# Polystyrene cutter to learn about resistance, current voltage and HEAT

#### Mellisa_K

Joined Apr 2, 2017
359
Polystyrene cutter to learn about resistance, current voltage and HEAT

Hello,

I have a need for a hot wire polystyrene cutter. (Poystyrene melts at 240 degrees centigrade or 464 degrees F.) The basic shape would resemble a fret saw with the teeth replaced by a hot wire. Please see attached jpg.

I can easily find a project on “Instructables” or some other maker site.

However, I don’t want to do this. I want to design the cutter myself from first principles. It would be fun to then construct it and see how well the maths predicts the actual performance of the tool. This way I can modify the length and thickness of the hotwire as I please

So, I want to take this opportunity to learn about the function: heat(expressed as wire parameters) = f(current, resistance, voltage), where f is functional notation.

Can you tell me please where I would find the equations of the above function so I can predict the parameters regarding what I need to design the cutter:

a) The diameter of the wire

b) The length of the wire

c) The electrical parameters of the cutter like current and voltage.

I don’t want you to tell me these parameters, even if you were inclined to do so. No, I want you to tell me a reliable source of accessible literature so I can find out about the relationship between heat and current in a wire for myself and then build it.

#### oz93666

Joined Sep 7, 2010
701
This is not an issue that is easily solvable by formula ...

I don't think you want to cut polystyrene , it's a hard clear plastic similar to acrylic , not easily cut with a hot wire. Expanded polystyrene is polystyrene with many bubbles in it , white , light , sometimes used in packaging.

The moment the hot wire touches the expanded polystyrene , it's temperature drastically drops , so does the resistance of that portion of the wire .... the amount of force you exert will also determine how much the temperature drops ...

Just too many unknowns , forget calculations , get a variable power supply and go to it.

A sensible size of wire (nicrome) is about 0.3mm dia ... current a few amps.... voltage dependent on length of wire , less than 10V

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#### Alec_t

Joined Sep 17, 2013
10,600
Almost any textbook on electronics will tell you the relationship between energy (heat in this case), power, current and voltage, but I agree with Oz above. Even knowing how much heat is generated won't tell you much about the temperature of the wire. Some of the unknowns include heat lost by radiation and by conduction to the frame holding the wire.

#### Mellisa_K

Joined Apr 2, 2017
359
Really?

Yes expanded polystyrene foam. Forgive me; I didn't realise the difference.

Okay I'll just give it a go as you suggest, but AC, yes?
This is not an issue that is easily solvable by formula ...

I don't think you want to cut polystyrene , it's a hard clear plastic similar to acrylic , not easily cut with a hot wire. Expanded polystyrene is polystyrene with many bubbles in it , white , light , sometimes used in packaging.

The moment the hot wire touches the expanded polystyrene , it's temperature drastically drops , so does the resistance of that portion of the wire .... the amount of force you exert will also determine how much the temperature drops ...

Just too many unknowns , forget calculations , get a variable power supply and go to it.

A sensible size of wire (nicrome) is about 0.3mm dia ... current a few amps.... voltage dependent on length of wire , less than 10V

#### jpanhalt

Joined Jan 18, 2008
8,335
Hi,

In answer to your question about heating, current, and wire sizes, I suggest you search on "ampacity of wire". Sizing PCB routing is basically a heat calculation. Here is one discussion: https://www.ti.com/seclit/ml/slyw038c/slyw038c.pdf Search on heating.

As for your design, be sure the heated wire is insulated from the frame at least on one end, and preferably on both ends for safety unless your supply is fully isolated..

#### oz93666

Joined Sep 7, 2010
701
.....Okay I'll just give it a go as you suggest, but AC, yes?
Alternating Current ??? The wire doesn't care if it's fed AC or DC ...both are fine .

The power-supply is the biggest component in this project , you really need a variable supply ... this is a cheap one from eBay 3A is enough current 12V model for short hot wire .....24 V long wire ...about $12 Thread Starter #### Mellisa_K Joined Apr 2, 2017 359 Alternating Current ??? The wire doesn't care if it's fed AC or DC ...both are fine . The power-supply is the biggest component in this project , you really need a variable supply ... this is a cheap one from eBay 3A is enough current 12V model for short hot wire .....24 V long wire ...about$12

Okay many thanks oz

#### Mellisa_K

Joined Apr 2, 2017
359
1) The one illustrated varies the voltage, keeping the current constant, yes?

3) Can you pls give the link?

#### Mellisa_K

Joined Apr 2, 2017
359
Hi,

In answer to your question about heating, current, and wire sizes, I suggest you search on "ampacity of wire". Sizing PCB routing is basically a heat calculation. Here is one discussion: https://www.ti.com/seclit/ml/slyw038c/slyw038c.pdf Search on heating.

As for your design, be sure the heated wire is insulated from the frame at least on one end, and preferably on both ends for safety unless your supply is fully isolated..
Thankyou for the pdf link, jpanhalt. It's a very handy book isn't it.

Yes, I omitted the insulation in my illustration. I am aware of this.

Thanks for your trouble.

#### shortbus

Joined Sep 30, 2009
7,237
Alternating Current ??? The wire doesn't care if it's fed AC or DC ...both are fine .
Here we go again, this was discussed in at least one other thread on hot wire cutters. If DC is the right form of current, why is it that all of the DIY and commercial machines use AC? There is no, or at least none that advertise, that they use DC for this purpose.

#### Dodgydave

Joined Jun 22, 2012
8,586
What about a Transformer from an old car charger rated at 5A and use a Triac dimmer to control the AC voltage to the Hot Wire..

#### Mellisa_K

Joined Apr 2, 2017
359

#### Mellisa_K

Joined Apr 2, 2017
359
What about a Transformer from an old car charger rated at 5A and use a Triac dimmer to control the AC voltage to the Hot Wire..
Yes, good suggestion. I've got access to one of those, thankyou Dodgy!

The triac dimmer goes on the output of the transformer, yes?

#### KeepItSimpleStupid

Joined Mar 4, 2014
3,760
We made heating jackets out of tantalum wire and Boron Nitride. They operated around 20A, 40V and could get hot enough to melt copper/
We also made flat heaters, about 200 W from tantalum foil. this stuff operated in a vacuum.

it was observed that DC heating resulted in better heater lifetimes for whatever reason.

The initial setup used phase angle firing into an autotransoformer with an homeade panel dedicated to metering and the autransformer.

i stuck my neck out and converted an entire system of 7 heaters to a DC power supply and programable temperature controllers. It reduced panel space and allowed real power computations which they did want.

It was like \$900 to add power measurements for two heaters for the AC system.

I make a "hot knife" at home to cut glass (Mirror tile). We did the same at work to cut 1'x1"x0.025" 7059 glass, Yes, tricks are involved/

#### Dodgydave

Joined Jun 22, 2012
8,586
Yes, good suggestion. I've got access to one of those, thankyou Dodgy!

The triac dimmer goes on the output of the transformer, yes?
No Triac dimmer on the Input side, Transformer will decrease the output voltage for you, if it's not going low enough, the dimmer can be modified for lower output.

#### oz93666

Joined Sep 7, 2010
701
Here we go again, this was discussed in at least one other thread on hot wire cutters. If DC is the right form of current, why is it that all of the DIY and commercial machines use AC? There is no, or at least none that advertise, that they use DC for this purpose.
I spoke out of common scene .... I can think of no logical reason why one supply would be better than the other , it's only heating the wire..

I can understand why commercial cutters use AC ... less components , cheaper to manufacture ....

Here's the link for the supply pictured above ... https://www.ebay.co.uk/itm/Adjustable-DC-Power-Supply-Adapter-Charger-Variable-Voltage-3V-24V-EU-Plug/323813192453?hash=item4b64c53305:m:mNe03ACW4AlGHpljT-KplRA

listings for nichrom cutting wire on eBay too ... you can see from the data below from one supplier , that at 0.56mm you need a higher current and hence a bigger and more expensive power supply ...

0.56mm Diameter / 24 Gauge
1m Coil length - Multiples purchases will be supplied as a continuous length unless requested
Smooth finish
5.02Ω/m @ 20ºC
Approximate current needed to heat a horizontal length of wire in free air to:-
205ºC 2.5A
315ºC 3.3A
425ºC 4.0A
540ºC 4.9A
650ºC 5.9A
760ºC 7.0A
870ºC 8.3A
980ºC 9.6A
1090º 11.0A

My experience is about 0.3 mm dia will work fine , produce less fumes , and will work with the 3A power supply I linked to.

here's a link for 30SWG (0.315mm) https://www.ebay.co.uk/itm/30SWG-Nichrome-Resistance-Wire-5M-Heating-Element/290937640510?hash=item43bd3c163e:g:FKIAAMXQrC9Sb4zS

Commercial cutters working constantly may use wire around 0.5mm diam as this will break less frequently ...thicker wire can also span meters in cutting length , since the thicker wire can tolerate higher tension keeping the wire taught and more strait..... , but this will a need power supply capable of delivering higher currents .

EDIT ...Ah I see you already have a supply ... that will work fine ...you may get away without a triac dimmer if you don't have one . Either adjust the length of the hot wire to get the right temp or put ordinary wire (like a 25m coil of ring main wire) , in series to get the current down.

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#### Mellisa_K

Joined Apr 2, 2017
359
I spoke out of common scene .... I can think of no logical reason why one supply would be better than the other , it's only heating the wire..

I can understand why commercial cutters use AC ... less components , cheaper to manufacture ....

Here's the link for the supply pictured above ... https://www.ebay.co.uk/itm/Adjustable-DC-Power-Supply-Adapter-Charger-Variable-Voltage-3V-24V-EU-Plug/323813192453?hash=item4b64c53305:m:mNe03ACW4AlGHpljT-KplRA

listings for nichrom cutting wire on eBay too ... you can see from the data below from one supplier , that at 0.56mm you need a higher current and hence a bigger and more expensive power supply ...

0.56mm Diameter / 24 Gauge
1m Coil length - Multiples purchases will be supplied as a continuous length unless requested
Smooth finish
5.02Ω/m @ 20ºC
Approximate current needed to heat a horizontal length of wire in free air to:-
205ºC 2.5A
315ºC 3.3A
425ºC 4.0A
540ºC 4.9A
650ºC 5.9A
760ºC 7.0A
870ºC 8.3A
980ºC 9.6A
1090º 11.0A

My experience is about 0.3 mm dia will work fine , produce less fumes , and will work with the 3A power supply I linked to.

here's a link for 30SWG (0.315mm) https://www.ebay.co.uk/itm/30SWG-Nichrome-Resistance-Wire-5M-Heating-Element/290937640510?hash=item43bd3c163e:g:FKIAAMXQrC9Sb4zS

Commercial cutters working constantly may use wire around 0.5mm diam as this will break less frequently ...thicker wire can also span meters in cutting length , since the thicker wire can tolerate higher tension keeping the wire taught and more strait..... , but this will a need power supply capable of delivering higher currents .

EDIT ...Ah I see you already have a supply ... that will work fine ...you may get away without a triac dimmer if you don't have one . Either adjust the length of the hot wire to get the right temp or put ordinary wire (like a 25m coil of ring main wire) , in series to get the current down.
This is very comprehensive and considered helpful advice Oz. I thankyou for your time and helpfulness.

As I do everyone who replied.

It's a generous community, committed to helping learners.