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30 meters of Cat5 cable inductance / back emf ?
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TwinScroll:
Hello.
I been requested to get 4 LEDs to be displayed about 30 Meters away from a control box (pre designed).
The LED interface PCB within the control box is also already designed. Normally these LEDs are on the front panel of the box.
There are constant current 20mA drivers fitted for each of the LEDs and the supply voltage to the constant current diodes is 5VDC.
I’m probably over thinking this, I know it’s low current but....
Q) In principle if driving a 20mA load (LED in this case) by using the pairs within a Cat5 cable, If the length to the LED is long, say 30 Meters, would the total inductance of the cable length create any significant back EMF when the load switched off ?
I’d need to use Cat5 cable due to the control box connectors available (there is a spare coupler).
I recall from many years back the cable properties will be distributed inductance / resistance and capacitance between the pairs.
I can’t recall if assuming a total inductance for the overall cable length is the correct thing to do here ( the use that in the normal di/dt calculations).
I try and avoid long leads to loads etc, it’s always a recipe for headaches, though I’m a bit stuck on this one !
Best regards
TS
mikerj:
Provided you use one of the twisted pairs in the CAT5 cable then inductance should be pretty small, from memory it's of the order of 500nH/meter. A fast diode across the driven end of the pair, connected so it's reverse biased should provide adequate protection.
ogden:
--- Quote from: mikerj on June 21, 2020, 12:54:03 pm ---Provided you use one of the twisted pairs in the CAT5 cable then inductance should be pretty small, from memory it's of the order of 500nH/meter. A fast diode across the driven end of the pair, connected so it's reverse biased should provide adequate protection.
--- End quote ---
Right. Introducing protection in this case is good idea anyway - even if you do not expect back EMF. There could be other kind of surges.
TwinScroll:
Hi, thanks for the input, just to clarify, where you mention ‘driven’ end - is this the end of the line ‘sending’ power out?
Each of the 4 LEDs will be making use of the twisted pairs available.
mikerj:
--- Quote from: TwinScroll on June 21, 2020, 03:14:25 pm ---Hi, thanks for the input, just to clarify, where you mention ‘driven’ end - is this the end of the line ‘sending’ power out?
--- End quote ---
Yes :-+
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