Werner's Miscellanea
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Source at commit 94e955811772fe342947424bd017073aa54b34e4 created 12 years 2 months ago. By Werner Almesberger, m1/ledravaganza/ramp.pl: exponential ramp generator for ledm | |
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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 |
18 | ---------------------------------- |
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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