Is there a 1-Wire(*) device that can detect the presence of mains? I
suspect that the DS2413 might do it but ideally I'd like it to be
galvanicly isolated so there is no chance of mains getting where it
shouldn't and frying things.
The application is to monitor the various pumps and demands on the
heating system here in conjunction with a scattering of 1-Wire
(*) "1-Wire" as in using the same system as the cheap temperature
sensors you can get from Dallas/Maxim.
Volts or amps?
Only, volts on mains wiring is really hard to detect reliably with a high
resistance isolation as it is so easy to pick up stray voltages.
How about something really simple, like a mains powered relay, an opto-
isolator, or a home brew optoisolator using a photo-device pointing at a
fully sealed mains LED unit?
On Fri, 06 Jan 2012 17:22:47 +0000, Tim Watts wrote:
Presence ie is something switched on? Loads are around 50W for
circulation pumps or oil boiler. I've been half looking at hall
effect switches to feed into a DS2413 1-Wire dual channel PIO device.
But I haven't a clue about the field strengths that hall effect
switches need or those that you'll find around a mains cable carrying
200mA or so.
I have a load of relays that could be liberated from the old control
box but that just seems a bit over kill and "old fashioned" but does
solve the isolation side. I can't help feeling that there must be
something from the process control area for this but perhaps they
don't come cheap...
Mains LED indiactor, opto switch into a DS2413 that's another way.
Might be a fair bit of messing about with heatshrink to make light
tight units though.
On Fri, 06 Jan 2012 21:42:06 +0000, Dave Liquorice wrote:
There are voltage and current sensors available, but you probably don't
want to pay the cost - especially for the current sensors! There are also
little hall effect sensors at a more reasonable price. The problem is
that they give an AC output around zero (and need a negative supply). I
once used a system of connecting an opto-isolator & resistor to watch the
voltage across series diodes (actually a bridge rec with + and -
shorted), but you get a 1.2v or so drop and a pulsating DC signal to deal
with. A relay to watch just the voltage is a lot easier!
Mick (Working in a M$-free zone!)
Looks like a very handy device, that...
The only way I can think of doing a reliable non contact volt-detection is
to do something like slip the conductor through a small bit of brass tube
(the detector), then oversleeve that with some plastic, then oversleeve
again with bigger brass tube which is earthed to provide some local
shielding from other stuff - but then you'd probably need to amplify the
signal which leads to a problem of power for the amp circuit.
Unless the DS2413 was sensitive enough to cope directly.
It mentions it has 1M input impedance and a max voltage of 28V on the IO
pins - wonder if a signal diode->tiny capacitor+clamp zener+high-ohms bleed
resistor could clean up the induced mains hum on the pickup (ie turn it into
a DC signal) and still provide enough oomph to the chip???
If it could be made to work reliably, it would be elegantly simple,
intriniscally safe and the pickup assembly could be made split for slipping
onto the conductor with a finishing turn of metal foil sticky tape to
complete the earth shield.
It's a massive conjecture because I've always been crap at analogue stuff...
That's effecitvely what I proposed, only the capacitor is formed between the
wire's insulation and the plastic between the sensing electrode and and the
I was trying to meet the objective of total isolation, hence no direct
contact with the conductor.
Interesting idea with the FET - that could help. Still need to convert the
AC signal to a damped DC one though otherwise the chip is going to have a
hard time deciding if the power is "on".
Coming back to common sense, I'd feel happier with applying a detector
sleeve over inulation that iteself meets the voltage requirement, rather
than directly connecting a class Y capacitor. It's not the capacitor that's
the risk, but engineering out other problems like, say, condensation getting
on the PCB where the capacitor is mounted.
If you were manufacturing these, you'd test the prototype and probably apply
coatings or potting to avoid this problem.
For a DIY solution, I'd much rather have a non contact solution as proposed,
or use a relay in a box with space.screw terminals where I can visually
confirm the isolation.
Saying the cap must be class Y is one thing, but it's meaningless unless you
can verify the installation of it.
Anything capacitive will be horribly subject to transmitting
any noise, usually many orders of magnitude more effectively
than the signal you're actually interested in.
Use an opto-isolator, which mostly give you 4kV isolation and
no differential grounding problems. If you select a part with
darlington output, you can switch it with as little as 1ma
driving the LED side, which you can easily derive from the
mains with a dropper resistor, and some rectification or a
protection diode, and a fuse for safety. If the DS2413 has
an internal pull-up or pull-down, you could connect the
opto isolator directly. Otherwise, you'll need to handle
providing a pull-up or pull-down and micopower from the
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