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Re: [T3] Voltage Regulator

On 20 Jun 2005 at 12:22, Iturzaeta , Joseph wrote:

> How exactly does a voltage regulator work?

There's so much misunderstanding here that I went ahead and wrote up a FAQ. 
Here it is. Anyone wanting a copy of it in the future can get one by just 
asking. Comments, corrections & typo mentions are always welcome.

How does the voltage regulator on your VW generator charging system work?

Inside your Bosch voltage regulator you will find 2 relays. Each of these has a 
completely separate function.

The cutout relay

The cutout relay contacts connect the generator output directly to the battery. 
Its only function is to disconnect the generator from the battery whenever the 
engine stops. If they stayed connected when stopped, the battery would 
discharge backwards thru the generator and probably burn up the generator as 
well as completely discharging the battery.

If you ever notice that your generator light has come on and stayed on after 
you have turned off the engine and removed the key, then you have a cutout 
relay which has welded its contacts closed. This is a fairly common problem 
with worn out voltage regulators.

The regulating relay

The second relay is the heart of the voltage regulator. It controls how much 
current flows thru the stationary field coils in the generator. It works by 
intermittently turning on and off with varying proportions of on/off time 
depending on demand. Basically, it sits there and buzzes, which is one of the 
reasons that it wears out.

The regulating relay has three states:

1) Full on

There is a coil with many turns of fine wire controlling the regulating relay. 
That coil is connected between the generator output and ground. When the 
generator voltage is low, that coil does not have enough strength to pull the 
relay armature in, so the relay contact, which is connected to the low side of 
the generator field (the other side of the field winding is connected to the 
generator output, inside the generator) is connected to ground. This puts the 
maximum amount of field current thru the generator field winding.

In this state the field current is I = Vg/Rf, where Rf is the resistance of the 
field winding, or 3.5-4 Ohms.

2) Intermediate (not touching either contact)

Once the voltage has risen enough to start to move the contact, it can float 
between the two endpoints and the current. 

At this point there is a resistor between the relay contact and ground which 
has R = Rf, so that now the field current is I = Vg/(2*Rf), or one half of the 
Full on value.

This resistor is the open coil, wire wound resistor on the underside of the 
voltage regulator. When it finally fails, there is nothing to absorb the 
inductive pulse from the field windings as the regulator contact opens from the 
Full ON state. This tends to make that contact arc, which rapidly leads to 
failure of the contacts and the voltage requlator. When this happens, the 
contacts tend to weld temporarily instead of buzzing quietly. These contacts 
will eventually break free and cycle again, but this leads to the clunking 
which is the other symptom of a dead voltage regulator.

3) Hard off

At high rpm, the intermediate state still gives too much voltage so the relay 
pulls in even farther and touches the other contact, which is connected to the 
generator output. At this point the generator field winding is connected to Vg 
at each end, so the voltage drop across it is zero and the current thru it is 
also zero.

In addition to the voltage windings around each relay, there are also current 
windings which modify the field in each relay. Both current relays "buck" or 
oppose the effects of their respective voltage relays.

The current winding on the cutout relay will open that relay if the generator 
current goes too high. 

The current winding on the regulating relay passes around a small shunt 
resistance and thru a diode which Bosch calls a Variode. Thus the regulating 
relay sees the effect of this winding only after the voltage drop across the 
shunt exceeds the forward voltage drop of the diode. The effect of this is to 
allow the system voltage to drop a bit as the current approaches the maximum 
rating of the generator.


Both relays are adjusted by means of little tabs which can be bent to adjust 
the spring tension of their return springs. Some of the relay contacts are also 
mounted on tabs which are also bent to get the right adjustment. Most of these 
adjustments interact, and it is hard to get them right once the regulator is 

Copyright, Jim Adney 6/2005

Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA

List info at http://www.vwtype3.org/list | mailto:gregm@vwtype3.org

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