Low voltage cut-off at Mains level - Page 1
Forum section: electronics / beginners

author: tribat
author_rank: Contributor
timestamp: January 25, 2011, 02:31:34 pm
content: My load is a 230VAC 28kohm (8.2mA) relay that on its own has a must drop-out voltage of 0.2Un which makes the relay open when voltage goes under 50VAC. But I need the relay to open much sooner, say when voltage goes below 200VAC. Is there a simple circuit I could use to achieve this?

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: January 25, 2011, 03:35:08 pm
content: Quote from: tribat on January 25, 2011, 02:31:34 pmPlease define the word " simple " .

author: FreeThinker
author_rank: Frequent Contributor
timestamp: January 25, 2011, 04:17:52 pm
content: You could try wiring a resistor across a nc contact in series with the coil and playing with the spring tension of the moving contact block. When switched on the full 230v is applied via the nc contact which then opens and holds the coil just above the desired dropout voltage, may work never tried it but the theory is sound.Some experimentation will be required and current capacity could (almost certainly will) be reduced.Also no two coils will be the same so some variation in the operating point is to be expected.

author: Zero999
author_rank: Super Contributor
timestamp: January 25, 2011, 06:35:47 pm
content: The easiest way is to buy a low voltage protection relay which will do what you want.

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: January 25, 2011, 06:48:24 pm
content: You might get some ideas from here ...http://www.brighthub.com/engineering/electrical/articles/68396.aspx

author: tribat
author_rank: Contributor
timestamp: January 26, 2011, 01:11:29 am
content: Quote from: Hero999 on January 25, 2011, 06:35:47 pmWhere could I get one?Quote from: FreeThinker on January 25, 2011, 04:17:52 pmProblem with that is the relay is not guaranteed to hold once the voltage goes under 0.8Un. So keeping it at say 0.3Un under normal used would probably mean a little vibration might make it lose contact.Quote from: JohnS_AZ on January 25, 2011, 06:48:24 pmThose circuits use a DC relay that needs a big bulky transformer to go with it, I wonder if there's a way to do this without a transformer.

author: FreeThinker
author_rank: Frequent Contributor
timestamp: January 26, 2011, 07:56:54 am
content: Quote from: tribat on January 26, 2011, 01:11:29 amExactly my point about the reduced current rating.Reducing the spring tension will help (while making the vibration issues worse) but without going to voltage sense relays or voltage monitoring circuits you are very limited.

author: Zero999
author_rank: Super Contributor
timestamp: January 26, 2011, 10:20:37 pm
content: Quote from: tribat on January 26, 2011, 01:11:29 amIt's a standard part which is normally used to protect AC motors which can stall and cook if the voltage falls too low.Here are some examples:http://www.schurterinc.com/Products/Circuit-Breakers/Undervoltage-Protectionhttp://www.schurterinc.com/en/schurter/link/pdf/datasheet_PG_Undervoltage_Protection_040191010000008762Although these might not be suitable for you since they operate when the voltage drops to 35% of the rated value which is 84V.I think you'll have difficulty finding something which operates at 200V, you might be able to get 180V.What voltage do you want it to turn back on at? Do you want manual or automatic release?If you go for the home made option, you can get some pretty small transformers. I'd probably use a small <3VA PCB mounted transformer, an LM311 and a suitable voltage reference. I'll post a schematic and suggest some parts, if you're interested.

author: tribat
author_rank: Contributor
timestamp: January 28, 2011, 04:05:59 am
content: Quote from: Hero999 on January 26, 2011, 10:20:37 pmYour right, a small PCB transformer would be pretty compact, but can it deliver enough current to power the voltage monitor circuit and a relay? A schematic would be really helpful!

author: Zero999
author_rank: Super Contributor
timestamp: January 28, 2011, 06:39:36 pm
content: That depends on how large the relay is which is determined by the voltage/current it needs to switch.Is the load capacitive, inductive or resistive?It's possible you could do away with the relay and use a TRIAC which would mean you could use a small capacitive power supply, rather than a transformer.

author: tribat
author_rank: Contributor
timestamp: January 28, 2011, 10:19:12 pm
content: Quote from: Hero999 on January 28, 2011, 06:39:36 pmLoad of the relay will be few computer PSU's (300W and 500W) and 3 x 12V 1A wall warts. I've never used a TRIAC so I wouldn't know if it could be used as a switch with a load like that.I was thinking a relay like Finder 40.52.9.012 which has a 12VDC coil at 220ohm so it would need about 55mA. Also checked out the PCB transformers at my local electronics store and the 2.3VA 1 x 12V seemed like it might be a nice choice. It can delived 192 mA at 12V and has a voltage of 21.4V when not loaded.

author: Zero999
author_rank: Super Contributor
timestamp: January 29, 2011, 12:24:15 am
content: Hmm can you get hold of a relay with a slightly higher resistance coil? I was going to suggest the LM311 but it's maximum output current is 50mA. It's not a big deal as you could use a buffer transistor, it's just a cleaner/cheaper design without one. If not, it's possible to configure the LM311's output as an emitter follower so it drops a couple of volts which will reduce the relay voltage to 10V and current to 45mA but it's nor best practise.I'll have a look tomorrow, I'm off to bed now.