angus67
Welder's penetrate deeper!!
If i read correctly, permanent magnet alts dont have this issue because they are self enegized.
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Alternator rotor Inductive Kickback
- Thread starter TwoManyXS1Bs
- Start date
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- Tags
- electrical
Hey, gggGary. I've seen some of those Pamco "spark at key-off" posts, and had the same thoughts. Figured that was Pete's department, he knows what supply line protections are in his products. But, this phenomenon is strictly limited to the "excited field" type of alternator, doesn't apply to PMAs.
Since we're unsure of the condition/quality of someone's solid-state rec/reg, TCI, aftermarket ignition, and exotic accessories in use, I'd recommend this mod to anyone who wants that extra layer of protection...
Absolutely. They're just bigger than what you need. The starter solenoid is much smaller than the rotor and would have much less inductive kickback. The 1N400x type diode is fine here, as per member Signal's post. Turns out that the highest rated, 1N4007 diode, is more plentiful and cheaper than its lower rated siblings...
Right on, Angus...
Tomterrific
XS650 Junkie
The Pamco throws a spark because the coil is energized and it is just like points opening when the key is turned off.
Tom
Tom
Nice work TM all those months ago. I notice the spike dropped to 1V and this will be the diodes forward voltage - I think the quoted 0.7V for silicon diodes is an RMS value. Pulled some LED rear brake lights apart to steal the COBs to make a super bright high level rear brake light. I noticed that they were all fitted with a diode, presumably to protect against this reverse induced voltage. I have now fitted a diode across my field winding. I realize I have an electronic regulator and for sure that must have internal diode protection, but this did not save me from blowing a small voltmeter last year.
I almost went for the USB scope (remote sampler, uses your computer display), but decided on this portable pocketsize standalone unit. Quite happy with it, for the lower-freq stuff...
The last one I used was the HP 54600A near the end of this walk down memory lane article 
We had two of them. I used it to debug timings.
http://hpmemoryproject.org/wc_pages/wall_c_page_01.htm
Only one I've personally owned was this Heathkit model that I bought from a radio guy for $20 about a hundred years ago. It was still working when we sold it at a yard sale about ten years ago.
https://www.ebay.com/i/323224134969?chn=ps
We had two of them. I used it to debug timings.http://hpmemoryproject.org/wc_pages/wall_c_page_01.htm
Only one I've personally owned was this Heathkit model that I bought from a radio guy for $20 about a hundred years ago. It was still working when we sold it at a yard sale about ten years ago.
https://www.ebay.com/i/323224134969?chn=ps
FB71
Expert Turd Polisher
ah, Heathkit! Had one near me growing up, my dad built several things from them, a scope included. My first scope was a Hitachi 20MHz analog 2-ch. Still have it in the basement...
These diodes are also referred to as 'clamping' diodes. Similar results can be obtained with a high value resistor, with wattage relative to the available current. I prefer resistors on lower current devices like starter solenoids, as they're not polarity sensitive, and less sensitive to elevated flyback voltages.
Incidentally, I had used a high brightness LED for a neutral lamp on the chop I built, and experienced failure immediately after the first shut down. I realized exactly what caused it right away. I paralleled a 10k 1/4W resistor and small ceramic capacitor across the LED, and that solved the concern. I hadn't considered flyback suppression during construction, as it was a simplified electrical system, using a 1980 charging sys, including vreg, and running points on a waste-spark coil with solid conductor wires, and no resistor caps, and Incandescent head and tail, no signal or instruments.
These diodes are also referred to as 'clamping' diodes. Similar results can be obtained with a high value resistor, with wattage relative to the available current. I prefer resistors on lower current devices like starter solenoids, as they're not polarity sensitive, and less sensitive to elevated flyback voltages.
Incidentally, I had used a high brightness LED for a neutral lamp on the chop I built, and experienced failure immediately after the first shut down. I realized exactly what caused it right away. I paralleled a 10k 1/4W resistor and small ceramic capacitor across the LED, and that solved the concern. I hadn't considered flyback suppression during construction, as it was a simplified electrical system, using a 1980 charging sys, including vreg, and running points on a waste-spark coil with solid conductor wires, and no resistor caps, and Incandescent head and tail, no signal or instruments.
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Sometimes referred to as a snubber circuit. I see them often in mains equipment with inductors. They use a rectangular 0.1uF capacitor that looks like a small match box in series with a resistor (I think 400 Ohms) connected parallel with the inductor. This circuit I believe changes the rate at which the inductor discharges so that the induced voltage is lower. That's my understanding anyway - could be wrong. You should see then in switched mode power supplies if you open them up. The obvious clue is the capacitor is stamped for something like 240V or 300V AC.
Look up "lc tank circuit". If the expected back EMF is significant, it can be more economical to use a damping tank than a high-current diode...
Very interesting thread 2M - I must look into this.
Pete
Pete
I made little brass forks and soldered them to a 1N4005 diode and fitted directly across the brushes. Tight but comfortable fit:
You can make out the diode's outline under the blue heat shrink.
You can make out the diode's outline under the blue heat shrink.
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Looks good, Paul. May want to test that 1N4005 diode after some usage, it could be a marginal implementation. I'd be curious to know if it holds up.
The 1N400x series handles 1 amp continuous, 30 amp surge currents.
Those values should be derated 50% due to being in a hot >100°C environment.
The published surge duration time limits are 8.3 ms.
From the scope trace, I'm interpolating about 12ms surge duration, max 3 amps.
Total drain duration of about 160ms.
The 6A10 will likely function much better in that location, a larger safety margin.
1N400x and 6A10 spec sheets attached.
The 1N400x series handles 1 amp continuous, 30 amp surge currents.
Those values should be derated 50% due to being in a hot >100°C environment.
The published surge duration time limits are 8.3 ms.
From the scope trace, I'm interpolating about 12ms surge duration, max 3 amps.
Total drain duration of about 160ms.
The 6A10 will likely function much better in that location, a larger safety margin.
1N400x and 6A10 spec sheets attached.
If the 6A10 is too big for that area (9mm dia), you could also try a 1N5408, 3amp, 1,000v, 200amp surge, 5mm dia. Pack of (10) going for $1.90.
https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.com/ulk/itm/173476430995
https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.com/ulk/itm/173476430995
There is a very good rational behind me using a 1N4005 diode that most will not be aware of - I own several! TM I will see what I can get hold of as an upgrade. If they are too big then I will fit one near the harness connector as you did. I will do a diode test to see how its holding out.
Thank you TM.
Thank you TM.
Haha, that's also MY battle cry.
Junk drawers full of stuff looking for a home...
Edit: Nothing wrong with putting a pair together in parallel...
I have pulled the Diode and tested. I have two meters, one gives Ohms on Diode test and the other gives mV on Diode test:
1N4005 from across alternator brushes: 454 Ohms ( 504mV ).
Four unused 1N4005 Diodes: 466, 468,469 and 471 Ohms ( 524,524,525 and 525mV ).
Note: All diodes are from the same batch.
Do I conclude the Diode across the brushes is slowly deteriorating?????
1N4005 from across alternator brushes: 454 Ohms ( 504mV ).
Four unused 1N4005 Diodes: 466, 468,469 and 471 Ohms ( 524,524,525 and 525mV ).
Note: All diodes are from the same batch.
Do I conclude the Diode across the brushes is slowly deteriorating?????
I just ordered a 10 pack of 1N5408 Diodes at £1.49 including p+p. Now what do I do with the 9 leftover???
