Werner's Miscellanea

Werner's Miscellanea Git Source Tree

1	Opto-isolated inputs
2	--------------------
3
4	The C8051F320 needs a current of at least 50 uA to drive an input low.
5	The current transfer ratio of the opto coupler is 50% or better.
6	Thus we need an input current of at least 100 uA.
7
8	If we allow for a loss of at least 100 mV (yielding a minimum input
9	voltage of 1.5 V), the protection resistor can be at most 1 kOhm.
10
11	If the maximum power dissipation of the protection resistor is
12	limited to 1 W, we can allow up to 30 mA into 1 kOhm +/- 10%.
13
14	The maximum input voltage would thus be 30 V.
15
16
17	Future changes for higher voltages
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19
20	The circuit is currently not designed for voltages exceeding about
21	30 V between any two points !
22
23	To allow the use with mains voltages, be it across the relays or be
24	it as a difference in "ground" potential of separate loops, at least
25	the following changes will be necessary:
26
27	- air gaps must be 2 mm or larger (6 kV)
28	- conducting elements closer than 2 mm must be isolated if high
29	voltage may exist between them
30
31	In particular, this means:
32
33	- the PCB must be coated
34	- opto couplers may not share the same socket
35	- the opto coupler headers (inputs and outputs) need wider spacing
36	- clearances need to be verified
37

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