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I have a 3D printer heated bed, instead of 3d printing I want to use it for reflow soldering. The bed is intended to operate at 70c, using 12v or 24v. It can be wired in series or parallel depending on the input voltage, it's 0.8 ohms wired in parallel, and 3.2 ohms wired in series.

I want to reach 260c, and the power demand will be at least proportional, so I think I'll need at least 90W * (260c-25c)/(70c-25c) = 500W. Lets say 700 to be safe. To push that through the 0.8ohm heated bed I'll need about 24v at 29A.

So that means I need either a large and expensive 24V PSU, or a large and expensive transformer.

But is there any way around that? For example, could I connect the heated bed directly to AC, and then use a triac to carefully ration the on cycles? I'm hesitant to do this because of the low duty cycle (15%), and high peak wattage (4.5kW).

Are there any other options? My first choice was to buy a cheap 30A 24v PSU on ebay, but that just blew up so I'd like to try something else.

Edit: To clarify what I'm asking: Is there a way to drive a high current, low voltage resistive load from 120v AC without using a transformer, or a full blown switching power supply?

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    \$\begingroup\$ This is not going to work. Something else is going to melt, or your heating element is going to fail. There are tons of ways people find to do reflow soldering, including using low-cost toaster ovens (maybe the cheapest). \$\endgroup\$ – mkeith Sep 8 '18 at 3:59
  • \$\begingroup\$ If you want to heat from below on a budget, buy a kitchen hot plate with a solid surface. That said, this method is perhaps better for pre-heating than for achieving the actual soldering temperature. \$\endgroup\$ – Chris Stratton Sep 8 '18 at 6:36
  • \$\begingroup\$ The "heating element" is an aluminum substrate PCB, so it can definitely withstand the reflow temperatures. ..at least a couple of times. \$\endgroup\$ – Drew Sep 8 '18 at 6:39
  • \$\begingroup\$ How critical is quality? Then you want uniform temps. with no gradients \$\endgroup\$ – Sunnyskyguy EE75 Sep 8 '18 at 10:28
  • \$\begingroup\$ I'm looking for better evenness than a hotplate, and controlled ramp rate. The heated bed I chose is a solid pattern of heating traces so it should be very even. It also doesn't have much thermal mass so it should be fairly easy to ramp up and down. \$\endgroup\$ – Drew Sep 8 '18 at 19:27
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They tend to use long quartz tungsten heaters for SMD reflow.

But you can use an industrial power 10kW dimmer and Halogen flood lamps positioned for forced air recycling as well as radiant heat. Some trial and error or review of existing commercial solutions is needed.

Then you can use multiple temp zone sensors top, bottom, centre, with exhaust air vented safely, forced air circulated inside and room humidity raised towards 50%. Each of these variables will affect final results along with your thermal profile for preheating ramp, solvent evaporation, liquidus duration and ramp down according to datasheet specs and solder+paste used.

High Temp Insulation will help reduce the room temp. if you can do this.

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Update: I've tested the solution I proposed and it does work.

So to summarize. I was able to drive a low impedance resistive load (3.2 ohms) directly from AC using a zero-crossing SSR at low duty cycles (<15%). This allowed me to pull 700W from mains without the need for a large transformer or switch mode power supply.

I included a 20A fuse in series to disconnect the load if the SSR fails. It didn't blow.

There was a negative side effect. The high peak current (over 50A), causes lights in the area to flicker.

I was prepared to just deal with the flickering lights, but ultimately my 3D printer heated bed failed around 240C (as mKeith predicted). I've since switched to a 1000W hotplate. It flickers the lights a bit too, go figure.

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