Lettin' the smoke out!

[flyboy122]

I was doing some rewiring on the aircraft, and accidently let a BNC connector for an antenna rest up against the bus bar. (My 1953 has been updated with CB's and an alternator.) So as you can imagine things got a bit interesting when I flipped on the master to check my handywork. After a few seconds of smoke there was a click and everything shut down. Turned off the master, and about 10 seconds later another click. Then everything worked again. I found and fixed the short, and everything seems to work without smoking, but the question is....what clicked?

I've been through system on a previous project and don't recall a fusible link or similar, and besides, shouldn't it have burnt up? An overvoltage relay trips over voltage, not over amperage, right? Obviously I'm missing something here, and I have feeling it's something obvious and I'm going to feel like an idiot when it's pointed out, but oh well.

Thanks,
DEM

[48RagwingPilot]

Battery contactor?

[Bruce Fenstermacher]

Depends on what was shorted to ground by the BNC. Normally the T&B and in our 170s and the stall warning indicator in B models is or was protected by a Klixon self resetting breaker. You can see in in the wire schematics if you look closely. On the later B model schematics figures 71,72 and 72A you can see it at the slit of the wire coming of the bus to the T&B and the SW. On Fig 72 and 72A it's labeled 2A for 2 Amps. On the 52 and earlier this Klixon is a mysterious round object with wires going to it mounted center above the piano keys under the panel.

If all your power went out and did not come on till the click, then you have another breaker of this type protecting the entire electrical system though I've not heard of such a thing in an airplane.

[Bruce Fenstermacher]

Or maybe as 48Rag points out you heard the battery solenoid click back in but what would have been causing that to hang up. A self resetting breaker on that circuit not normally found?

[c170b53]

If you had the battery supplying the buses, the master must have been closed. If you had a good short and a weak battery could the resistance across the coil for the master be sufficient for the master to open and then to close once the BNC ground was removed?

[c170b53]

Love it, Which eye do you open first ?

[Ryan Smith]

I let a little smoke out of 56D this evening. After showing my work off to several folks, one of my friends, who is an IA and former shop manager of the avionics shop at Air Service (bought out by Landmark), was asking to see how well the rheostat worked. I didn't have the false panel installed, so there were five cherry red lights staring us in the face. He told me to turn the lights down to a realistic level - I did, but while I was explaining to him that the lights were going to be muted some by the false panel, I hear my wife say, "uhhh..." and I look down to see a nice whisp of smoke rising from the rheostat, and the smell of burning insulation.

Master - OFF.

I was discussing this with a friend of mine a few minutes later, and while he was giving me pointers on what it could have been and troubleshooting, we had the lights back on and couldn't get the rheostat to smoke. After some more poking around, I decide to call it a night and start fresh tomorrow.

Battery - UNPLUGGED.

Hair - REMOVED BY THE FISTFUL.

Lights looked good though! That alone was enough motivation to keep me from throwing in the towel. This little project has been a pain in the ass.

[Bruce Fenstermacher]

Ryan, it could have been nothing more that a contaminant on the rheostat. It burnt off and you won't see it again. You may know but others don't, a rheostat is a coil of wire with high resistance. The knob you turn moves a wiper over the wire coil making contact on the coil at different lengths in this resistance wire. Longer length equal more resistance and your light bulbs will be dimmer but the power has to go somewhere so it is given off in heat at the rheostat.

It is normal for a rheostat to get warm, even hot and be within it's design limits. This is why the rheostat is mounted in the open under the panel. It is also why you must have a rheostat of a large enough wattage to handle the load.

With the rheostat adjusted full on there is little to no resistance hence heat created, the power all goes to the bulbs (which give off the heat). This is why you didn't see smoke till you adjusted the rheostat down and the farther down you go the more resistance and heat given off at the rheostat.

If I was doing this circuit, I'd have already figured out the load of each and every bulb or device being powered through the rheostat and I'd already be confident in my design and the ability of the rheostat I selected. So if it where me, I'd do nothing more than bench test the rheostat by allowing it to hang where it can be monitored. I'd turn on the lights and turn the rheostat to it's lowest level. And I'd monitor it. I'd feel how warm it gets and wait till I felt the heat leveled off. Today I'd use my nifty IR temperature sensor to do this if I remembered it, if not I'd swag a careful touch. Ultimately I would decide if the max heat is acceptable for me.

BTW I can't say when it was I last flew my 170 and intentionally turned on the lights. Because stuff like fuzz can get into the rheostat and scare the hell out of you as it burns off if someone unintentionally turns on the lights, I intentionally have my dimmer set full on. This way I'll only have the hell scared out of me after I intentionally turn down the lights should I ever find I'm flying and need lights.