BUTTER WARMER

One of my pet hates is rock hard butter in winter. This obsession finally got the upper hand and compelled me to invent a device for keeping butter at a constant temperature of around 23ºC (or 73.4ºF), soft enough to spread without totally destroying the bread, but not warm enough to make the butter oily. The prototype is still in daily use and still going strong and remains in prototype form, fifteen years later. Perhaps this is a good time to revamp it.

This new edition has improved temperature control accuracy, it has a better housing and is rather more aesthetically pleasing I think.

The circuit is quite straight forward with no oddities and shouldn’t present any difficulties for anyone who has some basic handyman skills. In the design I have specified inexpensive and easily obtainable parts throughout and the whole thing shouldn’t cost more than about $20 including the housing and all hardware. I have specified through-hole components as opposed to surface mounted since the parts are easier to handle and solder.

BILL OF MATERIALS:

CAPACITORS:

1x 2200µF, 25V radial electrolytic capacitor

2x 10µF, 25V radial electrolytic capacitor

1x 0.47µF, 50V polyester capacitor

RESISTORS with colour codes:

R5, R9, 2x 1k, resistor, 5%, 1/4W, axial, brown, black, red, gold (all 5% resistors have a gold band)

R6, 1x 1k5, “ brown, green, red

R10, 1x 4k7, “ yellow, purple, red

R3, 1x 6k8, “ blue, grey, red

R1, 7, 8, 3x 10k, “ brown, black, orange

R2, 1x 22k, “ red, red, orange

R4, 1x 1M5 , “ brown, green, green

2x 2k2, 10W wirewound resistors for 120V mains, OR 2x 7k5, 10W resistors for 240V mains, colour code not applicable.

SEMICONDUCTORS:

1x DB101 bridge rectifier or equivalent

1x 7812 voltage regulator, TO220

1x LM335 temperature sensor, TO92

1x LM358 dual opamp, DIP-8

1x 2N7000 N-channel FET, TO92

1x 1N4148 silicon diode, DO-35

1x 5mm green LED, standard brightness, diffused

1x 5mm red LED, standard brightness, diffused; try to get red and green of similar brightness intensities

1x 5mm blue LED or any colour you prefer. Use standard brightness LEDs for the temperature indicators as they seem to show relative brightness more clearly than hi-bright types.

HOUSING:

2x 370x160 melamine clad chipboard (Top & Base)

2x 160x130 ” (Ends)

1x 370x98 “ (Rear)

1x 285x130 “ (Door)

1x 130x117 “ (LED Panel)

1x 132x98 “ (Partition)

Melamine edging strip

MISCELLANEOUS:

1x 12V, 80mA, 40mm fan or similar

1x 15V, 3VA transformer to suit your mains voltage

1x J107F1AS1212VDC.45 ‘sugar cube’ relay or equivalent, also known as JQC3FC (T73) DC12V, any contact rating over 200mA

1x 20mm panel mount fuse holder

1x 500mA, 20mm fuse for 120V, OR 1x 250mA, 20mm fuse for 240V

1m, 3mm heat-shrink sleeving

Copper clad PCB, glue, hinges, door catch, 3.5x40mm chipboard screws, mains power cable, hook up wire, etc.

TOOLS NEEDED:

Soldering iron, up to 30W max., or temperature controlled

Flux-cored solder wire

Digital multimeter

Wire cutters

Long-nosed pliers

Basic hand tools for electrical and woodworking

A simple DC power supply is provided by transformer Tr1, bridge rectifier BR1 and smoothing capacitor C1. This gives about 19VDC and following that, U1, a 7812 voltage regulator providing a stabilised 12V for the temperature controlling circuitry.

Referring to the schematic diagram, R1 provides power to a very clever chip, U2, the LM335 temperature sensor. The chip gives out a very convenient voltage that is directly proportional to its temperature in degrees Kelvin and varies at the rate of 10mV per degree Celsius (or centigrade). Why is Kelvin used? The Kelvin scale has at its ‘absolute zero’ temperature, 0ºK, -273.15º degrees Celsius. ‘Absolute zero’ is that temperature at which all molecular motion ceases, the lowest temperature possible. Actually it’s not possible so it’s the theoretical lowest limit when absolutely zero energy exists, something that’s not even possible in deep space. So if it’s not possible, why use it? Well, one has to start somewhere and Mr Kelvin decided that this is the way it should be done. Fortunately he adopted the Celsius arrangement so there’s no need for complex conversions; a ten degree Kelvin change is also a ten degree Celsius change and vice versa.

So, using the Kelvin scale, zero degrees Celsius is expressed as +273.15ºK. And at 10mV/degree the LM335 gives out a voltage of 2.7315V at 0ºC. Confusing? Yes, it is a little. But one doesn’t have to know this; all one needs to know is that the LM335 produces a 10mV/degree Celsius change, and at zero Celsius, the chip gives out 2.7315V. The boiling point of water at sea level, 100ºC, is therefore 273.15 + 100 = 373.15ºK.

In this application we require a temperature of about 23ºC (or about 73.4ºF). This temperature was deduced by extensive, dedicated, empirical testing on numerous slices of bread, a most scientific process. 23ºC is 273.15 + 23 = 296.15 or rounding off, about 296ºK. After conversion by the LM335 a voltage of about 2.96V at 23ºC is obtained.

The output of the chip is fed into the non-inverting input of U3a, a standard LM358 dual operational amplifier. Resistors R2, 3, 4 and VR1 programme U3a to be an amplifier with a gain of about 275. This means that a small change in temperature will result in a much larger change in the output voltage at pin 1 of U3a, about 275 times greater change. This large amplification factor allows for quite accurate temperature control so the butter is always just right.

This output feeds an arrangement of resistors R5 and 6 as well as LEDs 1 and 2. One LED is red and the other is green, these lighting up to signify the approximate temperature inside the butter warming cavity. A brighter red LED signifies that the oven is slightly warmer than has been set, and when the green LED is brighter it means it’s slightly cooler. The actual desired temperature is set with VR1 and when that temperature has been achieved inside the warming cavity, both red and green LEDs will light up with equal intensity.

U3b, in conjunction with resistors R7, R8 and R10, is arranged to be a ‘switch’ so that it is either on or off and no in between. This is necessary to switch relay RE1 on or off decisively. When the temperature in the butter warming cavity is lower than what has been set, the output from U3a triggers U3b which drives the gate of field effect transistor Q1. Q1 switches on and energizes relay RE1, LED3 and the air circulating fan. The relay contacts close and connect the mains voltage to the load resistors R11 and R12. These resistors warm up and provide about 15-16 watts of heat, ample for gently warming a pound of butter.

Going back to the LM335 for a moment, one should note that I have made no provision for calibrating the chip. When used without calibration the chip may have about a one degree error. I purposely didn’t provide for calibration because it would complicate matters needlessly and it’s simply not necessary. Just set the temperature to around 23ºC and from there you can adjust to suit your needs. The exact temperature doesn’t matter; what matters is that the butter is the right consistency.

There are many methods of making your own printed circuit board, all of which are given fair coverage on several YouTube channels. There are also companies offering to manufacture a one-off board at reasonable cost. I use a number of different PCB programmes but for this project I used Pad2Pad and you can download the PCB layout here in printable *.pdf format. Pad2Pad.com offers the software free, and also offers to manufacture, which isn’t. I am not affiliated with Pad2Pad.com.

So if making your own it’s hardly necessary for me to explain the process here since there’s ample coverage on YouTube. But the method that’s most convenient for DIY purposes is ironing a laser print onto copper clad board. It’s a little tricky at first but with perseverance it can be done passably well. With a little ingenuity the circuit could also be built on perfboard or similar.

A little tip if you’re making your own: When the board has been made and the etch resist has been cleaned off, give the copper tracks a thin coating of soldering flux. Then, using a soldering iron and a very tiny bit of solder, run the solder over all the tracks; this is known as ‘tinning’. The tinning helps to protect the board from corrosion, it assists in soldering and I think it looks better, too. After tinning, be certain to clean off the residual flux with a solvent such as thinners or acetone, or, with some fluxes, water; if not it can affect the circuit’s operation. If you do tin the board it’s better to tin before drilling so you don’t have raised copper burrs interfering with the soldering iron tip.

After processing the PCB, populate the components on the PCB in their respective locations by referring to the component overlay diagram, but for now don’t install the 1M5 resistor, R5, just yet, because you will need to set the temperature first with VR1. I would not recommend substituting VR1 with a standard, panel mounted potentiometer. Sticking with a multi-turn control pre-set potentiometer allows for a much more precise setting.

When the board is assembled - without R5 - temporarily connect all the peripheral components as shown in the layout diagram but don’t make the LINE and LOAD connections yet. Power the circuit up and using a digital voltmeter set to the 20V range, connect the black probe to the ground or zero volts line, ie, the ‘-‘ pin on BR1, this being marked on the diagram as ‘TP1 0V’. Connect the red probe to the ‘+’ pin of BR1, TP2. You should be reading about +19V if the fan is off, or about +17.5V if the fan is running. The drop in voltage is normal and correct and anywhere between about +16V and +21V is probably acceptable.

Keeping the black probe on the ‘-‘ pin of BR1, connect the red probe to the output of U1, the 7812. It’s connected to the junction of R1 and R2, labelled ‘TP3 +12V’, and here you should read very close to +12.0V. If either of these voltages differ greatly you will need to find out why before proceeding. Be sure you have all components in the right way around although resistors can go in either way; it makes no difference.

If the power supply voltages are correct, and still with the black probe on the negative pin of BR1, connect the red probe to pin 2 of U3a, ‘TP4 2.96V’. If all is well you should read something fairly close to that voltage. Adjust VR1 so that the voltage on pin 2 is +2.96V; it doesn't have to be exact, just get it as close as you can. Now switch off and solder in the 1M5 resistor, R5, and switch on again. If it’s a cool day your fan should be whizzing around and the green LED1 should be on and the red LED2 should be a faint glow or fully off. If it’s a warm day, the red LED will be bright and the fan will be stopped.

Try putting a hair dryer onto the LM335 for a few seconds. Watch the LEDs and you should see the green one dimming and the red one getting brighter. As the green LED dims and the red one gets brighter, the relay will click, LED3 will go out and the fan should stop. Remove the hair dryer and as the LM335 cools, the red LED will gradually get dimmer and the green will get brighter and finally the fan will start up again after about 10-20 seconds or so.

The circuit has built in ‘hysteresis’, ie, it acts as an accurate thermostat so that the fan and heating are turned on or off as required with about a 2ºC overlap. The pre-set potentiometer VR1 adjusts the temperature setting of the controller and can be varied over a wide range from less than 20ºC to over 35ºC (59ºF to 95ºF). Some experimentation may be necessary to set the warmer to the temperature that suits you best. Make small adjustments at a time but do allow several hours for the temperature to settle between tests before checking the butter softness. A temperature probe with a digital meter would be advantageous but not essential.

Note: The butter warmer will only warm cold butter; it will not cool warm butter on a hot day. That may be my next project, putting in a Peltier module for cooling. Maybe even a digital temperature display.

The PCB should be fixed to the floor of the unit using 3x20mm wood screws. To stand the PCB off the base here’s a little trick; knock the centre pins out of a couple of 4x10mm pop rivets then drill them out to 3mm diameter. The screws can be passed through the PCB and the rivet sleeves so that the PCB stands off the base. If you prefer imperial measurements, use 3/16” rivets and drill out 1/8”.

The original was built using wood-grained hardboard but it was soon discovered that more thermal insulation was required since the unit was cycling on and off too frequently. Corrugated cardboard was glued over all external surfaces so as to keep the warmth in. This worked surprisingly well but I thought possibly something more elegant could be built using chipboard. But anything suitable that offers thermal insulation characteristics could be used. I settled for 16mm melamine clad chipboard but 12mm would work just as well. The hinged front panel can be fitted with a magnetic catch or similar to keep the door closed. I epoxy glued a 6x4mm neodymium magnet onto a tiny bracket with a mating piece of flat steel glued to the door. These fell off shortly afterwards since epoxy and melamine don’t go well together it seems. Hot-melt glue adheres very well, however.

Referring to the diagram, the butter is enclosed in a compartment with a hinged, front panel opening section. A partition is placed between the butter compartment and the section housing the electronics. A fan is mounted at one end of the partition with a suitable circular cut out; I used a 12V, 80mA, 40mm diameter fan, but anything similar can be used. A gentle stream of air is all that is needed, no need for a howling gale. At the other end of the partition a 12mm gap is left so as to allow air to flow and circulate around the butter and then back into the controlling section. From here the air passes over resistors R11 and R12 where it is warmed, then gently blown back into the butter section by the fan and the cycle repeats until the correct temperature is reached. At this point the fan and heaters are switched off but switch on again when the temperature in the cavity drops to below the temperature setting threshold.

The heating resistors should be positioned near the fan suction side so that cool air flows over the resistors and the warmed air flows from there into the butter cavity. A simple arrangement of mounting the resistors on a couple of pieces of copper clad circuit board can be used or even just screw terminal blocks could be employed. The only constraints are that the resistors must be electrically insulated from their surrounds and the mounting should be somewhat heat resistant, and of course, they must be near the fan inlet side. Keep them apart so that ample air can flow around them so they don’t overheat and don’t put them too near the fan or the plastic may be affected. From the drawings you will see that I glued two scraps of copper-clad PC board onto the partition wall. The resistors were soldered onto them but with the resistor legs kept long so that they stand out and away from the fan. Above the resistors I glued on a small piece of 3mm polycarbonate sheeting. An equally effective insulator would be a tiny piece of drywall material. The purpose is to cover the mains connection so that it is safe. Please also note that the neutral wire should be at the upper connection to the resistors and the live at the lower connection point. Keeping it like this makes it safer as the live is out of the way and not easy to touch, although if the connections are reversed it won’t affect the circuit’s operation. Again, ALWAYS KEEP IN MIND THAT MAINS VOLTAGES ARE DANGEROUS.

LEDs 1 and 2 should be mounted on the front panel by drilling 3 holes, 5mm in diameter, then counter-boring 6.5mm diameter. The LEDs are pushed in and held in position with a blob of hot-melt glue. LEDs 1 and 2 provide a quick visual confirmation of the correct temperature. LED3 provides instant verification that the heating cycle is operational. When connecting wires to the LEDs, cut the legs short, solder the wires on and insulate by sliding a short piece of plastic, heat-shrinkable sleeve over each joint. Repeat this wherever there is a danger of connections shorting together. Be sure to get the LED polarities right; they won’t light up if you get them backwards and they might even be damaged, likewise the LM335. The diagrams explain how to determine their respective polarities.

A panel mounted fuse holder should be mounted on the rear and there must be provision for a mains power cable entry point. I mounted a 3-way screw terminal block on the inside of the rear panel for connecting the incoming mains power to the fuse holder, the transformer and the heating resistors, R11 and R12.

Once again, it shouldn’t be necessary to warn of the dangers of working with mains voltages. Nevertheless, always be extremely cautious with the mains; it bites first time without warning – please take heed.

Referring to the wiring diagram, make the connections from the terminal block to the LINE connection point on the PCB and also to the transformer. Then connect the LOAD point on the PCB to the heating resistors. Connect the neutral wires to the transformer and the heating resistors. Connect the power cable to the terminal block as shown.

The temperature sensing chip, U2, the LM335, is mounted in the upper rear section of the butter compartment where it is warmed by the air flow and senses the average temperature in the butter cavity. The moulded plastic encapsulation of the chip should not be touching anything and should be kept slightly away from the rear wall of the cavity. If it is glued onto the wall of the housing then false temperature readings will affect the performance of the warmer. I used a small cable clip to secure the wires and insulating sleeves to the rear panel, and another blob of hot-melt glue keeps it in place.

When putting a block of very cold butter in the warmer it will take several hours before it has warmed sufficiently to soften the butter fully. But once it has warmed, and as long as the butter is returned to the warmer after use, it will stay warm and soft until the butter is used up.

When I originally had the idea of creating the butter warmer my wife was rather sceptical. But now, after 15 years, she won’t live without it; it is now in the same league as the microwave, the dishwasher and the kettle.

Maybe I'm sticking my neck out but if you have questions or are having trouble in this build you can email me direct, burischroland@gmail.com. I can't promise to answer everyone but I'll do my best.

Roland.