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Outboard Rectifier Testing

This video is on the techniques for testing a bridge rectifier.

Dangar Marine

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Outboard Rectifier Testing – Video Transcript

Hey, Dangar Stu with you. Today's video is about how rectifiers work and is proudly sponsored by MarineEngine.com.

Before we get start with that though of another viewer t-shirt viewer fellow this one is Tim Yates from Seattle in the US who's a longtime viewer and commenter and a fellow YouTuber, thanks Tim. I found that the powerful combination of the chalkboard and electrical theory is a really good way to garner dislike so we'll start there. So, the idea of a rectifier is simply to take an H C curve which is the drawing up a top here as a bit of a sine wave. So, it starts off with the current flowing one way and then positive and negative keep swapping as it alternates which is what the AC current stands for, Alternating Current. The trouble is you can't put AC current into a DC battery. So, you need to rectify it first and that's what these rectifiers do.

The rectifier you'll find most commonly on an outboard is a bridge rectifier and this is the circuit diagram for one of those down here which we'll go through in a minute so the simplest rectifier you can have is just a single diode that only allows current to flow in one direction so instead of alternating you're simply allowing only go in one direction what this gives you though is half wave rectification. So, what we're saying is we're taking the sine wave here and we're only allowing the pulses that go in one direction to pass and the rest just gets chopped out if I put a diode in the circuit now I'm wiping out the bottom half of this wave now so all I've got is the current that's passing the right direction to go through the rectifier this is kind of wasteful though because you're losing essentially half your power.

The advantage to this slightly more complicated bridge rectifier is it allows you to take both halves of the AC current and send it in the right direction to be this sort of DC current for the battery. With a bridge rectifier, what you end up is something like this, the section that used to be flowing in the opposite direction is now all made to flow in the right direction. It's not quite the same as a normal DC current you get from a battery, with a battery you've got a steady DC voltage.

So, you've got 12 volts here say, and it goes straight across. But, with your rectified AC current it actually comes in sort of pulses of this DC current heading in a certain direction, same direction but of a varying voltage. That's okay when it comes to running certain components lights, no dramas at all, that kind of thing. But, in order to use other components it's best to have that battery in line. Because these pulses can charge a battery and then from the battery you can receive a nice steady DC current of a solid 12 point 5 volts or whatever. What we'll do now is have a closer look at the circuit diagram and then, we'll go through testing one at the bench. Each one of these purple circles on the diagram is a terminal that a wire either can be connected to or comes from on the rectifier.

So, in this case this one's about positive to the battery, this one's negative to the battery, then these are the two green wires that go off to our charge coil. So, what happens with these is as this current alternates. We'll say for example during this little cycle this green wire's the positive, and this one's a negative. You can see here that the positive comes in to this point it can't flow through this diode that's the wrong way. The arrow shows which way it flows, this triangle arrow so it comes up and the positive gets to here at this stage this was the negative the negative can't flow this way, this is the way the positive current flows. But, it can flow this way. So, we've got our negative going to a battery positive going to the battery. Then, on the next cycle this becomes our negative, this becomes our positive, in this case the positive comes in here, it flows up to there, the negative comes in here, any current flow to there.

So, as you can see every time this AC current oscillates the positive is always sent out the positive terminal here and the negative is always sent out the negative. And that's the essence of how it's rectified. So, that's how it works normally. But, if any one of these diodes stops working, it either doesn't allow current to pass in the right direction or it does allow current to pass in the wrong direction the rectifier won't work. So, we'll go to the bench and test a real rectifier. Now, this is an example of a rectifier it's a Yamaha part number. So, you can see here coming out of it we've got the positive here, the negative here, and the two greens going to the charged coil you'll also notice the body of these is a built-in metal heatsink because it generates a bit of heat and that needs to be dissipated. In order to test that a diodes working properly you need to check that current can flow in the right direction and that it's prevented from flowing in the wrong direction. Needs to have both those things to be a good functioning diode. You can do this with a resistance check.

You should have an open circuit full resistance going the wrong way, and a low resistance going the right way. It's not a great way to go though it's not a particularly definitive test. But a lot of multimeters have a diode test mode. So I'll show you that on this one. So, you can see here this is where you set the multimeter to test resistance. You can see the symbol, but it's also got modes for testing the diodes. You can see the diode symbol there, and testing capacitors and things. So, if I press the mode button now. So, now I've pressed the mode button, you can see it's gone into this diode testing mode. Now, what we can see here is that we should be able to get a reading from either of the two green wires flowing to the positive.

So, let's do that test. What I've done here is put the negative of the multimeter to the positive wire, so negative to here, then I'm going to put a positive to the green, and current should be able to flow across this diode from either green wire. So, you can see here if I touch the positive to the red wire I get a reading here of 0.5 2 volts and that's the forward voltage across that diode, which is the value we expect. I can then do that to the other green wire and get a very similar value. If I didn't get that value, that 520 million value instead, I just had this sort of open circuit like I had when I don't connect the terminals. Then, I know there's something wrong and those diodes that isn't allowing current to flow in the right direction and that means that none of the charge current coming from the coil at the outboard is making it to the battery. Now. I can look at things the other way around I can put the negative onto the negative terminal here, instead of a positive and then put my positive onto both of these green wires and I should see that current can't flow.

I can't get a positive current coming either way here to this negative terminal. Now, what I'm going to do is take out this negative lead and put the positive lead of the multimeter into the positive lead of the rectifier, and then if I touch both of these green wires, you'll see I've got an open circuit. What that's telling us is both these diodes are allowing current to pass to the positive correctly and this is stopping current from flowing in the wrong direction so these two diodes we know are in perfect condition and working properly. Now, what I got this time is the positive from the multimeter going to the negative of the rectifier, and then will touch the negative to the two green wires. So, going there getting our 530 millivolts and then here, getting the same. So, what that tells us is that these diodes are allowing current to pass here through up to the green wire and then through here to the other green wire.

The last test I'm going to do now is the negative terminal of my multimeter to the negative of the rectifier, and then I'm going to put positive to both these green wires and we should get no connectivity and that's because we've got a negative here, and we're seeing whether current can flow backwards through these diodes to this terminal. So, we go red to this green wire and we've got an open circuit and then red to this green wire and we're getting a reading 0.95 volt. I don't know which green wire is which really but what that means is one of these two diodes is allowing current to pass the wrong way with a slightly higher resistance. Which is why we have a higher voltage drop. But one of those diodes appear to have failed in this rectifier.

There is one other thing you can do and let's see where the current can flow from the negative to the positive. Because theoretically, you know all these point in this direction. So, if I have the negative into the positive there, and the positive going to the negative and you'll see I'm getting about a 1 volt reading. So, current can pass, positive current can coming, this negative run up here and come out here, come in here and come out here. There you could see because we're getting this point nine volt drop you can see it's actually heading up here through two diodes, on its way up well actually four. I can go either way but then you've got a parallel circuit as well. So, I presume that kind of halves the resistance again. So, these are all the tests you can do for these rectifiers. And as far as I can see, this one is bad, one of the diodes is allowing current to flow a way that it should be blocked.

Now, I know these tests probably seem a little bit confusing. It's like a positive-negative, negative- positive, you know all this sort of thing. But, if you've got a multimeter and you've got a rectifier like this, I think drawing the diagram picturing these little triangles the diodes, they're just little one-way gates, you should be at our positive flowing in one direction and not in the other. And, if you look at the diagram and do the test and look at the results you get. It'll sort of gradually sink in I think of how it's working and what you're testing and what types of failures you can have current can't flow in the right way or it can flow in the wrong way they're the two things that are bad this was all working quite nicely in theory until I opened a new regulator and got the exact same result so I'll show you what I'm thinking.

I'm thinking the reason I'm getting this resolved is that this isn't just a rectifier this is a regulator rectifier. So, I'm wondering whether some part of the circuit when you're just doing a static test like this gives you this result. I've just gone rummaging through the space box to find some more rectifiers and the first one I got gives the expected results. So, it leads me to think that there's something wrong with both those rectifiers or that it is the regulator circuit that's giving me this other result that I don't understand. So, just to prove to you I'm not making it up here's this other little rectifier and I've got the positive going to the negative here. So, I should be able to get current to flow to both green wires and there we got 0.56 and on this 1.56. Now, if I swap the negative to the negative like I had on the original brand-new one. You can see here we've got open circuit on one and open circuit on the other.

Which is what I would expect this one to do. I don't know what the answer to this little quandary is. So, this little rectifier passes as healthy, and shows those tests you need to do why that other rectifier gives those results. I'm not sure it's hard to call it bad when a brand new one does the same thing. Was it a dodgy batch from Yamaha that has faults? Maybe, interesting to find out. Or, is it something to do with the regulator circuit that's giving that result. It is interesting to note that the rectifier that I used to do this test first up, I had actually put on a boat that was having trouble charging. The rectifier on that boat tested bad, so I put a new one in, wasn't working, started thinking maybe it's the coil under the flywheel. You generally test that by looking at the resistance at the coil, and the resistance was good.

Something hmm good coil, brand new rectifier, so maybe that's more evidence than actually use a fault with both those rectifiers, these new ones anyway, time will tell. I would love to do all that homework before I upload this video. But then it'd be no video this week. So you see where I'm coming from, once I do find out what's going on though. I'll write about it in the description of this video. So you can find out what the outcome was. All right well take care. I hope this video answered more questions about rectifiers than it raised. But sometimes it goes that way. all right well take care, catch you soon.


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