Oscilloscope on electrics

Hi Peter. I have a pretty good oscilloscope, but hardly ever use it nowadays. If you have a perfect recifier, you should see a classic full wave rectififier output, consisting of half sine waves joined together, all on the positive side of the neutral line. However the old selenium diodes had a very high forward voltage so you will get steps in the waveform as the voltage crosses the zero line. With modern Silicon rectifiers these steps should be much smaller. Without any load on the recifier such as the battery you may well see switching spikes that are not damped off. It will be very apparent if any diodes have failed, so quite a good test and better than just a simple voltage check.

Les

Hi Pete. It seems strange that now the fault has gone you are getting only half the charging current as before.If you were getting some AC component the ammeter would be more likely to show a smaller deflection as the reverse current flow would cause a negative pull on the ammeter andthus partially nullify the charging current indication. If the readings were the other way round; 2 amps with fault and 4 amps with fault cleared then youmight have some chance of being correct with your AC assumption, but it seems at odds with your theory.

It may well have been a carburation fault, a small air leak or pilot jet blocked that caused the the original misfire.

If you can get to analyse the output of the rectifier with a 'scope you will see straight away what is wrong. otherwise you can block any possible ac component by fitting a single silicon diode on the rectifiers output and check that the charging current is the same. You will have to make certain the diode is correct polarity wise but no damge will be done if reversed. Best to have at least a 10 amp device, but they are very chep nowadays. In fact a 25 amp bridge rectifier can be brought from MAPLINS foronly a few quid so you could replace the original and get a better charging voltage as the forward voltage drop will be less and the internal resistance is virtually zero compared to the old selenium one.

Les

Hi Peter. Yes a silicone junction requires about 0.65 volts before conduction takes place. I'm not exactly sure of the selenium forward voltage but the selenium diode suffers from a high resistance and this will manifest as a higher and higher voltage drop across it as the current increases andwill waste energy as heat . Thedrop across a silicone junction will hardly increase at all as the current rises, so less energy is wasted.

Les

Previously wrote:

Hi Les, thanks for that, my navigator is +ve earth, so the opposite would apply, but all makes sense.

The reason I'm asking is that my bike tended to miss a bit when coasting, ie when the throttle was just closed a small amount from cruising, and I couldn't track the fault. I tried points, condensers, and changing over the coils. It wasn't until I removed the rectifier (a flat thing about 3" x 3" so its probably an original selenium type), cleaned all the contacts checked each diode for forward and reverse current, that things improved. Although the ammeter now shows a charge (no lights) of 2A whereas before it was 4A, and I was wondering if I had some AC component going to the battery, which would certainly mess up the ignition, and get worse as the battery drained.

Cheers, Pete.

Les Yes it is highly likely that there was AC coming through the Selenium rectifier, they were poor when new, when old!!! Please throw it away and I can sell you a decent silicon bridge rectifier for £5 with a proper specification. Now the ammeter reading high is due to the fact that these cheap ammeters we have on British bikes are moving iron and they respond as well to AC as well as DC so it would seem very likely that you did have AC giving you a miss fire and apparently excessive battery charge.

Use of an oscilloscope might not show up anything useful as you will have large spikes on the battery line due to ignition-will this be cause or effect??

Hope this helps. Al Osborn.

In reply to Al Osborne.

Car and motorcyle ammeters ARE NOT moving iron meters. They are what is known as ZERO CENTRE meters. they differ from moving iron meters in that that they have a small bar magnet sytem and the needle is centralised with a spring arrangement.

As you will know, magnets have a north and south pole and from Flemmings left hand rule Theory, when the current is reversed the force is reversed.If Auto ammeters were made of the moving iron type they would not have any ability to show a discharge ie: when the current flows ina reverse direction through the meter, it would always move in a positive direction. Not very useful for a battery charging system!

If you apply an AC current through a ZERO CENTRE ammeter you will see NO deflection as the needle is pulled one way and immediately pulled the opposite way with the same force. With a very low frequency you may see a small oscilation corresponding to the ac frequency.

Therefore, if a rectifier was passing a certainAC component combined with the DC current, the meter will only register the DC current and not indicate theAC current.As the alternator will only be making the same current at any given revs, anyAC part that has not been rectifiedwill result in aLOWER total current shown by the meter, by the fact that the AC current has not been registered by it and only the DC part. The effect will be that the indicated current will be LOWER...just the DC and not the AC.

Les

Previously wrote:

In reply to Al Osborne.

Car and motorcyle ammeters ARE NOT moving iron meters. They are what is known as ZERO CENTRE meters. they differ from moving iron meters in that that they have a small bar magnet sytem and the needle is centralised with a spring arrangement.

As you will know, magnets have a north and south pole and from Flemmings left hand rule Theory, when the current is reversed the force is reversed.If Auto ammeters were made of the moving iron type they would not have any ability to show a discharge ie: when the current flows ina reverse direction through the meter, it would always move in a positive direction. Not very useful for a battery charging system!

If you apply an AC current through a ZERO CENTRE ammeter you will see NO deflection as the needle is pulled one way and immediately pulled the opposite way with the same force. With a very low frequency you may see a small oscilation corresponding to the ac frequency.

Therefore, if a rectifier was passing a certainAC component combined with the DC current, the meter will only register the DC current and not indicate theAC current.As the alternator will only be making the same current at any given revs, anyAC part that has not been rectifiedwill result in aLOWER total current shown by the meter, by the fact that the AC current has not been registered by it and only the DC part. The effect will be that the indicated current will be LOWER...just the DC and not the AC.

Les

I have found in practice that having a failed (in this case shorted) rectifier on a bike gave a very high reading on the bikes ammeter and when compared to an AVO the real DC reading was half (correct). Replacing the rectifier got the ammeter reading correct. The DC voltage appeared correct before and after the rectifier change. The alternator checked out as being correct also. So both myself and the bikes owner were puzzled with this excessive reading, until the rectifier was changed.

By the way going back to the original misfire on this bike when cruising, re reading it makes me think that the owner has a worn/rich carb. When accelerating we need a rich mixture (you have it) and you need best spark, that works, when cruising you no not need such a good spark nor do you need the extra richness so it missfires. But then by running on battery only you have shown that it was/could be AC via the selenium rectifier-bin it.

Al Osborn.