Posted by Morris Dovey on November 1, 2011, 8:53 pm
On 11/1/11 3:02 PM, amdx wrote:
You're probably right - it's probably going to depend heavily on being
able to put together a good op-amp circuit.
If you asked if I can do that, I'd have to give you the same answer I'd
give if asked if I can play the violin: "I don't know, I've never tried
Posted by Dan Coby on November 2, 2011, 5:22 am
On 11/1/2011 10:51 AM, Morris Dovey wrote:
a fairly low duty cycle to avoid melting the wire.
One table says that NiCr melts at either 1350 C (NiCr C) or 1400 C (NiCr A).
Another table says that #40 will heat to 1093 C with .79 amps.
You are planning on running 2 amps through the wire.
My guess is that the wire will melt rather quickly.
You will need to use either:
1) A larger wire to handle the current (which would have to be longer to keep the
total resistance the same).
2) Several #40 wires in parallel to share the current (and they would have to be
longer to keep the total resistance the same).
3) A very short duty cycle. (However the first program bug that leaves the
on will melt the wire.)
P.S. I would be very surprised if this works. If Rossi had what he claims he
would be presenting real information.
Posted by Morris Dovey on November 2, 2011, 2:41 pm
On 11/2/11 12:22 AM, Dan Coby wrote:
I think you're right. The only practical option was #3. Debug could have
been done using a scope, but to handle the strict duty cycle
requirements I'd have had to give up more programming flexibility than
I'm willing to sacrifice.
I've ordered a 1/4" diameter 3" long 115VAC 200W cartridge heater that
should fill the heating requirements, but precludes co-locating a
thermocouple for closely tracking heater temperature. :-(
This means that I'll have to either attempt to monitor heater resistance
using a DPDT relay and op-amp circuit or settle for measuring the
reaction chamber temperature via thermocouple and amp with an
uncomfortable latency from application of power to measured temperature
I'd be delighted to hand you a surprise. :-)
Rossi suffers from a severe case of geekiness. I think he's spent so
much of his life learning to deal with technical matters that he hasn't
developed other skills. I suspect that he's so worried that someone
might steal his work that he'd prefer to limit his statements to "I did
it - GFY." Geeks are notoriously poor salesmen - DAMHIKT.
If it does work I'll be pleased - and if it doesn't I won't be - but
either way, I don't think I'll be terribly surprised.
Posted by Jim Wilkins on November 3, 2011, 2:32 pm
I don't lack interest in your project, only power and phone lines thanks to
our little snowstorm.
This laptop is finally operating off a very marginal Harbor Freight solar
panel kit and Virgin Mobile wireless "broadband". If you want I could mail
you a few feet of thermocouple wire, and I picked up considerable experience
with op amp measurement circuits working as an analog test engineer.
Posted by Morris Dovey on November 3, 2011, 5:07 pm
On 11/3/11 9:32 AM, Jim Wilkins wrote:
I heard it was a mess.
I don't know yet if I need the wire yet - but I'm grateful for the
opportunity to tap into your experience.
As you probably read, I've ordered a little (1/4"ODx3"L) 200W 115VAC
heater and would like to be able to capture heater temperature in real time.
AFAICT, the "real time" aspect would pretty much require turning off the
heater power briefly at intervals long enough (1 msec?) to capture the
heater resistance to determine the temperature.
[ The thermocouple approach would be much simpler electronically, but
would create other problems (with construction and loss of real-time
capability) that I want to avoid. ]
I'd like to be able to adjust a trim pot to set the lower limit (what my
A/D converter considers "zero") to room temperature, and to adjust
another trim pot to set the upper limit (what my A/D converter considers
its maximum conversion value).
I've been looking at schematics on-line but haven't found one that
allows adjusting both low- and hi-end readings. Can you help?