How To Make And Use A Slow-Cooker Temperature Control

An add-on slow cooker temperature control for less than $10? That’s the cost in parts, because it’s a do-it-yourself project, a re-purposed light dimmer I discovered at The Frugal Filmmaker site. This article is about how I built it and how I reworked it as a slow cooker temperature control.

To someone who does a lot of LTB in a slow-cooker oven (as I do), the most important accessory after a nice set of analog and digital thermometers is an external temperature control for fine tuning the amount of heat inside the cooker. Most slow cookers have only 3 built-in heat settings: warm, low and high, which can be switched in some sequence to maintain a target temperature. Although I prefer slow cookers as my LTB oven (it’s more versatile and has a smaller carbon footprint than an oven or a toaster oven), all that switching to steady the temperature can be exasperating.

In LTB, a relatively stable oven temperature is important to minimize the formation of acrylamides and other toxic compounds in baked foods as well as for testing recipes. Beyond baking, a precise, steady heat control is mandatory when the slow cooker operates as a food dehydrator or a yogurt incubator or a sous vide water oven. Home cooks experimenting with sous-vide techniques pioneered rigging a slow cooker as a water oven by plugging it into a light dimmer to fine-adjust the amount of electrical current flowing in the cooker’s heating coil, and hence fine-adjust the temperature inside the crock.


There are two types of add-on temperature controls for slow cookers: thermostats and dimmers. Dimmers are the same thing as ordinary household light dimmers except that a slow cooker replaces the light bulb. Turning the dimmer’s knob (or moving a slide) varies brightness in lighting or heat in a cooker. At a stable temperature, the heat going into the cooker is balanced with the heat being dissipated through the lid and the crock. Because dimmers don’t have the ability to measure temperature, the temperature inside the cooker can drift (the dimmer has to be adjusted periodically), but the heat is relatively steady and continuous.

Thermostats, like the SousVideMagic from Fresh Meals Solutions and the Cooking Controllers from Aubrey Instruments, are basically dimmers with a thermometer probe to read and automatically regulate the cooker’s temperature. The user plugs the cooker into the thermostat, enters the target temperature, hangs the thermometer probe in the crock and the thermostat takes care of the rest. Thermostats regulate heat by cycling the cooker on and off. A thermostat maintains an AVERAGE set temperature; the actual temperature can range above and below the target temperature.

Thermostats can cost 5 times as much as the cooker itself, and at that level of investment, I would seriously consider purchasing a high-end countertop oven with an accurate internal thermostat. Ovens heat faster (and consumer more power), although a slow cooker is still more versatile (those ovens can’t do sous-vide cooking, for example).

Commercially-made inline lamp dimmers and dimmer boxes (sold in lighting and home improvement stores) can function as temperature controls, if they are rated to handle AT LEAST the amount of power the cooker will draw. A lot of those desk lamp slide-type dimmers, for example, are rated for 300W. Since the average large slow cooker is rated just under 300W, those dimmers are not recommended for large cookers, because the safety margin is non-existent. Plus, the travel on slide adjusters can be as little as 1 inch, so fine adjustments are difficult or impossible. A homemade dimmer box, on the other hand, can be assembled with more robust parts and at less cost.


A search online pulled up several light dimmer projects, but the Frugal Filmmaker’s inline dimmer switch was the cheapest and didn’t compromise quality. It costs less than $8 US to build (2010 prices), and ALL the parts can be sourced from a Home Depot store (there’s a Home Depot a few blocks from where I live). Other home improvement stores with an electrical section should have the identical or similar parts.  It’s so easy to build that the author (Scott Eggleston) has posted a YouTube video of how to do it here. The dimmer box is rated for 600W (excellent safety margin with most slow cookers) and is very rugged.

Heat Selectors On Manually-Operated Slow Cookers

Automated Cookers Are Incompatible With Dimmers And Thermostats

Caveat: Dimmers and thermostats are compatible ONLY with manually-operated slow cookers. They will not work with (and may damage) slow cookers with automatic features like timers, programming and preset cooking sequences. Manual slow cookers have just one control, a rotary switch with 3 or 4 settings: OFF, WARM, LOW and HIGH. They don’t have even a power indicator light and usually don’t have a separate on/off switch.

For detailed plans of the dimmer control, see the Frugal Filmmaker’s project page, his instructable and video linked above. This article supplements those guides with information about constructing the dimmer as a temperature control. My pictures below record how I assembled and tested the dimmer and how I incorporated it into my slow-cooker-based baking station.


1. Putting It Together:

Warning: Anyone who is uncomfortable or unfamiliar with electrical safety should ask a knowledgeable person for help or buy a pre-built dimmer.

Examples Of Electrical Labels With Power Consumption Numbers

1. This dimmer box is mounted “inline” box on an electrical extension cord. I bought a generic “light duty” extension cord rated at 120VAC and 13 Amps, so it can handle a power transfer of over 1000 watts,  more than three times the power consumption of an average slow cooker. To be safe, check the cooker’s electrical label near the attachment point of its AC cord.

Dimmer Box Wiring Diagram

The wiring diagram above shows how the dimmer box is embedded along an AC extension cord. When dividing the extension cord, I paid attention to the length of the cord with the plug. My complete slow-cooker oven setup is a jumble of wires and a long cord would only contribute to the mess. I cut the cord so that it it can reach the AC outlet on the kitchen wall with the dimmer box sitting on the counter next to the cooker. If I were to move the dimmer box for another purpose (for example, to dim a lamp) not convenient to an AC outlet, I’d plug it into a power strip or another extension cord.

Marking The Neutral (Ribbed) Side Of The Cords

This dimmer box is wired for 2-prong plugs (there are 3-prong dimmer projects on the net, although this dimmer box could be modified for 3-prong plugs too). All slow cookers I’ve owned have 2-prong plugs. The polarity on the extension cord MUST be preserved during wiring. After the extension cord is divided into male and female sections, examine the insulation along the cord of both sections. Each cord will have a smooth side and and a ribbed side. The ribbed side is the “neutral” wire. Mark the ribbed side below the exposed wire on both sections for easy identification later.

Popping The Cutout At The Top Of The Work Box

2. The cutout at the top of the work box popped off by tapping it with a nail and hammer.

Strain Relief Knot

3. I made the strain relief by holding the ends of both cords together and knotting them in one big knot.

Connect Neutral (Ribbed) Wires

4. Twist the stripped neutral wires together and cap them with a twist connector. The connector must be twisted firmly over the wires so that it won’t come loose. Three orange twist connectors are included in the rotary dimmer package.

Dimmer's Green Ground Wire Rolled Up And Away

5. The instructable doesn’t list a specific rotary dimmer model. Home Depot sells more than one brand. I chose a Levitron 6681 600 watt dimmer. Other brands should have similar wiring schemes.

After snipping off the exposed part of the green ground wire on the rotary dimmer, roll it up out of the way.

Dimmer Wiring Finished

6. Connect each black wire from the rotary dimmer to one of the remaining white wires and cap them with twist connectors. It doesn’t matter which black wire goes to which white wire. The connectors must be twisted firmly over the wires, so that they don’t come loose.

Dimmer Set Into Work Box

7. Carefully push the rotary dimmer into the work box and align the screw holes at the top and bottom. Install screws (included in the rotary dimmer package) to hold the dimmer in the work box.

Faceplate Attached

8. Position the faceplate over the work box so that the shaft of the dimmer goes through the faceplace’s rectangular opening and align the screw holes. Install screws (included with the faceplate) to secure the faceplate.

Knob With Indicator Line Drawn In

9. Press one of the knobs (the dimmer comes with a choice of 2 knobs) on the dimmer’s shaft. Turn the knob all the way to the left to the minimum position. Draw an indicator line on the side of the knob. The indicator line will help mark the knob settings later when the control is calibrated.

Parts To Test Dimmer Box

10. To test the dimmer box, plug a lamp with an incandescent bulb (or a dimmable florescent bulb) in to one of the dimmer’s receptacles. If such a lamp isn’t available, make a test lamp with an incandescent bulb and a 2-prong bulb adapter. These parts are inexpensive. A box of 4 100W incandescent bulbs cost me about $1.20 and the bulb adapter was $2.00. Both are found in the electrical section of Home Depot.

Dimmer Box Controls The Light Bulb

11. For testing, the dimmer’s AC plug should be plugged into a power strip with a circuit breaker. Press the dimmer knob down to activate (if the light’s not already on). Turn the dimmer knob to the right, and the light intensity should increase. Turn the knob to the left and the light intensity should decrease. End of test.

2. Calibrating The Temperature Control

Large Slow Cooker With Temperature Control And Probe Thermometer

To calibrate the temperature control is to ascertain the knob setting(s) that will produce a stable temperature inside the crock. In the picture above is my full LTB oven set up: slow cooker, dimmer temperature control and a remote probe digital thermometer to monitor the temperature inside the crock. In lieu of a digital thermometer, if the crock has room and if the lid is transparent, a simple analog oven thermometer would be as suitable (see Low Temperature Baking: A Journey Of Three Paths).

Aside: all this may seem like a lot of equipment, but together it cost me about $50 US, and the system can operate as an LTB oven, a sous-vide oven, a dehydrator, a plain slow cooker. Not shown in the pictures is a small appliance timer I bought from Home Depot for $4, so the system can be programmed to turn on and off at specified times.

Diagram Of LTB Oven System

The diagram above clarifies the connection of the temperature control. Before powering the system for the first time, I recommend that the temperature control be plugged into a power strip with its own circuit breaker, as a safety precaution. The relationship between the knob settings and crock temperature isn’t linear, so it’s important to determine the settings for temperatures frequently cited in recipes. That said, the actual temperature inside a crock may change day to day or hour to hour due to ambient room temperatures and food load, regardless of the settings. However, a ceramic crock is very good at holding steady heat, so once stabilized for the food being baked, it should stay close to temperature for the full baking cycle.

DIY Heat Protection Coils For The Thermometer Probe

Thermometer Probe Hooked To Crock

If the thermometer is a digital remote probe thermometer, the probe should be protected from hot spots inside the crock by sheathing it in a wire coil. The first picture above shows 2 coils I made from 16-gauge brass craft wire. The long one protects the full length of the probe in large cookers. The small one is for my 1.5-quart cooker. In the second picture, the probe is hooked to the crock so it doesn’t fall. I made the hook from the same brass craft wire as the heat coil. The hook is positioned as close to the probe’s cable as possible, so that it props the lid from crimping the cable.

Do NOT make the coils by wrapping wire around the probe itself. The probes are delicate and easily damaged. A wooden dowel or metal rod of similar diameter would be good former. I didn’t have a former and wrapped the coil free form with mini pliers. The coil should be longer than the probe, so that the probe’s tip never touches the bottom of the crock, and should fit loosely, so that the probe never touches the sides of the crock. On the large coil, spaced every 2 inches or so, I wrapped one large round, about 3/4-inch in diameter, to further cushion it from the crock.

On my controller, I have marked settings for 250°F, 225°F, 140°F and 100°F. 250°F is the top baking temperature in VaporBaker recipes (foods may be exposed to higher temperatures only briefly). 225°F is for baking without (or very little) browning. 140°F is the maximum temperature for dehydrating foods inside the crock (beef jerky must be pasturized at 160°F to kill bacteria before drying) and 100°F is for rising bread dough.

I started with the setting for 250°F and worked my way down. Here’s the process to calibrate the temperature control for an LTB oven, which can take several hours to complete, if the cooker heats up very leisurely. If the system will be a sous-vide oven, then crock should be filled to the desired level with cold water before calibration.

1. Turn the slow cooker heat selector to HIGH. The crock should be empty, except for a thermometer or thermometer probe. Place one or two dish towels over the lid to block heat loss, especially at any point where a thermometer cable exits the crock. For an alternative to layers of towels, try this crocheted draft stop. It has good insulating properties, applies very little pressure to the lid and won’t damage the thermometer probe or cable.

2. Turn the temperature control on and set the knob to maximum current (full clockwise). The rotary dimmer in my box has a push-on/push-off switch, and it’s impossible to tell visually whether it’s on or off. I wait a minute and touch the side of the cooker. If I feel warmth, then it’s on.

3. Let the cooker to heat up to about 240°F. My large cookers need about 40 minutes to get this hot. Turn the temperature control knob back to the middle position and wait about 15 minutes. If the temperature climbs beyond 250°F, turn the knob down (counter-clockwise) a little. If the temperature doesn’t reach 250°F or falls, then turn the knob up. Continue adjusting the temperature until it stabilizes at 250°F (or within a degree of it). Mark the knob setting on the faceplate.

4. Repeat step 3 for the other target temperatures, going from the highest to lowest.

Note: If the temperature control will be swapped out to other slow cookers, it must be re-calibrated for each cooker.

3. Baking In A Slow Cooker With A Temperature Control

The temperature control calibrations are only approximations. The crock temperature will factor in the ambient room temperature and the type and quantity of food being baked. In my experience, the calibration marks are very good approximations, under a variety of conditions.

1. Turn on the system with the slow cooker heat selector set to HIGH and the temperature control set to maximum (full clockwise). Place rolled up dish towels or a draft stop around the lid to block any heat leakage, paying careful attention to any gap created by the presence of a thermometer probe.

2. If the recipe must be baked as soon as it’s prepared, then wait until the pre-heating is almost finished before proceeding.

3. Let the crock temperature rise until it reaches about 10 degrees ABOVE the target. For example, if the recipe says to pre-heat to 250°F, I let the crock temperature go up to 260°F with the temperature control on full.

The higher temperature will help compensate for heat loss, when the lid is opened to put in the food. The longer is lid is off, the more heat will be loss, so any pan stands or trivets or other oven accessories should be in the cooker as it pre-heats.

4. Remove the lid, put in the food, put the moisture absorbing towels over the cooker, and put on the lid. Move quickly to minimize heat loss. If the recipe requires moisture-absorbing towels under the lid, have them ready at hand’s reach.

5. After the lid is put back on, if the crock temperature is still above the target, turn the control down to the calibrated setting for the target temperature.

If the crock temperature has fallen below the target, wait for the temperature to rise back up to the target and then turn the control down to the calibrated setting for the target temperature.

6. Check temperature after 15 minutes. Adjust the control, if necessary, to bring the crock temperature closer to the target. It may take several attempts to stabilize the temperature. My digital remote probe thermometer features alarms that can be set to sound when the temperature approaches, reaches and exceeds the target. The alarms free me from having to watch the thermometer continually.

This temperature control is not a thermostat. Sometimes the crock temperature will refuse to stabilize when baking very wet foods, possibly because dramatic changes in water content alter the thermal characteristics of the food. Crock temperature is steadier when baking bread dough, for example, than a wet cake batter. In the course of baking, I will let the crock temperature fluctuate by as much as +/-10°F, so long as the average temperature is at target. Overbaking is unlikely, because the food absorbs heat gradually and there is time to turn down the heat.

DIY Presto Chango Dehydrator With Temperature Control

A slow cooker with a temperature control has adequate precision to incubate yogurt or dehydrate foods directly in the crock in accordance with raw foodist requirements (although my DIY Presto Chango dehydrator shown above with its own temperature control is more thermally stable, has more flat drying space and is expandable). For less than $10 US to build, the temperature control is money well spent.

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