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edpav8r(at)yahoo.com
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 Posted: Mon Nov 17, 2014 9:12 am Post subject: Flyback Diodes |
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The following video was just posted by a fellow on YouTube who I follow. It does a very nice job of illustrating the purpose and effect of using flyback or catch diodes on inductive loads like relay coils.
Since the topic has arisen here a number of times, I thought this might be of interest.
http://www.youtube.com/watch?v=c6I7Ycbv8B8
Eric
[quote][b]
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Mauledriver(at)nc.rr.com
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| Posted: Tue Nov 18, 2014 6:24 am Post subject: Flyback Diodes |
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So, the diode we find jumpered between the coil contacts on let's say, our starter relay, prevents a high voltage shooting back thru our starter switch and arcing the contacts when we let go of the starter key/switch?
Or does it prevent a power surge from hitting the entire electrical system?
Bill "waking up brain cells in disuse since early adolescence" Watson
(nice video, thanks!)
On 11/17/2014 12:07 PM, Eric Page wrote:
[quote]
The following video was just posted by a fellow on YouTube who I follow. It does a very nice job of illustrating the purpose and effect of using flyback or catch diodes on inductive loads like relay coils.
Since the topic has arisen here a number of times, I thought this might be of interest.
http://www.youtube.com/watch?v=c6I7Ycbv8B8
Eric
[b]
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nuckolls.bob(at)aeroelect
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| Posted: Tue Nov 18, 2014 11:23 am Post subject: Flyback Diodes |
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At 08:19 2014-11-18, you wrote:
So, the diode we find jumpered between the coil contacts on let's say, our starter relay, prevents a high voltage shooting back thru our starter switch and arcing the contacts when we let go of the starter key/switch?
Yes . . .
Or does it prevent a power surge from hitting the entire electrical system?
Not at all. Contactor/relay coil flyback, catch or suppression diodes have been part-and-parcel of the system designer's toolbox for quite a few years. While I was a tech writer at Cessna (65-69), our chief scientist, Gordon Wood, Phd, purchased this nifty peak-reading voltmeter (Hewlett-Packard I think) and did some battery and starter contactor behavior studies in the labs.
"My gawd Mabel . . . did you see the size of that spike?!?!?! Gee whiz . . . over 400 volts . . . that sounds dangerous."
This was exactly the time when Aircraft Radio Corporation, recently purchased by Cessna, was producing Cessna branded 300, 500 and ultimately 400 Series radios and autopilots.
When the radios began to show up with transistorized audio/modulator and dc/dc converters for vacuum tube high voltage, we began to experience a rash of un-explained failures in the transistors of newly installed radios. We were building 10,000 airplanes a year back then . . . needless to say, even a few percent failure rate in radios was a SIGNIFICANT impact to corporate bottom line.
The quest was on . . . if not to understand what was really happening . . . then at least to stop the failures. About that same time, Cessna hired Wichita University to craft a course on "The Industrial use of Transistors," which was well attended by about 20 folks from both Cessna plants and your's truly.
The instructor was extremely well versed in the topic being that he was both a college prof and a sought after consultant. Not a great teacher . . . but Jerry Wedel I learned how to 'pick his brain' in useful ways by the questions we asked. We sat on opposite sides of the class and stroked him to our great advantage.
Back then, the power transistors of choice were germanium, mostly PNP devices with max operating voltages on the order of 30 volts. Hmmmm . . .
Mean while, back at the airplane farm, Gordon was poking around the electrical systems of our airplanes in search of clues. It was quickly discovered that adding the diode across the contactor coil completely eliminated the gawd-awful spike. Whew! Slew that dragon. But that still didn't fix the radio failures. About then, flight test pilots discovered that if the radios were OFF during engine start, the did not suffer the failures.
You know what came next. The avionics master switch was born. Simultaneously, a new line of thought was launched into the aviation consciousness . . . and a new phrase was oft repeated on avionics benches across the world, "Damn, I think a spike got it."
It took about two decades before anyone really began to sift the simple-ideas for properties of materials and management of energy (it's called engineering) and figure out what was really going on. Turns out that transistor failures in the radios were not suffering the effects of high voltage spikes . . . but second breakdown effects in relatively fragile transistors due to LOW voltage effects, i.e. brownout during cranking.
Batteries back then were pretty sorry things by today's standards. Our 'airplane patch' east of the plant would have hundreds of new airplanes parked out there awaiting ferry pilots. The batteries were NEVER attended to in a manner consistent with their physics . . . hence, more that a few airplanes needed to be propped, jumped and/or battery charged . . . but only AFTER the pilot had attempted to start the airplane perhaps with one or more radios turned ON.
It was also not well understood that those spectacular contactor coil spikes never went anywhere . . . at least not out onto the ship's electrical system. 99% of energy stored on the contactor's inductance was dissipated in the air-gap of the spreading switch contacts as the battery master was turned OFF or the starter switch was released.
The diodes across the coil were a good thing, it saved wear and tear on switches . . . but had no significance on the life span of a radio or any other appliance.
In still later years, there was some earnest debate about OTHER effects of adding diodes across relay or contactor coils. Simple measurements demonstrated that adding the diode caused there to be a DELAY in relay/ contactor drop-out . . . the diode cause coil current to be sustained for some milliseconds after the switch opened as energy stored in the coil was dissipated in circuit resistance.
Hmmm . . . if the contactior's release is delayed/slowed, does that also translate into a slower contact spreading velocity which in turn aggravates the arcing? Intuitively it seems likely . . . indeed, many articles have been written, some by folks working for big-name companies that thoroughly described the diode induced drop-out DELAY . . . and then extrapolated that good information into an erroneous assumption that influences on drop-out delay were equally applicable to contact spreading velocity . . . without making a single measurement.
On page 4 of this document
http://tinyurl.com/n296nl6
there are 'scope displays that demonstrate approx 5x increase in dropout delay for having added a plain-vanilla diode across a relay coil.
At the same time, the contact spreading times are very similar demonstrating no significant difference in the arc signature.
Bill "waking up brain cells in disuse since early adolescence" Watson
(nice video, thanks!)
The youtube tutorial was nicely done and confirms the value of adding SOME form of coil spike suppression . . . but to extrapolate the demonstration's significance for other features of relay performance is fraught with opportunity for error.
Bob . . . [quote][b]
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