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Here is my circuit. It works well, as described in a book, but I want to understand it better, so I want to check voltage on capacitor. But when I do it, LED doesn't blink and voltage goes up to 4V. When I turn off multimeter - it works again. Мultimeter is connected to capacitor directly. What did I do wrong?

circuit

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  • \$\begingroup\$ Is this an analog multimeter or a digital one? \$\endgroup\$ – Brian Drummond Nov 29 '14 at 21:03
  • \$\begingroup\$ If it goes up to 4V, input impedance of the multimeter is 1 MOhm, it is a digital one. You should try with another multimeter, a better one. \$\endgroup\$ – Roger C. Nov 29 '14 at 21:10
  • \$\begingroup\$ 1Mohm is a bit low for a DMM. \$\endgroup\$ – Ignacio Vazquez-Abrams Nov 29 '14 at 21:11
  • \$\begingroup\$ Try using a scope. \$\endgroup\$ – Leon Heller Nov 29 '14 at 21:13
  • \$\begingroup\$ To understand simple circuits I always recommend using a simulator; it rarely suffers from similar issues. \$\endgroup\$ – PlasmaHH Nov 29 '14 at 21:25
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Obviously your multimeter influences your circuit. To measure the voltage across the capacitor some current must flow through the multimeter. The amount of current through the multimeter is defined by the measured voltage and the internal resistance of the multimeter.

With the high resistor value of \$470 K\Omega\$ even an internal resistance as high as \$1 M\Omega\$ has a remarkable impact on your circuit.

When you have the parts at hand, you could change your circuit to use \$47 K\Omega\$ and \$22 \mu F\$. The time from flash to flash shouldn't change. However the flash duration would be 10 times longer.

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  • \$\begingroup\$ Another quick solution could be to swap 15k and 27k resistors, maybe voltage is not enough to light the LED though \$\endgroup\$ – Roger C. Nov 29 '14 at 21:40
  • \$\begingroup\$ It doesn't work for me, because of transistor. The LED lights without blink( If I understand right, the voltage on capacitor in my case is only 2V? \$\endgroup\$ – Ishayahu Dec 6 '14 at 20:08
  • \$\begingroup\$ @Ishayahu - Transistor??? We are talking about a thyristor. Where do you have the 2V from??? What have you exactly done that doesn't work for you? \$\endgroup\$ – Kitana Dec 6 '14 at 20:51
  • \$\begingroup\$ @Kitana sotty, thyristor) I use multimeter to measure voltage on capacitor. In that case LED doesn't blink and I don't understand why \$\endgroup\$ – Ishayahu Dec 7 '14 at 9:32

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