RailPro > RailPro Specific Help & Discussion
Stay Alive Circuitry
nodcc4me:
I"m with Kevin. :-\
Alan:
--- Quote ---Wouldn't the diode bridge prevent feedback of power from the capacitor to the rails anyway?
Answer
Could eliminate the resistor if not too many locos on the layout. Large amounts of capacitance may make it difficult or impossible to startup the power supply/breakers.
--- End quote ---
Sorry Bill. I totally misread your question. My eyes read diode. My brain heard resistor. You must be like Huh???
Yes, the diodes in the bridge rectifier do prevent the capacitor from discharging back to the power supply. So the bridge performs two functions - blocking and polarity rectification.
William Brillinger:
Phew. I thought I was losing it there.
So then for clarity: in light of the diode bridge, the resistor is not needed - correct?
Alan:
--- Quote ---So then for clarity: in light of the diode bridge, the resistor is not needed - correct?
--- End quote ---
For a single loco on the layout the resistor is not needed. As more locos are added to the layout you will eventually reach a point where the combined inrush current of all the caps is so high that the power supply can't startup.
A cap is essentially a dead short at the instant of power up. It's resistance climbs very fast so the "short" is extremely brief. Power supplies are designed to allow excessive current to flow for a very brief moment during power up. If there are a lot of caps then the amount of time that excessive current must flow exceeds the power supply's ability to deliver. Hence, the power supply will shut down. This all happens so fast that it will appear the power supply refuses to turn on. Naturally higher output (amps) power supplies will turn on larger cap loads.
It should be noted that old style iron core transformer power supplies (below) don't suffer from this problem. They will start any cap load or send your wiring up in smoke trying!
William Brillinger:
--- Quote ---For a single loco on the layout the resistor is not needed. As more locos are added to the layout you will eventually reach a point where the combined inrush current of all the caps is so high that the power supply can't startup.
--- End quote ---
Ah! it all makes sense now.
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