Posted by *vaughn* on October 31, 2011, 1:29 pm

*> That will resolve the fused contacts issue, but leads me to ask a related *

*> question: If/when an SSR fails, does it fail open or does it fail closed?*

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.

Vaughn

Posted by *Morris Dovey* on November 1, 2011, 3:17 pm

On 10/31/11 1:55 AM, Morris Dovey wrote:

*> Now can we please go back and figure out the heater itself so there's*

*> something to measure?*

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

it behaves.

Thanks to all for sharing ideas and suggestions!

--

Morris Dovey

DeSoto Solar

http://www.iedu.com/Solar/

Posted by *amdx* on November 1, 2011, 5:06 pm

On 11/1/2011 10:17 AM, Morris Dovey wrote:

*> On 10/31/11 1:55 AM, Morris Dovey wrote:*

*>> Now can we please go back and figure out the heater itself so there's*

*>> something to measure?*

*> 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*

*> it behaves.*

*> Thanks to all for sharing ideas and suggestions!*

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.

Thanks, Mikek

Posted by *Morris Dovey* on November 1, 2011, 5:51 pm

On 11/1/11 12:06 PM, amdx wrote:

*> On 11/1/2011 10:17 AM, Morris Dovey wrote:*

*>> According to my calculations, a 12" length of #40 NiCr wire should*

*>> provide about 250W of heating and draw about 2A at 115VAC.*

*> 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.*

*> Thanks, Mikek*

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

http://www.wiretron.com/design.html

and the tables at

http://www.wiretron.com/nicrdat.html

Note that the second table specifies ohms/ft for the wire sizes listed.

--

Morris Dovey

DeSoto Solar

http://www.iedu.com/Solar/

Posted by *amdx* on November 1, 2011, 8:02 pm

On 11/1/2011 12:51 PM, Morris Dovey wrote:

*> On 11/1/11 12:06 PM, amdx wrote:*

*>> On 11/1/2011 10:17 AM, Morris Dovey wrote:*

*>>> According to my calculations, a 12" length of #40 NiCr wire should*

*>>> provide about 250W of heating and draw about 2A at 115VAC.*

*>> 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.*

*>> Thanks, Mikek*

*> 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*

*> http://www.wiretron.com/design.html *

*> and the tables at*

*> http://www.wiretron.com/nicrdat.html *

*> Note that the second table specifies ohms/ft for the wire sizes listed.*

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.

Mikek

> That will resolve the fused contacts issue, but leads me to ask a related> question: If/when an SSR fails, does it fail open or does it fail closed?