Quote:
Originally Posted by Qiaozhi
Note that D1 limits the voltage across the "meter + R24" combination to 0.7V. Without R4, the current through the meter would be: 0.7V / 675 = 1.04mA. This is clearly too high for the 250uA meter. Hence R24 (3k) is used to limit the maximum current to: 0.7 / (3k + 675) = 190uA. The slightly lower then 250uA value stops the meter needle banging against the meter stop when a large signal is received.
All you need to do is replace R24 with a suitable value for your meter. Let's say (for example) your meter is 100uA full-scale deflection (FSD) with 1k5 resistance. A total resistance of 0.7V / 100uA = 7k. Hence, R24 needs to be 7k - 1k5 = 5k5 (or slightly higher). A simple solution would be to replace R24 with a 10k multi-turn preset, and manually adjust it to suit your particular meter.
It should also be obvious that your chosen meter needs to have an FSD of less than 1.04mA.
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Thank you very much Qiaozhi. You are a great help.
