matejcek@parvax.DEC (04/04/85)
>A while back, I posted a problem to this net, concerning the habit of my >'65 C-150 (the last of the straighttails) to turn the power back on after >the master was turned off. I got a lot of good suggestions from netters >regarding the problem, which turned out to be water getting into the master >power solenoid and shorting out the contacts. > >Now the solenoid has failed completely, and I'm into a bit of a parts >problem. I went to a local aircraft parts place to find a replacement, and >the guy looked it up in his Cessna parts book, and gave a price of $14.95. >As I had paid $100 for one to replace the original last summer, I was >a bit shocked. I gave him the parts number off the original one, and he >said, "Oh, yeah, the parts book lists a second type of solenoid... it's >refered to as 'solenoid w/diode,' and it costs $190!" > >Gulp. $175 for a diode seems a bit steep. Both solenoids were listed as >good numbers for 150Es. Maybe the $190 one is meant for the military version >of the 150 (the famed F-150). :-) > >Anyone have any ideas why that diode might be necessary? From cursory >examination of the old diode, it's across the coil, not the main contacts. >I've heard of putting a reversed biased diode across the coil to protect >IC circuitry (preventing transients into the driving chips caused by >the collapsing field when the coil is turned off), but can't see why it >would have been necessary in this case- the plane was originally delivered >with a Mk IV tube-using "coffeegrinder." The only other possibility would >be to protect the coil, somehow. Assuming the replacement solenoid I put >on last summer didn't have the diode, that might explain why if failed aft >er 30 hours. > >I have ordered the cheaper solenoid, and, depending how my thought processes >go, may install a power diode externally. I've been told I don't need an STC, >as the change is not on a flight-critical item, and I have a friendly A&P >willing to sign it off. Any thoughts on this option? > > First, be careful about assuming that the ONLY difference in the two solenoid assemblies is that one has a diode across the coil or the contacts. It's a logical assumption, but not necessarily true. As for the purpose of the diode -- yes, it's certainly there to dampen inductive spikes when the power is removed. The coil of the solenoid is an inductor, and when you remove the supply of current, the magnetic field begins to collapse, inducing a current in the coil and a potentially (pardon the pun) large voltage across the coil. That induced 'spike' can do all sorts of interesting things. It can cause an arc across the contacts of the solenoid, eventually pitting them and causing them to fail (stick closed, make a poor contact, et cetera). That spike can also get into whatever electronics are on board and wreak havoc. Even if you have all tube type avionics -- which are far less susceptible to damage from transients than are solid state components -- your voltage regulator is at risk. If it's solid state, you may fry a component; if it's mechanical (relays) you may burn contacts a la the contactor solenoid. A large diode across the coil gives the induced current a harmless path. If you decide to install one yourself across the more inexpensive solenoid, be sure that it has a low enough forward resistance that only a small potential will develop when the field collapses. Use a real hulking component that's rated at least ten times what you think it should need (sorry -- I don't know what that would be). You want it to be massive (relative to the application), because if it fails, the first symptom will be when the next item fails. Have fun!