Current operated auto switch?

[manatbandq]

however

That's the contact rating.

I think the question was do they work with an Alternating field as you would get by winding mains supply to a power tool around one?

MBQ

[manatbandq]

How do you rectify the feed to the reed? It's a magnetic field from a few turns of the supply wire to the load?

[Rolyata]

If you wanted a steady feed, then I guess you could put a power diode in series with the coil and a similar one the opposite way round across the coil and first diode. The one in my utility room hasn't been told about that sort of luxury and just keeps on working.

I've also seen systems that use about three suitably rated silicon diodes in series in line with the live feed ( presumably with reverse diodes across them to provide the path for the opposite polarity.

As each diode drops about a volt, you'll get 3V dropped across the three of them. That's enough to operate an LED plus limiting resistor. Obviously the master load will receive a slightly lower operating voltage than usual.

In reality, the LED will actually be part of an opto-triac which will switch the slave power when the master load operates.

I tried to make a similar system for an A/V system once, but the standby current ensured it never switched off and I had to use a different solution.

[Dave Plowman (News)]

It's possible it is simply vibrating at 50Hz and this is fast enough to keep the power relay engaged. Wouldn't have thought this would do the life much good, though.

[Bob Eager]

That sounds feasible. However, as someone mentioned, I also need a time delay before the vac goes off!

[bigcat]

Put it thourhg bridge rectifier, ensuring its rated to take surge current and voltage spikes.

NT

[Lurch]

On 26 Jan 2005 22:27:41 GMT, "Bob Eager" strung together this:

Get a delay off timer as well then. I'm sure ther'll be one in the CPC kits, or butcher an electronic time lag switch.

[Dave Plowman (News)]

I'm confused. The original idea was to use the winding on the reed 'relay' to current sense. If you start introducing diodes, surely this will effect the supply to the device you're sensing? Unless you use a shunt resistor and sense off that via a rectifier.

[bigcat]

resistor

The only effect on the load item is 2 diode drops, ie about 2v. The load still gets ac, its only the reed thats sees dc.

I'll see if i can draw it...

OK... you replace this:

______________ ) ) reed relay coil ) ) ______________)

with this:

____________

• /\ ) / \ ) ________/ BR \___ ) reed relay coil ~ \ / ~ | ) \ / | )

- \/____ | ____) | _________________|

BR is a bridge rectifier, 4 diodes. Also add a cap across the reed coil to stop it buzzing at 100Hz.

NT

[Dave Liquorice]

Might have looked OK on your screen but it was rubbish on mine. If you are going to "draw" somthing use a fixed pitch font and don't use tabs only spaces.

[bigcat]

i did

i did, im not a dummy. Looks like google beater stripped out all the leading spaces.

Lets try again: And if it doesnt work tough shit.

[bigcat]

hmm, sorry for being rude :/ i was in a right grump over something.

NT

[Lurch]

On 29 Jan 2005 04:12:46 -0800, [e-mail] strung together this:

Could it be that you use Google?

[Rolyata]

If anybody hasn't been able to work out that ASCII diagram, I'll try with words.

He suggests using a suitably rated bridge rectifier. They have four connections, labelled plus, minus and two marked with the AC symbol ~

The idea is to connect the reed coil and smoothing capacitor across the plus and minus.

One AC terminal connects to the incoming mains while the other AC terminal is the connection to the load. This arrangement interrupts the live connection to the load.

[Andrew Gabriel]

I kind of doubt a smoothing capacitor would work with what is essentially a neglibable impeadance load; there's negligable voltage to charge it up and very low impeadance to discharge it instantly. The capacitor would have to be enormous (but could be very low operating voltage). With no smoothing capacitor, there's no point in the bridge rectifier.

[Dave Plowman (News)]

Well, yes. But you'll end up with rectified mains. Which will fry any reed relay I've ever seen.

[bigcat]

This shows the trouble with asking a tronics question on a non-tronics newsgroup.

NT

[Dave Liquorice]

Er, how? The reed contacts are not in this circuit only the coil around it...

What Mr Gabriel said about the smoothing C is true though. It effectively has a dead short across it through the coil windings.

I guess you could add a series R and up the number of turns to compenstate for the lower I or maybe just upping the turns and using much thinner wire for the coil would do it?

[Dave Plowman (News)]

I'm talking about the coil. Haven't seen any high voltage ones.

[bigcat]

The coil doesnt see high voltage, youve misunderstood the scheme.

NT