Jaymom

http://nsrca.org/technical/tip_trick...wer_supply.htm

If you have built this foam cutter power supply would you please help me out. I just finished mine, havn't built the bow yet. Could someone tell me what readings I should be getting with switch in "long" and "short" with no bow attached ? Any help will be much appreciated. Thanks

Jaymom

bullseye000

The long short switch is for the resistance differential in the long vs short bow size. The voltage reading will most likely be the same or close to the same. There will be difference in current readings. You also may not see this until the circuit is placed under load. By the way the circuit indicates nichrome wire for the bow...forget this and get some stainless steel wire.

Jaymom

Thanks Bullseye000, I'm asking because I have no readings in DC. I do have readings of 27.5 in AC. Not too sure of what is going on right now.

BMatthews

What sort? Safety wire?

I tried music wire one time but it always had a "hot spot" and would burn out within a few minutes. Went with some of the pre-packaged nichrome from the LBS and it's still working the last time I cut cores. Must have cut about 2 dozen for control line combat models.

If it's just stainless safety wire you can get a one lb spool at racing supply outlets that cater to actual racing car and bike builders. Or some of the commercial machine shop supply outlets may have it. And I "suppose" homebuilt aircraft places would have it too... Handy for lots of other stuff. And a good thing too. A one lb spool is a LOT of wire. At least 2 lifetime's worth.

raiki

A quick look at the circuit shows that this is a variable voltage unregulated AC power supply. You will not measure any DC.

With no wire connected, and because of that no current flow, you will measure the same voltage with the switch in both positions.

No load 27.5V AC sounds spot on to me.

(PS I am an electrical engineer and and electrician)

Jaymom

Raiki.....Many thanks for the info at least now I feel a lot better as I gather more parts and get ready to start building my bow. Planning on building one 44 inches wide. Christmas is too close to start on it right now, too many other things to be done. Appreciate it Raiki, have a nice and safe Christmas Season.

Jaymom

Jaymom

Hello out there ! Anyone else out there that has built this power supply ? Would love to hear from you if you have. Any comments on how it worked out for you ? Have you improved it in any way ? Thanks

Ed Smith

I guess I have spent too much time in my basement. I have been using the same Nichrome wire bow for twenty years now. I purchased the wire from RC modeller for $2.00!!

My questions are, what is the advantage of SS wire, what size is used and where does one get it?

Ed S

G1K

I have buit this supply also, but I have not used it yet. I am nearing completion of a CNC foam cutter, which should be done in ~2 weeks. Once everything is together, I'll let you know how it works..


Ryan

Jaymom

G1K... Have you checked the output voltages from your unit yet? I'm talking about no load, no bow attached readings. I'm real anxious to find out what other people's readings are !

Scott Smith

My Feedback: (4)

I just cut my first set of cores with this power supply and was very happy with the results. I used .020 SS safety wire from Aircraftspruce.com ($9.00 for a life time supply.)

This is the table/bow I made:


BregmanCutter


3 or 4 practice cuts will reveal the correct setting for the rheostat.

Rudeboy

Take a look at this: http://www.electronic-circuits-diagr...robotck1.shtml
It is a simple controller circuit to control unipolar stepper motors.
Would unipolar stepper motors move in small enough step to drive a CNC cutter?
And also, in the article they are talking about using a 555 multi vibrator to control stepper motor speed. Do you really need speed control in an application like this?

This is just a hypothetical question as I am not planning on building my own "CNC foam cutting factory" in the near future. I was just wondering...

AV8TOR

My Feedback: (2)

I had a friend build me one of these and I am very happy with it. As for any mods we talked about putting a small computer fan inside the case, as it does get warm. For the money you simply can’t beat it.
Does anyone know where to get the soft wires like feathercut uses? That would be another change I would like to do especially if you use gravity cuts.

Jaymom

AV8TOR ... I'll ask you the same question, can't seem to get an answer from anyone.....Have you checked the output voltages from your unit yet? I'm talking about no load, no bow attached readings.

Scott Smith

My Feedback: (4)

Short Bow - 6.4 to 27.0 VAC (no load)
Long Bow - 6.5 to 27.3 VAC (no load)

Jaymom

Thank you Scott, looks like Im doing something right with mine because I'm getting the same readings. I have yet to put together a bow, thats the next step for me. I just couldn't believe the readings that I was getting. Thanks again.

AV8TOR

My Feedback: (2)

No I have not as it did not matter to me because it worked precisely as intended. If you like I will check them later.

Steve Lewin

With no load connected there is no current through the 5ohm resistor so no voltage is dropped by it. Therefore the voltages are the same. It will be different with a load connected.

Steve

Jaymom

Thanks AV8TOR, Steve.... I was expecting a DC reading and was surprised to see AC . Thought that I had assembled it improperly or goofed up the PC board . I did try a 6 ft. piece of wire across the output terminals and it soon blew a fuse.
I have a new 120V extention cord laying around and am wondering if I can use that for the lead wires to the bow. Will that work out ok? I live in an appartment and its Christmas time so I can't experiment too much yet. Thank you for the help guys.

G1K

Jaymom,
I'll check what I am getting tonight.

Ryan

AV8TOR

My Feedback: (2)

You can use standard power cord for powering your bow. If you do gravity cutting though it is to stiff of a wire and will influence the cut. I also have a feather cut and their power supply leads are very soft and do not affect the dangle of the bow during gravity cuts. I would love to find another source for those, as Tekoa wants too much money for them.

Jaymom

Thanks guys....Well this will give me something else to look for as I visit the stores. There's got to be something in one of the hardware type stores that will fit the bill. 6 ft. long "test leads" would sure be nice would't they !

onewasp

Jaymom,

Have built and used this power supply for the last year. My readings are about the same as those listed. My bows use Sig's nichrome wire and all is right with the world . The power supply has all kinds of adjustment so just use a scrap piece of foam and nail the proper setting for your foam and desired bow movement.

I can't think of anything wrong with the design. It very simply works just as they indicate in the plans.

Jaymom

Thank you onewasp appreciate the words of encouragement. I was afraid that I had wasted my time and money on this power supply. But with words like yours I feel a lot better about it. Its going to be a couple of weeks before I will be able to try it out due to the holiday season but when I do I'll sure let everyone know how it went. Thanks again.

fprintf

Wow, the parts for this power supply sure add up quickly! $40 later and I am back at home with most of the parts. Unfortunately RS was missing a couple of the 10k 10 watt resistors. Hopefully this power supply will come together easily and make my $50 worth it -- I suppose for this price I could have done the Battery charger + dimmer switch route for about the same, and still gotten a battery charger out of the deal.

Oh well, at least it'll be fun putting it together!