Tuesday, March 4, 2014

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

V = 8.7amps  * 0.5ohms
V = ~4.4v

V = 12amps * 0.5ohms
V = 6v


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?


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.

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