AC current relay

Some threads I remember pointed to COTS stuff e.g. Sears current-sensing boxes. The posts by DaveM were the most useful, but I can't spot those now.

default wrote in news: snipped-for-privacy@4ax.com:

Put a full rectifier bridge in series with the mains power of the tv, short the dc side with a wire, wind some of that wire around a reed relay, let the reed relay switch a heavier relay, and you are where you want to be.

=2E.....brilliant......really brilliant.....sincerely......

mike

Another possibility. Our new TV has USB inputs. I'm using one to get the +5 and trigger a solid state relay to turn on the audio amp.

I was wondering if this will work. Most of my electronics devices don't turn on just by having power applied. You have to turn each item on separately, after the power is applied.

Bill

I like the concept. Have you tried it? Seems to me a reed would buzz at 60 hertz, and filtering a high current low voltage (what you'd expect on the added coil to the reed relay) would take a very large capacity cap.

Then something would have to protect the cap from an overvoltage with a transient like turn on or degauss (this is a CRT TV)

Worth playing with though.

It's rectified, so it would be 120Hz

you can use a hefty shorted turn around the coil (eg: a length of copper or aluminium tube) instead of the cap.

aluminium tube)

Well my preliminary study indicates that it takes a fair number of turns to energize a reed even with the original coil removed and it does buzz.

The killer is the small wire size it takes to get the turns in there. That would have to be fused separately.

I do have a solution that does work. But was looking for something easy and cheap. I take split bobbin power transformers cut out the secondary and rewind a couple of turns of heavy wire, use the primary as a secondary and power a FWB to energize a solid state relay, or optical coupler. On my water heater and AC compressor - but it can work down to 40 watts with 5 turns of wire and a 240 volt primary.

Low and behold:

There's already 3 different brands of this sort of thing - had I known what to search for.

yeah, the original reed relay coil is sized to a certain number of ampere-turns, if you're running full appliance power through a modified coil and want it to energise with 1A but survive 15A it's going to need a much larger coil.

this may explain why the first electronic load detector circuit I encountered used a triac instead of a relay as the detector, L ------+-->|-->|--+-----> appliance | | +--|

I didn't try the shading coil/tube to stop the relay from buzzing. Now I'm curious to know how that would affect the turns of wire it takes to energize.

I developed transformer the idea years ago using a pair of 12 position rotary switches with 47 ohm resistors to put a load across the secondary then calculated the output, voltage and current, with various resistance's from 47 ohms to 4700. Peak power is close to the DCR of the secondary (as expected) but resistance equal to or higher still transfers a "lot" of power (in the milliwatt range).

For safety I've used diacs to crowbar any spikes on the high turns ratio windings - seems to be working... water heater >10 years, AC >9 months.

The water heater sounds a piezo buzzer when it comes on and flashes a light while on and sounds the buzzer again when the thermostat turns it off.

The AC just flashes a light in proportion to the current it pulls and tells me the compressor is running - it is a variable speed rotary compressor, and works over close to a ten to one range.

I was secretly hoping that someone had found a way to modify a GFI circuit to do what I want, and maybe do away with the effort (and cost and size) in cutting up transformers.

I posted a modified GFCI circuit that does what you want in 2005. If you want, I'll email the schematic to you, or re-post on ABSE. However, I did not use the current transformer that was part of the GFCI, and I don't recall why I used a different one. I was squeezing the whole thing inside a Belkin power strip, and it might be that the CT that was originally part of the GFCI was too big to fit.

Ed

Great. I sent an email removing nospam. I do have ABSE on the server...

Got your e and sent the schematic.

Ed