Dryer Boost Heater

WARNING: Making a boost heater using this model is entirely at your own risk.  All safety aspects are on you.  Don't make a boost heater unless you fully understand and accept all risks, including but not limited to the risks of using a power supply to intentionally heat up power resistors with no inherent temperature limit.  See also the warning at the end.

Be careful not to over-power a boost heater, since it could easily melt stuff if the power is set too high, and/or if fans fail.  Make sure to test that the heater power won't cause trouble even if (all) fans completely fail, even after many minutes of fan failure while the boost heater is still powered.

PC Blend or Polymax PC-MAX is strongly recommended (I've tried both; both work for me, though the PC-MAX seems more prone to ripping PEI off the textured sheet when I don't use a separating agent (oops)).  Lower-temperature filaments probably won't be able to handle the heat.  Some CF Nylon filaments would also likely work, but those tend to be more expensive.  Don't attempt to make this out of PETG (and definitely not PLA); the deformation could easily get bad enough to start shorting wires together, which could easily bypass some of the resistors, increasing the power output far beyond the intended power level, creating a safety hazard.

This is not intended to try to get the dryer to go above its 70C max temp, as that would require the boost heater to do all the heating despite the main heater being off.  The boost heater is not intended to provide that much power, and is only intended to help the main heater just a little bit more to get to ~60-65C reliably when running with extension walls and an internal circulation fan.

This is posted as a remix of the extension walls.  This model is based on the extension wall geometry, so that this fits perfectly within the front or back extension wall section.  This model requires the extension walls in order to fit, and even with the walls, this model sticks up above the top of the extension wall a bit.

Low voltage only, ≤ 48V.

Experience with soldering and calculating voltage, current, power for simple (symmetric) resistor networks required.  A fairly high temp / high power soldering iron helps.  If you've got an iron that can handle soldering big RC connectors with lead-free solder, that's hot enough.

Before making this thing, please consider how much you really care that your dryer isn't quite reaching the 60-65C for drying PETG optimally (especially when running with taller walls and an internal USB fan to mix the air better), and weigh that against the time to make this thing, the risk involved, and other options like just getting a bigger dryer (like maybe the PrintDry or a food dehydrator).  This mod is admittedly a bit silly just for a few degrees C, and probably someone can think of a better way, but … this works for me.  I'm posting in case it saves anyone else some time modeling basically the same thing; I'm not trying to claim this is the best way overall, nor am I really trying to recommend that anyone else build one of these.  Feel free to comment with your better ideas (please do).

Directions:

• Consider a test print in PETG to check fit in the dryer and to use as a jig.  Note the height of the boost dryer in case you think it's too tall and don't feel like proceeding.
• With some 150mm x whatever x 1.75mm thick proto board, score both sides with a hobby knife, 2-3 rows at a time (roughly matching slot height), put the 2-3 rows in a vice, and bend the board back and forth until it starts to give, then gradually fails along the score line.  It's more effective to bend gradually back and forth until it starts to fail than to try to snap it off in one go.  A rubber mallet can help, but hit just enough to get it to barely fail to spring back at first (break the glass fibers gradually).  Make 4 thin protoboard strips this way.
• With the 4 proto board strips, solder in several power resistors (don't skimp on the number), wired to your preference according to whatever variable voltage power supply you have handy.  Make sure the power resistors are suspended between the protoboard by their leads, so that the resistors won't touch anything but air directly.  Use the test print as a jig to get the height correct.  Make sure each resistor will be running far below its max power rating.  As an example, I use 16.8 V powering (series) 5 groups of (parallel) 3 10 ohm 10 watt resistors, to get a bit less than 20 watts spread evenly across the 15 resistors.  Make sure you wire them so that every resistor gets essentially the same current.  Given the choice, it can be good to choose to use higher voltage and lower current, to allow for thinner power wires, but don't go above 48V.
• Solder on the power wires.  I use red+black 18 AWG BTECHNGO since that's quite flexible and has 200C insulation.  Ensure the power wires are thick enough to easily deal with the current you're planning.  We want to heat the resistors, not the wire.
• Feed the power wires through one of the bottom small triangles in the grille, and out of the dryer (however you want to do that -- I carefully cut a narrow slot in the bottom of the back wall in the middle of a rounded corner).
• Carefully insert the ends of the protoboard strips into the slots, trying not to bend the resistor leads much.  The bottom slots can be a bit tricky, but a screwdriver or similar can fit through the grille and help position the strips.  Make sure the resistors are suspended in mid-air after this.  Optional: Use the little diamond holes to wire the strips into place using needle-nose pliers - or just rely on friction fit if the protoboard strips are the full 1.75mm thick.
• Install the boost heater in the dryer.  I have mine in the front, with a USB fan in the back, but the opposite would also work fine.  Which is best may depend on which direction you tend to feed filament.  I don't hit any issues feeding filament under the bottom, past the boost heater, then up through the top hole.  Filament doesn't melt when it touches the protoboard strips.
• Attach the power wires to a variable voltage power supply (I use a DROK 120V to 0-36V supply, but there are many fine options), and set the voltage to whatever will get around 10 watts to start, up to maybe ~20 watts max (safety is entirely on you to figure out).  Double check all your calculations (maybe have a friend re-do them separately too) and be careful.
• Assuming you're drying PETG, tune the power to the boost heater to barely reach 60-65C.  If you start to hear the dryer's relay clicking to turn off and back on, don't turn up the boost heater any further (regardless of what temperature was reached).  You want the main heater being off to drop the temperature back down so that the dryer's temperature limit still works; setting the boost heater power too high might prevent the dryer's temperature limit from working.  If the dryer still doesn't reach the desired temp, try moving the dryer's temperature sensor directly into the airflow of a USB fan, and a little higher up than default.

I use an 80mm AC Infinity USB fan (claims up to 70C and seem to work fine at 65C) at the other end of the dryer (blowing across the bottom under the spools toward the boost heater) to help mix the air a lot more.  Running without an additional internal fan may result in an overall increase in hot spots that could soften the filament (vs. without the boost heater).  Don't rely on the extra fan to necessarily prevent overheating or hotspot trouble if the main fan fails.  Test that the boost heater is able to run at the max power you'll ever set it to, without any fans helping mix the air, without causing any big problems.  You want the max boost heater power to be able to dissipate in steady state without any fan help, without damaging the dryer itself, and preferably without causing filament to stick together (but maybe that last criteria is optional; if you're ok with both fans failing implying a ruined spool or two).

WARNING: Feeding voltage to resistors has no inherent temperature limit.  In contrast, the main PTC heater is much safer, since it will basically refuse to go any hotter even if airflow stops; it'll automatically increase its resistance and drop the watts to avoid getting hotter.  So, when feeding voltage to resistors, you need to keep the power level fairly low (20 watts or less), spread the power across many resistors to avoid any one resistor getting insanely hot, and it's on you to test whether complete fan failure will result in the resistors getting too hot and starting to melt or burn stuff creating a safety hazard.  It's also on you to use an IR thermometer or similar to check whether the temperature of the resistors over their ambient surroundings inside the heater (~70C) will put the resistors above their max temperature spec (sometimes ~150C for cement power resistors, but can vary).  Do not put the variable voltage power supply anywhere that children (or drunk adults) could potentially turn it up too far.  I recommend posting a warning note on the supply.