How to Build a Diode Generator Cut Out Replacement

[DB26]

I’ll have to probe around again and see if I get ground on that connection

 

if thats the case, it makes things much simpler, I hope. 

[DB26]

5 hours ago, TerryB said:

Ok, the fuse is just a special type of current sensitive wire that connects the field coils to ground.  It has zero resistance for all intents and purposes and is there to provide an overload protected path to ground for the generator field coils.  The cut out thin wire coil wire is getting a path to ground where it is connected to the fuse. If the thin wire is connected at the same point the field wire is connected, it means the thin wire coil will have no path to ground if the fuse blows.  If it’s connected where the fuse is grounded then the fuse has no impact on the small coil operation, it’s just a convenient place to make the connection.

 

The small coil, from what I can find on cut out operation theory, is the part of the relay that starts the cut out to attempt to close the contacts as the generator output is increasing.  When the generator output is up to snuff and able to charge the battery it’s the coil with the thick wire that keeps the cutout pulled in to charge the battery.  Theory says the battery charge current is going through the thick wire coil which makes for a strong contact closure.

 

So with that information it seems there is no need to worry about the thin coil connection to the fuse.  If the voltage at the diode is now the value that makes the whole show work the small coil and it’s connections should not matter.  The field coil connection to the fuse must be maintained and one side of the fuse connected to ground for the generator to operate.  The fuse can be replaced with just plain wire but then you risk a generator meltdown if an open circuit develops in the charging circuit.

 

Note:  all this is based on my background in electricity and what I have read on cutout operation on the internet.  I have not actually done what you are proposing.  

You are RIGHT, it is a common ground. The fuse and relay just happen to use the same location. I poked around with a meter and it has continuity with ground. So, problem solved. Thank you. You gave me the clarity I needed. 

 

Onto making the diode circuit. 

[DB26]

I have made the diode replacement and reassembled my generator. Preliminary tests seem to show a success. I will come back at you with photos and results soon. 

[DB26]

Here are a few photos of the new cutout. Uses a 25 amp diode. Installing back on car in a few days. 

 

 

[DB26]

Okay, it’s installed and I have some results. I have NOT done any adjustments yet. 

 

I bench tested the generator with a drill and got it to output around 7 volt at a medium drill speed. 

 

After I installed the generator back onto the car I did a baseline reading of the battery which was at about 6.15 volts. 

 

After starting the vehicle, at idle, there was no change in the voltage. 

 

When I increased RPMs, the most I saw at the battery was 6.35 volts. 

 

I did a reading at the diode cutout and saw only .5 (half a volt) coming from the generator at high RPMs. 

 

I assume I need to move the 3rd brush now.

[maok]

Before adjusting, do you have an amp meter installed in the car, if so check what it reads. it should be on the charge side of the gauge.

 

The voltage should increase slowly and reach up to high 6's (volts) as charge goes into the battery from the  generator. Do note, this would only occur well above idle speeds.

 

Are you measuring across the diode? If so then your reading of 0.5v is about right.

Edited July 28, 2019 by maok (see edit history)

[DB26]

1 hour ago, maok said:

Before adjusting, do you have an amp meter installed in the car, if so check what it reads. it should be on the charge side of the gauge.

 

The voltage should increase slowly and reach up to high 6's (volts) as charge goes into the battery from the  generator. Do note, this would only occur well above idle speeds.

 

Are you measuring across the diode? If so then your reading of 0.5v is about right.

 

I just re polarized my generator, and got a different set of readings for you. 

 

Generator output before diode, at idle:

 

Ranging from 3.80 to 4.40 volts, and after a few minutes dropped to 1 volt. I think the idle settled down. 

 

Generator output before diode, at higher RPM:

 

About 7.00 volts

 

Reading after diode at idle:

 

6.05 volts

 

Reading after diode at higher RPM:

 

About 7.00 volts

 

Reading at battery negative post at idle:

 

6.15 volts

 

Reading at battery negative post at higher RPM:

 

6.60 volts. 

 

The ammerter is at a slight discharge at idle. And jumps into the charge portion when it’s at the higher rpms. 

[maok]

They looked to be ideal readings.

 

While driving, monitor your charge rate on the ammeter and adjust the third brush if you need more or less charge. Turning it with the rotation gives you more current output and anticlockwise gives you less.

[maok]

If your generator runs off the fan belt then you could change the pulley on it for smaller diameter one which will give you increased output at idle. If driven from the timing chain then ignore the above and increase your idle speed on the carb for more output at idle.

[ply33]

3 minutes ago, maok said:

They looked to be ideal readings.

 

While driving, monitor your charge rate on the ammeter and adjust the third brush if you need more or less charge. Turning it with the rotation gives you more current output and anticlockwise gives you less.

 

+1 to this. Another thing to do is periodically check the battery water. If the water level is low then some has "boiled off" (not really boiling but rather splitting the water molecules into separate hydrogen and oxygen) indicating overcharging. And check the density to see if it is undercharged. In the third brush systems the battery was used to even things out, you just want to make sure that on average the battery ends up fully charged when you park the car but not over charging on the road.

 

Back in the old days when these cars were driven year round on a daily basis the third brush needed to be adjusted fairly often to account for season and driving style. For example it typically takes more power to start an engine in freezing weather and in winter the days are shorter so your commute has the lights on more. The reverse for summer.

 

1 minute ago, maok said:

If your generator runs off the fan belt then you could change the pulley on it for smaller diameter one which will give you increased output at idle. If driven from the timing chain then ignore the above and increase your idle speed on the carb for more output at idle.

 

Not necessarily a good idea. The third brush mechanism "kind of sort of" turns the generator into a constant current device with the peak output, based on pulley sizes, final drive ratios, wheel size, etc., usually between 25 and 35 MPH. So you may be trading better charging at idle for worse charging while actually moving.

[DB26]

Thank you both for the encouraging words. I will monitor the battery and adjust as needed.

 

On a side not, the generator runs off a flexible disk mounted off of the water pump. 

 

[trini]

I might be going offline here but it is nice to know what to expect. I remember in the old days to quick check if the generator is charging, just use a jumper and ground the field. When I first started my 28 DB the generator was charging 10 amps and the next time it was charging 20 amps steady . I went on this forum and do not remember getting any help. WHY WAS IT CHARGING 20 AMPSTEADY ?  Well the generator burnt out. The restorer , I think A and E offered to restore it for me  No trouble since . The armature was burnt to a crisp and so was the field coil.  The field coil was grounding on the frame. One end of the field coil is attached to the cut out and the other end is attached to the frame through an insulated screw from which the flat brass strip forms the fuse holder. It so happens the  insulator was broken thus grounding the field apart from blowing the fuse. I am happy I had the generator rewired . After so many years the insulation breaks down  and causes lots of headaches. 

[Fordy]

Been reading this out of interest and nobody seems to mention if these are Zener diodes being used. I googled the NTE numbers quoted in one of the posts and they seem to be rectifier diodes. Excuse my ignorance but I don't quite understand how these are meant to work as they pass through one way and block the other whereas a Zener has a breakdown voltage in reverse bias that will allow it to pass current and act as a regulator?  

Is the intent of the rectifier simply to prevent the battery from trying to "motor" the generator at low engine speeds when output is not higher than battery voltage? If so, then surely a regulator circuit of some type would be a beneficial addition.

Edited December 11, 2022 by Fordy
forgot to add a thought (see edit history)

[JFranklin]

The diode is there to stop the flow in one direction to replace the action of a cut-out.

[TerryB]

6 hours ago, Fordy said:

Been reading this out of interest and nobody seems to mention if these are Zener diodes being used. I googled the NTE numbers quoted in one of the posts and they seem to be rectifier diodes. Excuse my ignorance but I don't quite understand how these are meant to work as they pass through one way and block the other whereas a Zener has a breakdown voltage in reverse bias that will allow it to pass current and act as a regulator?  

Is the intent of the rectifier simply to prevent the battery from trying to "motor" the generator at low engine speeds when output is not higher than battery voltage? If so, then surely a regulator circuit of some type would be a beneficial addition.

Not Zener diodes, just standard diodes to block the battery from draining into the generator.  The adjustable third brush setting does the regulator function.

[Bloo]

Cutouts are not regulators. A rectifier diode is what is commonly used. Usually some sort of alternator diode. Those are completely normal Silicon diodes, with a voltage drop of about 0.7 volts. Since these replacement cutouts are used with third brush generators, and there is no voltage regulation anyway, the generator just runs 0.7 volts higher than the battery and never knows the difference.

 

Know why alternators don't have cutouts? Because the diodes are already there.

 

In my opinion, most of these cutout conversions I have seen are not heatsinked near well enough for the current they are expected to carry. If you want to get fancy, you could use a big Schottky diode. The voltage drop is much less, maybe 0.2 volts, so you would have less than half the heat to get rid of at the same current.

 

6 hours ago, Fordy said:

Is the intent of the rectifier simply to prevent the battery from trying to "motor" the generator at low engine speeds when output is not higher than battery voltage? If so, then surely a regulator circuit of some type would be a beneficial addition.

Yes, and that is exactly what the electromechanical cutout did, and all it did.  Third-brush-and-a-cutout systems have some real downsides. Voltage regulation would improve things a lot. How to add voltage regulation to them is a bit of a rabbit hole.

 

Edited December 11, 2022 by Bloo (see edit history)

[edinmass]

I agree with Bloo……..every one I see had overheated and caused lots of problems. Cut outs work fine. They are cheap, and easy to install. A solid state cut out conversion is a solution looking for a problem that doesn’t exist. 

[dfsdfszdfdsz]

One of the things that will drag a battery down is a leaking diode. Model A Fords did not have diodes back in Henry’s day. Today many owners have installed a diode in the generator cut-out, or replaced the generator with an alternator. An alternator has six diodes in it. If you drive the car daily you might not even be aware you have a leaky diode. However, if you let the car sit for a long period of time it will pull the battery down. After many years of service the cutout contacts can become stuck together, this is especially true with the poor quality reproduction cut-outs being offered on the market today. Thus became the reason for the cut-out diode modification. The diode does not have contacts, but it can still fail. Many times the failure is subtle and is manifested in a leak instead of a dead short.

Anyway, I usually buy batteries from here.. http://electronicx.de

Edited December 24, 2022 by dfsdfszdfdsz (see edit history)

[Frank DuVal]

It all goes back to read the specifications of what you are working on. The specifications of the diodes are easy to find on the internet, or through the supplier (DigiKey, Mouser, etc.). This will show how much heat sink area is needed for the diode to pass its maximum rated current. Now trying to get that inside the stock cutout housing and get some air flow in there is going to be fun!

[edinmass]

Just convert you cut out to solid state, and watch the armature toss solder………….too many people have a solution looking for a problem that doesn't exist.

[ply33]

On 12/11/2022 at 11:12 AM, edinmass said:

I agree with Bloo……..every one I see had overheated and caused lots of problems. Cut outs work fine. They are cheap, and easy to install. A solid state cut out conversion is a solution looking for a problem that doesn’t exist. 

I’d agree with you except. . .

 

Back in the late 1970s I went through electro-mechanical cut outs on a regular basis. My theory at the time was that the new cut outs I was getting from places like NAPA used aluminum in the contacts while the vintage ones used tungsten and were just not as good as the ones that were available in the 1920s, 30s, 40s and even 50s. I would have to keep an eye on the ammeter every time I stopped the engine and, if (usually when) it showed a discharge, open the hood, pop the cover off the cut out and pop open the contacts. I would then replace the cut out and wait for it to happen again. My solution in 1978 was to gut a failed cut out and install a rectifier diode in it. That same diode is in there today, over 40 years and many, many thousands of miles later it is still working fine. In my case it was a successful solution to what was at the time a very annoying problem.

 

Maybe the replacement cut outs that are available today are better than those I was buying in the 1970s but since my diode is still working fine I haven’t bothered to investigate.

[edinmass]

PLY33..........I understand in some cases it may work fine. That said, I work on pre war cars full time, and service a huge amount of cars. I see a 20 to 1 ratio of them working vs melting. Also, very few people actually drive their cars. I have never seen a solid state unit make more than one or two thousand miles before failure......sometime minor, sometimes cooking the generator. My 36 Pierce V-12 has something like 25k on it since I restored it. It's running the same points, condensers, coils, plugs, cut outs, ect..........only have installed new tires and batteries. I got 11 year out of my last lead acid unit from Napa. I just don't see the value in them when original equipment works fine. Interestingly I actually carry a solid state unit in my tool bag when on tour to help out others. In the last 20 years I haven't had to use it. Best, Ed

 

I'd also my understanding you can get solid state diodes that will handle much more amperage today.......that may make them a better option than in the past.

Edited December 22, 2022 by edinmass (see edit history)

[kuhner]

In our model Ts we had Fun Projects selling a good working voltage regulator. They sold the business to another company and they are no longer available.  I had one and it worked well, didn’t boil my battery on long trips.

Several members on the Model T forum have been working on a design, schematics are posted ver there.

https://www.mtfca.com/phpBB3/viewtopic.php?f=2&t=25609&hilit=Regulator