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