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scr polarity 06 Nov 2016 14:19 #747428

  • nads.com
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1985 honda big red. No spark. Exciter coil has50+ ac volts. Pulse coil zero. Took off engine cover and the magnet on the outside of the wheel for the pulse coil was dead or near. From ebay pics, it looked like a magnet used to sit on top the raised steel block i mistook for a magnet. I put my own magnet on it and now how have apulse thats .3 or +. Thats what was qouted as the spec. Still no spark and 2v to the charge wire to the coils. My question to all is.... Is .3v enough to trigger or open the gate of an scr, or thyristor, or whatever is in the cdi? Is the scr polarity sensitive and wont turn on if i installed my magnet with n and s poles opposite the orginal magnet? And if polarity of the pulse is now reversed can it burn out the new cdi. Dave i know u know what i need to know here and hope to hear from you! Hondas need lovin too.

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Last edit: by nads.com. Reason: add words

scr polarity 07 Nov 2016 08:15 #747487

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nads.com wrote: 1985 honda big red. No spark. Exciter coil has50+ ac volts.

Trikes will get you killed.

nads.com wrote: Pulse coil zero. Took off engine cover and the magnet on the outside of the wheel for the pulse coil was dead or near. From ebay pics, it looked like a magnet used to sit on top the raised steel block i mistook for a magnet. I put my own magnet on it and now how have apulse thats .3 or +. Thats what was qouted as the spec.

Do you have photos? Usually the magnet on the rotor passes by the stationary pulse coil. Is that what you have now? How do you know if you have the magnet polarity correct? Have you tried both ways? Do you know if the poles are facing the correct orientation (not just polarity, but actual pole directions as they pass the pulse coil)? You probably need to somehow get a factory magnet to know how it is oriented with regards to poles.

This link will show how to determine a magnet's north/sout polarity.
gpzweb.s3-website-us-east-1.amazonaws.co.../PickupAndRotor.html

nads.com wrote: Still no spark and 2v to the charge wire to the coils.

This pulse happens once per rotation? Measured with a regular DC voltmeter? DC Voltmeters measure *average* voltage, so it is difficult to judge what the actual *peak* voltage spike was to the coils... but 2v seems very low, even for an average. This could mean the capacitor ("C" in "CDI") may be bad, or the trigger is happening at the wrong time (due to wrong magnet polarity or orientation), or the SCR is bad, or other things I haven't thought of.

nads.com wrote: My question to all is.... Is .3v enough to trigger or open the gate of an scr, or thyristor, or whatever is in the cdi?

0.3v seems pretty low. But again, are you using a typical DC voltmeter? If so, then you don't really know what the peak pulse was. Also, the pulse could be AC, in which case the .3v DC reading is almost meaningless.

nads.com wrote: Is the scr polarity sensitive and wont turn on if i installed my magnet with n and s poles opposite the orginal magnet?

SCR's have a polarity for their main current, but I assume you are asking about their trigger signal. The trigger needs to be the correct polarity to trigger the SCR. If the trigger signal is not rectified (normally they are not), then the pulser signal is AC. So if the polarity is wrong, the pulse timing may be off, and the peak value may be different from what is expected, so it may or may not trigger depending on the details of the signal.

nads.com wrote: And if polarity of the pulse is now reversed can it burn out the new cdi.

Typical CDI systems for dirt bikes are crude, and there is not a whole lot to them. So normally, having the polarity wrong should not harm the SCR.

You can possibly test the CDI system by using a 1.5v battery in series with a 100 ohm resistor.
Disconnect the pulser, and put the 1.5v battery and resistor in its place. Rotate the crank a few times and connect the battery to the CDI input. This should trigger the CDI and you should see a spark.

The battery negative goes to ground. The positive goes to the resistor, and the resistor feeds the CDI module (where the pulser would connect). If both wires of the pulser went to the CDI module, then don't ground the battery. The battery negative will go to the second wire, and you have to try the battery in both directions to get the polarity correct.

If that works, then there is some problem with the pulser and/or magnet.

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Last edit: by loudhvx.

scr polarity 08 Nov 2016 18:39 #747602

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I removed the pulser coil and with my hand passed a magnet across it back and forth. The reading was .3v ac vs 0v ac when the pulser was on the moter. I checked the mag fields of the magnet by flipping every direction. I insttalled the magnet onto the flywheel the so that it attracted itself to the platform the old magnet was on. The platform was slightly magnetized and if i flipped the magnet over it would repel. I put the pulser in and 2 to 3 revolutions and it showed .3v ac consistant. But i only got 2v ac to the coil and i remember testing a an 80's articat and it had 90+. I think the capacitor is shorted, if the newcdi box doesnt create spark, ill use a battery to determine wether the polarity of the magnet is right. How many volts is a 1.5amp battery with 100ohms of resistance? And yes it has a pulser in the cover and external flywheel magnet. People say the honda mag only shows ohm tests, someone said .3was the spec, but i cant go by that. So a cdi module does not convert ac from the pulse trigger to dc and then the thyristor? I wish i had a flywheel from another moter so i could check the polarity.

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scr polarity 08 Nov 2016 22:33 #747612

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The trigger can receive AC, but will only be triggered by the positive part of the signal (relative to ground).

As I said, DC voltmeters don't measure peak voltage, only average voltage, so it's hard to say if 0.3 is high enough or not.

I'm not sure what you are asking by this question:

"How many volts is a 1.5amp battery with 100ohms of resistance?"


If you are referring to my suggestion of using a battery to trigger the module, I mentioned a 1.5v battery. Not a 1.5 amp battery.

It can be a AAA or AA battery.

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scr polarity 09 Nov 2016 05:53 #747620

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I meant 1.5v for the battery. Dvms cant measure peak voltage, avereage rms only, dvms arent fast enough, etc. Even my harbor freight dvm will read very low ac, and a dvm worked better than my analog olsen meter. So im not sold on a dvms inability to catch voltage fast enough, or high enough. I can always tell whats going on by the readings. i was releived to see u posted, i was hoping you would. Your knowledge is indispensible, what you told me is going to be the saving grace. Thank you so much for sharing what u know. Ill let u know what happens when i plug new cdi box in.

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scr polarity 09 Nov 2016 18:17 #747677

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I wish I had more specific info for your application.

It's pretty common knowledge that standard DC voltmeter's measure average voltage.
But I also confirmed it using function generators and oscopes. IT was all part of an investigation on AC voltmeters. AC voltmeters use different methods of approximating RMS voltage. Some methods are better than others.
Here's my write up on it:
gpzweb.s3-website-us-east-1.amazonaws.com/ACvoltmeter.pdf

If you still doubt it, you can test it yourself. Get a 9v battery and measure it with a DC voltmeter. Then touch the DC meter very briefly, like 1/100th of a second. The meter will read a much lower voltage.

My point is, the meter will not give you a peak value of that pulse. But if you know the time value for the meter's averaging function, you can certainly get a very rough estimate based on the reading, if you use a known peak as a reference.

when I do that test on a Harbor Freight cheapo meter, a 9v battery measures about 9.3v when held steady, but with a glancing strike on the battery terminal, I get far below 1 volt, and it varies with how quickly I touch the battery.

Regarding the CDI system, I have a page describing various ignition types and their general circuit topography. Scroll down about 1/4 of the page to see the section on CDI.
gpzweb.s3-website-us-east-1.amazonaws.co...esAndCoilWiring.html
If you have a specific schematic, that would help to give more specific troubleshooting advice.

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scr polarity 12 Nov 2016 13:59 #747826

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I couldnt find a cdi box so i banged on the old one. Cranked the engine kick start and had 20v or so to the coil. Grabbed the plug wire and could feel a tickle. Put the plug in and spark. Put the carb on and with no exhaust pipe, fired it up. Sounded good. Think its n pole passes then south. Ill check with a compass since i took the cover off again, but my magnetic instincts tell me its cool all up there. Thanks again loudx. Though shalll let no mystery go unsolved. Onward

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Last edit: by nads.com. Reason: add words

scr polarity 13 Nov 2016 08:19 #747860

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Nice job! B)
Though shall not let any post go without being entertaining! :cheer:

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