Fridge temp control fix, for AC and DC operation. (ANY model, not just N300.3)

[rickst29]

When you open the door, for (variable) amounts of time, with (variable) mass of cold contents preserving the low temperature, while (variable) amounts of "hot" air, at (variable) humidity ....
Then all bets are off. I will probably have similar 4-degree rises (subsequently recovered) when I similarly poke around in my Fridge on a hot day, before preparing dinner.

But I changed to a longer control period when I saw that the system shuts down PID/Relay output power while doing analysis for about 3 seconds. If your Control Period is only 20 seconds, then you are losing a considerable portion (15%) of your maximum input energy over some multi-period amount of time. (My corresponding energy loss is only 1.5%, just 1/10 as much.) Although I haven't attempted to quantify my "recovery" from an extended open door period, it might be quite a bit faster than yours, because I sustain my input power level better.

Being more "nice" to the heater wires, and eliminating a bunch of small clicking noises from the Automotive Coil relay (DC side), are secondary benefits. (In the recommended implementation, the DC coil is switched together with the AC SSR. So it creates those clicking noises, even when you're running 120VAC on shore power.) But my #1 reason was increasing total power over time (== energy input), by reducing the frequency of those 3 second "dead" periods.
- - -
Since you are (probably) using a different mode of "settings" than I am, our software will be different. My "software mode" drives Fridge temp down to a lower-than-expected temp (near the bottom of the range, rather than the set "desired" value). I'll SWAG that it doesn't do much of any trend analysis. And since our "heater" doesn't have a great deal of overshoot, a more simple ON/OFF method still works fine.

Perhaps it turns on the Fridge, and then slightly overshoots the "desired" temp via heater "overshoot" (lacking PID logic). Or, more likely, it stays ON past the desired set point and goes a bit further towards the minimum value, based on a programming assumption that heat flows in at a pretty high rate, like an open grocery store freezer case.
- - - - -
If you have your temp probe in a place other than recommended (about one inch above the bottom, 1/4 to 1/3 away from the LEFT wall as you're looking inside) then you will see bigger swings than I do, even if the PID is working the same. (This location is the coldest and most stable spot in the fridge, the one where things will accidentally freeze first. That's why it is the best measurement location.)

Now, with the long explanations done If your PID does support my setting mode, and your PT100 probe is located correctly, then you'll want to switch to my settings. A 0.3F range of door-closed temperature range IS better than a 2.0F range, and more continuous power input IS better than allowing a bunch of extra 'dead' periods to occur while the Fridge is too warm, and should be running.

UPDATE: PT10.0 mode doesn't work when the Fridge temp exceeds about 47 degrees, so I switched to PT100. Except for "door open events", it still holds somewhere in the 34F degree range -- but the PID doesn't show exactly where, because the display switches from 1/10 to full degrees when you change to PT100. (It just sits at 34F ... I no longer get the 34.6 versus 34.9 type of readout.)

[ThePair]

I see the points. I will enlarge the time, and check my other settings as well. My temp is in the bottom right corner -- it was the easiest/clearest place for me to drill through the back of the fridge without having the heating elements in the way!

I'm fairly satisfied with what I've got -- far better than stock! -- and I'll let you know if the settings change make any kind of noticeable difference in function. I have added fans on the cooling fins internally, drawing air in to circulate the cold better, as well as fans on the top vent on the outside, to better allow for convection behind the fridge. These two mods should improve efficiency, now it's just a matter of fine-tuning the master controller...

[rickst29]

Wow, when I forget to turn on the Fridge "fan switch", that upper exterior compartment (with the hot condenser fins) absolutely sizzles on a windless day.
This can be switched ON/OFF via the PID, instead of a forgettable switch. In my configuration, where two SSR's are connected in parallel to the "Relay Output" circuit. (Pins 4-to-5) I can simply "Y" the +12V fan wire, connecting it in parallel with the "DC-Direct" power lead from PID. This way, it will work when either the exterior "DC-Direct" power is turned on (for towing), or when the inside-the-TM "Reefver Fan" in switched on.

I can't connect it directly to the PID, because the PID runs continuously as a "phantom load" in storage. If I find some "extra" computer fans lying around, I might attach them as well. (copying your upgrade). After making that change, I'll post my PID settings.

[ThePair]

I have my stock fan wired to turn on with either my external switch, or the internal switch as well--quite worthwhile as I have been known to forget the inside switch on rare occasion, and this way I don't have to open everything up, the fan will still operate when the DC is switched on. Or, I can start up the fridge with the TM collapsed if I want to get a head-start on cooling.

[rickst29]

I switched from "Inty" type P10.0 to P100.
And, in editing this post, I show values one degree higher than I started from - a bigger "safety from freezing" factor.
- - - - -

In section "0089":

Inty = P100 (P10.0 is more precise, but won't take measurements and relay activation at temps above about 48F. Do NOT USE P10.0, you'll never be able to perform an initial cool-down.)

Outy = "1" (Disabled SSR; I'm using the using the "J1" PID Method).

psb = "12" (correction factor for PT-100 probe resting in a water/ice bath. My own thermocouple was off by 12 degrees, yours will have a different correction factor.)

rd = "1" (Cooling, rather than Heating. Important!)

Hy = "0002" (This is the new software, which replaces "Caty" algorithm selection with a number to indicate how close around "SV" you want it to stay. The value "3" does a great job of keeping the Fridge stuck at 35F.
- - - - -
In section "0036":
Ot = "0190" I set a much longer Control period.
Filt = "1"
- - - - -
In section "0001": (EDITED)
SV = "0035" (Target temperature)

If your model does not have an explicit setting for "Hy" in section 0089, then it is equal to AH - AL. So you will need to set the following values:

AH = "0036" (Might be important, but the range might actually be controlled by "Hy" above).
AL = "0034" (Might be important, but the range might actually be controlled by "Hy" above).

With these numbers, the system instantly shuts OFF when the temperature fall to the value "34". (That occurs at "34.9", rather than "34.0".) At 36.0 or higher, it turns ON - and stays ON continuously, until it has reached back down to "34.9". It's showing "35", either ON or OFF, more than 95% of the time. (Except after 'Open-Door-And-Looked-Around-For-Stuff' events.)

I removed the factory-original "Cable-TV and wire-phone connector plate", replacing it with a blank plate. (On the TM outside, I removed the factory-original cutout (with the flip-up connectors) and replaced it with my ON/OFF/ON switch for DC. (New note: I have removed my backup wiring; a simple ON/OFF switch would now be sufficient for me.)

Per the wiring diagram, 120VAC can only run when the PID works. The "Green" side of the exterior ON/OFF/ON switch activates PID-controlled DC (the desired mode for driving or roughing it). Inside the TM, the Fridge Selector can be in any position - "OFF" runs DC alone, "Propane" runs the gas heater (using the DC to "top off" the amount of heat with high precision).

I recommend against using the interior switch on "120VAC" with the exterior switch in any position except "OFF" - because you will be running both the AC and DC heaters at full power, and you'll possibly burn the ammonia solution (in particular, it's rust inhibitors). Extra exhaust fans, blowing out from the upper Panel, are almost certainly required to do this safely.

Attached photo shows the PID (and blank plate above), reading 33F - the instant after it shut off power, when my settings were one degree lower than now. For ThePair's configuration and similar, where the PT100 thermocouple is not in the absolutely coldest part of the Fridge, you should use settings even another degree or two higher than my new ones.

[rickst29]

Originally Posted by ThePair

I have my stock fan wired to turn on with either my external switch, or the internal switch as well--quite worthwhile....

The TM supplied fan works when TM is set up, and also works when shells are down (towing configuration). Our new upper-panel-mounted "blower" fans aren't effective during towing, because the fan exhaust is trapped between weather-sealed shells and the TM "frame box".
The TM-supplied fan spins really fast, moving lots of air but making too much noise for camp use. I consider it useful (and more or less mandatory) only for towing.
My panel mounted blower fans, and I currently have two, cannot be heard from inside the TM, and cannot even be heard from outside at a distance greater than about a foot (750RPM @ 12V).
- - - -
I am concerned, however, that one of my low-speed blowers might actually be hurting performance. I have one on each side of the TM-supplied flex-tube leading down to the "standard" exhaust fan, and the blower which I've installed above the heater stack tube may be pulling too much heat from that un-insulated tubing before it reaches the radiator.

I also think that activation and de-activation of my low-speed blower(s) should not require any "guesswork" and manual button pressing. So I am inclined to buy another PID and thermocouple, and use the second set to make my "blower fan" run and stop according to actual temperature conditions along the radiator (probably locating the thermocouple near the underside of the radiator center point).

[ThePair]

I took the opposite tack: I put two computer case fans ziptied on the upper panel directing airflow outward, so that the convected air is blown out, and therefore cooler air can come in from below. Same end result, different technique. Yes, this means my upper panel is permanently wired in, but I left a lot of slack if I ever need to open it, so it's no big deal. Plus, I can always uncap the splice if necessary. It does work well, silent, and I can feel the warm air leaving the chamber when its on.

Thanks for the settings, I'm going to have to check mine. I think yours are a few degrees cooler than mine.

I thought I would post my reply here although my issue does not exactly fit this thread, but here goes.

My DW and I are on day 4 of a 14 day trip and our Norcold fridge will not cool properly. Before I give give details, I want you to know that we precooled the fridge for a day before we initially departed, cooled or froze everything we put in, got a small battery operated circulation fan, bought a 40-gallon thermo-electric cooler at Walmart (thanks to an earlier post) so we wouldn't stuff the fridge and have tried AC, DC, and propane with and without the fridge exhaust fan all on setting 5.

Now the issue: the fridge will not stay cold and there is a 10 to 15 degree variation between the top rack and the base of the fridge. We have erred on the cautious side because the temp has gotten as high as 60 in the bottom and 45 to 50 degrees in the top rack so some food has been thrown away. The freezer will freeze, but my Klondike bar was soft.

Outside temps have ranged from mid 70s at night to low 90s during the day. I even set up an umbrella on the fridge side of the TM so that area would be shaded.

I'd appreciate any suggestions or thoughts.