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Old 12-29-2012, 04:35 AM   #31
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wonder what the life expectancy of that resistor is tho...

 
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Old 12-29-2012, 02:12 PM   #32
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wonder what the life expectancy of that resistor is tho...
Well I'm not an electrical guy so I have no idea. But what I do know is that a 7.5 Kohm 8W resistor will draw 32mA @240V and that equates to 7.7 Watts (just under the 8W rating). So I would think with just a few hours a month use it should last a while. But like I said I really don't know for sure.
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Old 12-29-2012, 03:11 PM   #33
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Quote:
Originally Posted by -TH- View Post

Well I'm not an electrical guy so I have no idea. But what I do know is that a 7.5 Kohm 8W resistor will draw 32mA @240V and that equates to 7.7 Watts (just under the 8W rating). So I would think with just a few hours a month use it should last a while. But like I said I really don't know for sure.
Life depends on several things including wattage, how much use, and temperature. The resistors we use at work have a wattage de-rating down to 50% at our normal operating temperature according to the datasheet.

 
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Old 12-29-2012, 09:40 PM   #34
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Life depends on several things including wattage, how much use, and temperature. The resistors we use at work have a wattage de-rating down to 50% at our normal operating temperature according to the datasheet.
How long would you guess mine will last?
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Old 12-29-2012, 10:12 PM   #35
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Quote:
Originally Posted by -TH- View Post
Well I'm not an electrical guy so I have no idea. But what I do know is that a 7.5 Kohm 8W resistor will draw 32mA @240V and that equates to 7.7 Watts (just under the 8W rating). So I would think with just a few hours a month use it should last a while. But like I said I really don't know for sure.

But it's not going to have 240 across it as it is in series with the leakage resistance of the SSR. Measure the voltage across it, then use V*V/R to calculate the power dissipated. It will probably last a long time.

[Edit]

Cancel that thought as it will have 240 across it when the SSR is on.

Have you thought about sensing element current i.e. with a current transformer?

 
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Old 12-29-2012, 10:36 PM   #36
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Or, maybe a series resistor and varistor in series with an LED.
Nobody liked the varistor in series with the LED? A 120V varistor, then the resistor, then the LED would probably work.

Not sure what's available at Radio Shack, but they probably have them. You could also scrounge one out of an old 120V power strip with surge suppression. Or, just buy from Digikey.
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Old 12-31-2012, 12:02 AM   #37
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No. The control side would operate normally, and the LED would turn on. The output would stay on (or off) forever.
Um, no. If the control side was operating properly, WHY would it ever command the SSR on, if the SSR was 'failed closed'?

 
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Old 12-31-2012, 12:11 AM   #38
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Um, no. If the control side was operating properly, WHY would it ever command the SSR on, if the SSR was 'failed closed'?
OK, you're right if the PID is in auto mode, where the temp sensor is the feedback channel.

However, in manual mode, where the PID ignores the feedback, I'm right.

I'll split the difference with you.
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Old 12-31-2012, 03:53 AM   #39
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Originally Posted by passedpawn View Post

Nobody liked the varistor in series with the LED? A 120V varistor, then the resistor, then the LED would probably work.

Not sure what's available at Radio Shack, but they probably have them. You could also scrounge one out of an old 120V power strip with surge suppression. Or, just buy from Digikey.
Since I know nothing about varistors you would have to explain to me how/why it would work, and then also why it would be better than a single resistor in parallel with the LED that I've already got working.
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