HV (500v) Voltage monitoring relay circuit help - Page 1
Forum section: electronics / beginners

author: andybarrett1
author_rank: Regular Contributor
timestamp: October 14, 2016, 11:05:39 am
content: Hi allThank you for reading.... I am looking to build a device as simple as possible that will indicate that a 500v AC line has decayed to a safe level (< 10v maybe) so as we can safely swap out test equipment.At first was thinking of a mains fail relay but it is only interested in 15% drop on the linesSame with an Opto isolator / relay of some description.Then got to thinking about op-amps in some configuration but again starting to over complicate things.Has any body had any need for this sort of application or similar to reiterate as simple as possibleThank you for readingAndy

author: Gyro
author_rank: Super Contributor
timestamp: October 14, 2016, 01:08:50 pm
content: If you think that opamps are getting a bit too complicated then you could always try the humble 555 timer. It gives you two comparators, a reference resistor chain, a flip-flop (with reset) and a reasonably strong output stage.It ought to be possible to come up with something, using resistive dividers obviously. It's a shame it's an AC source, as you would need to rectify and (minimally) smooth the input first. You would also need something in the 5-12V range to power the 555 / relay.Just a thought anyway.P.S. You could maybe also use it in re-triggerable monostable mode to detect the loss of AC zero-crossings. It's unusual to have an AC signal with a decay time - if it's an AC supply then it's normally either there or not.

author: oldway
author_rank: Super Contributor
timestamp: October 14, 2016, 07:07:13 pm
content: A relay with 230Vac coil as FINDER 55 series with 2 MOV's (varistor's) type S20K175 EPCOS in // with coil This must be feeded by a power resistor 15K 11W in serie with the 500Vac.

author: Seekonk
author_rank: Super Contributor
timestamp: October 14, 2016, 10:24:31 pm
content: We built a commercial product that did this and it was antipolar diodes into an opamp. A LM431 makes a great relay driver, combination volt reference - opamp - and driver. Just use a high resistance voltage divider to get the 10V detect and protect input with diodes. Detecting only half cycle should be sufficient.

author: andybarrett1
author_rank: Regular Contributor
timestamp: October 19, 2016, 01:37:29 pm
content: Hi allThanks for all the replies...Seekonk .... Would love to know a little more about your design if possible.BRAndy

author: max_torque
author_rank: Super Contributor
timestamp: October 19, 2016, 07:32:32 pm
content: Most commercial testers consider over around 50volts to be "dangerous". So you need a device to indicate the bus is less than 50v, perhaps with a little leeway, say under 40vdc. My suggestion is a simple LED, driven through a power resistor and zener to delay the switch on a bit.For fail safer, fit two such devices and include a checking procedure in our operational notes (ie ensure both leds are lit when bus IS greater than 50v)

author: Jeroen3
author_rank: Super Contributor
timestamp: October 19, 2016, 07:42:52 pm
content: Use something like this to power a small signal relay. With sufficient heatsink and parts you can create an 500V capable linear regulator.TL431 can be replaced by a zener. You get the idea, right?You'll need to get rid of ~10 watt perhaps. Negligiblecompared to the complexity of the alternatives.You can mount iton this.Source:http://myelectrons.com/mosfet-tl431-ldo-linear-voltage-regulator/

author: R005T3r
author_rank: Frequent Contributor
timestamp: October 19, 2016, 08:37:29 pm
content: if you don't feel comfortable with 500V lines and you don't want to messing around making your own PCB circuit , you need a minimum voltage relay. Something like this one:They are not cheap, but you can set options such as the delay time and a precise range of voltages to operate with it. For 500VAC, if the relay can't handle the voltage or current, you can cascade the output with a suited relay, if you are in a 3-phase environment you need a "contactor" or power relay. Also, I wanted to mention that "safe voltages" are from 0 up to 49V max in AC at 50Hz (which in my country are required to be used by law in industrial applications and signaling).This solution is probably the simplest and safest: 3 relays and you are good to go and that particular relay is guaranteed it will work. Another simpler option could be a rotary switch: they ensureat least4mm of space from the live contacts and yep that's why rotary switches are used: to ensure that it's not live or goes live in any way unless the rotary switch is used, for safety reasons, NEVER use breakers for sectioning a circuit: they do not provide a sufficient safe distance between the switch contacts, because they are designed to break the circuit if something malfunction.

author: julian1
author_rank: Frequent Contributor
timestamp: October 19, 2016, 09:34:25 pm
content: Why is there a decay time - measurable in seconds/minutes/hours? There must be some other engineering principles in play - since my first reaction is that the only thing I would trust at that voltage is a multimeter and a (secondary) mechanical isolation switch on the unit.

author: andybarrett1
author_rank: Regular Contributor
timestamp: October 21, 2016, 09:00:37 am
content: Hi JulianDecay time maybe in Seconds.... There because of the presence of some large LC Filters on the lines...BRAndyThank you for all other Ideas and suggestions all ..... Liking the use of the shunt regulator at the moment !

author: Jeroen3
author_rank: Super Contributor
timestamp: October 21, 2016, 09:36:26 am
content: Don't forget the 20mm varistors. It'll catastrophically fail if you surge it too much.Meanwhile, why don't you comply to safety regulations and install anisolator switch.

author: WackyGerman
author_rank: Regular Contributor
timestamp: October 21, 2016, 09:42:33 am
content: The easiest way is just to put in a voltmeter in parallel to the source . As an option you can put in an indication lamp that lights when voltage is present as a second safety feature if the voltmeter fails