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Thread: Old Jeep Voltage Regulators; An Introduction: Section I

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    Super Moderator LarrBeard's Avatar
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    Ft. Wayne, IN

    Old Jeep Voltage Regulators; An Introduction: Section I

    Part I of this series was a discussion of Old Jeep Generators. Part II here is an attempt to try to describe the innards of the three-relay voltage regulator. There are five illustrations, but I haven't found a way to embed them in text where I want them, so I will tack them on at the end of the writing. I wrote too much, so I have to break it up into sections to get it to post!

    The Voltage Regulator

    Before we jump into the innards of the Voltage Regulator, let’s take a little time to talk about electromagnets and relays – the things that make the voltage regulator work. A relay consists of a solenoid and a set of contacts. The solenoid consists of coils of wire wound around a metal core. When an electrical current flows through the coils, a magnetic field is produced that opens or closes the relay contacts. If current flowing through the coils causes the contacts to go from an OPEN to a CLOSED state, the relay is called a NORMALLY OPEN relay (normally open unless current flows through it). If, on the other hand, contacts open up when current flows in the relay, the relay is called a NORMALLY CLOSED relay.

    Relay contacts are held open or closed by springs under tension. It takes a specific amount of magnetic force to open or close the relay contacts. The magnetic force produced by a solenoid is related to the “Ampere-turns” of the relay coil. Ampere-turns are simply the number of turns in the coil multiplied by the current flowing through the coil. If a set of contacts requires a magnetic force developed by some number of ampere-turns, that force can be developed by a few turns and a lot of current (e.g. 35 amps x 7 turns = 245 ampere turns) or by a lot of turns and a little current ( 1/5 amp x 1,225 turns = 245 ampere-turns). These examples are taken from the coils in a typical Jeep regulator.

    If we want a relay to respond to the current flowing in a particular circuit, we use a few turns of heavy wire, with a lot of current flowing through them; we will see this in the current limiter relay in a bit. But, if we want the relay to respond to a voltage, we use a lot of turns of wire and connect the coil across the circuit – we will see this in both the cutout relay and voltage regulator relay – also discussed later.

    Relays are basically electromagnets, and like any magnet they have North and South Poles. If a relay has two coils, with current flowing the same direction in both coils, the North and South Pole forces add, making the magnetic force stronger. But, if currents flow in different directions, the magnetic poles oppose each other and the stronger force (yep, generally the one with the most ampere-turns) will overcome the lesser force. Why go into this? Because those old three-relay regulators use this to make things happen at the right time.

    Now, let’s talk about Jeep voltage regulators. All of the regulators we are going to talk about are three-relay regulators. You may find one and two relay regulators on old tractors, but not on Jeeps. We’re not going to go there. In the hey-day of generators and regulators, there were literally dozens of different regulator units with only minor differences in specifications and performance. But, as time passed and regulator manufacturers dropped product lines, the design of regulators became more standardized. Today, if you go looking for a 6-volt, 35-amp, negative ground regulator for a civilian Jeep, you will end up with a Standard VR-2 or a generic 923131 part number from just about everyone. These standardized voltage regulators may not have the performance enhancements that an automotive designer thought were needed for a 1949 Buick Roadmaster, but they will keep the lights lit, the battery charged and won’t boil the water out of the battery by overcharging. In three simple words; “They’re good enough”. The military version for the MB or GPW is a different regulator and is a bit harder to find (and a lot more expensive).

    As we talked about in the Generator section, Type A and Type B generators require different regulators, so one size does not fit all in this case. Now, what’s inside that black box with three connections?

    INPUT RELAY (Cutout Relay/Circuit Breaker/Reverse Current Relay)

    I am a visual person and I learn best by being able to see something being described. When I look at diagrams of the three-relay voltage regulators I am confused by the fact that they are most often a combination of a mechanical assembly drawing and an electrical hookup diagram. Neither of these presentations gives me an insight to the schematic diagram of the device – the tool I as an electronics guy have grown up with. So, bear with me as I try to redraw the various functions of the regulator as a more modern schematic diagram that may be easier for many of us to understand.

    I will talk about a Type B Generator/Voltage Regulator in the first part of the description; I will modify things to show a Type A Generator/Voltage Regulator at the end.

    See attached Figure 1. Forum protocols do not support embedded pictures


    See attached Figure 2. Forum protocols do not support embedded pictures

    Voltage Regulator Partial Schematic; Cutout Relay

    We will start with the input relay of the regulator. In many of the older descriptions, this relay is called the “Circuit Breaker”. In today’s terminology, we think of a circuit breaker as a device that opens a circuit due to a current overload. That is not what this relay does and it confuses us as we try to understand it. A different name for this relay is used in later descriptions; in the picture above it is called the “cutout relay”. Then, to make things a bit more confusing, it has also been called the “reverse current relay”. If you take the cover off the regulator, this is the relay on the left (terminals toward you) and is generally right behind the BAT terminal of the regulator. Look at the schematic and you will see that it is a single pole, single throw normally open relay – an ON-OFF switch for the battery.

    Its function is simple. When the generator is turning over and making enough voltage at the ARM/GEN terminal of the regulator to charge the battery, it connects the ARM/GEN terminal to the BATT terminal on the regulator. When the generator stops making a sufficient voltage at the ARM/GEN terminal, it disconnects the BAT terminal (and the battery) so the battery does not discharge through the armature windings of the generator.

    Now, how does it do this? The circuit breaker relay is fed through the winding of another relay, the current regulator. For now, don’t worry about the current regulator; just consider it a way to connect the circuit breaker/cutout relay to the ARM terminal of the generator. On the schematic, you can see that there are two windings on the relay; W1 and W2. W2 is only a few turns of very heavy wire – it is connected in series with the generator and all of the generator current flows through W2; W2 is a current coil. W1 is connected across the output of the generator (at the output end of W2) and is a coil consisting of a whole lot of turns of fairly small wire; W1 is a voltage coil. The other end of W1 is connected to vehicle ground, the frame and chassis. At engine start the battery provides energy to the starting motor and the ignition; things go rur, rur, rur, putt- putt, and vroom! (Most days, anyway).

    As the engine comes up to speed, the generator develops a voltage from the armature spinning in the residual magnetism in its field poles and this voltage, supplying current through W1, allows the relay contacts, which are held open by a spring, to close. W1 can close the contacts at a fairly low voltage (typically about 6.4-volts) - the generator does not have to be at full output for this to happen. Closing the contacts connects the second winding (W2) to the regulator BAT terminal.

    Now, notice that there are two dots on W1 and W2 – this is important. Those two dots mean that so long as current enters W1 and W2 at the dots, the magnetic fields of the two windings reinforce one another. (For the real tech weenies, this is called series-aiding). So long as the generator voltage is high enough - the current through W1 and W2 generates enough magnetic force to overcome the spring tension and keeps the contacts closed.

    Now, when the engine is shut off and the generator stops, (or runs so slowly that the generator cannot provide charging current to the battery), for an instant current from the battery flows backwards through W2. I’ll give you a hint – the battery can drive a lot of current back through the generator BAT terminal (as those of us who have had this relay stick can tell you). The magnetic fields from W1 and W2 now oppose each other (another tech weenie term, series-opposing) and the opposing field from W2 wins (a few turns, typically only about a dozen, but a LOT of Amps). Its magnetic field overcomes the field set up by W1 and forces the relay contacts open, disconnecting the generator from the battery. (The relay contacts are pulled open by the spring). When the generator speed increases, the ARM/GEM voltage increases again, the process repeats and the contacts reconnect the battery.

    That’s all this relay does; act as an ON-OFF switch for the generator and battery. Kind of sneaky how those old guys used those two windings to aid and oppose each other wasn’t it?
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