It is a known limitation that the Model T Ford can suffer from an overcharged battery because of the absence of any voltage regulation. While the 3rd brush generator allows one to set the charge current, which remains fairly constant over a wide generator speed, there is nothing to reduce the charge when the battery is full. So, if one has set the charge current to say 10A in order to run the headlights continuously, when the headlights are off the battery will receive this 10A, even after it is fully charged. That's not good for the battery, apart from it losing more electrolyte than normal. In practice, the best compromise with the stock standard set up is to set the charge current to 5A which won't damage the battery, and will allow moderate headlight usage.
Principles of Automotive
The principles of voltage regulation for automotive electrical systems were known back in the 1920's, but Ford with its economy focussed ideals, ignored it.
The method used, which has remained to the present time, is to reduce the generator (or alternator) field current when the battery has reached full charge.
Until solid state regulators started to become common in the 1970's, the regulator was an electromechanical device built along the lines of a relay. One set of relay contacts was placed in series with the field coil supply. Shunted across the contacts was a resistor; the purpose of which set the minimum field current and also protected the contacts against the arcing that would result from the rapidly collapsing magnetic field of the field winding.
This set of contacts was pulled in by the voltage coil. Effectively, the voltage coil is connected across the battery and its pull in power is obviously dependent on battery voltage. Therefore, the contacts are set to open when the battery has reached about 7V for a 6V system, or 14V for a 12V system.
The charge current is severely reduced, and the battery does not overcharge. Along with the voltage coil and contacts, are also a current coil and cutout with their own sets of contacts. The current coil senses battery charge current and its contacts open, again reducing field coil current, if the charge current is too high - such as when the battery has a low charge. This is required to protect; a) the battery, b) the generator brushes and commutator, and c) the generator windings from excessive current. Finally, the cutout coil senses current direction. Its contacts are between the generator output and battery. If current flows from the generator to the battery the contacts close, and if the current from the battery tries to flow back into the generator (e.g., when the engine is stopped) then the contacts open. This prevents the battery discharging into the generator when the car is not in use.
The design of alternators is such that they are inherently current limited, by means of winding inductance and that they generate AC. The rectifier diodes used to change the three phase AC to DC automatically prevent the battery discharging when there's no output. Hence, alternators do not require the cutout and current coils in the regulator assembly.
Electronic regulators which replaced the earlier mechanical types use a power transistor to control the field current. This is controlled by a zener diode voltage reference and comparator circuit. As alternators were standard by this time, there was no need to provide current control or a cutout in the electronic regulator assembly.
Regulation and the
An immediate problem trying to connect an accessory voltage regulator to the Model T, or any other car using a 3rd brush generator, is that there is only one terminal on the generator which is for the output.
The field winding is energised from this internally. So, to use a standard regulator involves having to separate this connection and provide a second insulated terminal. Indeed, this has been done. However, it looks out of place and involves wiring modifications. For those who just want a drop in regulator with no wiring modifications, an electronic regulator was developed by Fun Projects.
Regulation with a 3rd
Because of the way the 3rd brush generator works, it is quite easy to shut off the output. We cannot merely open circuit the connection between the generator and battery because of the excess voltage generated under no load, which can burn out the windings. But, because the generator is a constant current source, we can short out the output. Taking the output terminal to earth immediately robs the field coil of supply. It is perfectly safe to run the generator continuously this way because as the field coil is not energised, then the armature cannot produce any output.
Shorting the generator output to earth would cause a huge current from the battery to flow if it wasn't for the cutout.
The obvious component required here is a diode, sufficiently rated to handle the charge current.
To take the generator output to earth, either a power transistor or a MOSFET could be used. To control this, a switchmode controller or a comparator could be used. A switchmode control would gradually increase its 'on ' time (i.e. earthing the generator) as the battery reached 7V, and give the appearance of a variable current control when observed on the ammeter. A comparator would provide full charge until 7V was reached then shut the charge off altogether, until the battery voltage dropped to say 6.5V. This would show more of a pulsing type of indication on the ammeter.
Note that the charge current is still determined by the 3rd brush setting.
The Fun Projects Design.
A well known and popular accessory for the Model T is the Fun Projects Voltage regulator. It can be purchased at http://funprojects.com. It eliminates the potentially unreliable mechanical cutout, and prevents battery overcharge. It is a drop in replacement for the original cutout, looks identical, and needs no additional wiring. A further advantage is the crowbar protection to prevent generator burn out if the battery should be disconnected.
However, there is no technical information available on how it works, or the the internal design. As I'm often asked about the Model T electrical system, it was necessary to investigate and provide my own information.
The particular regulator in my possession was given to me on the premise that it was faulty, and could I fix it? As I discovered, the regulator was actually quite OK, but other wiring in the car was at fault. Nevertheless, I had been curious about the design and wanted to confirm my theory as to how it worked. So what better way than to examine the unit, and trace out the circuit. It could be of use for others who have a faulty regulator, or just want to build their own. Opening the unit simply involved a slight grind into a couple of spot welds. The cover then slips off.
I used a variable, current limited power supply, in series with the secondary winding of a 14V power transformer (primary not connected), to test the unit, which indicated it was functional. Upon raising the input voltage, a high frequency tone could be heard from the power supply, and connecting a CRO showed that indeed it was rapidly switching on and off.
Waveform at generator terminal. As battery voltage rises, the pulse width shown here decreases so that the generator output is earthed for a longer period.
Inside the unit, the cutout diode and switching device (both TO220 packages) are mounted on the chassis, the diode between the generator and battery terminals, and the switching device between the generator terminal and earth. This immediately confirmed the regulator is based on the well known 'grounding switch'. Mounted above, is the PCB with the other components, including a 16 pin IC. Remarkably, the IC, diode, and switching device were devoid of type numbers. Thankfully, that need not present a serious obstacle.
The cut out diode.
Obviously, the diode was a dual Schottky type. I've serviced enough switchmode power supplies to have recognised it as such. Plus, logic tells that this would be the ideal type to use here. The reason being that a Schottky diode has less of a voltage drop across it than a silicon type. This means less heat dissipation for a given current, and less heatsinking is required. In fact, the body of the regulator housing is sufficient for this purpose. Typical types would include MBR30H100CT, rated at 30A and 100V.
The Switching device.
A bipolar power transistor could have been used, but given the current it would have to switch, and the lack of heatsinking, a MOSFET was the likely candidate. As the 'on' resistance of a MOSFET is a fraction of an ohm, it means very little voltage drop, even at high current. Hence, low power dissipation. Besides, MOSFETs are easy to drive straight from regulator IC's, as this one was. A further clue was the 470R gate pull down resistor between gate and source terminals. A bipolar transistor would not need such a low value of base resistor because the the base is current driven. With a MOSFET however, the gate presents as an open circuit, and the drive source needs to be low impedance to switch off the MOSFET very rapidly. A slow turn off will cause overheating due to operation over the linear part of the curve.
Indeed, a quick test confirmed it was a MOSFET. A good choice would a type such as IRF640. One important specification of the MOSFET used is that the gate voltage required to turn the device on is no more than about 5V.
The Integrated Circuit.
As to the IC, several options suggested themselves. But first, before getting carried away with guessing, we need to trace out the circuit. As there are few external components, this was straightforward. Having done this, several clues became apparent. Two things indicated it was a switchmode regulator IC - this being gleaned from the .0047uF and 49.9K resistors - obviously the time constant of an RC oscillator. The one and only zener diode was 15V, and with a voltage that high, was clearly part of the crowbar protection. It was obvious that the voltage reference was internal to the IC; another feature of purpose designed switchmode regulator IC's. And of course, with my previous testing of the unit, the high frequency oscillation present was another confirmation.
Now, to get a block diagram of this IC to analyse operation requires knowing the exact type number. 16 pins immediately narrows it down to a few types. Then by looking at the function of the pins we can match them up. For example, the timing resistor and capacitor need to be on pins 6 and 7 respectively.
Also the output to drive the MOSFET needs to be pin 14, and/or 11. We can also see the output voltage is monitored at pin 2 via a voltage divider - obviously the input to the error amplifier. Using the values of resistors connected to this terminal, and knowing the regulation should commence at about 7V, the reference voltage of this IC was calculated to be around 5V. This immediately eliminated several types from the list of possibilities. Pin 9 would appear to be able to shut down the IC should the 15V zener conduct.
Type SG3524 fits these specifications.
The MOSFET and Schottky diode have already been discussed. The IC receives supply to pin 15 via the chain of diodes D1 to D3.
Apart from reverse polarity protection, this chain of diodes, and their associated 100uF capacitors, also function as a voltage multiplier. Once the battery voltage reaches about 7V and the MOSFET starts switching, there is effectively an AC component present at the generator terminal. By multiplying this, we can get sufficient voltage for the IC to perform properly. Apart from increased drive to the MOSFET, the internal voltage reference is more accurate. There is about 13V present at pin 15 once regulation commences.
The charge retained in the 100uF connected to pin 15 would keep the IC powered up during 'dead time'; i.e., when the MOSFET is switched on.
The output of the IC drives the MOSFET gate from two emitter follower output stages, at pins 11 and 14. In a push pull type power supply, these two outputs are used anti phase, but in a single ended regulator they can be paralleled. The supply also has to feed the collectors of the driver transistors, via pins 12 and 13.
The battery voltage is sensed via a voltage divider feeding pin 2; the non inverting input of the error amplifier. This is compared to the reference voltage (5V) internally generated in the IC, and made available at pin 16. Note that the input to the voltage divider is not the actual battery voltage, but the battery voltage plus the drop across the Schottky diode and D1. We can work out the voltage at which regulation commences. With 5V across the 240R resistor, there will be 7V at the junction of D1 and D2. Adding 600mV onto this gives 7.6V at the generator terminal, and losing about 400mV across the cutout diode (Schottky) means about 7.2V at the battery, which is what we want. It would appear D1 and the associated 100uF decouple the voltage sensing input from commutator noise, and again provide reverse polarity protection to this part of the circuit.
The ramp generator to generate the variable
pulse width relies on the resistor and capacitor at pins 6 and 7 to set
the oscillation frequency. In this application their values are not critical,
and they appear to have been merely selected as being mid way of the specified
range of recommended values.
Crowbar protection for the generator (and the IC) is performed by the 15V zener. If the battery should be disconnected, the generator voltage will rise to an excessive figure. The zener conducts, and pin 9 starts to go high. This then forces the MOSFET to switch on and short the generator to earth. It may be wondered why pin 10 is not used despite being the official shutdown pin. This is because taking pin 10 high actually turns off the switching device, as would be required in the conventional kind of regulator circuit. Here, we want it to turn on the MOSFET to shut everything down. Pin 9 is the inverse of pin 10.
Not that I support 12V being used in Model T's, the 6V regulator could be simply modified should the owner wish to change to a 12V system, having already purchased the 6V model. The 100R resistor would need to be increased to 413R. The 100uF connected to the junction of D2 and D3 should be removed as we don't want the voltage multiplier functioning at 12V.
The most vulnerable component is the MOSFET and cutout diode. If the negative earth version of regulator was installed in a car that had been converted to positive earth, the cutout diode would immediately conduct. D1 and D3 will prevent the reverse polarity reaching the IC or the 15V zener (which would fully conduct on reverse polarity). However, there is an internal diode between drain and source inside the MOSFET. This would conduct. Assuming no fuses between the battery and regulator, a rather high current will flow until one of the diode junctions is blown open. So, expect to replace the cutout diode and MOSFET if this has happened.
Another scenario is if the generator should be 'flashed' after this regulator has been installed. If the MOSFET should be conducting when the battery and generator terminals are bridged out, the MOSFET will be blown apart internally.
When installed correctly, neither a short circuit to earth at either the generator or battery terminal will harm the regulator.
There is a positive earth version of this regulator for Model A owners. As I have been asked about this, I am presenting a theoretical circuit of what needs to be done to change the negative earth version to positive earth.
The circuit is essentially flipped upside down in the electrical sense. Essentially, the cutout diode and 2200uF electro need to be reversed, as well as the supply connections to the PCB.
Note that I have not tried this so cannot confirm if it works!