Posted by vaughn on October 31, 2011, 1:29 pm
The normal fail mode is closed. I have used literally thousands of these things
in traffic signals controllers. In practice, they seldom fail randomly and are
at least as reliable as a mechanical relay (which can fail open OR closed)..
High current from a short circuit or a zap of lightning were the usual culprits
that caused failure.
Posted by Morris Dovey on November 1, 2011, 3:17 pm
On 10/31/11 1:55 AM, Morris Dovey wrote:
Following up - I just ordered a 100' spool of NiCr wire on eBay and
picked up a piece of 3/8"OD (1/4"ID) copper tubing and piece of
HardyBoard (work surface for brazing this stuff together) from Home Depot.
According to my calculations, a 12" length of #40 NiCr wire should
provide about 250W of heating and draw about 2A at 115VAC.
I'm going build /two/ heaters so I can try both the DPDT relay with an
op-amp circuit and the SSR/thermocouple/TC amp approaches. I'm
interested in learning how many cycles a relay can switch 2A before it
welds the contacts together; and I'm interested to see how much
crosstalk there will be between 115VAC and 5VDC pairs inside a 1/4" cavity.
When the wire arrives I'll connect a 12" length to the mains and see how
Thanks to all for sharing ideas and suggestions!
Posted by amdx on November 1, 2011, 5:06 pm
On 11/1/2011 10:17 AM, Morris Dovey wrote:
Sorry, I need to ask, Will the NiCr handle 250 watts per 12".
Just thinking about my toaster and I know the NiCr is longer than
12". I know it is different wire, but still want to ask.
Posted by Morris Dovey on November 1, 2011, 5:51 pm
On 11/1/11 12:06 PM, amdx wrote:
Nothing to be sorry about - it's a good question.
I think so, but won't really know until I try it out. I suspect it'll
require a fairly low duty cycle to avoid melting the wire.
I worked from the formulas at
and the tables at
Note that the second table specifies ohms/ft for the wire sizes listed.
Posted by amdx on November 1, 2011, 8:02 pm
On 11/1/2011 12:51 PM, Morris Dovey wrote:
I see a 1.5% resistance change from 400C to 600C. Is that right?
If so that's like 50 ohms at 400C and 53.75 ohms at 500C. If I'm
close with my numbers, that will be tough to use as a thermometer.
I hope my quick check is wrong.