trini Posted September 17, 2019 Share Posted September 17, 2019 Here are some pictures of generator cut out. It has 10 amps diodes in parallel. I could not find a single 20 amp diode. so I used 2 in parallel 2 Link to comment Share on other sites More sharing options...
DB26 Posted September 17, 2019 Share Posted September 17, 2019 Nice Link to comment Share on other sites More sharing options...
trini Posted September 17, 2019 Author Share Posted September 17, 2019 The one thing all of us missed. The diode does not regulate the amperage coming out of the generator. The cut out does. The diode will prevent feed back in case the cut out malfunction so it has to placed in line with the output circuit. Cheers. Link to comment Share on other sites More sharing options...
DB26 Posted September 17, 2019 Share Posted September 17, 2019 55 minutes ago, trini said: The one thing all of us missed. The diode does not regulate the amperage coming out of the generator. The cut out does. The diode will prevent feed back in case the cut out malfunction so it has to placed in line with the output circuit. Cheers. So are you saying eliminating the cutout altogether would be detrimental to the generator? That’s what I did with my cutout. I have the same cutout unit as you do. I removed the copper coils and contact points and put one big 25 amp diode. This one to be exact: https://www.amazon.com/gp/aw/d/B008UTTHNE?psc=1&ref=ppx_pop_mob_b_asin_title Link to comment Share on other sites More sharing options...
Bloo Posted September 17, 2019 Share Posted September 17, 2019 (edited) You guys must have a lot of trouble with cutouts. I am always baffled by these threads, and the fact that diode conversions are so popular in the Model A Ford community. These must stick all the time for other people and not for me. The most common failure mode of a silicon diode is a dead short, so a failure would have exactly the same effect as stuck points. With that out of the way, there is no reason a diode wont work fine, as long at it is sufficiently large (and has a heatsink, if a heatsink is necessary to meet it's rating!). To set the record straight, the cutout does not regulate. It is just a switch. The reason it needs to be there is that in a vehicle electrical system, the source of current will be the highest voltage point in the system. That would be the generator when it is charging, and the battery when the generator is not charging. When the generator is not charging, if it were still connected to the system, it would try to run like a motor, helping to discharge the battery rather than charging it. If the engine were also stopped, in addition to discharging the battery the generator would also draw excessive current through ONE winding, most likely burning the armature up. These are all really undesirable effects, so the cutout is arranged in such a way that the generator has to be generating, at least a little, before it gets connected to the system. That is really all there is to a cutout. Regarding regulation, "third brush and a cutout" (that is what we are talking about isn't it?) systems only regulate current, not voltage. The current regulation is accomplished with the third brush in the generator. It limits the field current, which limits the output current. These systems rise and then fall in output current as engine RPM rises. So, there is a magic vehicle speed that gives you the most output, and if you are going faster or slower you get less. The absolute limit is set by the position of the third brush, and it is usually adjustable. Voltage is not regulated at all. A third brush system will do it's best to charge at the current it is set to, no matter what happens with voltage. If it was charging at 5 amps, and the battery gets fully charged, it will continue charging at 5 amps, and the voltage will just continue to rise. The battery will boil, the bulbs will not be happy, and so on. This is why third brush systems are so small in current capacity, and why the third brush cannot be set very high. This unregulated voltage characteristic is why you can get away with using a diode for a cutout, and it makes no difference. It takes about 0.7 volts to make a silicon diode conduct, and once it does, there will be about 0.7 volts dropped (or lost) across the diode. Since the voltage is completely unregulated, the generator just sails up 0.7 volts higher than it would have if it were directly connected by a cutout relay. The 0.7 volts is dropped at the diode, and neither the generator nor the battery (nor the driver) is aware of a difference. Edited September 17, 2019 by Bloo (see edit history) 1 1 Link to comment Share on other sites More sharing options...
trini Posted September 17, 2019 Author Share Posted September 17, 2019 Thank you very much for that DB26. I thought the diode will work as a current limiter. I will go your way. Now I see the logic. Now without the cut out the generator is set to produce10 amps or so .( with the position of the brushes) The cut out prevents the feed back only. apart from cutting in. The diode comes in here to do the job of the cut out . Makes sense. Cheers. Link to comment Share on other sites More sharing options...
trini Posted September 17, 2019 Author Share Posted September 17, 2019 Thank you Bloo. I learned something today and also refreshed my memory. One question Bloo Can a zener diode eliminate the need of the third brush ? Just a thought. Thank you. 1 Link to comment Share on other sites More sharing options...
DB26 Posted September 17, 2019 Share Posted September 17, 2019 50 minutes ago, Bloo said: You guys must have a lot of trouble with cutouts. I am always baffled by these threads, and the fact that diode conversions are so popular in the Model A Ford community. These must stick all the time for other people and not for me. The most common failure mode of a silicon diode is a dead short, so a failure would have exactly the same effect as stuck points. With that out of the way, there is no reason a diode wont work fine, as long at it is sufficiently large (and has a heatsink, if a heatsink is necessary to meet it's rating!). To set the record straight, the cutout does not regulate. It is just a switch. The reason it needs to be there is that in a vehicle electrical system, the source of current will be the highest voltage point in the system. That would be the generator when it is charging, and the battery when the generator is not charging. When the generator is not charging, if it were still connected to the system, it would try to run like a motor, helping to discharge the battery rather than charging it. If the engine were also stopped, in addition to discharging the battery the generator would also draw excessive current through ONE winding, most likely burning the armature up. These are all really undesirable effects, so the cutout is arranged in such a way that the generator has to be generating, at least a little, before it gets connected to the system. That is really all there is to a cutout. Regarding regulation, "third brush and a cutout" (that is what we are talking about isn't it?) systems only regulate current, not voltage. The current regulation is accomplished with the third brush in the generator. It limits the field current, which limits the output current. These systems rise and then fall in output current as engine RPM rises. So, there is a magic vehicle speed that gives you the most output, and if you are going faster or slower you get less. The absolute limit is set by the position of the third brush, and it is usually adjustable. Voltage is not regulated at all. A third brush system will do it's best to charge at the current it is set to, no matter what happens with voltage. If it was charging at 5 amps, and the battery gets fully charged, it will continue charging at 5 amps, and the voltage will just continue to rise. The battery will boil, the bulbs will not be happy, and so on. This is why third brush systems are so small in current capacity, and why the third brush cannot be set very high. This unregulated voltage characteristic is why you can get away with using a diode for a cutout, and it makes no difference. It takes about 0.7 volts to make a silicon diode conduct, and once it does, there will be about 0.7 volts dropped (or lost) across the diode. Since the voltage is completely unregulated, the generator just sails up 0.7 volts higher than it would have if it were directly connected by a cutout relay. The 0.7 volts is dropped at the diode, and neither the generator nor the battery (nor the driver) is aware of a difference. Wow. This right here. This is what took me weeks of research and asking many people many questions to finally understand. Very well said. 1 Link to comment Share on other sites More sharing options...
ArticiferTom Posted September 17, 2019 Share Posted September 17, 2019 34 minutes ago, trini said: Thank you Bloo. I learned something today and also refreshed my memory. One question Bloo Can a zener diode eliminate the need of the third brush ? Just a thought. Thank you. No , must still regulate . A diode simply allows power to go only one direction .After it rises enough to overcome the circuit itself (0.7 v.) . There are aftermarket add ons that can eliminate the third brush EVR's (electronic Voltage Regulators ) . Fun Projects and Petersen device are two common ones used in model T-A community . 1 Link to comment Share on other sites More sharing options...
Bloo Posted September 17, 2019 Share Posted September 17, 2019 (edited) No, there are systems that regulate with a zener (like some old UK-built motorcycles), bu that is completely different. They have no field to control the output current, and so charge at all the current they can manage all the time. The zener bleeds all the excess current to ground! It has to be gigantic, with an even bigger heatsink. I see no way to apply that method here (nor would you want to). It is possible to add a voltage regulator to a third brush system. "Peterson" ones hide inside the generator (no personal experience with that one, but I hear positive reviews). Edited September 17, 2019 by Bloo (see edit history) Link to comment Share on other sites More sharing options...
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