Critic a simple relay drive with Neon bulb and LDR - Page 1
Forum section: electronics / beginners

author: Qmavam
author_rank: Regular Contributor
timestamp: May 16, 2023, 12:44:29 am
content: I'm working with high Voltages. I want a red light to blink when the potential is on or is still discharging. I plan to use a neon bulb illuminating a LDR. I haven't quite got the neon limiting resistor figured out because my output voltage can vary. I have a working circuit, but would like a critic, for any suggested changes I should make. I think the base voltage is a little high. It's simple, but it is a safety issue, I want the red light to blink when the voltage is not discharged to a safe level.https://www.dropbox.com/s/s8pra9x5bdw7dq/Neon%20tube%20Relay%20Driver.jpg?dl=0Thanks, Mikek

author: antenna
author_rank: Frequent Contributor
timestamp: May 16, 2023, 01:07:16 am
content: I believe a neon bulb can extinguish before the voltage drops to a safe level. I have read that the maintaining voltage is 10-20v below starting voltage, and since that is around 80-120v, that tells me the voltage can still be high enough to hurt you after it goes out.An LED light can handle high voltages if the current is limited. Maybe a zener and LED in series with a current limiting resistor would be better allowing the lamp to turn off at a lower voltage.

author: james_s
author_rank: Super Contributor
timestamp: May 16, 2023, 06:30:21 am
content: You don't really need the analog nature of the LDR here either. I would suggest using an off the shelf phototransistor type optocoupler, while not super efficient, you can use a simple resistor to limit the current to the LED up to around 100V or so, if you need to go higher than that then a regulator circuit is probably a good idea in order to supply enough current at the lower end of the range without dissipating an absurd amount at the high end of the range.

author: barshatriplee
author_rank: Regular Contributor
timestamp: May 16, 2023, 08:33:58 am
content: If you are planning to use the neon lamp as a constant voltage source, you have to add a ballast resistor in series. If the input voltage of the neon lamp is 220V, you have to select a 220K resistor. If the voltage is variable, it is better to add a voltage regulator too.

author: Terry Bites
author_rank: Super Contributor
timestamp: May 16, 2023, 09:18:21 am
content: You could put a 555 flasher into reset when the HV drops to an acceptable level.Its possible to adapt this ciruit to operate an HV discharge relay then the 555 reset pin goes low.An opto may work just as well. I'd use a high gain Darlington type.

author: Berni
author_rank: Super Contributor
timestamp: May 16, 2023, 09:28:19 am
content: The trick for doing this is a depletion mode FET current source.You can easily get depletion FETs up to 1000V rating. Then you just give it a resistor to set it up for the LED current you want. That way you can have say 5mA flowing trough a LED while supplying it with any voltage from about 5V to 1000VSince this is a linear regulator means that at the higher voltages it can burn a fair bit of power in the FET, so might need a bit of heatsinking. Ideally you want to use a very high brightens LED so that you need less current to light it up to a clearly visible level. Even 1mA can be rather bright on modern efficient LEDs.

author: EPAIII
author_rank: Super Contributor
timestamp: May 17, 2023, 06:26:19 am
content: Your Dropbox link does not seem to work.Perhaps you could better define what you mean by, "high Voltages".100V?500V?1,000V?5,000V?10,000V?50,000V?100,000V?More?AC?DC?I have worked with high DC Voltages, like 40 KV DC. The way safety was ensured was a combination of resistors that were designed to bleed it off in a second or two or switches that automatically applied grounds when power was turned off AND a manually operated ground cable attached to a hook on an insulated pole that could first touch all the HV points in the device and THEN be HUNG on the circuit point you were going to work on. The cable should be sized at least 10X what is needed for any currents that the HV power source can generate. So, it is going to be a heavy, copper braid. Perhaps a real heavy one.Capacitors and other elements that act like capacitors in HV circuits can recover the charge after being discharged. So a light that simply says the circuit has reached a safe value may not indicate such a recovered charge some time later. The ground hook hanging on the HV conductor while you work will constantly discharge any residual charge.When working with high Voltages, I would not trust any device that depends on any active circuit elements like FETs or transistors or LEDs or relays or even neon bulbs to protect MY LIFE. I have seen each and EVERY one of those types of devices FAIL in circuits with no more than 12VDC in them. And some of the above circuit suggestions have more than one of those devices.The simpler the better and you just about can't get any simpler than an extra heavy, grounded conductor.AND, of course you INSPECT that ground probe BEFORE EACH USE. Not each day. Not each hour. But BEFORE EACH USE.I have worked in electronics for over 60 years and have only been shocked twice by HV. And that was two times too many. BE SAFE, PLEASE!