Building the Hofer Thermal Shield
Theory of Operation Parts List / Cost Estimate Seven Construction Steps Calibration Procedure Resetting Procedure Final Words
Shown above is the simple circuit of the Hofer Thermal Shield (HTS). The hot terminal from wall power, shown in red, is in series with the relay's activation coil and the Ebo Jager heater. Given the strong current-limiting property of the relay's activation coil, the heater approximates a closed circuit when on, activating the relay and, thus, closing its normally open switches (technically a double-pole, double-throw switch). When the switches are closed full wall power is sourced to the full-power outlet. The beauty of the design is that the relay activation coil limits current through the Ebo Jager heater to only 11.4 mA thus preventing it from functioning as a heater in practical terms yet still sensing its state, off or on.
The cool-operating Ebo Jager heater is placed in tank water. If the water rises to the critical point, as dialed in on the heater itself, the heater will turn off, thus approximating an open circuit, turning off the relay activation coil, and opening the relay's normally open switches. The open switches will cut power from the full-power outlet, turning off any devices attached to it.
Parts List / Cost Estimate
Estimate prices--some online prices--and parts list
(given as shown in picture from left to right, top to bottom):$5 14-gauge 9' power cord (Jameco part #100271 shown)
$12/$2 15-amp GFCI OR non-GFCI square-faced decorative power outlet.
The GFCI outlet provides protection against electrocution but
is optional. However, the GFCI is not recommended if the
HTS itself is to be plugged into a GFCI outlet.
$1 15-amp standard outlet (does not require ground)
$1 Dual outlet box (one as sold at Home Depot is shown)
$15 Ebo Jager 50W heater - any wattage will work. I recommend
the Ebo but whatever heater is used cannot be an
electronic model for this project.
$0.25 Cable tie
$1 Outlet box faceplate (one as sold at Home Depot is shown)
$7 Relay with 110V AC activation coil and 15A+
current capacity (Jameco part #175572 shown)
$0.25 2 x 14-gauge wire connector cap
$0.25 3 x 3.5-inch 14-gauge solid copper wires
$0.25 3.5-inch 14-gauge stranded copper wireTOTAL COST: $43 with GFCI / $33 with standard outlet
Step 1
Drill 3/8" hole through dual outlet box. Take loose-wire end of
power cord and push through hole in direction shown in picture.
Step 2
Step 3
Thread and solder the 3.5-inch wires exactly as shown in picture using 2 solid and 1 stranded wire. I recommend a 60W soldering iron but a smaller one might work with patients. If you haven't soldered before, you should practice on something before trying this step. It's absolutely essential that the bottom four solders shown in the picture are excellent solder joints as they must deliver up to 15 amps through the circuit. The top-right solder bridge and top-left solder joint are far less critical as they need carry about 1/1000th that amount of current.
Step 4
Bridge the outlet neutrals as shown. Outlet neutrals have a note on the plastic to put the white wire into them. They also are on the side with the longer prong holes in the front.Connect the power cord as shown. The green and yellow (ground) wire may need to be stripped further and goes to the green side screw. The black (hot) lead should be stripped and cut of any solder. It should be twisted with the stranded wire coming from the relay. The white (neutral) wire may need to be stripped a bit further as well but leave the solder on the tip. The white wire may or may not fit in the quick connect hole as shown. In general DO NOT use the quick connect hole for spring-loaded outlet designs like those of non-GFCI outlets because they are not designed for stranded wire leads. Instead use the screw side mount. You need a really good connection here, so give the white wire a good yank to make sure it's tight.
Connect the cable tie as shown about 1 to 2 inches from the lose wires.
Step 5
Connect the remaining two-solder solid wire from the relay to the GFCI hot connection. The hot
connection is on the outlet side opposing the neutral connections. Connect the one-solder solid
wire from the relay to the hot side of the temperature-sensing (non-GFCI) outlet. The hot side
is that with the shorter of the two slitted prong holes in the front.
Step 6
Carefully position the relay and outlets into the outlet box as
shown. Tighten four screws on the outlets into the box.
Step 7
Screw on the decorative faceplate as shown above.
First why must we calibrate the heater? Isn't it already calibrated from the factory?
Yes, the heater is calibrated from the factory but not for our purposes here. Here's why: in the ideal case, a factory-calibrated heater will turn on when the water temperature just falls below the temperature set on the temperature control dial. But have you ever stopped to wonder when it will turn off? The answer is a heater turns off when the hot coils inside make the thermostat sense a temperature several degrees warmer than the kick-on temperature. My tests reveal, for example, that an Ebo Jager turns off when the thermostat senses a temperature about 6°F (varies around +/- 1°F from heater to heater) warmer than the switch-on temperature.But wait a minute! I said before that running on the HTS the sensing heater is usually on (but running cool) and only turns off if our water gets too hot. And I said that what we need the heater dial to show what temperature the heater (and hence our power to the full-power outlet) will switch off. We don't want the dial to show, as it does fresh from the factory, the turn-on temperature, but rather we need it to show the turn-off temperature. So this means we must calibrate our heater dial to show what we really want. And here's how:
1) Plug your heater into the sensing-side outlet and a small lamp into the full-power outlet. Instead of a lamp you can plug in any appliance as long as you can tell when it's on.
2) Put your heater in sump water of a known temperature for 30 minutes.
3) Turn the heater dial clockwise until the lamp turns on.
4) SLOWLY turn the dial counter clockwise until the lamp just turns off.
5) Using a flathead screw driver pry the calibration button on the top, as clearly shown in the picture above, until the button pops up.
6) Turn the heater dial so it points to the known temperature of the water the heater was in and forcefully push the calibration button back in.
7) Turn your dial all the way clockwise and turn it back until it just reads one degree warmer than you ever expect your reef tank (i.e. this is the temperature the HTS will turn the full-power outlet off).You're done!
So, you come home and notice your reef tank lights are off too early. You check the temperature of the house and discover that you forgot to open that kitchen window, your tank overheated, and the HTS saved your tank by turning off your lights. That's great, but now what? First, of course, you have to fix the issue that made your tank too hot and allow the tank to cool below the trip temperature. After that, you must reset your HTS via one of two methods:
Simple But Slow Method
1) Take the sensing heater out of the sump and let it sit out in the open, still plugged into the HTS.
2) Wait about 5 to 10 minutes. The evaporation of the water on the heater will cool it down to several degrees below the trip temperature, turning it back on, effectively resetting the HTS.
3) Put the heater back in the sump.Slightly More Complex But Faster Method
1) Take the sensing heater out of the sump.
2) Turn the dial clockwise until the lights turn back on.
3) Turn the dial back counter clockwise until it just reads the critical temperature.
4) Put the heater back in the sump.If you fail to reset your HTS--say you are out of town--the HTS will automatically reset itself once the tank temperature falls approximately 6°F (varies around +/- 1°F from heater to heater) from the trip temperature set on the dial.
Well, that's about it! I hope your HTS will never trip, but if it does, it may have saved every coral and fish in your tank! In my opinion, that's a lot of peace of mind for about $40. If you have any questions, feel free to e-mail me at domitron@yahoo.com. Enjoy!
