The International Cessna^® 170 Association

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In my electrical junk box I found this and-- until today -- had no idea what it was. In studying SK172-22M item #9 is p/n CL1-18. Googling the p/n came up with the part I recognized from my stash as well as this picture: My part is polarity sensitive, i.e. it acts like a diode. The cylinder pictured looks more like a capacitor to me. Years ago Miles wrote

cessna170bdriver wrote: ↑Wed Sep 23, 2009 11:59 pm... If you feel you need to protect avionics from this small spike, and the spike generated by the collapsing magnetic field in the aircraft wiring, place an identical diode from the power bus (or the bus side of the master contactor) to ground. Orient the diode with the band on the bus end. ...

Here's a magnified shot of the "suppressor", p/n CL1-18 from SK172-22M, (#9 in the hexagon):Take a look at the direction of the diode. It appears to be opposite of how Miles suggests installing it. Wouldn't one have a dead short to ground installing it as depicted?

Cool, an old selenium rectifier. If the schematic really shows the "arrow" pointing left, then it's wrong, as you said. The diode symbol should point to the right, so current will flow if the bus voltage drops below ground.

The color convention in a selenium diode is: current flows yellow to red. The selenium diode material is the deposited stuff on the red side of the plate.

The cylindrical thing is a capacitor.

The diagram does appear to have the diode symbol backwards; misprint maybe? I know nothing about selenium rectifiers but if what Dave says is true then connecting the colored terminal as depicted is correct.

Thanks guys! I'm always amazed at the depth of your knowledge in abstruse electrical subjects such as this.

The reason I'm back to studying Cessna's "Generator to 60 Amp Alternator Conversion" Service Kit SK172-22M is that I need to rectify (pun intended ) some paperwork for my 170. Back in 1985 an A&P/IA filed a 337 field approval with the FAA in OKC but, as I recently noticed, he neglected to get the local FSDO's approval stamp in block 3. On the back of the form he referenced the C-172 drawing 0500062 in the '62-'68 Cessna 100 series Service Manual. I asked my PMI what to do about this and he said to file another 337 for him to approve. So I'm comparing drawing 0500062, the drawing shown in SK172-22M, Zeftronics' diagram for the installation of their R15V00 regulator, and the nineteen 337 form examples of this conversion in our Maintenance Library.

Looking over all the S.E. Cessna wiring diagrams I have, I don't find this "supressor" anywhere. So my question is should I install it as shown in SK172-22, leave it out, or replace it with something more up to date? Is it a solution to a problem or a potential problem? In this video the guy says selenium rectifiers are, "... notoriously prone to failure which can result in a fire and emission of toxic fumes."

Regarding the orientation of the rectifier, it appears that the Cessna drawing might not be a misprint. Very confusing. A commenter in the above video writes, "IMPORTANT! Selenium rectifiers almost always have their polarity marked incorrectly. With modern diodes, the cathode (striped end) is negative and the anode (non-striped end) is positive. Seleniums, however, have their cathodes marked as positive and anodes marked as negative. Please consider this when replacing selenium rectifiers and consult the schematic for your device for proper diode orientation."

It’s easy enough to check diode polarity with a VOM.

n2582d wrote: ↑Wed Jul 03, 2024 5:48 pmRegarding the orientation of the rectifier, it appears that the Cessna drawing might not be a misprint. Very confusing. A commenter in the above video writes, "IMPORTANT! Selenium rectifiers almost always have their polarity marked incorrectly. With modern diodes, the cathode (striped end) is negative and the anode (non-striped end) is positive. Seleniums, however, have their cathodes marked as positive and anodes marked as negative. Please consider this when replacing selenium rectifiers and consult the schematic for your device for proper diode orientation."

Yes, very confusing. In the days of vacuum tube electronics, current was thought of as the flow of electrons, i.e., from negative to positive. An antique-radio hobbyist friend tells me that the drawing convention for diodes changed when solid-state electronics came along.

Have you ever seen a diode connected directly to an alternator bus as shown in the SK schematic? I haven't researched this specifically, but I've never seen one on any of the five airplanes I've owned. Maybe it's more applicable to generators than alternators? I'd be inclined to leave it off, but if I did install one it would be a high-current silicon diode. Sudden release of toxic smoke seems like a bad idea in an airplane.

Here is a chart of various/common diode symbology…(triangle points to direction of flow):

GAHorn wrote: ↑Thu Jul 04, 2024 6:21 pm Here is a chart of various/common diode symbology…(triangle points to direction of flow):

Common diode symbology for semiconductor devices, but not for selenium rectifiers or tube diodes. Their symbols point the other way, as in SK172-22.

Regardless of the markings, if you connect a diode across the coil of the master contractor it should NOT conduct with the coil energized. Its purpose is to suppress the large negative voltage spike generated by the coil when it is denergized. That negative spike can be upwards of 200 volts across an open circuit.

Perhaps the confusion lies with the difference between “conventional” current-flow …versus “electron” flow…

The “conventional” symbology follows the logic of “positive” being a surplus and “Negative” being a deficiency… so the “conventional” thought is that current flows from positive to negative.

However, electrons are “negatively” charged…. and the “electron” flow is actually from tne “-” or negative-post to the “+” post. (the Right side of the illustration is the “+” side and the Left side of the illustration is the “-“ side.

Hmmmmmm! Must have been designed by Lucas, what with their positive earth.

johneeb wrote: ↑Fri Jul 05, 2024 4:59 pm Hmmmmmm! Must have been designed by Lucas, what with their positive earth.

LOL….

Actually…. it was probably Ben Franklin and his experiment with Wax and Wool that created the backward-conventional-thought on current-flow.

I once owned an almost pristine ‘51 Ford P/U (surplussed out from the Dept of Agriculture with a new AER motor in it) that had a 6-volt Positive-ground battery mounted beneath the pax-side floorboard. In order to add a decent radio and C.B. …I added a 12-volt generator to the V-belt system and tied-in a motorcycle battery to it under the hood.

My idiot brother decided to go into real estate selling desert lots outside El Paso and needed a “first sale” to keep his new job…and I’d broken my leg hitting a deer with a motorcycle…. lost my job because I was in a cast for 3-months….. and couldn’t drive it anyway……so I let him talk me into selling that cast-iron P/U for $150 so he could pretend he’d sold a down-payment to “Horizon Properties”.
I guess my family had TWO idiot brothers…..