Induction heater for graphite - Page 1
Forum section: electronics / projects

author: Rachie5272
author_rank: Regular Contributor
timestamp: November 20, 2015, 08:35:18 pm
content: I'm doing some science experiments, and I'm looking for an induction heater. I intend to heat small 2-3 mm diameter graphite rods to 200-300 °C as quickly as possible. Does anyone have any experience with thesecheap ebay units?I suspect I won't need anywhere near a kilowatt, but I can water cool it just in case. How do I control something like this? PWM the power supply?

author: AF6LJ
author_rank: Unknown
timestamp: November 20, 2015, 08:47:37 pm
content: You might be surprised how much power it does take.Here is the experience of one person who used the same one you linked to.

author: Someone
author_rank: Super Contributor
timestamp: November 22, 2015, 05:23:19 am
content: Resistive heating of the graphite is used in some processes., why do you want induction?

author: miguelvp
author_rank: Super Contributor
timestamp: November 22, 2015, 06:29:01 am
content: Quote from: Someone on November 22, 2015, 05:23:19 ammaybe some cold heat soldering iron implementation?

author: rikkitikkitavi
author_rank: Contributor
timestamp: November 22, 2015, 07:06:18 am
content: Graphite has quite high resistivity, relativivly seen and therefore it will be difficult to induce sufficient currents for heating an object made from graphite.Skin depth is high, so it will heat evenly though.I would rather look into resistance heating by current.You control it by controlling the frequency, it is a resonant converter.

author: Ian.M
author_rank: Super Contributor
timestamp: November 22, 2015, 11:45:43 am
content: Quote from: blueskull on November 22, 2015, 11:03:27 am+1. Its almost invariably the substrate of the die and electrically connected to the thermal pad so you can bet its *NOT* the base/gate without even looking at the datasheet.

author: AF6LJ
author_rank: Unknown
timestamp: November 22, 2015, 01:14:21 pm
content: Quote from: Ian.M on November 22, 2015, 11:45:43 amIndeed.

author: SeanB
author_rank: Super Contributor
timestamp: November 22, 2015, 01:24:49 pm
content: If you are willing to pay for the specialised new punch set for the leadframe, and are willing to order 100k of parts, pretty much any manufacturer of power semiconductors will make you a custom leadframe, with the leads in any order, so long as it conforms to a JEDEC outline, and if you want to pay more they will make them non conformal to that as well. The centre substrate leadframe is just a convention, it can really be any one of the leads, using the right punch tool to stamp out the copper alloy sheet used in manufacture.

author: Rachie5272
author_rank: Regular Contributor
timestamp: November 22, 2015, 06:11:38 pm
content: I want inductive heating for two reasons: not having to touch the graphite with electrodes, and being able to heat only a small section at a time. I'm working with thin but very long pieces of graphite, and need to move the heater along the length to stimulate a chemical reaction.I originally thought of using hot air, but induction seems so elegant, and can be done in a vacuum.

author: SeanB
author_rank: Super Contributor
timestamp: November 22, 2015, 06:14:22 pm
content: Doing zone refining then.

author: Rachie5272
author_rank: Regular Contributor
timestamp: November 22, 2015, 06:45:58 pm
content: Yes, that's an apt description. I'm not working anywhere near the melting point of carbon, but I want the reaction to proceed in a similar manner.So, does anyone know of a cheap induction heater which won't explode when first turned on? I can't quite afford a $1000 unit for a proof of concept until we get more funding.

author: calexanian
author_rank: Super Contributor
timestamp: November 22, 2015, 06:59:29 pm
content: Some thoughts about induction heating graphite. First graphite is a good material for induction heating. It actually heats very well. We heat 2 inch by 2 inch cylindrical anodes in high voltage rectifier tubes to red heat regularly in production. Zone heating as you are mentioning is hard to control however due to the high radiation factor of graphite. if you are heating small pieces of consistent dimensions I would just build my own unit for heating or investigate those hand held both heater units. You can send the company a sample and specs for how hot you want the piece and they will make a cool to suit.

author: JoeN
author_rank: Frequent Contributor
timestamp: November 22, 2015, 10:09:50 pm
content: Quote from: AF6LJ on November 20, 2015, 08:47:37 pmHoly cow, who is this guy? What is his deal?

author: AF6LJ
author_rank: Unknown
timestamp: November 22, 2015, 10:18:29 pm
content: Quote from: JoeN on November 22, 2015, 10:09:50 pmhttps://www.youtube.com/channel/UChWv6Pn_zP0rI6lgGt3MyfAHe is funny.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 22, 2015, 10:25:17 pm
content: Off the shelf?There are few options. Induction heating below 1kW is not useful, so I don't know of any real products. There is a wand type device for automotive purposeshttp://www.theinductor.com/but it's above 1kW, and I think near or above $1000, too.Your only other choice is to build it yourself, or buy one of those silly toy circuits. Same thing, really, as a proper induction heater circuit needs frequency tracking, control and feedback, and protection circuits, and those take time. Anything you'd throw together would be just a toy. (So, hey, if you have the parts on hand, you might as well make something basic, instead of waiting for delivery.)The other problem is frequency. A few mm graphite rod probably won't heat very quickly for frequencies under 100kHz. Most toys aren't intended to run that fast. So you'll probably be building it yourself, but also having trouble dealing with stray inductance, burning transistors, etc...For power control, you'd probably just vary the supply voltage/current into one of these things, yeah.Tim

author: JoeN
author_rank: Frequent Contributor
timestamp: November 22, 2015, 11:17:00 pm
content: Quote from: AF6LJ on November 22, 2015, 10:18:29 pmI was wondering if anyone who watched his channel knew what his overall bio is. Is he an engineer, tech, scientist (not likely, right?), or just a self-taught bubba? Why does he do his channel? Anyone know? I will watch a few of his other vids but I wondered if anyone had a one-sentence summary in advance. I'm not sure I like his shtick or not, still up in the air on that, but he seems to know what he's talking about.

author: LaserSteve
author_rank: Super Contributor
timestamp: November 22, 2015, 11:58:50 pm
content: Perhaps get a OEM Fluxeon board and pay Neon John to optimize a coil for your load?http://www.fluxeon.com/buyflux/index.php?route=product/product&path=33&product_id=52Steve

author: calexanian
author_rank: Super Contributor
timestamp: November 23, 2015, 12:50:00 am
content: You might be able to get enough power out of an 807 tube in a driven oscillator configuration and a output air transformer to heat it. I suspect you will be in the few mhz range for such a small diameter object to heat.

author: calexanian
author_rank: Super Contributor
timestamp: November 23, 2015, 12:52:06 am
content: Wait wait wait. WHy bother with induction for such a small mass at such a low temp. Just wrap a loop of nichrome loosely around it and heat it up. What would heat up the rod pretty fast.

author: ajb
author_rank: Super Contributor
timestamp: November 23, 2015, 03:48:42 am
content: Quote from: JoeN on November 22, 2015, 11:17:00 pmI think he's some sort of heavy equipment mechanic or similar, he's made references to spending weeks at a time fixing equipment in middle-of-nowhere mining operations. He's definitely sharper on the mechanical side of things than the electronics, but he does a lot of fun projects and his tool reviews involve ripping the subject to bits in an informative and entertaining manner.

author: dkozel
author_rank: Regular Contributor
timestamp: November 23, 2015, 04:35:30 am
content: Check out this guy's setup. The video description has the full parts list.

author: Rachie5272
author_rank: Regular Contributor
timestamp: November 25, 2015, 05:46:16 am
content: I was searching around for cheap induction heaters again, then realized the Metcal soldering iron I use is just that. I think I'll try making a new coil, and see if it will heat the graphite. It even uses a standard DIN plug.I have no idea how the driver will respond, but it shouldn't be any worse than potentially melting the coil, right?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 25, 2015, 06:24:13 am
content: Well... blowing up the power supply is always an option. Somehow I doubt the user manual says what range of SWR it's safe to operate into...Tim

author: AF6LJ
author_rank: Unknown
timestamp: November 25, 2015, 02:03:40 pm
content: Quote from: Rachie5272 on November 25, 2015, 05:46:16 amI think I would advise against that.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 25, 2015, 03:18:43 pm
content: You could always forget induction & use an Oxy/acetylene torch or a blacksmith's forge!

author: AF6LJ
author_rank: Unknown
timestamp: November 25, 2015, 03:30:20 pm
content: The one problem I see with melting graphite is...Won't it combine with the oxygen in the air and turn to a gas???

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 25, 2015, 04:27:55 pm
content: As graphite is very black, how about a laser ?

author: calexanian
author_rank: Super Contributor
timestamp: November 25, 2015, 06:23:50 pm
content: Quote from: mikeselectricstuff on November 25, 2015, 04:27:55 pmAnother much easier idea.

author: Rachie5272
author_rank: Regular Contributor
timestamp: November 25, 2015, 08:54:15 pm
content: I tested a propane torch last night. It sort of works, but it's hard to control. If it stays in one place for too long (half a second is too much), the graphite will get too hot and ruin the reaction. Oxidation isn't really a problem, since I'm only trying to heat it to 200-300 °C.I also thought of a laser. It certainly could work, but there's a safety issue. Hmm, maybe I can use a CNC laser cutter for testing.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 25, 2015, 10:39:39 pm
content: Heat lamp and reflectors would work. Cheaper than lasers.tim

author: calexanian
author_rank: Super Contributor
timestamp: November 26, 2015, 04:20:30 am
content: I still like my loop of nichrome wire idea. So easy to control the temp.

author: Wolfram
author_rank: Frequent Contributor
timestamp: November 26, 2015, 09:44:52 am
content: The resistivity of graphite is roughly three orders of magnitude as high as that of copper, so the skin depth for any frequency is going to be around 30 times that of copper. The efficiency of induction heating starts to decline rapidly when the workpiece diameter is smaller than four skin depths (due to eddy current cancellation within the workpiece). This suggests that the operating frequency for induction heating should be in the 10 MHz range or above. Since graphite is so much more resistive than copper, going below this frequency won't be disastrous for the electrical coil efficiency (the ratio of the reflected workpiece resistance to the total AC resistance of the coil + workpiece), but the amount of reactive power needed in the tank circuit will go drastically up. Personally I wouldn't go below a couple of MHz in this application.This suggests that repurposing a metcal soldering station might not be such a bad idea afterall, if it provides enough power for you. I would calculate how much power you need given the thermal mass of graphite at the speed you want to move it, while pessimistically assuming that 50% of your RF power ends up in the workpiece.For example, assuming you use a 3mm diameter rod and move it at 5mm/sec, you need to heat up pi * ((3 mm)/2)^2 * 5 mm = 35 mm^3 ~ 80 mg of graphite a second. The specific heat of graphite is around 0.72 J/gK, so if you want to raise the temperature by 300 K then you will need 0.72 J/gK * 300 K * 0.08 g/s = 17 J/s = 17 W of power delivered to the graphite. This suggests that a common 50 W metcal station would have enough power, as long as you can get the impedance matching reasonably good. If you need more power, you could use a higher powered RF amplifier to drive it, like those used by radio amateurs.If you want to go in this direction, I would suggest repeating my calculations as they were done quickly and may contain errors.

author: GNU_Ninja
author_rank: Regular Contributor
timestamp: November 26, 2015, 10:09:56 am
content: Quote from: rikkitikkitavi on November 22, 2015, 07:06:18 amActually, Graphite is anexcellentmaterial for induction heatingbecause of its high resistivityand is often used as a susceptor material to transfer heat (by conduction) to other non conducting materials. You can heat Graphite to temperatures of the order of 3000 C and it's easily machinable. The stuff is widely used in the induction heating industry.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 26, 2015, 10:29:19 am
content: Quote from: GNU_Ninja on November 26, 2015, 10:09:56 amYabbut...Ain't no body makes their susceptors all of 3mm in diameter.3mm might be a typical wall thickness, but the diameter might be 25 or 50 or 500 mm, which makes things quite a lot easier to heat!Tim

author: GNU_Ninja
author_rank: Regular Contributor
timestamp: November 26, 2015, 11:07:56 am
content: Quote from: T3sl4co1l on November 26, 2015, 10:29:19 amDepends on the frequency. At 1 MHz the penetration depth for Graphite is on the order of 1 to 2 mm.