Circuit for HV-DC alert

[diegoperez]

Hi, I'd like to ask for advice in a circuit that I'm trying to build.

The function of the circuit is to indicate the presence of high voltage (from 50V to 1000V DC approx.)  lighthening a led.

I'm trying to design a solution which does not need extertnal power. Also, I'm looking for a simple circuit with low parts count and safe (the point were the voltage is sensed is a high voltage and high energy point)

Here is a draft of a design that I just came up with. It would be interesting  pointing out possible flaws or problems of this circuit and also suggesting other circuits.

Thanks for helping!

https://www.dropbox.com/s/tty7xcultqiueo5/circuit_draft.jpg?dl=0

[richard.cs]

Make a neon lamp blinker. A neon with a few kiloohms in series, paralled with a capacitor of a hundred nanofarads or so. Ground one end, connect the other to your high voltage via a resistor of some hundred kiloohms, you'd find it starts blinking slowly at about 90V and faster as the voltage increases. Your high-value resistor needs to be able to take the full voltage so is probably best as a specialist high voltage resistor or as many normal ones in series.

[rickey1990]

You could use something like a spark gap,bit basic,properly not the most accurate but two metal electrodes which can be screwed closer or further apart. Will spark at certain voltages depending on the distance of the electrodes.

[NiHaoMike]

A 47k or so HV rated resistor (or a whole lot of normal ones in series adding up to enough voltage and power rating) connected in series with a pair of blue LEDs in inverse parallel. Blue LEDs will glow quite brightly with little current.

[diegoperez]

Make a neon lamp blinker. A neon with a few kiloohms in series, paralled with a capacitor of a hundred nanofarads or so. Ground one end, connect the other to your high voltage via a resistor of some hundred kiloohms, you'd find it starts blinking slowly at about 90V and faster as the voltage increases. Your high-value resistor needs to be able to take the full voltage so is probably best as a specialist high voltage resistor or as many normal ones in series.

Good answer! It was just what I was looking for. Truth be told, it is not the safest and fault tolerant circuit (as evb149 pointed out). However, as Dave says, It is good enough for Spain 

Thank you!

[richard.cs]

Good answer! It was just what I was looking for. Truth be told, it is not the safest and fault tolerant circuit (as evb149 pointed out). However, as Dave says, It is good enough for Spain 

Thank you!

Nothing stops you using several such circuits to give some redundancy. I have been known to make two with different time constants so that by the time one is near-continuously lit the other is blinking at a useful speed, gives a better way of estimating voltage. Best practice would be to check that the lamps blink with HV present before trusting them to indicate its absence, make sure you've waited the expected number of time constants required to hit a safe voltage, and then to follow safe-discharge procedures anyway just in case it is charged.

This circuit has the advantage that it can give useful indication with far less current. I use it at 10s to 100s of kV (with appropriate resistor chains) where 100uA for an LED would be ten Watts wasted. I can then have 3 independant bleed chains of 10 uA or so, two with neon blinkers (the third without ensures that it discharges to 0V rather than stopping at the neon strike voltage).