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Old 03-17-2009, 03:41 PM   #1
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Default Switching power between elements/PIDs possible?

Hi All,

I am in the planning stages of converting my direct fire rig to an electric HERMS. Although I would love to run a 240v element in both my HLT and kettle at the same time, currently it's not an option.

My plan is 4500w 240v in boil kettle and 2500w 240v in HLT. I will heat my strike water in the BK, and then move to the HLT. Both elements will be controlled via PID and 40a SSRs. I would like to have a switch on my control panel that allows me to switch power between the 2 sides of my rig (from HLT to BK). This will allow me to operate on a 30a circuit. Later down the road, when my wiring is upgraded, I would be able to remove the switch, and power both sides at the same time.

Since the amp draw of the 4500w is 19a, could I use a 30a DPDT switch? Is a DPDT the correct way to go, or is there any other way to accomplish this?

Thanks,

Rhino

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Old 03-17-2009, 04:29 PM   #2
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Quote:
Both elements will be controlled via PID and 40a SSRs. I would like to have a switch on my control panel that allows me to switch power between the 2 sides of my rig (from HLT to BK). This will
The no cost option:
Put one of the temp controllers in the no run mode or off, depending on brand and model.

My edit.
Option two:
Install a 3-position selector switch between the PID out and the SSR.
Position 1, left = PID #1
Position 2, center = Both PID's
Position 3, right = PID #2
No high current selector switch required.



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Old 03-17-2009, 04:47 PM   #3
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My control box has a dryer outlet and each of my elements has a dryer cord attached. When I want to use the HLT (the one that has the temp probe in it) I plug that in. When I'm done heating strike/sparge water I plug in the boil kettle and use the manual control.

I have a 30 amp switch on the box to turn off the current when I switch cords for extra safety.

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Old 03-17-2009, 05:52 PM   #4
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Wait, if you never plan to have both energized at the same time, why would you waste the money on two controllers and SSRs? Operate a 3PDT switch on the elements themselves and the two temp probes. With one throw of the switch, all the associated electrics including the temp probes will be switched from the BK to the HLT.

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Old 03-17-2009, 06:21 PM   #5
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Operate a 3PDT switch on the elements themselves and the two temp probes. With one throw of the switch, all the associated electrics including the temp probes will be switched from the BK to the HLT.
If he uses thermocouple probes the switch contacts should be made with thermocouple-grade calibration alloys.



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Old 03-17-2009, 06:34 PM   #6
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Wow, they're really that sensitive huh.

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Old 03-17-2009, 07:13 PM   #7
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Quote:
Originally Posted by ClaudiusB View Post
If he uses thermocouple probes the switch contacts should be made with thermocouple-grade calibration alloys.

Cheers,
ClaudiusB
That is exactly correct.


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Wow, they're really that sensitive huh.
Yes, they are. That is why I would shy away from using thermocouples if possible. They can be a real pain in the tush.
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Old 03-17-2009, 07:18 PM   #8
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Yup, they are. Thermocouples are comprised of two different materials joined at one end and separated at the other. The separated ends are considered the output, and they generate voltage which is proportional to the heat they are measuring or monitoring. That is, the hotter the temperature, the higher the voltage. The fact that two metals generate voltage is known as the Seebeck effect. The voltage they generate is very small, usually milivolts. If you were to connect the output leads of the thermocouple to a switch, the switch would have to have contacts made of the same material as the thermocouple, otherwise the two dissimilar metals of the switch and the thermocouple leads would make another thermocouple and skew the output of your measurement thermocouple. PID's and thermometer are calibrated for specific thermocouples. If you were to intoduce another thermocouple by intoducing a switch of dissimilar metal the calibration of your PID or thermometer is comprimised. It's readings would not be accurate any longer. However, there is such a thing as a special thermocouple switch but they seem to be costly. I have seen a couple of thermocouple switches on Ebay for under $100 recently.

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Old 03-17-2009, 07:31 PM   #9
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You don't need a probe in the boil kettle, though I suppose it would be nice. I didn't switch between two thermocouples because I did figure that putting a switch in line would screw up it reading the temp right.

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Old 03-17-2009, 07:40 PM   #10
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So, Conpewter - You have a single PID+TC in the HLT, controlling a SSR, then when you switch to the boil, you just choose a different SSR (or plug for manual control) and set the PID to manual...?

I think that is doable. I know that the PID has to have the TC to have the circuit complete.

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