Freezing your you-know-whats off

Last winter, the prep work I did on the exposed RV plumbing worked well. Nothing froze. Well, almost nothing. The drain valve froze.  Annoying but not catastrophic. 

It's time to see if we can cobble something up to tackle that. There are commercial products available, but now that I have more time than sense, I thought I'd see if I could make something.

I found some 12V heating pads on Amazon. Four 20 watt pads, each about 6x2". Fifteen bucks.

When they arrived, I plugged them up to a 12V battery and was immediately shocked, figuratively not literally, by just how hot they got. Within minutes, they were over 200° F!  That's way too hot!  

Reducing The Heat

Rather than return them for something with lower wattage, I scrummaged around the parts drawer and found my DC-DC buck converters. I could use that to lower the wattage and measure the resulting temperatures.

Quick experimentation at 9V, 7.5V and 6V showed that the last one -- 6V -- gave the best (least plastic melting) temperatures. The pads maxed out at about 125° F. That should be safe.

On Only When Needed

Now came the issue of controlling the pads, I'd like them to come on near freezing and turn off when things are warm enough.  The scrummaging in the parts bin revealed that I had bought two W1209 Temperature Controlled Relays from Amazon. I had used one to turn on some additional fans in the RV's refrigde compartment when things got hot.

The W1209 Relays have a "cooling" and a "heating" mode. In the cooling mode, the relay turns on (energizes) when temperatures go above a Trigger temperature. In heating mode, the relay de-energizes at the trigger temp, but energize a temperatures below.

Note, figuring out the exact settings to get Heating Mode to work was a bit of a puzzle. In the end, you program the device: set the Trigger Temperature to the temperature to turn off. In my case, I picked 15° C. Then I set the P1 (hysterisis) value also to 15° C. This results in the relay energizing at 0° C. (Trigger Temp - P1 value = relay-on temp). 

So I have a relay which energizes at 32° F and turns off at about 60° F. 

The temperature probe accuracy is astonishing. Clearly within a tenth or two degrees C.

The Prototype

Here's the prototype:

The temperature probe is in ice water, and is reading -0.1° C. The W1209 relay has energized and is feeding 12V to the LM2596 buck converter which is dropping it down to 6V before sending it to the heating pads.

Note, each pad was consuming 0.84A at 6V.  Both pads, together, took 1.7A at 6V. Well below the rated 3A of the buck converter.

The results

Some temperature measurements with the prototype.

Time    Pad 1        Pad 2

T0        68° F        68° F 

T1        95° F        105° F 

T2        106° F      114° F 

T3        112° F      123° F 

T4        119° F      124° F 

T5        120° F      126° F 

T10      126° F      129° F 

After five minutes they both leveled off at around 120-125° F.

Next up, a case to hold the electronics and installation on the RV.