Meters, Power Supply and Wire

Just a back'n'forth with a user via email. I'm in orange. This is a really common series of questions I get from people making their own cutters and power supplies. Hope it helps someone out there.

Could I wire in a digital AC voltmeter to output connectors, where cutter plugs into, to see what voltage I'm adjusted to with dimmer?

Yea, you can do that.

If memory serves me correctly… A voltmeter assesses the voltage differential across two points (positive & negative terminals). Therefore essentially being placed in parallel with your circuit. Where as an ammeter assess the flow of electricity through the circuit, needing to be placed in series with your circuit. 

The voltmeter should come with instructions for installation, as it might not work if you switch the leads (negative to positive, instead of positive to negative). 

I made the same power supply as you with the XFR-1006E.

Okay, I think you mentioned that previously and I forgot :) 

I purchased the 16 gage wire from Jacobs. Didn't see then u recommended the 20 for rigid cutter like I made. I created one with an adjustable are to use as a straight cutter or rigid, depending on wire. It can adjust from 2 inches up to like 20 inches. Have u used the 16 gage wire with ur same custom power supply? I don't wanna hurt my supply if I don't use right wire to voltage ratio.

Remind me, did you set up your power supply so that you can interchange between 12v/8.3a and 6v/16.7a? If so, then you shouldn't have a problem using the XFR1006e with 16ga.

This diagram shows the acceptable wire gauges and length with the XFR1006e. Just a REALLY rough gauge.. Using the 6 volt setting, and 16 gauge wire, you need to range between about 5" - 25" of workable wire. 

I looked at Jacobs calculator and it's confusing. Depending what I solve for, it's different I seems. I set temp to 600, gage to 16, choose button to solve for volts and adjusted length for wire. What do you think the min and max of wire I can safely use without burning up transformer. I'm using the same custom power supply as shown on ur site using the XFR-1006E transformer.

Check out the Electrical Power page. I have a tutorial video on how to use Jacob's calculator. 

Here's what I would do

Set the button to solve for "Temperature"

Move the slide to your wire length, let's say 5"

Set the Gauge to 16

Now here's what you're going to look at.

As you slide the voltage to the right, you should see the temperature, amperage and wattage change. You'll want to pay attention to the amperage and temperature.

You know that your power supply when set to the 6v setting, can max out at 16.7a. Typically I recommend not exceeding 90% of your power supply's total. But let's just say 16amps. 

Moving the voltage slide, you'll see you can not exceed 1.7-1.8 volts before over drawing your power supply. 

Next, move the "length" slide to the right, lets choose 10". Slide the voltage, and now you can increase to about 3.4-3.5v before over powering your power supply (~16 amps)

Since you're using a dimmer switch, 100% on is 6 volts.. so 3 volts is half way on… 

Does all that make sense?  It was a long winded explanation that you can certainly use 16g a with that power supply, it just has to be on the 6v setting. You'll also have to make sure you use AT LEAST 5" of wire, but 10" is probably easier to adjust your power supply with.

I haven't got the resistance reading yet. Been trying to think how to calc it. If I knew ohm of circuit, how would I manually calc for max voltage if I don't know any other variables?

If you attach a multimeter and give me the resistance reading, in addition to what gauge nichrome you used, I'll send you a range of working voltages. 

Ohms Law states

Voltage = Amperage * Resistance

Amperage is essentially an equivalent of Temperature when using NiChrome wire. 

Select a temperature, find it's corresponding amperage, and multiply it times your circuit resistance.

Here is a PDF found on the website with relevant NiChrome Data

Ok, but I still want to learn how to calculate 

Using 16G = 8.7amps @ 600 deg

Do I take my length of cutter and divide it by 12 and then multiply that by my meter reading to get R?

Then multiply that R by 8.7 amps to get max V?

Short answer, yes!

Long answer...

Your meter reading IS the circuit resistance. You don't have to worry about measuring and dividing anything. Any math related to resistance calculation is only ever theoretical resistances. But if you have a multimeter, set to read resistance (ohms) then there you have it. Total circuit resistance.

Take that reading (my guess is it will be less than 1 ohm) and multiply by 8.7 to yield your power supply voltage setting to reach 8.7amps. Using the 6v power supply setting, your MAX output amps using the XFR1006E is ~16a. So if you want to find your MAX volts, just take the resistance reading and multiply by 16. 

In my experience, your calculated voltages/amperages/resistances are always a little different than what is seen in person. So just be prepared for about a 10% difference. Not that it will matter for cutting, but just incase you get really specific and want your readings to match your calculations. 

Used 15 inches of 16Gage and measured 0.50 ohms

Voltage = Amperage * Resistance

600°F

V = 8.7amps  * 0.5ohms

V = ~4.4v

800°F

V = 12amps * 0.5ohms

V = 6v

So…

If you used the calculator and only ever looked at theoretical values… 15" of 16 gauge nichrome is only 0.32 ohms. But with all the other metal components, you picked up a little resistance and totaled out at 0.5 ohms. At this point, the only thing that matters is the 0.5 ohms, and no longer the theoretical 0.32 ohms of your 15" NiChrome wire. 

From here on out, using that exact set up and knowing that you have 0.5 ohms… whatever amperage you want to achieve, just half the voltage. Want 2 amps? Set 1 volt. Want 8 amps? Set 4 volts. So on and so forth.

It all changes if I use shorter or longer wire though, right?

yup

whatever your measured resistance is (based on length of wire) you can plug it into ohms law.

Volts = Amps * Resistance

Amps = Volts / Resistance

Both equations work for whatever you may be doing…

Ok. Think I can do it now. Only other question was I see with the chart is the 600 deg with 26 gage was 8.7amps. The 800 deg shows 10.9 amps. How did u get 12amps for the 800 deg you sent me?

This is that fudge factor I was talking about. You're starting to get too literal between theoretical, calculated and actual temperatures. 

Technically 12 amps, with 16 gauge wire, is theoretically 880°F. That temperature is provided by the jacobs calculator. 

But again, in my experience, that which is calculated is different than what is seen on your bench.  Not to mention, as the foam sucks heat away from your wire, the wire will cool and often slow cutting. So it's a fine balance between wire heat, wire transfer, wire recovery (getting hot) and foam cutting speed. 

The calculations are merely so you select a power supply strong enough to supply your circuit. Then get everything on your bench, and turn up the dial until it cuts foam.

I see. Are ammeters accurate? Could you attatch it over one wire to make sure you never overdraw Amps that could harm transformer? Do you test it before / input or after / output on transformer?

Yea you should be able to add in an Ammeter. I think the AC ammeters are expensive though. 

You would want to attach it to one of your "secondary wires" which are the after/output side of the transformer. You can make it easy though, and simply attach the ammeter to the terminals where your foam cutters plug into the power supply. That way you always get an accurate reading, regardless of which power supply circuit you have activated.

Large Multicutter Design

Another fairly common question, large multi cutter design

I'm planning to build a prototype for a large foam block multi-wire slicer.

Scenario:

1. I will need 5-8 wires

2. The length of the NiChr are going to be 8-9 feet in length

3. I will be cutting different densities of foam, 1lb to 3lb

4. I will be using 220V power

What do yo recommend in this situation?

With a cutter that large, there are going to be a few difficulties.

1. Power Supply

2. Wire Tension

1. Power Supply

The size and power of your power supply will solely depend upon the wire you choose and the overall resistance of your circuit. Over those lengths, I'm not sure that NiChrome wire is going to be your best option. You may want to consider stainless steel wire.

The biggest factor with resistance and necessary amperage to heat your wire, is how you lay out your design. If you go with stainless steel wire, chances are you're going to want to set up your wires in circuit series, rather than parallel. But if you use NiChrome wire, you'll probably want to use a combination of series and parallel to achieve the best combination of voltage and amperage requirements. 

This is discussed in further detail on the website, and should help explain that recommendation a bit more.

http://hotwirefoamcutterinfo.com/_MultiWireCalc.html

Don't even both trying to find a DC power supply. For something like this an AC Step Down Transformer is going to be your cheapest option for overall output power. If you're using 220v power, I'd assume you're not in the US? If so, I do not have a ton of international options for power supplies. But feel free to scan through my power supply section for some US suppliers that can ship to you.

http://hotwirefoamcutterinfo.com/___BenchTop.html

2. Wire Tension

For lengths over 24" I typically recommend using 24 gauge NiChrome wire. But if you're looking at 8-9 feet, you're going to need something quite strong that will respond to both the pressure of cutting as well as the linear applied wire tension. I still think stainless steel wire is going to be your best option, but I do not have an idea of wire size. Good news is stainless steel wire is cheap, so buy a few rolls and see what gauge wire works best for your design. Keeping in mind that wire gauge and length have a bearing on circuit resistance, and require amperage to heat. You're going to want to find the smallest gauge wire, that can hold up to cutting pressure. Probably 24-18 gauge.

Next, I'd recommend designing your cutter so that at one end you have a rigid extension spring and at the other end some type of winding device. Even if that winding device is just an eye bolt you turn to increase wire tension. Keep in mind that as a wire is heated it will expand, and with expansion comes a saggy wire. If you're cutting a grid out of large foam billets, the last thing you want is wavy lines. I suppose it all depends on your application, but you're going to want wire tension. I have actually seen a design were someone used a FULLY threaded pipe, from end to end. And all they had to do was twist the entire pipe, and it increased resistance on all the wires. It was basically a huge, large diameter machine screw at that point. Simplistic design, just requires you to know someone who can thread pipe. 

I’m thinking of using the Transformer from MastechPowerSupply.

Volteq 5KW Transformer Variac 5000VA 0-250V 220V Input, with maximum current output of 20amps (@250V)

or do think I can get away with….

Volteq 2KW Transformer Variac 2000VA 0-250V 220V Input, with maximum current output of 8amps (@250V)

Not just because I'm American.. but I'd have to say "bigger is better." When it comes to non hobby large scale cutters, especially unique set ups, you do NOT want to be 100% ready to go and have an inadequate power supply.  With the 5kW unit, you have the same voltage regulation on the dial but have 2.5 times the power. Using the 5kW model, you'll have the best power supply, just in case you need to wire in series on the fly...

Just remember… Greater than 45 dry volts will penetrate human skin. If you're moving 5kW across something like that… there is a pretty high risk of shock. Just be incredibly careful. 

I’m also thinking of having them wired is Parallel setup, just so I can hook them up on two shafts, and have adjustability on the thickness of the slices, just by moving the Springs up and down along the shafts.

I love the idea, and would love to see it in action. Send some pictures when you get it set up. 

Just remember your electrical properties when placing a circuit in parallel. An easy way to think of it is like this….

Placing four wire in series, is like making one wire four times as long

Placing four wires in parallel is like making one wire four times as thick

When you have a long wire or wires placed in series, the resistance dramatically increases, the voltage required to heat dramatically increases, but the amperage pretty much stays the same. 

When you have a larger gauge wire or have wires placed in parallel, the resistance dramatically decreases, the required voltage decreases but the necessary amperage to heat the wires dramatically increases. 

I hope that wasn't confusing. 

Long story short… depending on how this pans out, you may end up needing a combination of series and parallel to find the right combination of circuit resistance, voltage and amperage. You may even consider having switches to "activate" a cutting line. You'll want something rated at 20amp max, which is usually a house circuit switch.

The majority of the time, I’ll be using one wire, but I want to make sure that I can do 5-7 wires without any issues.

With the Transformers I have listed above, what do you think there limits are as far as the number of 8-foot wires they can handle?  We’ve been using two sizes of wire, 0.010” and 0.014”, in 8-foot lengths for many of our cuts, usually lettering and shapes, so we need smaller wire gauge.  Do you think that Stainless Steel is better to use that NiChrome?… with the wire gauges we are using, 30 AWG and 27 AWG, which one would be best for heat and length? Right now we have it automated to cut at around 45in/min on 1 lb density foam.

Well… There are a few ways to answer. 

For lengths greater than 2-3' I typically tell people to shy away from NiChrome. Its a good wire, don't get me wrong. But often the resistance is too high at those lengths. Where as, for a small cutter <2' I exclusively recommend NiCHrome wire and NOT stainless steel because NiCr's resistance is higher and makes power supply selection easier. So both are good options, it just depends on your application. 

BUT… 

Jacobs-online.biz JUST started to carry 27 gauge Rene wire…. I think you might like this… Rene wire is considered a super alloy, and virtually indestructible. It is an "up and comer" in the foam cutting world, and a lot of guys use it to cut large (8') foam wings. You might like it a lot, I know Gary Jacobs really likes it. Personally, I am looking into using it for my combination foam cutter as a good straight wire cutter. 

27 gauge Rene wire has similar electrical properties to 28 gauge NiChrome wire, so you can use that reference for calculations. As you would imagine, high wire resistance will be counteracted when placed in parallel. 

Just for kicks, lets see what we can do with a little math. 

A single 8' length of rene (34ohms) would take about 150 volts to heat to 1800° and only draw about 4.5 amps from your power supply.  If you put six 8' lengths of rene wire in parallel (5.667 ohms), my guess would be it's similar to 20 gauge based on length and resistance. Here's the difference… you'd  require about 80 volts to heat to 1800° but drawing close to 15 amps from the power supply.