Silicon Chip Energy Meter Kit (Jul/Aug 2004)

F

Franc Zabkar

Guest
I'm building the Silicon Chip Energy Meter Kit (Jul/Aug 2004). I can't
see the point of R2, an 0.01 ohm resistor in series with a 1K 1%
resistor in the AC current sense circuit. For perfect resistances R2
would make perfect sense (impedance matching on the inverting and
non-inverting amp inputs), but 1% of 1K is 10 ohms, so the design
logic escapes me.

Another observation is that the placement of VR1 (Contrast) on the
display PCB is inappropriate. IMO a better position would be on the
solder side of the board where it would remain accessible after
assembly.

The design of the battery backup circuit could also be improved by the
addition of an enable/disable switch. AFAICS, the battery would be
very quickly drained if the unit is stored for even a short time.

Lastly, I would thread a stiff wire link through the common terminals
of switches S1-S4 to provide resistance to movement.


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.
 
"Franc Zabkar"
I'm building the Silicon Chip Energy Meter Kit (Jul/Aug 2004). I can't
see the point of R2, an 0.01 ohm resistor in series with a 1K 1%
resistor in the AC current sense circuit.
Click to expand...

** The pic on page 71 of the August SC shows R2 as a wire link.





............... Phil
 
On Thu, 16 Dec 2004 10:29:29 +1100, "Phil Allison"
<philallison@tpg.com.au> put finger to keyboard and composed:

"Franc Zabkar"

I'm building the Silicon Chip Energy Meter Kit (Jul/Aug 2004). I can't
see the point of R2, an 0.01 ohm resistor in series with a 1K 1%
resistor in the AC current sense circuit.


** The pic on page 71 of the August SC shows R2 as a wire link.
Click to expand...
.... which is what I've used. However, my DSE kit (K7217) did not
supply the special "length of 0.2mm enamelled copper wire" called out
in the parts list. Instead I have temporarily used a piece of the
0.5mm tinned copper wire that was supplied for the other PCB links.
Upon closer reading of the SC article, it states that "R2 is necessary
to mimic the noise picked up by R1", R1 being the 0.01 ohm 3W load
current carrying resistor (which looks like a heavy wire link). The
amp inputs of the ADE7756 chip are filtered by a 1K/33nF RC LPF, which
gives a 3dB point of around 5kHz, so I can't imagine this noise is RF.
Instead I suspect that R2 is intended to detect radiated AF noise, and
to ensure that this noise appears as common mode which can then be
cancelled by the amp???


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.
 
"Franc Zabkar"
"Phil Allison"
** The pic on page 71 of the August SC shows R2 as a wire link.

... which is what I've used. However, my DSE kit (K7217) did not
supply the special "length of 0.2mm enamelled copper wire" called out
in the parts list. Instead I have temporarily used a piece of the
0.5mm tinned copper wire that was supplied for the other PCB links.
Upon closer reading of the SC article, it states that "R2 is necessary
to mimic the noise picked up by R1", R1 being the 0.01 ohm 3W load
current carrying resistor (which looks like a heavy wire link). The
amp inputs of the ADE7756 chip are filtered by a 1K/33nF RC LPF, which
gives a 3dB point of around 5kHz, so I can't imagine this noise is RF.
Instead I suspect that R2 is intended to detect radiated AF noise, and
to ensure that this noise appears as common mode which can then be
cancelled by the amp???
Click to expand...

** If induced hum pickup ( from the power tranny) in the loop formed by
the low value resistors is the issue - then the exact value of R2 is
irrelevant. Any low value will do.





............. Phil
 
Franc Zabkar wrote:
I'm building the Silicon Chip Energy Meter Kit (Jul/Aug 2004). I
can't
see the point of R2, an 0.01 ohm resistor in series with a 1K 1%
resistor in the AC current sense circuit. For perfect resistances R2
would make perfect sense (impedance matching on the inverting and
non-inverting amp inputs), but 1% of 1K is 10 ohms, so the design
logic escapes me.
Click to expand...
A case of following an application note and not realising it has no
effect in this design implementation perhaps?

Dave :)
 
David L. Jones wrote:

A case of following an application note and not realising it has no
effect in this design implementation perhaps?
Click to expand...
Yes, I recall reading it has something to do with the inductance of said
path, designed to match the current sense.

So if you're going to leave the kit as supplied, it's pointless.
--
Linux Registered User # 302622 <http://counter.li.org>
 
On Thu, 16 Dec 2004 17:34:06 +1100, "Phil Allison"
<philallison@tpg.com.au> put finger to keyboard and composed:

"Franc Zabkar"
"Phil Allison"

** The pic on page 71 of the August SC shows R2 as a wire link.

... which is what I've used. However, my DSE kit (K7217) did not
supply the special "length of 0.2mm enamelled copper wire" called out
in the parts list. Instead I have temporarily used a piece of the
0.5mm tinned copper wire that was supplied for the other PCB links.
Upon closer reading of the SC article, it states that "R2 is necessary
to mimic the noise picked up by R1", R1 being the 0.01 ohm 3W load
current carrying resistor (which looks like a heavy wire link). The
amp inputs of the ADE7756 chip are filtered by a 1K/33nF RC LPF, which
gives a 3dB point of around 5kHz, so I can't imagine this noise is RF.
Instead I suspect that R2 is intended to detect radiated AF noise, and
to ensure that this noise appears as common mode which can then be
cancelled by the amp???



** If induced hum pickup ( from the power tranny) in the loop formed by
the low value resistors is the issue - then the exact value of R2 is
irrelevant. Any low value will do.
Click to expand...
I considered the transformer after I posted, but I dismissed it
because I would think that for proper hum cancellation the two
resistors would need to be located symmetrically with respect to the
transformer body. As it is now, the distance to R1 is about twice that
to R2. In any case I would think that the amount of hum pickup would
be negligible.


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.
 
On 15 Dec 2004 23:48:57 -0800, "David L. Jones" <altzone@gmail.com>
put finger to keyboard and composed:

Franc Zabkar wrote:
I'm building the Silicon Chip Energy Meter Kit (Jul/Aug 2004). I
can't
see the point of R2, an 0.01 ohm resistor in series with a 1K 1%
resistor in the AC current sense circuit. For perfect resistances R2
would make perfect sense (impedance matching on the inverting and
non-inverting amp inputs), but 1% of 1K is 10 ohms, so the design
logic escapes me.

A case of following an application note and not realising it has no
effect in this design implementation perhaps?

Dave :)
Click to expand...
The only app note I found was this one:
http://www.analog.com/Analog_Root/static/pdf/library/applicationNotes/an564.pdf

The application circuit on page 12 uses a transformer for current
sensing.


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.
 
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