Couple of hopefully simple questions

gstopp at fibermux.com gstopp at fibermux.com
Fri Oct 25 22:53:59 CEST 1996


     I tend to use 24 gauge solid wire on my panels, and 22-24 stranded for 
     PCB-to-panel and power distribution. The solid wire on the panel can 
     be bent into neat paths like plumbing and it stays put. I run all my 
     voltage divider pots in a daisy-chain, and just apply power to the 
     whole front panel at some easy access point, usually near one edge.
     
     28-gauge is a little too thin for my tastes. Also, I try to use 100K 
     pots for dividers, especially if there's 30 volts across them, to keep 
     current draw to a minimum and to minimize impedance effects on the 
     power busses. If you mess with one pot and it affects another separate 
     setting, you probably have a power distribution impedance problem (not 
     true zero ohms) or a mis-wire somewhere.
     
     10K is as low as I'll go for a divider that's across -15 to +15. That 
     calculates out to 30/10k = 3 milliamps, and E*I = 90 milliwatts. The 
     pot element will probably be running warm at this power. So beware of 
     this effect as well. My motto is to always calculate out your power 
     dissipation anytime there's voltages over 5 volts involved. Keep 
     things in the microamps if possible, except for LEDs and stuff.
     
     For power busses I usually go way overkill. Many times in a 3' wide 
     modular my transformer/bridge/cap raw D.C. section could melt a coat 
     hanger. So with this manly-man attitude I usually run 14-18 ga. 
     stranded all over the place, using twisted 2/3/4 wire bunches (twisted 
     with a drill motor) and solder lugs (remember, fuses are our friends).
     
     - Gene
     gstopp at fibermux.com
     


______________________________ Reply Separator _________________________________
Subject: Couple of hopefully simple questions
Author:  Bob Zimmer <bzimmer at voicenet.com> at ccrelayout
Date:    10/25/96 10:36 AM
     
Can anyone answer a couple of what I hope are simple questions?
     
1.  I'm planning to use 28 gauge (stranded) ribbon cable for power 
distribution through my modular.  These are going to be connected directly 
to the modules through IDC edge connectors.  I have seen them listed in 
Digi-Key's catalog as capable of 2A, but in a book I bought, they are rated 
under 200ma.  This is quite a difference in ratings.  The question is, just 
what current are they capable of carrying?
     
2.  When delivering +15/-15 to the two sides of a series of pots, can this 
simply be daisy chained or is it better to bring each pot back to a central 
point?  I'm trying to solve a wiring problem where I have about 8-10 pots on 
a daisy chain and it seems that changing one is having an effect on some of 
the others.
     




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