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 Trains / 20185
20184  |  20186
Subject: 
Re: DCC for the RCX
Newsgroups: 
lugnet.trains
Date: 
Thu, 29 May 2003 12:45:38 GMT
Viewed: 
2504 times
  
In lugnet.trains, James Powell wrote:
  
   As I run 12v stuff I dont know the 9v motor good enough to comment on the capacitors but I dont think it’s that good an idea to use a resistor to drop the voltage. Use one or more (zener)diode’s instead.

Resistors work fine due to the relative difference in cost. The accessory outputs of the earlier decoders are 100ma or so @28V or less, so at most you are dealing with 2.8 w loss (or 100 ma current draw). In order to drop from 14V to 9V, you are loosing 5V/.5W at 100ma, (and 50 ohms) which is not really a signifigant load. The more recent decoders with 500 ma accessory outputs are trickier, since you are getting into more serious power draws (like 14 w, which is darn’d near what my soldering iron is...)

James Powell

Huh??!? Your basic resistor can only handle 1/4 watt (0.25W), so your worst-case 2.8W is more than ten times the rated maximum current. Even dumping an “insignificant” 0.5W across a resistor is probably not adviseable unless you are also trying to heat your house. With fire. And don’t forget that all of this power is being supplied by the motor driver circuit of your precious RCX which has limits of its own.

Is it worth trying to save a few pennies using a resistor to do a diode’s job at the risk of doing dammage to your RCX?

-Chris.



Message has 1 Reply:
  Re: DCC for the RCX
 
(...) I don't know about you, but I tend towards .5W or bigger for most of my resistors, and diodes have exactly the same problem if you use them as a voltage drop. Load is not really signifigant on a 5A booster (soon to be 10 amps total...2 (...) (21 years ago, 30-May-03, to lugnet.trains, FTX)

Message is in Reply To:
  Re: DCC for the RCX
 
(...) Resistors work fine due to the relative difference in cost. The accessory outputs of the earlier decoders are 100ma or so @28V or less, so at most you are dealing with 2.8 w loss (or 100 ma current draw). In order to drop from 14V to 9V, you (...) (21 years ago, 29-May-03, to lugnet.trains)

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