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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. Adjustment 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 worn. 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 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~