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120v self resetting 5 minute kill switch

timsrv

Jun 22, 2014
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Jun 22, 2014
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I'm happy to find this forum. Seems like I'm always building something, so this will definitely be a valuable resource. My current project is modifying a pellet smoker to automatically reignite if it goes out (which happens from time to time). I've got everything worked out with the project & it's mostly done except for a time based kill switch (in the event it fails to reignite). The temperature controller I'm using will activate the element in the event temp drops down to 140 deg F (temp only drops this low if it's out or going out). When temp rises back to over 150 controller will shut off the element. The controller I'm using is electronic and programmable, so I can adjust these parameters as required at a later time. My concern is in the event it fails to reignite. I thought about using a NC Thermo switch with a relay to kill power based on temp, but since the element itself produces heat, I'm thinking it would be safer/better to shut things down based on time.

I could do this a couple ways using low voltage controllers, but I don't have a lot of room for components & don't wish to run a voltage converter. What I'd really like is a device that would automatically cut 120V to the element anytime the circuit is powered for more than 5 minutes (more than enough time for ignition to occur). I figured such a device should be easy to find, but I have been scouring the internet for hrs and cannot find anything that fits my needs. Is anyone here aware of such a switch? The element is 500w. I will need this device to automatically reset when 120vac is removed & reapplied.

Any suggestions or advice would be appreciated. Thank you. Tim
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
25,510
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25,510
What you want is some sort of one-shot (or single shot), or monostable. When triggered it gives 5 minutes of power then turns off. It is triggered by turning the power on. Obviously it is reset by turning the power off.

I wonder if there are time delay relays with options like this? Looking at the descriptions of time delay relays, this seems to be an available option.

This might be an option. I'm not exactly sure how you trigger these, but you want a trigger to be applies as long as power is present. It can switch 8A, and the timing period can be set between 0.1 seconds and 100 hours, so that seems OK.

There may be other options.
 

timsrv

Jun 22, 2014
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Jun 22, 2014
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Thank you for such a fast reply. Almost infinite possibilities when you consider possible combinations of function, trigger, voltage, time range, amp rating, etc. This can make some things almost impossible to find. Terminology is key. Single-shot/one-shot were the search terms I was missing (thank you). So now I've actually found a couple possibilities that seem suitable, but like you I'm a little confused about the trigger. In my case voltage will be applied to trigger and held there indefinitely until the unit is reset (might be hrs). I wonder how I can find out if the timer/shut down cycle will be started with continuous voltage applied to the trigger. I'm thinking maybe trigger power needs to be applied and removed before the shutdown sequence begins......Any thoughts? Thanks in advance for your assistance. Tim
 

timsrv

Jun 22, 2014
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Jun 22, 2014
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I went ahead and ordered the 88827125 One-Shot from Digi-Key. If nothing else I'll have fun testing it. Hopefully it will start the sequence when power is applied and held. I'll report back with my findings once it arrives. Tim
 

timsrv

Jun 22, 2014
6
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Jun 22, 2014
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I think I just found one that looks like it fits my application better. It doesn't specify the terms "single-shot" or "one-shot", but in reading the description it sounds to be exactly what I'm looking for as it specifies "upon application of power to the terminals, time delay sequence is initiated. Reset is accomplished by removing the power from the terminals". Here's a link to the datasheet: http://datasheet.octopart.com/Q1T-00010-341-NCC-datasheet-10797.pdf. Thanks again for your help. Tim
 
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