AC regulator?

[cs.dk]

If you need a "large" light-dimmer, i have some in my stove - The only cost for you is the shipping. It's output is some squarewave, or pulsed sine. I used it to run some fans, but they become unstable with the pulses. A VFD replaced them, so as said, they are here for free.

[stev.dk]

I have 16.4 feet / 5 meters 20 AWG nichrome wire. I dont think i'll be needing 400C to bend acrylic? At least not the thicknes i'll be working with. I think i'll be bending 5mm tops...

I cant remember the specs of my toroid transformer, but i'll post, when i get back home, tommorrow.

[stev.dk]

Ok, so my torrild transformer spec. is: primary 230v, secondary 24v 80VA. I have 16.4 feet of 20 awg nichrome wire. The chart says 5.1 amps is needed to heat 2 feet of straight wire to 315C, but devided by 2 if element is coiled. I should have enough wire to coil it, as my heating strip only needs to be about 9 inches long. Will my transformer be big enough for this job? Im planning on rectify the ac to dc, and use a 555 timer to pulse a transistor, that will power the heater element.

[Simon]

No it won't, 80VA is just over 3A at 24V and you want 5.1. I'm not sure how your current will go down if you coil it unless it is very tight. If you plan to PWM control it at much less than 3 A then you can use the transformer if it has a large enough input capacitor, I once did this to run a 80W fan off a 30W power supply.

[dentaku]

I wonder if this is useful for you?
http://jacobs-online.biz/power_supply_design.htm

[stev.dk]

Ohhh. the table http://www.wiretron.com/nicrdat.html is based on 115VAC. That in mind, it will take 586,5 Watts, to heat up 2 feet of straight nichrome wire to 315C... Im assuming, i'll need the same amount of watt, using 24VAC, which will result in 24.4 Amps... way above my transformers capability. Think i'll have to rethink this completely.

[SeanB]

Depending on duty cycle your transformer might work. Bar sealers use a 56VAC transformer running grossly in overload, but as they typically are on for 5 seconds every minute they last quite well. The transformer does get very hot if the user does 10 seals in a minute though, but they do at least come with a thermal trip to protect the transformer. Not always effective though.

[jlmoon]

To the original op.. Stev.

This might be a tad off topic but still pertains to a portion of your overall project:
Following this thread, I notice you're employing nichrome wire for your heater.  Might I suggest a much better solution for heating wire?  Take a look at Rene Wire.. or 'Rene 41'  This is considered a superalloy with outstanding tensile strength when hot.  Some of my business dealings involve foam, plastics cutting and forming which requires a much more robust wire than nichrome.  You will be pleasantly surprised with the performance of this stuff. 

my 2c worth! 
Jon

[jakeypoo]

Ohhh. the table http://www.wiretron.com/nicrdat.html is based on 115VAC. That in mind, it will take 586,5 Watts, to heat up 2 feet of straight nichrome wire to 315C... Im assuming, i'll need the same amount of watt, using 24VAC, which will result in 24.4 Amps... way above my transformers capability. Think i'll have to rethink this completely.

Just Look at the first two charts, that's all you need.
Ditch the transformer and PWM a DC source. It will make your life easier.
If you need denser energy output, use a coil as others have suggested.

[stev.dk]

Im thinking about modding a computer psu, that can deliver 24 amps at 12VDC, and pwm this. But will 12VDC be enough?

[jakeypoo]

Im thinking about modding a computer psu, that can deliver 24 amps at 12VDC, and pwm this. But will 12VDC be enough?

You can calculate all of this with the first two charts. Check my first post.

I have 16.4 feet / 5 meters 20 AWG nichrome wire

Look at the first chart, under 20AWG:  5.1A at 315C, 6.3A at 427C
Looking at the second chart: 20AWG has an R of 0.66 Ohms per foot.

Your voltage requirement is then 6.3A * 0.66Ohm/foot * feet   = Round up to 4.5V per foot.

Now lets go the other way around:
If you end up using a 12V supply, use about 30 inches of wire that will give you about 1.65ohms of resistance (cold) so peak current is 7.3A.   

[stev.dk]

Im thinking about modding a computer psu, that can deliver 24 amps at 12VDC, and pwm this. But will 12VDC be enough?

You can calculate all of this with the first two charts. Check my first post.

I have 16.4 feet / 5 meters 20 AWG nichrome wire

Look at the first chart, under 20AWG:  5.1A at 315C, 6.3A at 427C
Looking at the second chart: 20AWG has an R of 0.66 Ohms per foot.

Your voltage requirement is then 6.3A * 0.66Ohm/foot * feet   = Round up to 4.5V per foot.

Now lets go the other way around:
If you end up using a 12V supply, use about 30 inches of wire that will give you about 1.65ohms of resistance (cold) so peak current is 7.3A.   

Thx, i understand now. I think i'll be modding my old pc psu, and use a 555 timer to control pwm I should have the parts for that.