Heated Bed Resistor problem

Hi im trying to construct a heated bed using 5 power resistors each at 6.8 Ohm, the resistors are coil based and can handle extreme high temperatures.

Ive constructed Alu heat transfers clamp for them myself and glued it all to the plate.

The problem now seems that when i connect them in parallel the ohm value drops wich doesnt make any sense according to rules of electronics as it should be the same but Wattage tollerance increased.

Can this Phenomena be due to some sort of inductance occuring in the resistors and cause them to act up as a form of coil? If so how can i correct this problem? or is this just a phantom of my measuring equipment? (ive tried meassuring using 2 independent multimeters)

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The problem now seems that when i connect them in parallel the ohm value drops wich doesnt make any sense according to rules of electronics as it should be the same but Wattage tollerance increased.

I don't know which rules you are referring to but when you wire N identical resistors in parallel the resistance will be R / N.

If you want the resistance to remain the same you need to wire then in series / parallel and have a square number of them. E.g. a pair of resistors in parallel gives half the resistance so two of those pairs in series give the original resistance. You can't do it with 5, but you can with 4 or 9.

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[www.hydraraptor.blogspot.com]

Ah i see,
well sorry my understanding of electronics is based in basic analog / digital electronic as a trained repair technican and years of experience and some hobby knowledge in other words not at enginering skill level

but since i dont work with it nowdays its kinda dim knowledge :/

Thanks Chris, and btw got any good ideas on how to make a simple voltage divider to stepdown an oldfashin 220V to 24V Transformer to 12V and not loose the amp rating? (and no i cant remove windings as its epoxyed to death by the manufacturer back in the 80's)

I suppose the obvious - that transformer in question doesnt have a center tap.

One bad but cheap way to do it is to put an adjustable voltage regulator, some are up to 5A, like the very old one LM338, although for 12V it would run probably very hot. If higher amps are needed its output can drive some chained power transistors in cascade or paralel. Even fixed regulators are somewhat adjustable if the gnd pin has the voltage raised accordingly, so it can be improvised. Old parts are cheaper and have a bunch of schematics and variants on the net, although newer regulators could be better but more priced.

If you need 12V, look for a dead UPS (uninteruptibile power supply), those have a nice hefty transformer inside+goodies+case. It may be my first choice to go. Btw, what u need the output for?


If its for heated bed, i think 24V should be much better than 12V, because it will require half amps at same power so it makes it more in the "normal" limits. E.g. 120W heater with 12V needs 10A kinda stretched out, but same power from 24V only drains 5A so it feels somewhat more normal.

Also on heated bed page i think its a fet control. I think if one wants to control the direct output of a transformer which is a.c., from ttl, it may require a solid state relay to zero crossing or the same thing unpacked, a combo of optocoupler+triac, e.g. moc30xx+bta140.

Edited 2 time(s). Last edit at 03/19/2011 06:02PM by NoobMan.

Hmmm, with a 24V transformer, like the ones from ups units, one secondary can go directly to heated bed, the other can go to a bridge+smoothing+2 adjustable regulators 5A each, one for extruder and one for stepper boards. So atx 12v can be dissed completelly.

Would have the advantage to run motors on higher voltages with less amps and less heating, and adjust the power going to extruder heater.

Something like that seems neat. Would make a nice reprap supply.

This one is rated 10A at 24V, and unless im not misstaken again ... i should be able to get around 20A at 12V (in ideal conditions of course)

The easiest would be to unwind some copper or center tap it but due to the 2kg of Epoxy surrounding it it makes it kinda overkill if not almost impossible, unless anyone got an idea how to solve epoxy (already tried aceton and it only scraped the surface)

There isn't a practical way to get 12V DC from 24V AC. When you rectify and smooth 24V AC @ 10A you will get ~34 DC & 7A. The only way to convert that to 12V & 20A would be to use a switch mode down converter or step down the AC with another transformer and then rectify it. Either way would be less efficient and probably more expensive than a mains to 12V switch mode PSU.

You could use 24V AC to power a heated bed using a solid state relay for control, but it is not much good for driving the rest of the electronics.

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[www.hydraraptor.blogspot.com]

The ups units i think have secondaries of 15.5 and 18.5Vac. Transformer rated at like 30-35Amps. Not sure i recall correctly. I should check tomorrow. But that would work?

Also, since it would supply reprap with voltage regulators, does those really need alot of smoothing close to Vpeak or can work with less - e.g. it wort raise the voltage too much, sort of speaking, could the voltage regulators work with minimal smoothing or none at all?.

Edited 1 time(s). Last edit at 03/19/2011 07:55PM by NoobMan.

Any suggestion for a (cheap and simple if possible), switching mode solution in the required range pls?

With no smoothing at all the voltage will go to zero every half cycle. So you need at least enough smoothing to ensure the voltage does not dip below the minimum input voltage of the regulator when at max load. Regardless of how much smoothing it will peak at 1.4 times the RMS. If you use a linear regulator the dissipation will be massive.

The cheapest way to get 12V is with a PC PSU.

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[www.hydraraptor.blogspot.com]

Well cheapest i can figure out is go to ikea or other lampstore, get a transformer for a 12V spotlight system. 220VAC to 12DC but im unsure what the load minimum is on them, the bigger set requires somewhere closer to 50W or more (about 4,2A at 12V) of load inorder to make the safety not to shut it down. (Ikea name TERMOSFÄR not cheapest i used to have 2 but since converted the setup to a LED system the transformers are only laying around.)

[www.reprap.org]

Last line says:
"Dibond is rated up to 80oC, so it may be OK for ABS (which should need about 70oC), but I haven't tried it. To get the bed that hot you'll probably need more power than the 100W that I was putting through it. I'd try 200 W. "

Probably with 6R8 in parallel making 1R36 @12Vdc drawing 8.82A -> 105.9W.

Assuming that trafo and resistors can handle and everything is ok, imo its the way closest to "official" page.

I dont think you can use a fet like it is on that page, because you will have a different supply & diff gnd, so you might really need a ssr. As a result, if you use a ssr anyway, you could use ac directly, i cant see any significant difference as it will still heat up. I might be mistaking tho. Good luck.

For ABS you need 100°C.
For PLA around 60°C is optimal.

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Bob Morrison
Wörth am Rhein, Germany
"Luke, use the source!"
BLOG - PHOTOS - Thingiverse

For the ABS from reprapsource.com on PET tape I need 140C for the first layer and 110C after that.

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[www.hydraraptor.blogspot.com]

I found it easiest to just run directly off of AC for the heated bed, and power the electronics and hot end of a separate cheap DC power supply. I'm running 5 35 ohm in series on 120VAC. I think your best bet would be use 4 of the resistors. 2 pairs of parallel resisors in series, for 6.8 ohms total resistance.

At 24VAC, thats 85W. I'm at 88W, and I've used it at 110C, which was enough for ABS on Kapton. It does take a while to reach that temperature though.