Re: rectifier -tach issue
Subsequent to my previous post I have put together a document on bridge rectifiers....
How to check a rectifier.
The rectifier on your outboard is a ‘bridge rectifier’ consisting of 4 silicon rectifier diodes, arranged in a ‘bridge’ pattern.
Let’s start with WHAT a diode is. Basically, a one way valve. It will allow current to flow in one direction but not the other. For those familiar with hydraulics it is exactly like a check valve. And just like a check valve it has a ‘cracking pressure’. In the case of these silicon diodes, that cracking pressure is about 0.7v, which means for us checking them, we need to use a meter designed to check diodes. The normal resistance measurements of a multimeter are not sufficient to check diodes. The ‘diode check’ function of a multimeter reads out the voltage drop around the diode being checked. We are looking for about 0.7v in the forward direction (the direction which allows flow) and infinity in the reverse direction. If the diode shows a short (0.00v) in either direction then the rectifier is faulty and needs to be replaced. Also if it shows infinity in both directions it’s faulty.
Explaining the circuit.
The circuit is quite a simple one. The stator (located under the flywheel) produces an alternating current... like this... (a ‘sine wave’)
Using the diagram below (and the number on the rectifier as reference) here is basically how is work.
When the stator is in the half of the wave that has the current flowing ‘UP’ the page. The positive part of current will travel to the battery through diode ‘2-1’ and back to the negative side of the stator through diode ‘4-3’. Because diodes only conduct in a positive direction, the other 2 diodes will block the flow of current. When the current flows in the other direction (DOWN the page) diodes 4-2 and 3-1 will conduct and the other 2 will now block...
Testing.
To test a rectifier you need to remove all the leads from it. Using a multimeter set on the ‘DIODE’ function, check each diode in both directions. (On some rectifiers there are only 3 terminals. The metal mounting pad is the 4th terminal). To check the ‘negative’ diodes, put the red lead on the ground terminal (pin 4 in our drawing) and the black lead on each of the ‘AC’ terminals (where the yellow lead would connect), and you should see our 0.7v on the meter display. That can vary a little, between 0.6 and 0.8v. Now swap the leads, black on ‘ground’ and red on each of the ‘AC’ terminals. You should see infinity on the meter.
Now put the black lead on the ‘+’ terminal (where the red wire would connect) and touch each of the ‘AC’ terminals with the red lead. Again we should see around 0.7v. Swap the leads (Red lead on ‘+’) and touch each of the ‘AC’ terminals with the black, and we are looking for infinity.
If you have all these readings correct, then the rectifier has checked out correctly. If any one of these readings is wrong (either infinity or 0v where there should be 0.7, or 0v where there should be infinity) then the rectifier is faulty and needs to be replaced.
I hope this helps you out,
Chris......