power supplies from microwaves

[gv71]

Are the power supplies from microwaves of any use other than making spotwelders?



Greg

[Patrick2by4]

I didn't know you can make a spotwelder with a microwave power supply. Do you have any plans or a website I can turn to?

[Bubba]

gv71
I made the transfromer for my power supply from a micrwave. I cut the 2000 V winding out (very carefully), wrapped the core with some cardboard (a tablet back) to protect the wires and did the same for the outside of the core steel the same way. Then I made a 1 turn temporary winding to see what kind of voltage I would get per turn. Using this and my desired end voltage, I knew how many turns would be needed. I got my magnet wire from MPJA - 14 ga (the biggest they have). Estimated how much wire would be needed and added some to that and started winding. I ended up a few tenths of a volt off but that was no big deal. Stripped the ends of the wires and ran them to a barrier strip to support that part and hooked up my rectifiers and capacitors. Been running the thing for about 2 years now and it is still going strong.

To remove the HV winding, I used a hacksaw to cut off one side of the coil and then supported the back side of the transformer steel and used a hammer and rod to punch the remainder of the coil out. Took about 30 minutes as I was carefull NOT to scar the primary winding. Also left the original filiment alone as this is wound in with the primary. Mine gives about 2.4 volts so it makes it to small to be useful for a 5v supply. Just tied the ends to a blank spot on the barrier strip so it won't short out and let the magic smoke out!

Bubba

[gv71]

patrick,

I got the plans off ebay but there is alot of other places to find it on the web for free. Its very simple and I have some pics of the one I made. I use it to spot weld .020 stainless sheet that I make the combustion chambers for my gas turbines http://www.5bears.com/welder.htm here is a link to one. There Are a lot of extras on this one that you don't need. If you want I could e-mail you some pics of it, and help you with this project if you want.



Bubba,

I don't know much about power supplies. I always wondered how you go from ac in to dc out. I guess I will have to learn this to, to save some coin on the power supply for my cnc router.



Greg

[Bubba]

GV71,
The basic basis:
AC voltage is measured in RMS volts so when you recitfy it, you get DC volts (peak voltage) = sqr rt(2) or 1.4 volts * AC volts. (example 1 VAC will give 1.4 VDC) after recitification. Then you add a capacitor to "smooth out the ripple" to give a "smooth" DC output. The bigger the capacitor, the smoother the DC voltage will be under load. The cap acts as a "small" short term battery to supply the needed capacity.

Therefore for the cnc supplys that we use for "power" (servos and steppers), the supply is NOT regulated and the output voltage may vary under load conditions.

For a regulated supply, you take the above supply and add voltage regulators to maintain voltage to a certain level under load. In other words, you take a higher voltage supply and and then "regulate" the supply to produce a voltage that will not deviate by +/- x Volts under load. (three terminal regulators are popular here!) These you will use for IC circuits etc.

Hope this helps in a simplistic explination.

Bubba

[Patrick2by4]

here is a website that shows the fundamentals of a power supply. It has diagrams which makes it real easy to understand whats happening.

http://www.kpsec.freeuk.com/powersup.htm#transformer

[gv71]

Thanks guys!!!
here is a pic of my spot welder

Greg

[BCwanderer]

Microwave transformers make very simple and cheap power supplies.

All transformers I have stripped produce 1 volt per turn on the secondary.

Keep the primary intact.

Rewind the secondary to the voltage you need.

You can place taps on the secondary or you can wind muntiple coils for power and control circuits.

If you are pulling a lot of current, you can use the fan from the microwave to cool the coil.

Here are some pictures of several successful rewinds.

[gv71]

OK I ready to start on my power supply from a microwave transformer. I'm planning on using rutex servo drives on a 3 axis wood router. I have 2 different types of servos with different voltage needs and encoders. The 2 bigger servos are 30v 20a the smaller one is 24v. I know I have to rewind the secondary what else do I need?


Greg

[Bubba]

Greg,
After you have wound the transformer (and tested it to make sure the magic smoke stays in), to each set of leads (for instance the 30V) you will add a bridge rectifier which is suitably rated. I would use at least a 50V 50 amp rated unit, but bigger is better especially considering the inrush current going to the filter capacitor that goes in line next (OBSERVE THE POLARITY).
You can then add (if desired) a resistor across the cap to bleed the voltage after turning off the incoming power. This would be a fairly high rating (say 10K). As you have 30V and a 10K rated supply, the power rating needs to be [P=EI] or using ohms law [P=E^2/R] or 30^2/10k=.09W; use a .25 watt. This will take quite some time to bleed down, but if you lower the resistance; you can figure the appropriate wattage.

You have indicated that you have two 30V servos, so now I would put from the cap a 20 amp fuse block going to the driver (in my case gecko) and then the leads as necessary to each servo. You do NOT want any fuse after the driver because in case of failure, you would blow the driver and let the magic smoke out!

A note of caution, when winding the transformer (I used regular magnet wire) be sure to protect the insulation from being punctured on the core, those corners are sharp. Wrap the core with some form of insulating/padding material and tape it on. On my original, I used the backing from a tablet and on the one I am doing this week, it will be a "glass" tape made for the electrical industry. When finished with a winding, it will be painted with an insulting spray paint. (both of which I got from MSC).

Hope this helps.

Bubba

[gmfoster]

I built a 20 volt or so tramsfprmer from a microwave transformer. I didn't use it very long because it was so inefficient. They usually weld the laminations togethere which is great at reducing noise but kills the efficency. In my case the transformer got to hot to olace your hand on with no load.

They are great for non-continous use and are basically free.

As some one else mentioned you can plan on 1 volt RMS per turn..

They are a great resource but I didn't need that much current and went with oower supply from an old HP think jet printer.

The nice thing about starting with one of these is you can get the voltage you want. Just add or remove a couple of turns..


Garry

[joecnc2006]

Has anyone used one with the Hobby CNC Board? I plan on doing #14g insulated copper wire, wind up to 24v and with his rectifier, and cap i should yield about 36vdc. Using a general 1.4 x factor.

And also a smaller wind for 12v to fan, I should be able to just wire directly

Does this sound right?

Thanks, Joe