Norcold 300.3 operation

[Mr_funnypuns]

I spent this morning repairing a broken connector on my Norcold that let the pilot go out whenever the trailer jiggled. In the process , I studied the wiring diagrams in the service manual. It seems the fridge is thermostatically controlled in AC mode, and in LP mode, the thermostat controls gas flow, and the temperature. However, unless I am reading the diagram wrong, it seems there is no control in DC mode.

How does one keep the fridge from freezing up during travel in DC mode?

[Bill]

Originally Posted by Mr_funnypuns

How does one keep the fridge from freezing up during travel in DC mode?

Turn it off ??

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That answer is not as dumb as it sounds, because you are right - there is no thermostat.

A couple years ago, I salvaged the thermostat and capillary from an old dorm fridge. It should work nicely here, except that it won't handle the current that the Norcold pulls when set on DC. A 30-amp automotive relay is cheap and would do the job, but I just haven't gotten to it. Thomas (LNT123), are you up for this?

Bill

[scrubjaysnest]

rickst29 has a long thread on this and a few changed out the temp control system with mixed reviews.

[tentcamper]

I was under the impression the the propane did not have a thermostat. The control knob only adjusted the flame size and its that size until you make a change via the knob.

The dometic fridge has a propane thermostat. Where you dial the knob once the temp is met it turns off that flame size and propane passes through a bypass to maintain a small flame

[Mr_funnypuns]

The gas setting isn't thermostatically controlled. I wrote that very badly. The Temperature control knob is indeed a valve, not a thermostat. Sorry.

[rickst29]

In the electric modes, the "fridge power" dial changes the ratio of "On" time to "Off" time. Therfore: The Norcold will tend to freeze your lettuce at 4 AM (and allow warm beer at 5 PM) if you don't adjust the setting to a lower number at bedtime, and turn the setting back to a higher number each morning.

(Aside: The Norcold N300.3 does not pay attention to internal temperature (in my model year), the one thermocouple is used to verify that propane is ACTUALLY LIT when the gas is on.)

The "problem" is emphasized when the Fridge is nearly empty, and it's also emphasized by travel to "high desert" places with low humidity and 40-50F daily swings in temperature. My "solutions" resolved the night-time freeze problem, but not the daytime warm-up. (Temps approaching 100F.) To create good performance in really high temps, I upgraded to a compressor fridge. But that route, even with ~$1000 invested, leaves you consuming electricity at all times (while the 3-way can run on propane in camp, because it uses heat - it's an "absorption" design, rather than a "compressor" design.) It's a different compromise, not "total freedom".

(Aside: Next weekend, I will achieve "total freedom" whenever the 4runner has gas - it will be my "12V" Generator, whenever clouds, smoke, or length-of-day issues cause the daily power output from my Solar system to be insufficient.)

- - - - -
My PID "solutions" for Norcold might work great in the Midwest and South locations. Day/Night temp swings are typically 30 degrees (and even less), because humidity is much higher. And the transfer of heat will work better at lower elevations than I tend to visit. But the Norcold and equivalent Dometic simply can't support a temperature differential above 40-50 degrees (depending on elevation), no matter how perfectly you control its ON/OFF heater state and ambient fan cooling.

My solution #1: Turn the dial up to "7", and make the Norcold electric modes switch On/Off according to an actual thermostat with great accuracy (mounted inside the Fridge). Recommended Thermocouple Setting: 35-36F.
My solution #2: Turn on BIG "intake fans" (intake, because the OEM fan is an exhaust), mounted in the upper vents, according to the temperature near the heat exchange fins. Recommended Thermocouple Setting: 91F.

To handle both AC and DC power sources, the wiring gets to be a bit complicated. But it does prevent exploded beers and frozen lettuce, while running the heaters continuously when the fridge interior temp is above 36F. (Note: you should use one of the "spread the cool around" interior fans as a prerequisite. Most run on D-Cell batteries.) I hate frozen lettuce, and I also hate drinking most beers 'too warm'.

[ThePair]

I worked with Rickst29 way back when on the PID upgrade. "Worked" as in, he came up with all the brilliant ideas, and then I just blindly installed stuff as a test bed. Ended up melting my DC relay until we figured out it needed a heat sink--oh well, no harm done but a burnt finger!

So, my DC side doesn't have the thermostatic control, because I didn't want to mess with the excess heat. The AC side works great. Honestly, for the 2-3 hours I'm on DC on the road, I'm not too worried about anything freezing. Any other time at home or at the campsite I'm on AC, so it's just not a big enough issue for me.

[rickst29]

I converted the DC Relay from SSR to a "coil-type" Automotive Relay.

All of that heat, generated by the DC SSR, was stolen from the battery and wasted (on burning fingers, and frying components). With the Automotive Coil Relay, virtually all the power is consumed by the heaters.

Quick calculations: An SSR consumes power in the form of a 1.5V Voltage drop. On the Fridge in AC mode at 120V, about 98.7% of the power goes to the Fridge heater. (The 120VAC SSR consumes a bit less than 2 watts.). That's good. But, in DC Mode at 12.7V, almost 12% of the power goes into the Relay - wasting almost 16 watts. That's a lot of wasted heat, and it's also unsafe.

In contrast, a typical DC Automotive Relay Coil will consume only 160mA at that voltage, consuming just 2 watts. No burned fingers, and virtually all of the power goes into the Fridge DC heater.
- - - - -
But your strategy is good: In camping with 120VAC, running with an SSR on the AC side works really well, and makes the Fridge respond to an actual thermostat reading (instead of running its stupid time ratio).
Meanwhile, the Norcold's "Mismanagement" of DC on the road, for moderately short trips, simply doesn't matter that much.

[Mr_funnypuns]

Seems to me that a pwm DC motor controller might allow one to adjust the supply voltage to the heater, and gain some control over 12V operation.

[rickst29]

Originally Posted by Mr_funnypuns

Seems to me that a pwm DC motor controller might allow one to adjust the supply voltage to the heater, and gain some control over 12V operation.

Yes, but "setting the dial position" according to a variable thermostat input (with extremely low voltage changes) seems quite complex, involving some kind of non-linear "thermocouple Voltage Booster" in the middle, and including a "Totally Off" limit at low Fridge temps. Rapid PWM pulses create no advantage for the battery, in comparison to a multi-minute "ON" cycle followed by a multi-minute "OFF" period with the same ratio of connect versus disconnect time.

In contrast, using an "Intelligent" PID as a simple On/Off control, based on thermocouple temperatures, can avoid all of the "thermocouple power to PWM speed setting" mathematical issues which a PWM Controller would require. They are built to do complex "Proportional–Integral–Derivative" temp management, but they also provide a couple of "stupid" modes. And one of those modes will (pretty much) do exactly what we want on a heater-based absorption fridge.

- - Rehash of the design follows - -

Forget about all the "PID" tuning parameters, and just set it up to behave as a simple On/Off Refrigerator switch. (That's an easy program "mode" selection.) The one I used directly attaches a P100 thermocouple, displays the temperature, AND includes a 3A On/Off Relay. The on-board Relay is insufficient to run the Fridge on DC, directly, but it's about 10x the power handling to switch the control coils on two other Relays:

"Power Relay #1" 12V Coil, rated 30A (or better) @12V on the Load terminals. The Load terminals disrupt the wire connecting Norcold "Thermostat" +12V output into the DC heater.

"Power Relay #2" 12V coil, rated 5A (or better) @ 120V on the Load terminals. (For this one, you cold also use an SSR. I personally used another Coil-Based Relay.) The Load terminals on this one disrupt the 120VAC "hot" wire connecting Norcold "Thermostat" 120V output into the AC heater.

Drill a hole for the Stainless Steel "food safe" thermocouple into the lower back of the Fridge, and gently coil the cable to make a smooth connection to the PID (no sharp kinks allowed!) Mount the PID into the side panel of the Fridge (next to 12V and 120V outlets.) Wire an unswitched 12V to both PID power input AND the PID Relay "Input"; wire from PID Relay "Output" to both "Coil +12V" inputs on Power Relays (terminals #85). Connect TM-grounding (TM-12V, the white ones) to the PID "-" and Coil "-" terminals (terminals #86). The PID will activate, ready to program.

Programming of my model, here.

Then: Make sure that the Fridge is powered OFF, and connect the cut "heater input power" wires onto both sides of the corresponding Relays. Done!