Small engine charging systems

[RS700L]

I'm new to electrical testing and could use some help.

I know how to tell if the charging system is working or not, but I don't know what makes it tick.

Say that I determine my engine is not charging the battery. Where do you go from here? I understand there can be a diode somewhere in the system? What does it do? What else do I need to know?

 

[AandPDan]

In a typical small engine there is a magnet on the flywheel. Under the flywheel there is a stator, a stationary coil of wire. When the magnet passes by it induces current flow in the wire.

It makes AC, alternating current. In order to charge a battery it needs to be rectified to DC, direct current, using a diode. A diode is like a one way valve.

Many small engines are unregulated. The voltage varies with the RPM of the engine. The larger sizes usually incorporate a regulator to prevent overcharging the battery.

There is also a "lighting coil" option, found on many snowblowers. This has the magnet and stator but no diode. The light bulb doesn't care AC or DC.

Some engines have multiple coils. One for the battery charge, rectified, and another coil for lighting.

Hope it helps.

 

[RS700L]

Thank you. I did not realize that it was actually making AC current and rectifying.

Can you tell me how a diode changes AC to DC?

 

[Nick R]

Think of a diode as a one way valve.

A/C flows forwards, then backwards, then forwards, then backwards (simplifying). A diode only allows it to go forwards. Think of it like a check valve, a one way valve. It flows forward, but doesn't go backwards. This would usually result in a binary type signal, on at the "forward" of the A/C current, and "off" during the "backward" There are usually several diodes linked together to provide a clean, linear DC power delivery.

 

[Nick R]

Edit, though his offical account was shutdown, he was the best at explaining things in laymans terms.

 

[AandPDan]

Originally Posted By: Nick R
There are usually several diodes linked together to provide a clean, linear DC power delivery.


Not in most small engines. They usually only have one diode, that's all you need to do the rectification. Yes, it does pulse, on and off, making it pretty "dirty." Putting a group of diodes in series won't smooth out the power, it'll still pulse.

In an automobile alternator, there are usually six diodes. These are different than the small engine in that they make 3 phase AC which is then rectified. It's much more efficient and provides for a very clean power output.

 

[RS700L]

Thanks, I did not know the difference between AC and DC.

Ah yes, Dave's Farm. I had forgotten he did that video...didn't really want to know about it at the time either.

Thanks again.

 

[brianl703]

Most small engines (that I've seen) use a bridge rectifier, which consists of 4 diodes.

 

[AandPDan]

Nothing wrong with the bridge rectifier, it is more efficient. Lots of Briggs and Tecumseh's have just a diode in the harness though, it's much cheaper. It really depends on how many amps you need. Many of the bigger versions in the lawn/garden tractors have regulators.
Briggs and Stratton small engine charging systems

 

[RS700L]

What is a bridge rectifier?

 

[AandPDan]

Basically it is 4 diodes arranged so that it can use both side of the AC waveform. One diode alone would just "clip" off one side to make DC, but it would be very "choppy." A bridge rectifier takes the other side of the waveform and uses it as well.
Bridge rectifiers

 

[RS700L]

Makes good sense, and I have seen those.

Thank you!