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# Bridge rectifiers

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I am asking for assistance with bridge rectifiers. Specifically, how to determine the voltage rating of a 20 amp bridge rectifier. The circuit that requires this rectifier is a non-regulated circuit. This circuit consists of a stator, 12 volt lead acid battery and rectifier. The stator produces AC voltages @ 180-200 DVA volts. The voltage going the battery cannot excede 14.5 volts DVA. I am getting 17.5 volts at the battery which is 3 volts over the allowable limit of 14.5 volts. What I do not understand is what part of the circuit limits the DC voltage going to the battery. Is it the voltage of the battery or the voltage rating of the recitifier. Is the 17.5 volts due to a faulty battery of faulty rectifier? Would the voltage rating of the 20 amp rectifier be that of the battery (12 volt 20 amp rectifier) or would it need to match the AC current being rectified (200 volt 20 amp rectifier)
Any help is greatly appreciated.
Cheers

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Hi Matthew,

Welcome to this forum.

I am not following you here, is it possible for you to post a schematic of your circuit?

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A schematic is something I don't have. So, please bear with me as I attempt a better explaination. The circuit that I am attempting to describe is for an outboard motor. The outboard uses a stator to produce AC current (alternator). The AC current then goes to a full wave bridge rectifier to produce rectified DC current to charge a 12 volt battery. The system is "non-regulated". Meaning there is no voltage regulator. The stator produces 180 to 200 volts of AC current. Some of this AC current is directed to the capacitor discharge ignition (CDI) and it's three coils which provide "spark" for the three cylinders via it's sparkplugs. The rest of the AC current that the stator produces is routed directly to the bridge rectifier to produce the "rectified DC current" in which the battery requires. Whatever DC or possibly AC current (I'm not sure which) the battery does not need is shorted to ground thru the rectifier. According to the repair manual for the outboard, voltage going to the battery should not exceed 14.5 volts. Any voltage readings above 14.5 volts with the motor running at or above a certain rpm the manual states is the cause of a faulty rectifier. With the motor running at the suggested rpm I get a reading of 17.5 volts. Going by what the manual says I am assuming I have a faulty rectifier. However, the out-of-range voltage reading of 17.5 volts may also be the cause of a faulty battery. For conversation sake though, I'd like to assume it is not. Electronic supply companies sell what I believe to be the very same type of rectifier for \$2.50 vs. Outboard parts dealers want \$60 plus dollars for a new rectifier. The manual states the rectifier has a amperage rating of 20 amps. However, the manual does not give the voltage rating for the rectifier. From what I am able to ascertain similar 20 amp rectifiers are offered with voltage ratings of: 50 volts, 100 volts, 200 volts, and 400 volts. What I would like to know is the voltage rating for a bridge rectifier based upon the input value of AC voltage of the circuit it is designed for. Or, is it based upon the value of rectified DC voltage the rectifier outputs regardless of the input AC voltage? Or, in otherwords do I need a rectifier that has a 200 volt rating or a 12-14.5 voltage rating. Or, perhaps a different voltage rating altogether?

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Matthew,

Ok, the 17.5Volts reading is on the battery connectors I assume? If so it means one of two possible reasons, first a faulty battery (most likely) and second an enormous charge current. Have you measured the current (ADC) going to the battery? Is the motor running nicely, no misfiring or uneven rpm:s at the presence of the 17.5V at the battery? The statement

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Thank you Ante,
Yes the motor does have a intermittent misfire. No. 3 cylinder is intermittently getting spark. Manual states possible causes as being blown CDI module and/or faulty rectifier and/or faulty stator. According to the manual the 180 volts AC being produced by the stator is correct. Exceptable values go as high as 400 volts AC. Anything above 400 volts going to the CDI modules the manual recommends replacing the stator. Since there is no voltage regulator the chance of burning out CDI modules, coils, batteries and rectifiers is extremely likely as well as costly. With what you have told me, and then putting two and two together, I guessing the stator is probably causing all the problems. Thanks again for your help. It is very much appreciated.

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Hi Matthew, Ante.
can you keep us updated with your progress on solving this problem. There must be some kind of voltage control, I am thinking that the CDI controls this, with my experience with early motorcross motorbikes, they had a high and low stater coil each coil would be switched on and off according to the RPM to maintain a reasonable voltage output.

So you should have a controlling mechanism either within your CDI unit most likely, or separately, that controls switching between stater coils to maintain a reasonable voltage output.

There may be more to your bridge rectifier then just diodes.

I think we really need to understand exactly how this system works, so we can work out exactly where the fault is.

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I know that the rectifier itself is a standard bridge rectifier consisting of 4 internal diodes. Radio Shack sells one exactly like it only it is rated at 25 amps and 50 volts. According to the Seloc repair manual for the outboard it states that the system is a non-regulated one. On the larger displacement motors that Chrysler/Force offered did come with a voltage regulator. The CDI circuitry does not have anything to do with regulating voltage to the battery. (as far as the limits of my knowledge allows me to assume) As it operates on the AC current being generated by the stator and it's role is strictly for sending signal voltages to the coils to produce spark. Since this is a three cylinder motor there are two seperate CDI modules. Both modules are identical to eachother. Except one module controls spark for no. 1 and no. 2 cylinder while the other module controls spark for no. 3. Either module can be swapped. There is a trigger mechanism on the flywheel that sends a timing signal to the correct module for spark on the appropiate cylinder. There is a blocking diode in each module. The blocking diode is to prevent a timing signal for no. 1 cylinder from also going to no. 2 at the same time. The CDI system consists of the two modules, 3 coils, the trigger assembly and the stator. The manaul also states that the battery only receives current from the rectifier when rpms are high enough to create the necessary amount of current. At low rpms the battery supplies power for everything except the CDI which, though operating on relatively high voltage AC current from the stator, actual current draw is very low. As far as being able to tell what limits the voltage going to the battery is the mystery I am trying to figure out. Could the battey voltage affect the rectified DC output voltage somehow by leakage across the diodes. (wild guess) sort of a feedback voltage. Other then that I am completely clueless as to what prevents the battery from receiving rectified DC voltage at the same AC voltage produced by the stator. That's why I didn't know if rectifiers voltage rating had something to do with the DC voltage of it's output or the input of the AC voltage. I apologize for only being able to supply you with information that has been dirived by much assumption. I realize it makes finding the answer that much more difficult.

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Hi Dazza,
I'm not certain if you are still interested in the developement of this thread but this is what I have been able to ascertain thus far. After consulting an outboard motor specialist on a forum specializing in outboard motors, I was informed that since my motor does not incorporate a voltage regulator on the charging circuit, the battery must deal with whatever DC voltage that gets sent it's way and why it is recommended that only high quality non-maintenance free battery's be used. High quality or not, batteries are not cheap so I intend to replace the OEM rectifier with rectifier with a built in voltage regulator. Larger displacment Chrysler outboards come with voltage regulators and installing one is extremely simple. The only difference in the wiring between those with regulators and those that don't use one is the regulated circuits use an extra wire connected to the ignition switch that supplies the accessories with 12 volts before being looped back to the battery. It doesn't get any easier then that. I do find it strange though that Chrysler would use a non-regulated system considering the potential for damage to the entire CDI system and the cost of repairs to replace damaged parts if and when a battery goes faulty from being overcharged since it is the battery that is the determining factor as to what the stator outputs as far as limiting AC spikes. I still don't have a clear understanding of how the how thing works but I believe it has to do with ohms law. As long as all the components stay within their respective tolerances as far as voltage and resistance goes the system works. A battery that begins to produce to high or to low a voltage or a short or open condition arises, resistance changes and the stator runs begins producing AC spikes which in turn may or may not cause further damage to the other components in the system. Go figure.

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You can use a larger filter capacitor to bring down the DC just a little bit.But you can add a series inductor and bring it down a little farther. And then you can add a series resistor that will take it the rest of the way. The voltage drop of the resistor will be determined by the current draw of the battery, but you should still get a charging of the battery if the resistance is low. The voltage that is applied to the diodes can be calculated by taking the value of the AC and dividing by .707. This is the voltage applied to both reverse biased diodes. If you don't know what the voltage will be, just use a fairly high one. I am missing something about the 20 A current capacity. Charging a battery should use much less than this. Also you should use diodes with a current rating to match the application, but the larger diodes give better regulation.

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Hi
Hi Kevin,
Since larger diodes provide better regulation then smaller ones and how most similar-in-amperage diodes (to the OEM rectifier used on my outboard) all seem tend to have a centered hole for fastening it down to a bracket or mounting location, are probably two factors they may have decided to use a rectifier with such a high amperage rating. Especially considering that the stator only produces a maxium amount of AC current of 7 amps. Plus, another reason maybe due to they purchase them in bulk so they probably pay less then \$1.00 each for them while selling them for \$45.00. US.
Using your example, the voltage rating for the rectifier I would require would be around 250-300 volts, correct?. From the manner in which you describe the regulator, it sounds like it is something realively simple for one to build himself/herself. The one problem that most concerns me would be how to determine the various ratings for the necessary components to build one. As well as a simple schematic. Might you know where I may find such information?

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I thought you were not going to use a regulator. Normally, battery charges are made to produce not only the voltage but the charging current. This charging current is produced by uping the voltage over the battery voltage. You can make a low current charger by practically not worrying about the current, but getting the voltage just higher than the battery. I don't think you need a regulator but it is a nice way of obtaining the correct voltage. Fortunately, a 12 volt regulator will give you the minimum voltage necessary. So use diodes that have let's say a 3 amp rating and a 12 volt regulator rated at 1A. You can stick your ammeter in there to read the current if you like. The filter capacitor might do well to be 1000uf.

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Using a regulator is something that I will no longer go without. Ruining batteries due to overcharging them is getting very costly and more then I can currently afford. Plus...replacement CDI packs go anywhere from \$160 to \$399 each. Loose or bad battery connections or a battery that is overcharging will almost always cause a circumstance in which blows a diode that is used to block the firing signal between the two CDI packs. I just want to have a system that will develope enough current to keep the starting battery charged as it also limits voltage to the battery at around 15 volts DC. I also have a deep cycle battery that I use to supply just my fish finder only. I would like to incorporate this deep cycle battery into the charging system via an isolator circuit.

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Matthew,

It is not possible to just raise the voltage to overcharge a battery. When you raise the voltage the current rises too (unless it

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Hi Matthew,

I'm sure you realise that you need to apply caution, when modifying your electrical system ;). You need to be certain that you have found the problem, before you try to modify/improved your electrical system. You want to be sure you are not actually applying a Band-Aid fix. You may have more than one component failing, which makes it a real pain in the B_M,

Do you have an auxilary engine, you don't want to find yourself drifting in the wild blue yonder,
well that's probably not so bad if you find yourself on the shores of down under, I could probably help you sort the problem out, but I think you may be a little hungry and thirsty buy then ;D ;D ;D.

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Considering that we have taken our little 17 foot Bayliner Capri some 60 plus miles offshore numerous times would make most have to question our intelligence out on the water. Such is the draw of the ocean. We do have a kicker motor on board just in case the main should fail us. One bit of reassurance out on the water is the fact that in San Diego CA. waters there is never a boat farther away then 5 to 10 miles and almost always they're within sight. Anyways, I have ran tests on the stator, trigger assembly, CDI modules, battery, and wiring. Everything is within spec. except for the deep cylce battery that I got from a friend who used it on his boat as a starting battery. And also, the last origninal CDI module needs to be replaced since it has a blown blocking diode. Preventing no. 3 cylinder from firing. I have studied all of the Chrysler outboards electrical systems that have regulators and all that is different between my motor and the regulated motors (of same year manufactured) is one wire that carries regulated DC positive current from the regulator, to the ignition switch then to the connection fuse block then to the accessory fuse block. Where as my system runs the postive DC wire directly off the rectifier then to the ignition switch where it eventually ends at the accessory fuse block. All else is the same. Same part numbers for CDI modules, stator and trigger assem. Same solenoids. All having the very same ratings as my engine. All I want to do is limit the voltage going to the start battery. We do enough starting and stopping and slow trolling to where the battery requires some current for recharging. Replacing the battery I have now with that of a correct starting battery instead of the deep cycle I have now, probably would have prevented the problems I am dealing with now. Mostly, having to replace a CDI module that died due to the bad battery. I have gotten to know my boats electrical system and wiring inside and out over the time I have owned her. And I am totally confident with adding a regulator. I would prefer to build my own if it isn't to difficult determining what the specs will need to be.

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Why not use a window comparator type circuit? You can adjust the ON limit and the OFF limit.
Actually, if you are dealing with any substantial amount of current, you will probably use the window comparator circuit to turn a transistor on and off.
One example is below:

MP

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Looks like Greek to me. lol
Perhaps adding a regulator is not what I should be focusing on.
I'd like to be sure I am grasping the concept behind my outboards charging circuit. The stator is rated at 200 volts AC , 7 amps. Between the stator and battery is a rectifier to switch AC into rectified AC...more or less turns AC into DC by filtering only one half of AC's signwave at any given moment. Now here's my grey area. What is limiting the voltage going to the battery. Meaning howcome the battery isn't getting 200 volts DC of the stators 200 volts AC output once it has passed thru the rectifier. Or, it does but it is relatively harmless because the battery isn't drawing any current. When the battery does draw current the increase in resistance creates a decrease in voltage. Ohms law. So, hypothetically, more then likely the addition of a voltage regulator on those motors that Chrysler installed them in is to regulate the voltage going to 12 volt accessories such as radio's, gps, radar, ect ect. Figuring (hypothetically again) that most boats using their 85 hp engine/s would probably only have one battery which is for starting purposes only.
So...close but no? Getting warmer? Not even close? Forget it he'll never learn? lol

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Matthew A. Concentrate your efforts on obtaining the electrical schematics for your outboard(and post them). This will be half your battle over. I'm sure someone will then be able to find a solution, contact your outboard manufacturer, someone anyone and get them from somewhere :).

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Thank you Dazza, MP, and everyone, for the time and help you all have given me. The knowledge each of you has shared has been of great help and value to me and is greatly appreciated. :)

Matt

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