Hello everyone

I am scratching my head on this one, I know it can be done because they do it on welders.

I am trying to build a hot wire cutting table. The Nichrome wire will be various lengths and gauges depending on the projects. 6" up to 60" 32 gauge up to 18 gauge.
I am using the Nichrome calculator and they are telling various voltages an amperage for different lengths and gauges to get the wire hot enough to cut the Styrofoam cleanly.

I know I need a transformer that drops line voltage from 110ac down to 28vdc. I would like (like they have on a welder machine) one dial that controls the output voltage and one dial the controls output amps that function independently. example 24v at 3 amps for 24 gauge and 12 v at 10 amps for 18 gauge @ 60"

How is this done, Please let me know
Brad.

 

I dunno about foam cutting but aren't they supposed to be heated with CC supplies.
Wait a bit to see what other members have to say.

Still you gonna need to a huge Tx if you need current like 10Amps continues and good regulator circuitry.

 

Looking at you topic on adjusting VI seperately, it is done using a CV, CC PSU.
You can make one if you can get the parts and if you can do soldering.
A SMPS is small, light and efficient. But designing with your suggested capacity is not for a n00b.
A linear supply is one thing you can make. I know I did. And I definitely cannot make a high power SMPS yet

I believe I can help you make one if you are up for it.

 

Hello,

You can take a larger power supply and use PWM to regulate the power to the wire.

Bertus

 

But you can't control the voltage and the current independently. When they say that you need a certain voltage at a certain current for a certain gauge and a certain length, what they are really saying is that you need this much current through this size wire to get the temperature you need and you need this much voltage to get that much current through a wire of this size that is that long.

What you think you are controlling on those welders are not the actual currents and voltages, but the current and voltage limits.

 

Ooops! I forgot to mention that both cannot be controlled at the same time.
For I and V, one is controlled while other set at a max limit.

 

As WBahn said---

Ohm's law is I = V/R, equals V = I*R, equals R = V / I.
You can independently control any two out of the three, but the third is determined by the equation.
Thus since the wire resistance is fixed by its diameter, material, and length, than you can vary the voltage across it to get a given current, or you can control the current through it, which will give you a given voltage drop, one or the other but not both independently.

 

The one I made has a simple TRIAC controller, many you see out there use a step down transformer and control the primary with a dimmer style from H.D. etc, I decided to feed the primary direct and Triac control the secondary, if you have various types of Nichrome Gauge/length etc, you could set up a indicated dial with markings for each settings.
Max.

 

Looking on YouTube, I could swear I saw someone made a transformer with a variable core. Yes, you actually have a manually adjustable core to control the current. I'm certain welding machines do NOT use that method, but if I can find that video again I'll post it.

I did some foam cutting when I insulated my basement. I used a solid wire, Chromoly, and my welder. Made the wire 9 feet and stretched between two points. The wire got hot enough to cut the foam without having the wire glow red hot. Cut nicely through every piece of foam I put to it. But as for numbers or calculating and designing - nope. Just did trial and error. The biggest piece of foam I could cut was an 8 foot length. Anything smaller, I just cut on the same jig. And one thing I learned was that when my wire got hot it got longer, meaning it would sag. So I made one arm fixed and one arm on a pivot with a strong spring (from an old dishwasher) to keep the wire tight enough without pulling the hot wire apart.

I'll still look for the video on that transformer. If I find it I'll post it.

 

I know I need a transformer that drops line voltage from 110ac down to 28vdc. I would like (like they have on a welder machine) one dial that controls the output voltage and one dial the controls output amps that function independently.

Welders don't work that way. They are either Constant Current (CC) primarily used for stick and TIG welding work or Constant Voltage (CV) for MIG/wire feed work.

Typically CC machines have a open circuit voltage of ~ 60 - 90 volts AC or DC and work by keeping the current stable at whatever setting is used by letting the voltage vary between zero at a dead short and the open circuit voltage limit.
Constant voltage machines are just the opposite . They try to maintain a constant output voltage while letting the current draw go anywhere it needs to within the power supply capacity limits of the unit.

For your application I would go with a simple variable voltage power supply and just adjust the voltage up or down to get the heating wire to stay at whatever temperature you want it at for whatever length you are working with which could be done with a simple triac based phase angle control (light dimmer of sorts) or a Variac (adjustable transformer) ahead of your main step down power transformer.

 

Looking on YouTube, I could swear I saw someone made a transformer with a variable core. Yes, you actually have a manually adjustable core to control the current. I'm certain welding machines do NOT use that method, but if I can find that video again I'll post it.

Actually most basic constant current welders use that method by having a set of adjustable magnetic shunts between the primary and secondary winding sets that shunt the amount of magnetic field power the secondary windings get away from them for low current work.

The design concept has been around for over 120 years now.

 

@born2dive00 any reason for not going with the Triac method?
If using the secondary control as I mentioned, you could also use a stepped secondary transformer for larger range if needed.
Max.

 

Hello guy

Ok here is the issue, Because I will be using variable lengths of wire depending on what I am doing, AND different sizes, I need something that is finely adjustable. to get the optimum temps of 600'F

I was considering using one of these http://www.ebay.com/itm/10A-30V-DC-...177148?hash=item3a9acc343c:g:0FsAAOSwhQhY5rhf
it is easily adjustable, but I don't know if by connecting the nichrome wire will cause the short protection to trip.

This unit is cheap in the states, however, where I am living and teaching in Taiwan, these units are being sold in the stores for $250.00
So I was looking at building one of these. it does not need to be pretty, just functional.

Any ideas how these units are built?

 

Like I said, you can built but that thing you showed could be a SMPS.
Can you get a Tx with a secondary rating of 30VAC @ 17Amps ?
That is the size you would need for an out put of 30VDC at 10 Amps continues.

 

Like I said, you can built but that thing you showed could be a SMPS.
Can you get a Tx with a secondary rating of 30VAC @ 17Amps ?
That is the size you would need for an out put of 30VDC at 10 Amps continues.

What is a SMPS?? and what is a Tx, (Tx for me is a transmitter on RC)

 

Like I said, you can built but that thing you showed could be a SMPS.
Can you get a Tx with a secondary rating of 30VAC @ 17Amps ?
That is the size you would need for an out put of 30VDC at 10 Amps continues.

Would this work for the transformer??
https://ae01.alicdn.com/kf/HTB1Qd4f...-Switching-power-supply-Led-Driver-font-b.jpg

or this?
https://www.aliexpress.com/item/Reg...lgo_pvid=e64d0ece-d5c9-4b30-906e-92c854516f43

 

Would this work for the transformer??
https://ae01.alicdn.com/kf/HTB1Qd4f...-Switching-power-supply-Led-Driver-font-b.jpg

or this?
https://www.aliexpress.com/item/Reg...lgo_pvid=e64d0ece-d5c9-4b30-906e-92c854516f43

That is a SMPS and you cannot control current or voltage the way you want

 

A tx is a transformer
A SMPS is a switch mode power supply.
Try Google sometimes.

 

I used the technique to cut glass at home and work. You have to have tension on the wire. A spring works.
It was really weird. I actually cut a large rectangular chunk (5" x 4") out of a corner,

At work we used a variable AC voltage. So a 0-120 V variac + a transformer to get the lower voltage and higher current.

I don't know if you can focus an IR thermometer on a thin wire, but that's a possibility or at least a way to set numbers.

With a lot of different sizes, having a metered DC supply may be your best choice. P = V*I I've seen only a few power supplies that have a power readout.

Electronic loads can do constant power, but they aren;t cheap. DC Modes: CC, CR, CV, CP means constant (current, resistance, voltage, power). e.g. http://www.programmablepower.com/electronic-load/SL/Overview.htm

DC supplies fall into categories of CC, CV, CC/CV and CV/CL The CL is current limit which isn't controlled current.

You can take a constant voltage power supply and add use a pulse width modulator (PWM) to control the average voltage. It's harder to measure.

The question is: How to you make things so you don;t have to fiddle a lot with controls.
1) Metered power supply so you can multiply V*I. Use one as your starting point.
2) Use a handheld IR thermometer to check temperature.

Actual temperature control would take some doing and that may not be enough.

 

Ok here is the issue, Because I will be using variable lengths of wire depending on what I am doing, AND different sizes, I need something that is finely adjustable. to get the optimum temps of 600'

I still think the easiest, cheapest and most practical is a dual or triple secondary mains transformer with a Triac control on the secondary. It is impossible to set a dial exactly for what you are doing as the resistance of the Nichrome varies with temp.
Max.