[tim rv415tp]

Yesterday I was going to give my wife,s uncle a ride, He,s 85 and hadn,t been in a small plane for about 50 years. When I started up and began taxiing I smelled somthing burning, electrical. So I shut down started looking. I found my fuel press. indicator foggy, along with oil temp & oil press. reaching underneath the panel, these three inst. were extremly hot, to the point that the cases were distorted. These are standard Van,s gauges. after isolating the problem I ran it, voltage was normal, it didn,t damage radios or dynon. any ideas?

 

[L.Adamson]

I wiped out the same three gauges, plus the voltage gauge. All positive and negative terminals were looped to each gauge plus a few others. I had pulled out the center panel with the Van's gauges to do some radio stack modifications, and had it sitting on the throttle/mixture knobs. With the large wire terminals on the back of the gauges, the center panel had moved enough to ground one positive terminal to the panel. In my case, it didn't distort the gauges, nor did they have time to heat up. The fuse blew rather immediately.

Any chance, that you had either a positive or negative lead touch one of these open terminals on the back of the gauges? And what about fuse size versus wire sizing.

L.Adamson --- RV6A (flying)

 

[tim rv415tp]

smoke in the cockpit

Today when I opened things up, I found nothing wrong with the wiring,terminals or anything else in back of the panel. I was thinking over voltage but after isolating everything ,I ran it,voltage was normal.

 

[lucky333]

Sometimes they pop

Tim,

Check out this thread. I got a look at the inside of some burned fuel gauges like yours and found out what went wrong but not the exact cause..
As posted earlier, and I recommend too, these should be fused on the power input.

http://www.vansairforce.com/community/showthread.php?t=10392&highlight=procoach

I traced out a schematic (with guesses on the burned parts) and can email you the PDF if you'd like.

John

 

[Doug]

John,

Does the zener power circuit have any RF bypassing? I don't see one in the photos. Disregard the electrolytic cap - it does nothing at RF frequencies.

I would have expected a low ESR capacitor to catch any high frequency instability in the power supply design - especially given the variability in the supply circuit to these gauges (and resulting inductance).

If my suspicions are correct then a 100nF ceramic capacitor across the zener should be all that is required.

Doug Gray

 

[lucky333]

Nope

+Vin goes in series through a 1N5819 diode through the big smoked resistor to the input of a 7805L Vreg. From that input to ground is the zener in parallel with the Al cap.

As you indicated, no RF protection to speak of.
John

 

[Doug]

I would add a 100nF (50 Volt) ceramic capacitor across the zener, at worst it will do nothing but may eliminate any HF/RF instability in the gauge's supply circuit.

Alternatively, putting a 100n cap across the gauge's supply terminals may be sufficient. I will do this myself.

Please note: without performing some bench tests to replicate the failure the forgoing is entirely speculative at this stage.

It is a pity the gauge designer has not responded to this fault. This is a simple circuit design and these gauges should be next to bullet proof.

Doug Gray