Thread Tools
Jan 22, 2007, 04:51 PM
S55
S55
Registered User
S55's Avatar
True, but if you have those amps available ...
S55
Sign up now
to remove ads between posts
Jan 22, 2007, 05:23 PM
Dismembered Member...
arx_n_sparx's Avatar
Quote:
Originally Posted by drcrash
I'm thinking that if the wire is mostly vaporizing the foam without touching it, this should be mostly independent of wire diameter---the kerf is more dependent on the watts dissipated than how the wire generates them, and essentially all of the heat is going into the plastic. (That probably won't be true with very big wires cutting on contact in a "hot knife"-typesetup.)
This only happens with a CNC cutter - unless you have very steady hands You can pretty much count on the wire "touching" the foam.
Quote:
Originally Posted by drcrash
Does this reduce the max amps at the higher voltages? If not, where do the watts come from? (Maybe making the transformer do more work between peaks, without overloading it?)
Paul - shoot me a PM with your email addy on it - you'll get a reply from "igor" I have some pix to attach on AC vs DC and what's happening. I dug out some old text books last night....

Brad
Jan 23, 2007, 09:26 AM
Impact Testing Specialist
drcrash's Avatar
BobK,

I've been looking at Ian Purdie's tutorial page on power supply design, and if I understand it correctly, it says your capacitor's voltage rating should be at least your output voltage times 1.414. For a 33-volt supply, that would be 46 volts. So if you run your 24AC-to-33VDC supply close to full-on, you risk frying your 35V capacitors.

Also, according to Purdie:

1. the bridge rectifier "should have a PIV rating of 2.828 times the Vsec", which appears to mean that for a 24V transformer, should be rated for 24 * 2.828 = 67.9 volts or better.

2. The rectifier should be rated for twice the load current maximum, so for a five amp system, 10 amps.

(I got a $2 50V 4A rectifier yesterday for my 24 V 4-amp system, which I guess I'll be taking back; I appear to need something rated around 70 volts and 8 amps.)

Here's the basic power supply page:

http://www.electronics-tutorials.com...wer-supply.htm

It has links to more advanced tutorials, but I haven't looked at them yet.
Jan 23, 2007, 10:12 AM
Impact Testing Specialist
drcrash's Avatar
Quote:
Originally Posted by arx_n_sparx
TANSTAAFL = there ain't no such thing as a free lunch.
It is an article of faith with me that there is, very rarely, a free lunch. Or at least a nice snack.

Quote:
Class 2 transformers (and doorbell/train tx's fall into this category) have a lot of winds on the secondary. This basically means that they are "safe" in a short circuit condition - the current is limited. Class 2 txfmrs are the only Tx's that don't need a fuse on the primary or secondary side (electrical code) because of this. They will deliver the current - as long as you don't pull them down too hard - they aren't much good for hot wire cutting though.
I always wondered what "Class 2" meant. Thanks.

If I wire two (same make and model) Class 2's in parallel, it will work fine, won't it?

(It'll no longer be Class 2, because it'll have enough amps to catch things on fire if you short across it with something just slightly resistive, but it will be fine for hot wire cutting, right?)

Paul
Jan 23, 2007, 12:38 PM
arx n sparx
I built one of Tom Weedon's power supply's last year and a few months ago I was using it, shut it off and later that day turned it back on & nothing. Found the Triac burnt out & replaced it with the heavier one on Tom's page. Now I only get a few volts if the 5K
pot is centered. Yes I put the Triac in the rite way ( had to turn it around ) I guess the center pin is the same as I only know the one thats marked gate. It worked fine before only thing was I had to put a 2 amp fuse instead of the 1 amp. I'm wondering if I need to change the 470 ohm gate resistor cause the replacement triad is heavier duty. Any idea's ? And what about me using my computer supply on the 5 volt post ? Can I use a fan control to adjust. I need to cut some wings. HELP PLEASE

Ken

BTW Tom's plans are found [ Foam cutting power supply ]
on MSN search.
Last edited by nubee 1; Jan 23, 2007 at 12:44 PM.
Jan 23, 2007, 05:08 PM
Registered User
L0keman's Avatar
A lot of this stuff sounds dangerous if you're not careful. I would just drop a few bucks on a pre-built.

http://www.dickblick.com/zz608/01/

I know I saw a few other models that were bigger too a year or so ago when I was looking into getting one.
Jan 23, 2007, 05:24 PM
Dismembered Member...
arx_n_sparx's Avatar
nubee: This is the problem when you start swapping in replacement parts. I had probs with mine when I tried an NTE triac, so I ended up getting the right one from Mouser. If you already have the transformer, I'd suggest just getting the dimmer from Lowes or HD and bypassing the rest of that circuit. I'm not terribly inpressed with it.

Paul/doc: You need a PIV (peak inverse voltage) of 34V and an If of 4 amps - keep the bridge you have. That voltage would apply to an electrolytic capacitor as well - I'd be tempted to go up to a 50V one, as you're very close to 35V there. I dunno about getting the juice out of a pair of class 2 txfmrs - why not just buy the right one in the first place?
Jan 23, 2007, 05:44 PM
I'd really like something in the box I have. I added a computer fan
and an amp meter to the Tom Weedon unit. I might try ordering a Triac from Mouser and giving it another try. I wish I had some voltages at some points to check. Like at the gate #1.

What about the computer power supply ? Can I tap off the 5 volt and go with it or do I still need to adjust and can I vari the voltage
going to it ? Thanks

Ken
Jan 23, 2007, 06:03 PM
Dismembered Member...
arx_n_sparx's Avatar
You can't vary the ooutput of a 'puter PS. Don't even think of trying to dim the input - you'll fry things for sure (it's a switchmode supply). You can try using the 12V or 5V rails - you may need to experiment with alligator clips to get the wire length right for the voltage.

Brad
Jan 23, 2007, 07:12 PM
OK I have been looking at the circuit and I beleive I can make it work using a differant Triac. I bought the NTE 56008 that was on the top of the sheet and Tom has added a note stateing the
NTE 56008 won't work as you stated you had problems also. I'm going to fight this dog a bit more and if it don't work maybe someone can give me some plans that I can use some of the stuff I have. The computer supply is for a CNC cutter I have the boards together and most the parts gathered. Happy Landings

Ken
Jan 23, 2007, 07:40 PM
Impact Testing Specialist
drcrash's Avatar
Quote:
Originally Posted by arx_n_sparx
Paul/doc: You need a PIV (peak inverse voltage) of 34V and an If of 4 amps - keep the bridge you have.
Purdie says you need a PIV rating of 2.828 times the Vsec, and I've been scratching my head trying to figure out why. I'm not even sure I know what PIV really means.

At first I just thought it was the peak voltage in the direction pushing back through the diodes, i.e., the backward voltage at its max.

Now I'm wondering if it doesn't mean that at all, but the maximum instantaneous voltage difference across the diodes.

If you have capacitors on the output (bow) side, that's going to be very different than if you don't. When you have a light load relative to your capacitance, and the capacitors stay pretty fully charged, they'll be continuously exposing the output sides of the diodes to a pretty high voltage. Meanwhile, on the transformer side of the rectifier, the AC voltage will be swinging all the way down to the negative peak value, so the difference across the rectifier will be about 2x the peak voltage of the AC, which is 1.414 times the nominal (RMS) voltage.

Basically, the rectifier is caught between something that's pretty much latched at the highest voltage and something that swings all the way to the lowest, so it's a positive-peak-to-negative-peak voltage situation. That's the voltage differential it has to withstand for a few milliseconds 60 times a second.

Then again, I may be wrong in interpreting PIV that way, and trying to justify Purdie's statement.

Paul
Jan 23, 2007, 08:20 PM
Dismembered Member...
arx_n_sparx's Avatar
Quote:
Originally Posted by drcrash
Purdie says you need a PIV rating of 2.828 times the Vsec, and I've been scratching my head trying to figure out why. I'm not even sure I know what PIV really means.

At first I just thought it was the peak voltage in the direction pushing back through the diodes, i.e., the backward voltage at its max.

Now I'm wondering if it doesn't mean that at all, but the maximum instantaneous voltage difference across the diodes.

If you have capacitors on the output (bow) side, that's going to be very different than if you don't. When you have a light load relative to your capacitance, and the capacitors stay pretty fully charged, they'll be continuously exposing the output sides of the diodes to a pretty high voltage. Meanwhile, on the transformer side of the rectifier, the AC voltage will be swinging all the way down to the negative peak value, so the difference across the rectifier will be about 2x the peak voltage of the AC, which is 1.414 times the nominal (RMS) voltage.

Basically, the rectifier is caught between something that's pretty much latched at the highest voltage and something that swings all the way to the lowest, so it's a positive-peak-to-negative-peak voltage situation. That's the voltage differential it has to withstand for a few milliseconds 60 times a second.

Then again, I may be wrong in interpreting PIV that way, and trying to justify Purdie's statement.

Paul

Look at basic rectifier cirtcuits and filter circuits here: http://www.play-hookey.com/ac_theory/ps_rectifiers.html

The 1/sin45*2 (2.828) applies if you use a 2 diode full wave rectifier. It doesn't if you use a full wave bridge (4 diode) rectifier. In the bridge, none of the diodes are seeing the full peak to peak potential.

PIV = peak inverse voltage - exactly what you thought it meant.
Jan 23, 2007, 09:10 PM
Impact Testing Specialist
drcrash's Avatar
Quote:
Originally Posted by arx_n_sparx
Look at basic rectifier cirtcuits and filter circuits here: http://www.play-hookey.com/ac_theory/ps_rectifiers.html
Ok, I did. I'm still thinking Purdie's right.

Quote:
The 1/sin45*2 (2.828) applies if you use a 2 diode full wave rectifier. It doesn't if you use a full wave bridge (4 diode) rectifier. In the bridge, none of the diodes are seeing the full peak to peak potential.
That would be reassuring if we were looking at ratings on the individual diodes, but we're looking at a rating on the whole bridge. Presumably it's telling us the maxiumum voltage we can have across the bridge. It wouldn't make sense to give us the rating of the individual diodes.

Quote:
PIV = peak inverse voltage - exactly what you thought it meant.
I know it means Peak Inverse Voltage, and assume that's at peaks in the AC cycle, but it's still ambiguous.

At first I thought it just meant the worst voltage against the rectifier in the reverse-flow direction. Implicitly I assumed that was an absolute voltage, relative to zero volts.

Now I'm thinking it's the voltage difference across the whole rectifier, not relative to 0 volts or neutral or something, but to whatever's going on on the other side of the rectifier at the worst point in the AC cycle. The rectifier PIV rating is like a DC voltage rating, and you have to figure out all the AC stuff yourself.

(That would make sense for any application of the device, including things where you don't have a normal sinusoidal waveform, but maybe an intermittent stream of pulses or something.)

In the case where you do have a sinusoidal waveform, and have a fully charged capacitor on one side and the maximum opposite voltage on the other, that low-level across-the-whole-part rating needs to be more than 1.414 RMS. It needs to be almost twice that.
Jan 23, 2007, 09:26 PM
Dismembered Member...
arx_n_sparx's Avatar
Quote:
Originally Posted by drcrash
That would be reassuring if we were looking at ratings on the individual diodes, but we're looking at a rating on the whole bridge. Presumably it's telling us the maxiumum voltage we can have across the bridge. It wouldn't make sense to give us the rating of the individual diodes.
Same difference
Quote:
Originally Posted by drcrash
I know it means Peak Inverse Voltage, and assume that's at peaks in the AC cycle, but it's still ambiguous.

At first I thought it just meant the worst voltage against the rectifier in the reverse-flow direction. Implicitly I assumed that was an absolute voltage, relative to zero volts.
It is
Quote:
Originally Posted by drcrash
Now I'm thinking it's the voltage difference across the whole rectifier, not relative to 0 volts or neutral or something, but to whatever's going on on the other side of the rectifier at the worst point in the AC cycle. The rectifier PIV rating is like a DC voltage rating, and you have to figure out all the AC stuff yourself.

(That would make sense for any application of the device, including things where you don't have a normal sinusoidal waveform, but maybe an intermittent stream of pulses or something.)

In the case where you do have a sinusoidal waveform, and have a fully charged capacitor on one side and the maximum opposite voltage on the other, that low-level across-the-whole-part rating needs to be more than 1.414 RMS. It needs to be almost twice that.
No. It *is* referenced to 0 because the other diode(s) is conducting in a bridge. The PIV on your bridge is 50 volts - that means 50/1.414 = 35.6VAC max. It's fine for your transformer - unless you have some higher voltage stuff you want to run. It's fine even with the filter cap. Purdie is referring to a 2 diode full wave rectifier, not a bridge with that 2.828.

Brad

Brad
Jan 23, 2007, 09:51 PM
Impact Testing Specialist
drcrash's Avatar
Quote:
Originally Posted by arx_n_sparx
Same difference
Purdie is referring to a 2 diode full wave rectifier, not a bridge with that 2.828.
Brad
No, he's not. He says
Quote:
The ac from the transformer secondary is rectified by a bridge rectifier D1 to D4 which may also be a block rectifier such as WO4 or even four individual diodes such as 1N4004 types. (see later re rectifier ratings).
And the picture shows a 4-diode bridge, with no center tap connection.


Quick Reply
Message:

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Digital Hot Wire Foam Cutter - Build Guide & Pics epp540 Foamies (Scratchbuilt) 45 Feb 14, 2017 06:16 PM
Discussion Got a Hot Wire Foam Cutter, Or What Do I Do Now? Murocflyer Foamies (Scratchbuilt) 23 Dec 28, 2006 06:54 AM
HELP: Need info on building hot wire foam cutter Chic The Builders Workshop 4 Jan 11, 2003 12:13 PM
hot-wire foam cutter with Astro 110d charger? esox Foamies (Kits) 5 Apr 25, 2002 03:28 PM
Please help: How do you hot wire styro reznikvova Foamies (Kits) 7 Mar 01, 2002 07:17 PM