Rectifier Results - Could someone interpret Please

[Biggles]

Hi<br /><br />I have carried out some tests on my rectifier due to no charging of the battery and intermitent Tacho working.<br /><br />The test results are as follows and have been done in accordance with some tests shown on this Web Site by a chap called Jerry.<br /><br />1) Try this: disconnect the battery cables from the battery. Disconnect all leads from the rectifier. Done this okay.<br /><br />2)Connect the positive lead of an ohmeter to ground of the rectifier and the negative lead to each stator terminal alternately. You should have continuity. This passed <br /><br />Connect The negative lead to the ground of the rec. and positive to the stator terms. Should have no continuity. This failed . I do have continuity.<br /><br />Then connect the negative of the ohmeter to the positive terminal of the rectifier and the positive alternately to each of the stator terminals. You should read continuity. This passed<br /><br />Then connect the positive ohmeter lead to the positve rectifier terminal and negative to each of the stator terminals. Should read no continuity. This passed <br /><br />My question is. The tests only failed on the Stator terminal to ground, it passed on all others. I have only seen this particuular test mentioned by Jerry. <br /><br />The reason why I question it is, a new rectifier in the UK is £60, about $90. In the USA they seem to be about $30, a massive difference. I don't really want to be buying bits and pieces I don't need. My car garage does that for me. <br /><br />Could any one explain to me in laymans terms what the failed reading means.<br /><br />Below is a copy of a UK quote for a replacement part.<br /><br />We have nothing second hand that I could say was definitely the right one, but possibly something that will do, most new ones supeseed to part no 816770T which is £56.20 inc VAT + £4.95 p+p. 3/5 days delivery<br /><br />Anybody know anywhere in the USA that ships to the UK at a reasonable price.

 

[SuperGlue]

Re: Rectifier Results - Could someone interpret Please

The fancy name for the rectifier is "Full-Wave Bridge Rectifier." It is made up of 4 diodes. What you have done is tested each diode both directions. A good diode will have continuity one direction, an no continuity the other direction. You have found either one or two of the diodes to have continuity both directions. This means that those particular doides are shorted(bad). The rectifier can no longer convert the AC signal from the stator to DC for the battery.<br /><br />phatmanmike pointed out that you can get a rectifier from radio shack that will work for around 4 US dollars. Here is the link for that:<br /><br /> Radio Shack Rectifier <br /><br />By the way, what motor do you have, year, size?<br /><br />CG

 

[chuckz]

Re: Rectifier Results - Could someone interpret Please

Two out of the four diodes in the bridge rectifier failed shorted. They are effectively shorting out the stator. You have no DC charge current.<br /><br />A bridge rectifier is a bridge rectifier. Go to an electronics supply house and buy a 20 amp bridge rectifier much cheaper than from a marine parts dealer. All epoxy encpsulated bridge rectifiers are hermetically sealed. The only corrosion points are where the wires connect. All the marine rectifiers I've seen are standard tinned leads.<br /><br />Look for a bridge rectidier that is the same mechanical size, shape and mounting. There are not that many variations to choose from.

 

[Biggles]

Re: Rectifier Results - Could someone interpret Please

LI Chuck<br /><br />Thanks for your help.<br />I don't mean to question your advice, but you say<br /><br />Two out of the four diodes in the bridge rectifier failed shorted. They are effectively shorting out the stator. You have no DC charge current.<br /><br />I thought the checks I did (Excuse my ignorance)showed that the diodes were working and it is a problem with the way the earthing is working. This is the bit I am having trouble understanding. Or deos it mean that two of these diodes go straight to earth, and because they are not stopping the current going that way it is taking the easy option and going to earth instead of the battery.<br /><br />Many thanks.

 

[chuckz]

Re: Rectifier Results - Could someone interpret Please

For reference look at this diagram:<br /> http://www.tpub.com/content/doe/h1011v1/css/h1011v1_106.htm <br /><br />Diodes CR3 and CR4 are shorted in your rectifier.<br /><br />The diodes should only conduct in one polarity, yours are conducting in both polarities indicating a short. As soon as they shorted they protected the other two diodes from shorting becaue the current followed the path of least resistance through the shorted diodes.<br /><br />A short in the boat wiring is the most likely cause of the rectifier failure.

 

[Biggles]

Re: Rectifier Results - Could someone interpret Please

LI Chuck<br />Many thanks for you help.<br /><br />I'm going to order a Rectifier now.<br /><br />Any ideas how I find out if I have a short.<br /><br />Having said that I have shorted the Nav Lights out a couple of times but the fuse blew with that.<br /><br />Am I looking at a major short some where. Surely a fault of that nature would have other evidence to look for?

 

[chuckz]

Re: Rectifier Results - Could someone interpret Please

Just because the fuse blew doesn't mean that the rectifier didn't. If the charging problems started happening after you shorted your nav lights, and the engine was running, that was probably the cause.<br /><br />A major fault is not necessary to damage the rectifiers. The amount of emergy dissipated can be measured in joules (watt seconds). 12V at 20 amps for a few milliseconds can damage the rectifier. 12 X 20 X 0.020 = 4.8 watt seconds, very little power and the 0.020 represents 20 milliseconds. In a short circuit condition the current probably went higher than 20 amps, momentarily. The rectifiers will fail before the fuse blows.

 

[Biggles]

Re: Rectifier Results - Could someone interpret Please

LI Chuck<br /><br />I haven't got a clue when the Rectifier failed. I've only had the boat a month and did,nt know it hada charging fault until the starter motor wouldn't turn over after a full month of boating.<br /><br />I guess I'll just cahnge the Rectifier and see how it goes.<br /><br />Once again many thanks.

 

[phatmanmike]

Re: Rectifier Results - Could someone interpret Please

yes, i LOVE those 3.29 dollars rectifiers, i bought a bunch of em, i figure they are kinda disposable at that low a price<br /><br />if you get it, make sure that you ground the NEG terminal to the engine block or another good ground, since the stock rectifiers ground to the motor through the mounting of the rectifier.<br /><br />good luck

 

[Clams Canino]

Re: Rectifier Results - Could someone interpret Please

Also use some heat sink goop on them. they tend to run hot. My 1961 Merc 800 has one in it.<br /><br />-W

 

[phatmanmike]

Re: Rectifier Results - Could someone interpret Please

or a small 12volt fan from a pc store

 

[SuperGlue]

Re: Rectifier Results - Could someone interpret Please

Clams, do you run a zener diode on your setup? I was wondering if that was a good way to go or not? I read your write-up on "staying regular" and was considering adding the zener. <br /><br />CG

 

[chuckz]

Re: Rectifier Results - Could someone interpret Please

A zener diode across the output will not work,it doesn't have the power dissapation capabilty. A solid state voltage regulator on a heat sink will work. You should be able to find a 10 amp 15 volt regulator.

 

[Clams Canino]

Re: Rectifier Results - Could someone interpret Please

There *is* a source of high dissipation Zeners that could be sinked to the front cowl support. I investigated this right before I moved and then never got back to it. I'll get back on it this month.<br /><br />-W

 

[Biggles]

Re: Rectifier Results - Could someone interpret Please

Well Folks,<br /><br />Eventually got my hands on a working second hand rectifier today. Fitted it and everything works hunky dory now. Charging at 15+ volts and rev counter is real smooth.<br /><br />Prior to all this seeing as I had a couple of weeks to wait, I checked all my engine wiring. I found one wire broken on the control loom and one wire very near to breaking, so I sorted all those out.<br /><br />I carried some more checks out on the old Rectifier and found that it was charging but at a very low rate, less than a volt and that was taking ages to build up. Also once the wiring had been fixed the tacho started to work but a bit slow to react when revved.<br /><br />Many thanks to everybody that helped. Keep up the good work.