Please don't try anything yet until I've checked over the spreadsheet.

I think there could be an error that made it work reasonably for a small range of wire sizes and voltages, but is wrong for what you are doing.

The method of calculating the resistance is not matching a lookup I've done on the web.

Hang fire....

Assuming a holding pattern. (I don't have bow built yet anyway).

~psguardian

Great, should have it straightened out by tomorrow. That amperage shouldn't stay the same, so there's an error somewhere in one of the formulas.

Ignore the earlier estimates, we will rework it. On the positive side, it may be possible to run the smaller bow on the PSU, ---- I think.

Originally Posted by vtdiy

Great, should have it straightened out by tomorrow. That amperage shouldn't stay the same, so there's an error somewhere in one of the formulas.

Ignore the earlier estimates, we will rework it. On the positive side, it may be possible to run the smaller bow on the PSU, ---- I think.

I was curious that the amps used always stayed similar. Good that it was found before many put it to use I guess lol. Thanks for ironing out the #s on the back end.

~psguardian

(7/31/2012 update: the latest file version is now attached to the first post. and will be updated there whenever needed)

Okay, spent a lot of time completely rewriting the Hot Wire Calculator, and I think it's looking good. Checked all of the earlier user data against it and the answers all worked.

I also added preliminary data for wire types other than stainless steel -- thanks gmwahl.

There's a new variable for this called "resistivity. Just plug 700 in there for most stainless steel. The spreadsheet should be self explanatory in this area. Regular steel wire looks like about 250.

I'll post it here now, but have more to say about it a little later tonight, plus will work out examples tried before.

So by the latest example a 12v 4.5a constant supply & .035" wire for a large bow?
(I have an old brick with these values, thought it to be too small)

~psguardian

Originally Posted by psguardian

So by the latest example a 12v 4.5a constant supply & .035" wire for a large bow?
(I have an old brick with these values, thought it to be too small)

~psguardian

That would be cutting it pretty fine since the calculated amperage is 4.1.

I would definitely put a 5 amp fuse in line with the bow, and maybe also an ammeter to check and make sure you're under the rated PSU output.

I'd also probably make the bow 50" but add a coiled air resistor of about 20" using the same wire. That would make the total electrical length 70" which would drop the amperage (and heat) a bit. Then with a lead and alligator clip gradually clip onto the coil closer in until I got the cutting temp I wanted (preferably), or my meter was getting close to the PSU limit, and go no further without using a higher rated power supply.

EDIT: maybe that fuse should be a Slo-Blo type. When the wire is cool it might draw more than 5 amps momentarily and blow a fast blow fuse. Or maybe use a 6 amp fuse. But then you're not really protecting the PSU very well. The PSU might also shut down if the initial momentary draw is over its limit.

I agree it would be close, should I start with the tap near the bow or at the far end of the coil?

~psguardian

The far end will reduce the current (and heat) the most.

Seems like I'm getting into wire cutter design and advice, and I hadn't meant to do that in this forum thread, just ask for some data input to work up an experimental, and hopefully helpful wire size calculator for others to use work out their cutter designs.

Personally, I think a fixed regulated power supply is not the best or easiest choice for wire cutters, and isn't what I use myself even though I have several of them lying around here. By using an air resistor, we're trying to make a regulated supply somewhat variable.

A wire cutter should be variable for lots of reasons -- initial testing at lower than maximum power, cutting different sizes or types of foam at different heats, etc.

Wire air resistors have several drawbacks like limited adjustment range, and the fact that they get hot themselves, etc.

Everything is probably do-able, with a fixed regulated computer supply but as with all homemade bow setups, it takes some electrical understanding and design foresight not to produce something that cuts poorly or is even potentially dangerous.

Maybe it would be best to open a new thread for this particular fixed supply design. I'm happy to answer questions here about the wire calculator, and certainly also grateful for user data to improve or correct it.

You might also look for threads including air resistors and PSUs. I remember seeing someone else's setup like this which included photos, here on the RCGroups forums.

I'll try to work out an example calculation with the spreadsheet for your smaller bow today, too.

Sorry about the side track clutter. Maybe have this exchange removed from here & off to the nw thread? I know this isn't the easiest path, but I think it could be useful for those who don't have readily available access to a true variable setup. I am grateful for the assist.

~psguardian

Put an LED dimmer on the output from the PSU. You can then vary the power from 0-8amps

http://www.ebay.co.uk/itm/1x-DC-12V-...item2a1dc13e58

Originally Posted by psguardian

Sorry about the side track clutter. Maybe have this exchange removed from here & off to the nw thread? I know this isn't the easiest path, but I think it could be useful for those who don't have readily available access to a true variable setup. I am grateful for the assist.

~psguardian

No, didn't mean it too seriously psguardian, and it wasn't chatter, so it's fine as is. I just mean that you'll probably get a lot more help and input from others (as well as me) in a thread titled something like "Hot Wire Using a Computer Power Supply?"

Originally Posted by frank48

Put an LED dimmer on the output from the PSU. You can then vary the power from 0-8amps

http://www.ebay.co.uk/itm/1x-DC-12V-...item2a1dc13e58

Now that's kinda cool! Never saw one of those -- just familiar with the AC light dimmers. Seems like it would do the job.

I just got one of theose dimmers, haven't tried it out yet.

Wanted to also say that psguardian wrote a pm reporting that the calculator was accurate after testing wire he had.

Originally Posted by frank48

Put an LED dimmer on the output from the PSU. You can then vary the power from 0-8amps

http://www.ebay.co.uk/itm/1x-DC-12V-...item2a1dc13e58

In the USA:

http://www.ebay.com/itm/1x-DC-12V-24...item231f17c5a0


Joe