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How'd they do that...

I ran a service call this holiday weekend on a machine with a small gas power burner. In the middle of the safety sequence I discovered that the Honeywell series 90 modulating valve was not working ( 7800 primary controller ). I replaced the valve and all was good. But now that I think back , the call for heat was from a Johnson A350 proportional control ( I remember seeing a note on one of the many diagrams on the control cabinet door that said 4-20 ma ).

I understand the " R-W-B " on the series 90 , but I'm missing something on my understanding of how it worked with the JCI control. I did measure the resistance at these terminals at one point and saw a 0-135 ohm range when I turned the dial , but then disconnected them to test the valve ( short RWB or RW or RB ). There was no other circuit board or unknown device in the circuit and the valve modulates very nicely , just wondering...if I ever make it back to the machine I'm sure it would make sense. Any thoughts on this ? -Mike

edit : just to clarify...I'm not confused by the sequence of the high / low fire ...just the 4-20 ma controling one side of the " bridge " ( 7800 and the series 90 end switches control the other ). Perhaps the milliamp output of the control is " negligable " and is based on a 135 ohm potentiometer anyway ? Just a guess...and kind of neat to see something different from the norm. I'll try and get a diagram of this. -M


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