Let's Make Robots!

Build a plastic heat bender or Nichrome controller

I am attempting to build a thermal plastic bender to create mounts and parts for my robots.  Basically a piece of nichrome wire heats up a small area of the plastic which allows me to bend/form it by hand.  The nichrome wire I have, came out of an old space heater.  It is formed into 1/4 inch coil.  I don't have a clue to the metal contact or diameter (pretty darn thick since it was an old heater.) 

 I tried connecting it directly to a 12v 2.6 Ah SLA battery.  A short section of about 6 inches coiled.  The direct connection proved to be too much current and almost instantly brought it to a bright glow.  Probably 2-3 seconds before I disconnected it for fear it would burn out. Much to hot to use for plastic forming... Smelting gold,  maybe :-) 

 The examples of the bender I  have seen use a variac transformer to control the current.  I don't  have one, so my question is,  will an adjustable power supply using a variable resistor / lm350 circuit have the same type of control 'dimming'/reducing the heat (or am I way off on the concept here?)  using a circuit such as this: http://diyaudioprojects.com/Technical/Voltage-Regulator/

I am learning as I go, so I may be totally off in this application.  But it seems it would act the same for a dc battery driven circuit (instead of an ac driven variac transformer.) 

Here is what I  think needs to happen. 

The original heater had a type of thermostat to control the heat.  Heat sensitive wire would allow contacts to close and send power to the coils.  A type of mechanical voltage control.  I can't place this 'thermostat' into the project.  It would interfere with the plastic being above the heat source.  So I  am attempting to control the heat radiated by reducing/increasing the voltage to the heating element. 

Thanks for any assistance. 

Ed

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You may want to take a look at 3d printer software. Its a bit overkill, yes, but something like Teacup will run on a regular arduino (328), can read a thermister (2, actually) and also control 2 mosfets to control the heat. All the PID stuff is done, it works great to control the temp of extruders and heated beds.

Again, overkill in that you would not be using any of the "CNC" part of the firmware, but it is an off-the-shelf solution, free, well documented and will do what you need it to.

I am eventually going to build a 3d printer from scratch (as opposed to a kit).  I've been collecting old inkjet printer and flatbed scanner parts with this in mind.

My next project is planned to ne a plastic vacu-form bed.  So I  will definitely be taking a look at this.  May just advance the timeline of my plans. 

ED

You got my hopes up. I saw "Plastic Bender" and went here:
e782_talking_bender_scale.jpg

If you have  a spare picaxe 08M or Arduino then the simplest way is to use them to read a potentiometer and generate a PWM output. Use the output to drive a high current FET and this will control the power through your ni-chrome coil. The FET can be any high current Nch FET although you should try to use ones with an "L" suffix as this indicates they can be driven by a logic level signal. The FET shown here can handle up to 45A with a suitable heatsink. If you can get a FET with a higher current rating then it will need a smaller heatsink or maybe no heatsink at all.

If you don't want to waste a Picaxe or Arduino controller then you can use a 555 timer IC to make a stand alone PWM generator. Just turn the knob until you get a temperature your happy with. The 100R limits the charge / discharge current of the capacitor but you will still get close to 0% and 100% duty cycles.

It's been a while since you posted this circuit,  but I  have built a couple of them and use them to experiment with all of the time.  I was just looking at the 555 version tonight and was wondering if I could disconnect it between pin 1 of the 555 and source of the mosfet,  connect to the common ground (ground to the source pin on the mosfet).  Also disconnect just after pin 8 of the 555 leading to the heater element (or load).  Insert a different positive power source terminal here, to power the load..

 A higher voltage that the mosfet and load can handle,  but the 555 is not capable of?  Say a 24v battery connection. A heater coil or motor capable of 24v as the load.  The 555 and the rest of the circuit would still continue to run on the original 9v-12v source?

   It should still switch the mosfet on and off.  The mosfet's gate will trigger a different power source through the 'source and drain' pins of the mosfet, completing the circuit to the load powered by the secondary power source? 

 If I understand how the components work correctly,  it should theoretically work as I described? 

Ed

This is perfect!  I think I will start with the 555 version, but may also try the other since I have a breadboard mount 328p nano that would work perfectly. 

   I just searched all my components and I think I have everything I need to try this out. I assume 50v instead of 15v would work for the capacitor.  And working with what I have on hand, I think the best bet for the Mosfet would be a 47n60f that I have (actually have dozens..) this is the first project I have used a mosfet for, so I  hope I am correct in this substitution. Others that I have on hand are buz11a, irf630a, 13n06L, irfz22. If one of these is better or I need to get the one you specified, please let me know. 

Everything else is exact to your specs.  I won't be putting it together until tonight at the earliest.  Allergies / migraines won't cooperate and I am down and out for a while. Forget about trying to figure out the principals behind this schematic right now.  But that is important to me, so I am putting it off till I can study it the attention it deserves so I understand why it works. 

Thanks Oddbot! 

 

Guess not.  Built the circuit with this mosfet.  Nothing happened.  So I guess it will not work with that mosfet.  Getting late, so I  will double check  my work tomorrow and then check if any of my other mosfets are close to the one you recommend. 

The circuit should work with almost any Nch Mosfet. Measure the actual resistance of the Nichrome heat element and calculate the current draw. That will help you check the FET is good enough. Ideally your FET should be rated at least 3x higher than the current draw otherwise you will need a huge heatsink.

To make it easy to fault find, solder a second capacitor (at least 100uF) in parallel to the 1uF capacitor. This will slow the PWM frequency down. Now connect pin 3 of the 555 timer to the anode of a LED and then connect a 1K resistor between the cathode and ground. The LED should now light up when the output (pin 3) is high. Set the pot to about half way and the LED should be blinking.

If the LED is not blinking then there is something wrong with your 555 timer IC or the circuit. If the LED blinks OK but the Nichrome does not get hot then maybe your FET is damaged or wired wrong.

Once you have the circuit working, remove the big capacitor and the LED should appear to change brightness as you adjust the pot.

Started with your original schematic and went back to the datasheets.  I took it for granted that the middle leg on the 47nf60 was the gate (like the diagram, told you it was first experience with them).  That was the entire problem.  Now it works, but still WAY too much current is flowing with no control. 

I start turning the 10k pot to get it started.  Not even a tenth of a turn and it is about as hot as it can get.  No pot resolution.  No real temperature control. I also notice the temp alligator clip getting power from the battery are REALLY hot. 

I've been experimenting with it, but am lost searching for  more control. The coiled nichrome was about .8 ohms resistance.  I temporarily fixed this by giving it another inch of coiled nichrome wire,  bringing it up to about 1.2 ohm.  If I add enough wire to bring it over 2ohms, the circuit will heat up but not glow. 

I plan to stretch the wire out, using less wire,  less resistance. The tight,  small coils are overkill for this project.  I would like to have more resolution/heat control over potentiometer for different types/sizes of plastic I plan to shape.  

What would I change to give it more control over the smaller resistance section of wire? Would it be a Smaller pot? I couldn't drive more than 2ohms worth of wire with this mosfet/battery combination? 

The heat of the alligator clips connecting the battery?  Too much current for the wire size standard hookup wires?  (think it's about 20 guage stranded.) 

Thanks for your help.  I'm learning am lot with this one.  (could take the easy route and drive it with a light dimmer/trasformer combo, but this is much more fun!) 

ED

 

Ok, 0.8Ω is to small for 12V and will give you a current draw of 5.44A. You need to increase your resistance as you discovered alread and possibly reduce your voltage.

Changing the value of the pot will make absolutely no different as you will still get from about 1% to 99% duty cycle. Your circuit is not the problem. Once you get the right amount of Nichrome wire you will have good control over the temperature.