relaxation oscillator

Jul 17, 2005 4 Replies
F

HI One of my first electronic projects back in 1959 was a relaxation oscillator, using a large neon bulb with and Edison type base. The bulb just flashes off and on. I have tried to draw the circuit using keyboard characters.



I would like to make one again, just to keep on my desk for nostalgia's sake. But here is the problem. I cant recall the values, and since I no longer have parts box around, I will have to buy the two components, and don't even know where to start with values. I vaguely recall that the capacitor was about .1 microfarad, and the resistor was several megohms.



I still have several adequately rated diodes I can use for rectification, but I hope someone can give me some approximate values for the resistor and cap., thereby saving me guessing and many trips to the parts supplier. If the circuit doesn't make sense, the capacitor and bulb are in parallel, and the diode and resistor are in series , connected to the top of the cap. and neon bulb . (I may not even have the circuit right - any help would be appreciated.)


------------[diode]---[resistor]---I-------- I I I I AC line I I [capacitor] [neon bulb] I I I I



-----------------------------------I--------I



Thanks in advance for your time



Felix


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V

The rule is that the charge time for the cap is resistance of R1 (your resistor) times size of cap in F so if r1 is 120 ohm, and c1 2µF:

120x2µF=charge time in seconds up to 4/5 of total charge.

In this circuit, the voltage would lie over the capacitor in the beginning of the cycle, this would be charged, the neon would light, on the negative cycle the cap will be discharged, and so it'll go on...

MVH, Vidar www.bitsex.net
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W

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You can find a NE-2 at Radio Shaft, but I doubt you will be able to find anything else. A NE-51 might work okay if you can find one. Nowadays just about everything uses LEDs, so neon lamps are getting scarce.

The resistor could be 1 megohm. You can buy a package of a few at Radio Shaft so if you need to, you could connect two or more in series for a higher resistance.

the capacitor should be rated at 200VDC or more, and any value around

0.1uF or so would work, it just depends on the flash rate you want. The higher the cap value, the slower the flash rate.

The diode should be a 1N4004, 1N4005, 1N4006 or 1N4007, which are rated for several hundred V. The original circuit used a 90V 'B' battery or a couple 45V batteries in series. But they are expensive nowadays. Just remember to take the high voltage precautions when working around 120VAC and DC. And remember that even if it is unplugged, the cap could have enough charge on it to 'bite' you.

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W

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There is no negative cycle. Assuming that the diode has the cathode to the right and the capacitor is non-polarized, the AC line is converted to positive pulses, during the negative halves, the diode is reverse biased and doesn't conduct.

But the circuit isn't polarized, so it should work with the diode either way.

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R

Hi, I have built such a device. The ne2 or ne2e is to be connected in parallel with the .1 or so cap . All three elements are connected in series. The 100k or more is connected in series with a single 1 amp 400v diode and they are in series with the cap/neon in parallel. The polarity of the diode is not critical because the ne2 and cap are not polarized. The flashing is from the ne2 ionizing and discharging the cap to a voltage below the ionizing voltage of the ne2. the resistor recharges the lost energy/ voltage to the firing point and the cycle repeats.

Ray King

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