for some weird reason, my circuit works only when i attached the
oscilloscope to the output. This is extremely weird. According to the
people I have consulted, I needed to add a resistor and capacitor
between the place the oscilloscope is attached to ground. However, I
am unable to tell what value is needed for the resistor and capacitor
If the scope probe is "x1", then to you can simulate it with
R= 1 Megohm
C= approx 10-30 picofarad (exact value not critical)
If it is "x10", then use
C= approx 1-5 picofarad
Connect them in parallel, from the point where the scope was connected, to
This does rather depend on the circuit and whether it needs the probe
tip to be connected, the earth clip, or both...
If the circuit is a 3kW inverter - then I would suspect that it is the
earth clip connection that makes the difference. If it is an oscillator,
the probe capacitance may be making the difference. If it is a dc
threshold detector, it is more likely to be the probe resistance to
Whatever it is, it presumably wasn't designed or intended to work like
that. Generally it is much better to revisit the design and correct it,
than make it work without reasoning why. Otherwise, if a component has
to be replaced, or a second unit built, the circuit may then need *two*
scopes to be connected before it works...
Don't worry. Some of the circuits I've built in the past only work when
I've attached my finger to a crucial point. OK for my own portable
equipment but a bit of a pain if the equipment is on sale to someone else.
I built a cable descrambler from a kit, and the only way it worked stable
was touching a certain piont with my finger.
After some experimenting it turned out to be the 60Hz signal I was injecting
(probably helped the PLL sync), so I soldered a wire to that point and then
wrapped (a few turns) the other end around the insulated 9v AC power lines
(see inductive coupling), stable for ever afterward.
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