Testing a rectifier

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

*** UPDATE ***

"I think you are on the right track in thinking the regulator component is messing up your test.

You see, when I saw your blackboard charging circuit I wondered what was limiting the charge current applied to the battery. In the simplest case a resistor is placed in series with the battery to limit the charge current.

But from what I can see the regulator inside the rectifier unit takes the different approach of shunting excess current away from the battery. If you take a look at the first circuit diagram on this page, you’ll see the bridge rectifier and then to the right the shunt regulator using 3 SCRs - silicon controlled rectifiers - which look like diodes with an extra connection:

(Note this is for a 3 phase system so the bridge uses 6 diodes and the shunt 3 SCRs).

So my guess is that you have a shunt regulator in your unit. It contains an SCR. When you measure from the green to black terminal and get 0.99v, I think you are measuring the voltage drop across the SCR. Ie you are making it behave like a diode.

According to Wikipedia “The device [SCR] behaves as 2 diodes are connected in series with reverse voltage applied.”. That would explain why you are seeing a voltage drop of about 1v, double that of the 0.5v your other diodes are dropping.

I suspect your smaller device is a pure bridge without a built in regulator."

[plgplgplg]

This fine vid, the follow up in description and Ron Powell's furthering knowledge has expanded this old man's chance of fixing his charging problem, much appreciated from Western Canada.

[TheGeorgecollier]

Great video like always, love the simplistic approach to explaining things. Also refreshing to see someone that isn't afraid to show when they don't know something.

[kaskontrol]

I have an old 1981 Kawasaki KZ305 motorcycle with a single phase rectifyer that has pretty much the same wire setup. This video actually helped me diagnose my electrical issues that I've been having recently. Thanks for this. =)

[ericmuhlbeier]

I love the chalkboard! Just like gradeschool!! Thats how teaching should be done! We thank you sir!!

[jimdrechsel]

Just a side note. My rectifier in a 1991 Johnson 225 hp had a bad rectifier. Part of the troubleshooting was if the tach was going crazy which it did. That rectifier was cooled by the water system. Thanks for jumping into electrical theory which most would avoid. Enjoyed it.

[scottmaschino]

The memories that brings back... I attended an automotive tech school right after high school. They had all the different sections broke down into separate classes, electronics I and electronics II being two of them. We wound up being able to design and build all kinds of circuits from simple park lamps to digital instrument displays, we learned a lot.

Might have to go dig out my old course books to reread.

[MortalWombat]

Finally found someone with good content in Australia.

Keep up the good work!

[brucecliffe]

Hi Stu, yep, they don't call it electrickery for nothing, do they.The depth of your knowledge never ceases to astound me..Good work and many thanks.

[ronpowell]

The big heat sink body gives a clue that the unit is a rectifier and a voltage regulator in one unit. The smaller unit is definitely a rectifier only. That .5 volt (often quoted as .6volts for silicon devices) is why a rectifier gets hot. Say the alternator has a rating of 20amps, to get DC watts (heat)  simply multiply Volts x amps, so of the power generated, 10 watts is thrown away as heat by the rectifier. How hot is 10 watts? Try holding a powered up 12watt car tail light in your bare hands - you wont be holding that globe for long! And as electronic devices are easily destroyed by heat, the designers work hard to get rid of the heat as fast as they can. That small rectifier body is about what I would expect to be able to handle 10- 20 watts. That big Yamaha unit is more like 150-250 watts when it is bolted to the engine. The alternators on a most outboards are permanent magnet types with the magnets being part of the fly wheel mass. There fore, the alternator makes full power ALL the time the motor is spinning. So if your battery needs 15 amps to charge and your boat electrical load is 5amps, that 20 amp output is all going to be absorbed and the regulator will barely get warm . But as the battery charges, the current it will absorb will drop, so at full charge your battery may only accept 1 amp, your boat load is still 5amps, so only 6amps is being absorbed. But the alternator always produces 20amps, and if you do not do anything with that unwanted 14 amps, the voltage will rise to some where around the 18 to 20 volt mark, which will blow up 12v electronic items, burn out pumps fast and kill your battery pretty quickly. So these alternators use  what is known as a series or 'bypass' regulator. Basically they turn the extra power into heat and throw that heat away. The amount of power turned into heat is controlled by the voltage needed on the output. In our case it is typically 13.8 to 14.2 volts for a 12 volt boat. So a device in the regulator ( another diode called a zener diode) is used. The zener diode is special in that it is designed to conduct  power  at a specific voltage. In our case, it would be a 12volt zener, hooked up to a big transistor (not an SCR) in such a way that once the zener passes a voltage (ie, the alternator output  is going higher than 12 volts ), the zener turns on, which turns on the big transistor a bit. That transistor then converts part of the alternator output to heat which absorbs the unwanted amps. In our example the zener would turn on the transistor pretty hard and turn 14 amps into heat . 14 amps at 13.8 volts is nearly 200 watts ( 2 x 100watt spotlights held in your hand!) - so that heat sink has to be big to get rid of the heat. Some outboards actually run cooling water through the heat sink to get rid of the heat . There is more than just a zener and a transistor in a regulator but they are the main working bits. And those extra bits is why a simple multimeter test is useless for a rectifier/regulator unit. You need a test rig ( 18Vac output transformer, a few globes as a switchable load would be simple to make) to test those. Some low out put alternators ( the Johnson 50ESL74M for example) are only 6 amp total output. So they run a rectifier only. And you act as the regulator by watching the voltmeter. If the voltage goes above about 15 volts you turn on more electrical load like your nav lights, step lights etc until the voltage stops going up.  Pretty crude and hard on your electrics if you get distracted. Many four strokes have a small car type alternator that works a totally different way  so if you have one of those, don't go looking for a big regulator  like the Yamaha one Stu shows in the video- there wont be one.

[carneeki]

Another piece of evidence that it has a regulator built in would be the input AC voltage. When you rectify AC the DC voltage will be about 1.4 times the AC RMS voltage (assuming single phase, it's sqrt(2), if 3 phase it's sqrt(3), but this is for a single phase rectifier ).

So setting the multimeter to AC volts and probing the alternator should give you an RMS voltage. If it's just a rectifier with no regulator, we would expect to see about 13.8/sqrt(2) which is about 9.7 volts.

If the input voltage is higher, dollars to donuts you have a linear regulator like a LM317, LM7812 (or similar) internally which is the part that will generate most of your heat based on how much current you draw and the voltage difference between input and output.

Thanks for sharing! I'm recommending this video to a friend :)

[Fireship]

Awesome Stu. I think out of all the different types of troubleshooting one can accomplish on a machine, the electrical portion is the one that’s most difficult for people to understand the theory of. You made this easy to understand with your drawings and thorough explanation. Excellent as always!

[keonfrederic]

Stu, I really like your style. Testing electrical component is the hardest for me. Most of them aren't that simple, they usually come with hidden electronic components making any reading erroneous. Rectifier is one of them with the embeded regulator, CDI is another one.
For rectifiers testing, what I am doing now is testing the AC out of stator and the DC to the battery. If the DC is all over the place but AC ok then I'll replace the rectifier. Even that isn't bullet proof as some tachometer take their input from the AC and a faulty tach could mess up the whole thing.
Thank you for posting that video, it feels real. Great job!

Cheers.

[Worsdier]

Remember you don’t get a perfect diode, there always is some reverse leakage current and that explains the reading you get when measuring between the red and black wires. Reverse leakage current differs from diode to diode as well, so even the one that gave no reading still has reverse leakage, your multimeter just couldn’t measure it.

You also have a forward voltage drop which is around 0.3V per diode, so for a full bridge rectifier you will drop around 0.6V.

The resistance that causes the forward voltage drop as well as the reverse leakage will cause heat hence the heat sink.

[Sopanewyork]

Simply the best explanation i have seen Yet. And i have seen videos on utube and school and this by far is the best

[TheKvass]

Your video Made it really easy to understand the rectifier I love the way you drew it out on the chalkboard and everything thanks a lot man appreciate it

[jimbogasoline]

Great video. I went through this when I first got my boat. The previous owner (who just hammer bashed lugs onto the battery leads) didn't have red heat shrink or even red electrical tape. And being a first time boat owner, when I took the battery off to charge it, I made the HUGE mistake of reverse polarity! I didn't know how bad it is for an outboard, and should have traced the leads before connecting. Lesson learned! My boat still ran for a while then all the problems came. Burnt wires exiting the regulator/rectifier, battery always weak, and Tach broken. Ordered a knockoff rectifier off Ebay and had the same problems. Ordered another and installed it and fixed my problems. I also had to cut and add several new wires that were badly burnt. The easiest test I found (besides seeing the tech working) was checking the battery voltage with a multimeter. Before it was in the 12 area. After the new rectifier the voltage was somewhere in the upper 13s. So I knew my battery was being charged. I've read stories where it gets REAL BAD. Basically destroying Stator and the whole electrical system and wiring.Thank God my mistake didn't end like others. I was pissed at the time dealing with all of this, but now consider myself lucky

Edit...Today I checked with voltmeter and with a good regulator rectifier I get mid 14s(showing its charging). I cant remember exactly what is was before I got it working but It was probably low 13s

[TimsWorkshopTJY]

Thanks Stu, I finally made the wall👍 Cool video on a rectifier. Allot gong on with outboard engines which is an amazing piece of machinery. One of the most important is keeping the battery charged up when running

[goaskgrandpa]

just wanted to say thanks, with your vids I was able to rebuild my carbs, water pump etc..keep up the great work. you're fellow boater in Michigan.

[johnmislow]

Hello from sunny south Florida. Please consider posting a Patreon link so that I can pledge to your channel. You are the best instruction on the web for outboards. Thank you.

[cybeer]

Beautiful !! This answers my question posted on the installation video of the rectifier.
(When testing without the battery.)
Thank you !