# 555 timer ?

Discussion in 'Datasheets, Manuals and Component Identification' started by bob.steel, Mar 1, 2020.

1. ### bob.steel

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Feb 27, 2020
Most 555 timers have an input voltage DC limitation of say 5V.-12V

Is it possible to place it in a 300 volt circuit in any way by limiting its voltage with resistors and the amount of current flowing to its sense triggers?

2. ### Harald KappModeratorModerator

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Nov 17, 2011
What do you mean by "input voltage"? Supply voltage or as your question suggests the input to the trigger pin?

For the supply voltage:
Resistors alone will not work properly. Due to changes in the operating current the voltage drop will vary. You'll need some kind of stabilization. A simple zener diode stabilizer can do the job. Add a capacitor of 10 µF or more at the output of the stabilizer to smooth the voltage and supply impulse current.
This is a linear type of regulator and as all these regulators it is very inefficient. The series resistor has to drop ~290 V (assuming 300 V input, 10 V output). At a moderately small current of 10 mA this means a power dissipation of 2.9 W in the series resistor.
A more efficient way would be using a switched mode step down regulator, but you'll be hard pressed to find a small and affordable one for this application.
The most cost effective solution would be to use a small wall wart with 5 V to 12 V output voltage.

For the trigger input:
A voltage divider can be used to scale down the 300 V input voltage to a level compatible with the supply voltage of the 555. As an additional protection a zener diode across the trigger input can be used to limit voltage spikes.

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3. ### hevans1944Hop - AC8NS

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Jun 21, 2012
It would help us immensely in suggesting a solution to your "300 volt" problem if you would tell us what you are trying to DO. Do you have a circuit diagram that would do what you want to do IF a "555-type" timer were available in a 300 volt version?

Sometimes all it takes is an LED-driven optical isolator to separate a low voltage circuit, such as a timer, from a controlling signal that happens to be a much higher voltage. This would be similar to what Harald suggested with the zener diode and its current-limiting resistor with regard to inefficiency, but you wouldn't have to worry about exploding the lid off a 555 timer IC if something goes wrong. OTOH, exploding optical couplers are cheap and easy to replace, but don't ask me how I know this.

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Nov 17, 2011

5. ### Kiwi

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Jan 28, 2013
Speaking from experience, I have to say that experience is a great teacher.