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11:33 am
June 3, 2012
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Hi all,
I've made my home DIY sous vide bath with a sestos PID controller, an SSR and a GFCI outlet that I can plug a heating element into. I was a bit nervous when I was building it because I don't have much practical electrical knowledge but I followed directions I found online. I have a few questions:
1. My GFCI is not connected to a ground source (I think most people use switches for a ground source?), is this OK?
2. I use a 25A SSR, if I use a higher rated SSR like a 60A will this be safer/it won't get as hot? I already use a heat sink and it has never even gotten to a noticeable temp when I touch the heat sink but the most I've ran it is about 8 hours.
3. On/Off or proportional, this is where I'm really out of my league: I've read a lot about how these PID controllers are either On/Off or allow a varying levels of electricity through and that the On/Off is not as good. Is this true? Should I be fiddling around with my PID to change it?
I've been using the box for months with no problem other than it always maintains a temp 1/2 degrees above the set value which I just factor in. Thanks for any help from those with more experience!
11:12 am
April 27, 2012
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1. In order for a gfci to work, the outlet needs to be grounded to the source. The heating element doesn't but the outlet does.
2. 25a SSR should be fine for most applications as long as you have the heatsink on. The rating is based on using a heatsink and having some ventilation. If you are using say a 1200w heater, the draw is 10amps @ 120v. this means the 25a is rated twice as high which is a good rule of thumb to follow. Most SSRs can handle 1/2 of their rating without a heatsink so 25 is safe. I'm using a 40a because it was only a couple of dollars more.
3. PID is like magic! The reason to buy the controller is the PID function. This allows you to reach temp without overshoot and maintain that temp. IF you are maintain a temp over long periods of time, are you sure you do not have it set to PID. I find on/off always overshoots and undershoots. If your temp is a bit off, you should be able to adjust the readout to match the actual temp. this is a common issue because of the variable resistant in wring and sensors.
2:44 pm
June 3, 2012
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What I don't understand is #1. My temperature controller works fine with a heating element plugged into the GFCI, but I definitely don't have the GFCI grounded. I wish I knew what was going on!
8:59 am
April 27, 2012
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What I meant is the the GFCI functionality of the outlet wouldn't work. This is true, sort of.
GFCIs work by sensing voltage change. When this happens the cut the power and ground the terminal to release an residual power in the circuit. The most important part is that it "cuts the power"
Without the ground connected, it will still cut the power. This is good, but grounding is better.
If you have your heater on and you hit the test button, does it turn off immediately? If so then it is cutting the power and you should be okay.
Another issue is that you don't have ground for your heater. This is not a big deal if you are using a non-grounded element. If you are using a grounded element without attaching ground, then this is a hazard.
How have you wired power to you setup? If you are using a 3 prong cord, why don't you just wire the ground and and then you will be killing two birds with one stone. Just run the green wire from the power cord to the ground on the gfci outlet.
This will give you the most protection and you will be set if you upgrade your heater.
9:05 am
April 27, 2012
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Here's a good description i grabbed from online:
The ground-fault circuit interrupter is a fast-acting device which senses small current leakage to ground and, in a fraction of a second, shuts off the electricity and interrupts its faulty flow to ground. The rapid response of the GFCI is fast enough to prevent electrocution and this protection is independent of the condition of the grounding conductor.
Though a GFI will activate if a grounded appliance develops an electrical short circuit to ground… such as when YOU touch a metal saw and become the path to ground… you will experience a momentary electrical shock. This could be a minor tingle or could be more catastrophic, especially if you are on a ladder or roof.
According to the NEC, it is allowable to install GFI's in ungrounded situations. This makes sense, since the GFI is not dependent of the ground to function. It does not measure shorts to the ground, it measures the current difference between the hot and neutral wires. A sudden difference, indicating that there is another path for the electricity to flow through… you, for example, causes the GFI to open the circuit and save you from permanently curly hair.
The NEC allows GFI's to be installed in ungrounded situations PROVIDED THAT the outlet is labelled "ungrounded". Though not "officially" approved in the NEC, the grounding hole in the GFI can be permanently defeated by using an epoxy or other adhesive to seal the hole.
Most safety-conscious electricians prefer not to install a grounded-type "three prong" outlet in an ungrounded situation. Think about it… once the outlet is installed, there is no way for anyone to know if the outlet is really grounded or not without testing it. Thus, there is a hidden shock hazard should an appliance or tool that needs grounding… has three-prong plug… is plugged into this outlet.
8:06 pm
December 31, 1969
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To answer your question on the relay, first you should calculate the amount of amperage that you system is drawing. Power = Voltage * current. So to determine how many amps you are drawing you solve for current and get I=P/V. So if you have a hair dryer rated at 1200 watts and you have it plugged into a wall socket of 120 V (I hope it is GFCI plug with a ground), then you are pulling 10 AMPs. A relay rated for 25A is just as good under these conditions as one rated for 60A except the 25A will be cheaper. Assuming the manufacturing of the relays are similar, the temperature will be the same in each relay. Just make sure the electrical wire from the plug to the unit is right-sized. You can find numerous on-line calculators to perform this calculation. Since heat is proportional to resistance, you want a wire from the plug to carry the appropriate amperage. Let’s assume you calculate for a blow dryer as above: 1) you need a circuit that is rated for at least 15A; 2) the GFCI should be rated for at least 15A (on a 15 Amp circuit, more is waste of money — spend it on making it grounded — always, always, always); 3) make sure the wire from the plug is at least #14 wire; 4) make sure your hair dryer pulls 20% (rule of thumb) less than the rated outlet.
9:36 pm
December 30, 2011
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The Proportional output is the P in the PID controller.
For sous-vide, proportional output increases the power to the heater relative to the error (i.e. temperature drop). With an SSR, the heating element can be switched on and off rapidly to simulate less than full power. Smaller time increments are generally better and can be adjusted in the controller set up. The 1 second interval is annoyingly close to an at-rest heart rhythm, but longer and shorter times should be perfectly fine.
This method is inappropriate for a mechanical relay, it will wear out with the constant cycling.
8:40 am
April 27, 2012
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Architeuthis,
While glen is correct about relays in general, tblockley's comments are important.
The SSR in your sous vide isn't like a traditional relay. SS allows for many cycles compared to mechanical relays, this is why they are used. But they can generate a lot of heat. The power rating is only for the SSR with the heat sink and the good ventilation. I've too many stories of people building DIY SV using a 25a and no heat sink. They hook up a 1000w heater and blow out the relay. If your using 1000W or less and have the heat sink and some ventilation, you should be fine.
5:47 pm
June 3, 2012
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Thanks guys! I appreciate the answers, they help me understand this stuff a bit more
9:29 pm
December 30, 2011
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Mheadroom brings up a great point. The flat bottom of the SSR is made to transfer lots of heat, but only if properly installed.
Another consideration is (as I've been told by someone very experienced with SSRs) that the usual failure mode is to the ON state. If that happens, there will be a runaway thermal condition in the sous-vide bath itself.
I PIDed my espresso machine after failed pressurestats left molten chunks of heating element in the bottom of the boiler (twice).
Putting some sort of redundant safety in place might prevent an unwanted sous-vide flambé. For example, most espresso machine PID modifications leave the original thermostat/pressurestat intact and in series with the SSR output. That way, if either device trips, the heating element goes cold.
Lacking that, if the PID controller has a relay in it, it is likely that it can be used to cut the heater power if the temperature gets out of range. Check the "alarm function" parts of the PID controller instructions.
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