs(at)ev1.net
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| Posted: Sat Oct 07, 2006 3:58 pm Post subject: Burned out switch |
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Very interesting! What can you tell us about the quality of the switches? I would think the opposite, with the master having more duty. At least the strobes have a bit of time to cool so maybe less of a load!
On 10/07/2006 6:16:48 PM, Vern Little (rv-9a-online(at)telus.net (rv-9a-online(at)telus.net)) wrote:
[quote] --> AeroElectric-List message posted by: Vern Little <rv-9a-online(at)telus ([email]rv-9a-online(at)telus[/email]).
net>
I just experience my second switch failure in 100 hours. The first time
is was the master switch (loose switch terminals), this time it was my
Strobe power supply switch.
Symptoms: tripping breaker at shut down, and burned fast-on terminal
insulation at the switch.
An inspection of the switch showed that the terminals were loose and
discolored, indicating high internal temperatures.
Bench testing with a constant current power supply indicated flaky
terminals and overheating.
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rv-9a-online(at)telus.net
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| Posted: Sat Oct 07, 2006 8:47 pm Post subject: Burned out switch |
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They are Carling switches with the riveted fast-on terminals.
My master switch was defective from first flight, but not 'burned
out'-just loose rivets leading to fluctuating field current in the
alternator.
The load on the master switch is only about 3 amps maximum.
The strobe has about a constant 7 amp load (nominal). If the voltage
drop, this current increases, as I mentioned... thus overloading the
switch and circuit breaker. Strangely, the Master switch driving the
alternator field has the same characteristics (a drop in bus voltage
increases the current through the switch).
I have a third switch that is suspicious as well... but I haven't
determined if it's the fault of my flap motor.
I've contacted the supplier (B&C). They replaced my Master switch, but
I haven't heard back yet on the other one. I'm not worried about a
replacement... I was concerned that they had a bad batch of switches.
Vern
A DeMarzo wrote:
| Quote: | Very interesting! What can you tell us about the quality of the
switches? I would think the opposite, with the master having more
duty. At least the strobes have a bit of time to cool so maybe less of
a load!
On 10/07/2006 6:16:48 PM, Vern Little (rv-9a-online(at)telus.net
<mailto:rv-9a-online(at)telus.net>) wrote:
>
<rv-9a-online(at)telus <mailto:rv-9a-online(at)telus>.
> net>
>
> I just experience my second switch failure in 100 hours. The first time
> is was the master switch (loose switch terminals), this time it was my
> Strobe power supply switch.
>
> Symptoms: tripping breaker at shut down, and burned fast-on terminal
> insulation at the switch.
>
> An inspection of the switch showed that the terminals were loose and
> discolored, indicating high internal temperatures.
>
> Bench testing with a constant current power supply indicated flaky
> terminals and overheating.
*
*
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europa flugzeug fabrik
Joined: 13 Feb 2006
Posts: 65
Location: North Coast, USA
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| Posted: Sun Oct 08, 2006 12:13 pm Post subject: Re: Burned out switch |
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| rv-9a-online(at)telus.net wrote: | | The strobe has about a constant 7 amp load (nominal). If the voltage drop, this current increases, as I mentioned... thus overloading the switch and circuit breaker. |
Uh, no, the current decreases with decreased voltage.
Fred F.
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rv-9a-online(at)telus.net
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| Posted: Sun Oct 08, 2006 3:12 pm Post subject: Burned out switch |
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Sorry, Fred... the strobe power supply is a "constant power" as opposed
to a resistive load device. If you decrease the voltage to the power
supply (subject to the lower operating limit), the current increases in
order for it to generate the same strobe output power.
That's why the current required at 28V is about half that required at 12V.
Vern
europa flugzeug fabrik wrote:
| Quote: |
rv-9a-online(at)telus.net wrote:
> The strobe has about a constant 7 amp load (nominal). If the voltage drop, this current increases, as I mentioned... thus overloading the switch and circuit breaker.
Uh, no, the current decreases with decreased voltage.
Fred F.
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=66562#66562
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retasker(at)optonline.net
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| Posted: Sun Oct 08, 2006 5:26 pm Post subject: Burned out switch |
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europa flugzeug fabrik wrote:
| Quote: |
rv-9a-online(at)telus.net wrote:
>The strobe has about a constant 7 amp load (nominal). If the voltage drop, this current increases, as I mentioned... thus overloading the switch and circuit breaker.
>
>
Uh, no, the current decreases with decreased voltage.
Uh, no, yourself. This is an electronic product and will try to keep
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the strobe firing voltage the same regardless of the input voltage.
That means that the power to the strobe head is relatively constant -
which is what you want. To do this the strobe supply draws more current
if the input voltage falls (power = voltage x current).
Dick Tasker
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europa flugzeug fabrik
Joined: 13 Feb 2006
Posts: 65
Location: North Coast, USA
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| Posted: Sun Oct 08, 2006 6:13 pm Post subject: Re: Burned out switch |
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| rv-9a-online(at)telus.net wrote: | | Sorry, Fred... the strobe power supply is a "constant power" as opposed to a resistive load device. If you decrease the voltage to the power supply (subject to the lower operating limit), the current increases in order for it to generate the same strobe output power. |
If you are referring to a strobe power supply designed for 14V/28V, well then yes we do have a different story. Whelen is like that, but I just now put a 4.0 A Whelen PS on the electronics bench, and the 4.0V spec is apparently peak current, which occurs in a very brief spike. At 12.3V, that spike is 4.5A, and mostly below 4.0 through the up/down cycle. At 14.1V, it is within spec at brief peak. For a 7A Whelen supply, that predicts to no more than an extra 1.0 amp at peak run off batt only, verses charging voltage. If we then look at the trip curves of a breaker, we will see that a 10A breaker (presume you have that) does not trip instantly at 10A, or even a notable amount above that, except after many seconds lapsing. So what’s causing your problem such as to burn up fast-ons, I don’t know. Perhaps others can help.
Fred F.
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rv-9a-online(at)telus.net
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| Posted: Mon Oct 09, 2006 9:12 am Post subject: Burned out switch |
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Many power supplies, especially switching regulator types, exhibit
what's called 'negative resistance'.
This means that if the input voltage to these power supplies increases,
the power supply input current decreases. These types of power supplies
do not work well when powered from a source that has a high
resistance... such as a malfunctioning switch.
In fact, they will oscillate quite nicely in certain conditions--
producing oscillating current and voltage at the input side... which can
really mess up other devices tied to the input bus.
I had to deal with this problem on power supplies that I designed for a
nuclear research facility (Cyclotron).
I would expect that the Whelen strobe power supply does not oscillate
under these conditions due to conservative design, but they still will
exhibit the inverse relationship between input voltage and input
current, and can therefore lead to higher than rated input currents as
the source voltage drops... and pop the breaker.
As a side note: This also proves that there is rarely a 'nuisance trip'
of a circuit breaker... it was trying to tell me something was not
right. In the end, the wiring was fine, but the terminals, switch and
circuit breaker were all toasted.
BTW, a new switch, terminals and breaker and everything is now working fine.
V
europa flugzeug fabrik wrote:
| Quote: |
rv-9a-online(at)telus.net wrote:
> Sorry, Fred... the strobe power supply is a "constant power" as opposed to a resistive load device. If you decrease the voltage to the power supply (subject to the lower operating limit), the current increases in order for it to generate the same strobe output power.
If you are referring to a strobe power supply designed for 14V/28V, well then yes we do have a different story. Whelen is like that, but I just now put a 4.0 A Whelen PS on the electronics bench, and the 4.0V spec is apparently peak current, which occurs in a very brief spike. At 12.3V, that spike is 4.5A, and mostly below 4.0 through the up/down cycle. At 14.1V, it is within spec at brief peak. For a 7A Whelen supply, that predicts to no more than an extra 1.0 amp at peak run off batt only, verses charging voltage. If we then look at the trip curves of a breaker, we will see that a 10A breaker (presume you have that) does not trip instantly at 10A, or even a notable amount above that, except after many seconds lapsing. So what’s causing your problem such as to burn up fast-ons, I don’t know. Perhaps others can help.
Fred F.
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=66644#66644
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