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How the Power Transfer Relay (PO) Works
The power transfer relay is one of the most important relays. This relay controls what source of power is sent to the bulbs. Its normally a AC relay or a DC relay with a rectifier attached. The following are several power transfer relays; GRS Type W A.C. relay, GRS Type K D.C. relay, US&S ANL-2 A.C. relay, and the US&S DN-22 D.C. relay. The GRS type W relay is a 110Vac relay. The GRS type K D.C. relay is a size 2 relay with a attached rectifier. The main concept of the power transfer relay is that when the AC power is on the relay is ON. When the AC is on the relay has the AC transformer connected to the bulbs. When the AC power is off the relay dies and connects the bulbs to the battery. The relay is normally either 110Vac or a tap off the AC stepdown transformer. In the GRS Cabinet from 1939, the power transfer relay is tapped off the 10Vac. When the 10Vac AC dies the relay drops. The transformer is feed right off the main power source. This relay means that the only thing running off the batteries when AC is present is the DC relays and searchlight mechanisms. On the photo to the left you see the diagram for the power transfer relay or PO. The dash line indicates that those are the contacts of the relay. The dark triangles are the normally open (NO). The white triangles indicate the normally closed (NC) contacts. BX10 is the positive 10v feed from the transformer and the NX10 is the neutral 10v feed. The BL is from the positive battery post. The CL is from the neutral battery post and called the common. EB is the positive lead to the bulb and the EN is the neutral lead. When the relay is energized the contact closes connecting the BX10 with EB and the NX10 to the EN. The photo on the right is a diagram of the taps off the transformer. The BX220 and NX220 is the main power feed. Most cabinets I have seen are all 110Vac not 220Vac. This transformer has a special tap for the 10volts going to the relay. The relay is the only thing connected to the POBX10 and PONX10. Not sure exactly why they gave it, its own tap but GRS did. The other tap is the BX10 and NX10. This is the lighting tap for the bulbs. This cabinet was designed to only light 2 bulbs at a time. Some cabinets had to light 4 and had much bigger transformers. This is the basic Power Transfer relay operation. Not complex at all and was fail safe. It prevented the bulbs from always running off the batteries. The only thing that runs off the batteries is the D.C. relays which may only take up .500 amps. A track relay at 2 ohms picks up and uses .125 amps. Not as much as one bulb.
All the pictures are off a 1939 General Railway Signal Company signal diagram. Used on the TRRA till September of 2006. 67 Years of operation!
Written By: Zachary Gillihan 2006