10>YouAsked
10>reduce line voltage down to low (DC) levels in order to measure the line
10>voltage with an AD converter.>>
Three alternatives come to mind:
transformers, caps, or "no connection".
You can use a very small transformer to isolate the mains while
maintaining great accuracy. For example, a simple 600:600 coupling
transformer with 120K in series then use an OpAmp with feedback resistor
of around 5K which can give you better than .1% initial accuracy.
The transformer is commercially available and can be less than 1/4 inch
cube. However, you could custom order it at around the size of a PCMCIA
card thickness. (Not that bulky)
Once the signal is across the barrier and referenced to the A/D power
supply, you can then do what you want with the signal. Put it through a
"two OpAmp" full wave bridge. Then do your A/D conversion if you like.
Or, you could use two AC mains approved .1uF caps with a 200K(two
100K's)/10K divider string that then gets you into the OpAmps referenced
to the ground reference of your A/D. But .1uF caps are fairly large.
If you up the impedance to 2M, you'd only need .01uF which are a decent
size (and commercially available.) Even if the caps shorted, you
couldn't get enough current to burn anything up.
Or, you could just run traces by a high impedance circuit and use the
parasitic capacitance for coupling. Then you could claim "no
connection" to the mains and approvals should be easy, since on the
schematic there would be no connection. <g>
- Robert -
robert.m...@engineers.com
AJM Electronics
---
~ OLX 2.1 TD ~ Rewrites of the Classics #12: "Le Morte de Elvis"
