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Zouave Mon 30 January 2012 20:07

Going insane... Need some help. Power supply questions
Okay, so I'm not as good with electronics as I would like to be, I'm trying to learn, but it isn't nearly as natural to me as I would like just yet. So, I went with the Motionking 34HS9801 motors, which are 4A motors, with 4.1mH inductance. The motors are rated at 4A, so for 4 motors, we're looking at (2/3 * (4*4)=10.7A) that needs to go into the drivers, since the Geckos only need 2/3rds of the Amperage to output the full amount.
And here is my first problem... Most of the transformers I'm looking at only output 8ish Amps. Does wiring my motors half-coil drop the A required down to 2.0? And then giving me a required A of only 5.33?

And while I'm muddling my way through that, I keep seeing the Toroidal Transformers either running off 110, or a pair of 110s(???) or the two legs of 220 wired in series. Am I going to be able to get what I need out of 110? It seems like all those transformers don't put out quite enough to reach my needs, while the 220 ones do. Am I missing something really obvious about this that I should be picking up on? I do have a 220 outlet immediately adjacent to the Mechmate, so I'm happy to go with the 220 option if necessary. But the 110 is much more flexible. Can anyone clarify the difference for me a little?

And, to confuse me further, looking at the specs for the transformers on Plitron's site, it looks like there are two leads coming out. Do I combine those two leads? The one I'm looking at looks like this:
500VA, Output V: 45 45 @ 5.56A

Which works if wiring the motors half-coil cuts the Amperage in half.

I really need to read the rest of that book I got on electronics....

So, if I'm thinking this through correctly... When the rectified 45V goes through the capacitor, it'll 'magically' go up *1.4, up to 63V DC, from the 45 AC coming out of the transformer. And given my communication with Geckodrives (recommended 60-65V), this seems like the correct transformer? But I feel like I'm really missing something. I have been reading through as many of the pages as I can, and will be going through CNC Zone's pages tomorrow when I'm not on dialup and can load them reasonably fast, but if anyone can clarify some of my questions, I would very much appreciate it.

bradm Mon 30 January 2012 20:47

Eric, you're doing great.

So first, you are correct that if you attempt absolute maximum acceleration on all four motors at the same time, you might need that hypothetical 10.7A. But you won't, at least not for more than a fraction of a second, and the capacitors will handle that for you. There is lots of reported success with 300VA supplies (and larger), and no reported failure with 300VA supplies, so you are well above what you'll need.

Most toroidal transformers these days have two primary windings, and two secondary windings. If you are going to drive the transformer with 110, you wire the two primaries in parallel, so you draw double the amperage. If you are going to drive it with 220, you wire the two primaries in series, and each one sees 110v. You end up with the same VA; as a contrived example, 3A * 110V + 3A * 110V = 3A * 220V. You do need to be careful to align the polarity of the paralleled primaries correctly. Note that if you only wire one primary, everything will tend to work, but you'll have only half the VA rating available. Similarly, with two 45V secondaries, you could wire them in series if you wanted 90V, but you'll want to wire them in parallel to get the full available amperage from the transformer. So you'll get 5.56A out of each secondary, for a total of 11.12A. That will be plenty.

Another way of thinking about this is that you have two transformers, each with a single primary and a single secondary, that happen to share the same toroidal core, and you're gonna use 'em in parallel to double your available current. This isn't strictly accurate, but it might help your visualization.

Ask again if I've been unclear!

Zouave Mon 30 January 2012 21:14

Wow, knowing that the transformers have a double set of windings makes everything so much clearer! For the transformer I listed, it didn't list a secondary voltage coming out. Is it possible that it just doesn't have leads attached to it, and the 2 45s are the two primary windings, and the secondary are completely unused? IE, would it be possible (not easy, but possible) to use those secondary windings to regulate down a separate group to eventually go to a 12v (for a fan) and a 5v (for the Bob)? I understand I would probably have to very, VERY carefully find which wires are which and then cut them at specific points to achieve the correct voltage, correct?

Thanks for your explanation, it helped clear up a lot.

bradm Tue 31 January 2012 07:53

Eric, I took "500VA, Output V: 45 45 @ 5.56A" to mean two 45V secondaries. 45 * 5.56a = 250.20 VA, so two secondaries would be 500VA, showing up as around 63 VDC after rectification.

You probably don't want to mess with the existing windings on the transfomer, but you can *add* additional windings. As long as you're drawing a trivial amount of power, it shouldn't be much of a problem. You wrap a suitable wire around for 5 or ten wraps, power it up, and measure the voltage. You can then calculate how many volts per wrap, and thus figure out how many wraps you need for your aux winding. You can also estimate the length of wire required by measuring how much those 5 or 10 wraps took. You then add the new winding, keeping to a single layer.

Richards Wed 01 February 2012 09:22

The confusion about AC being 45V and DC being 63 volts is caused by the way voltage is measured.

A.C. is normally measured RMS (Root Mean Squared) which is basically the "mean" or average voltage of the A.C. sine wave.

D.C. has no sine wave (except a minor amount of "ripple"), so the PEAK voltage is measured.

If A.C. were measured half-wave rectified (so that there would only be a positive "sine" wave) and if it were measured PEAK to PEAK, the voltage seen from a 45V transformer would actually be 63V.

All that happens is that the bridge rectifier converts the (+/-_ sine wave into a (+) only sine wave, then the capacitor filters or smooths out the ripple so that the voltage seen is the same as the PEAK to PEAK voltage. That's why a large capacitor is required. If the capacitor is too small, the motors will pull too much current from the capacitors between each 100 cycle or 120 cycle "charge" and there will be excessive A.C. "ripple".

D.C. = A.C. X 1.414

Capacitance = 80,000 X Amps / Volts

Because the actual PEAK to PEAK voltage is 1.414 X higher than the RMS A.C. voltage, always select a capacitor whose working voltage is rated AT LEAST 1.414 X higher than the A.C. RMS voltage of the transformer.

Zouave Fri 03 February 2012 12:42

Okay, so as I understand it, I'm looking for a capacitor with a rating of ~14,000uF, and 65+ V (going with 100 for safety probably). I am having a horrible time finding one around here, and I'm not sure if its just because I'm messing up the units. I happen to have on hand, two 1000uF, 200V capacitors. Is there just a units issue at work here, and the 1000uF in America is what everyone else is calling a 10000uF? Or does this thing not have nearly the capacitance that I'm looking for? (Its off a washing machine, btw, not sure I'm going to use it, but looking at something in my hand makes more sense to me than just reading numbers).

My calculation is as follows so far: 80,000 x A / V = (80,000 x 11A)/65V = 13500 (uF??)

And next question, do I operate off the incoming or the outgoing? Do I combine the two leads after the capacitors (and thereby use two capacitors, rather than just one??)
If so, if I have 2 leads of 45V @5.56A coming in, my calculation would be 80,000x5.56A/45V which gives me right around a 10,000 (uF??) capacitor, rated for say, 100V, for safety margin.

Which one is correct?

WTI Fri 03 February 2012 14:46

10,000uf 100v is a pretty common value.

Gang 2 of them together and you would be all set ($25 a pair):

WTI Fri 03 February 2012 15:00

1 Attachment(s)
When you gang the two caps together, you need to wire them in parallel - that is to say + to +, Neg to Neg:

Zouave Fri 03 February 2012 16:54

Wow. That diagram should get stickied.

Thank you guys for all the help, I am slowly winding my brain around this. But I think I am getting closer!

And thank you for the link, James.

WTI Fri 03 February 2012 17:07

That picture will make more sense when viewed in this post:

Gerald D Fri 03 February 2012 22:57

That picture is not correct - there is no 50V DC. At that point in the circuit you already have 71V DC. The capacitors cannot change the voltage in a wire going over the top of its terminals. That wire has 71V DC along its whole length. It is the rectifier that causes the (effective) change of voltage.

WTI Sat 04 February 2012 01:33

Actually, a digital volt meter will read only 50v DC until the caps are installed.

I put the 50v in the picture so that no one is surprised at the reading they will get if they are testing each step of the way during construction.

Richards Sat 04 February 2012 08:37

James and Gerald, you are both right.

Depending on the meter that you used, without the capacitors, you would get about 50VDC from a 50VAC transformer because the meter would "see" 120 positive "cycles" per second, assuming North American power source. If you switched the meter to AC, it would show the RMS (Root Mean Square) value of the "ripple", which would be 1/(sqrt 2) or 0.7071 X the peak to peak AC voltage. Many meters, on the DC setting, will also show the "average" DC voltage. However, if you used an oscilloscope, you would see that the sine wave was actually 71 Volts, peak to peak.

After the capacitors are installed, almost all of the ripple is eliminated, and a DC meter will read the voltage as 71VDC.

WTI Sat 04 February 2012 19:30

Mike, like always, you have some of the most knowledgeable and eloquent explanations - thank you.

smreish Sat 04 February 2012 20:16

...Mike, Gerald and others always make me feel like I missed something in college! Great lesson refresher today.

Zouave Mon 06 February 2012 15:41

You guys have done a fantastic job of explaining all of this. I am incredibly thankful, and appreciate the time and effort each of you put into helping me understand something I was struggling with. THANK YOU.

Zouave Wed 20 June 2012 13:15

So, interesting thing today. I'm putting together my table-top setup, and my transformer is supposed to have 2 45V outputs (among others, but that's not important). So I hooked it up to check what voltages were being output. The blue wires registered 80V AC, the green wires registered 47V AC. When I hooked them up to the recifier, I am getting the expected 47VDC reading from the rectifier, since it has not gone through the capacitors yet, so its still reading RMS, and not peak.
So I'm curious why I'm getting a reading of 80V AC from the blue wires, and 47VAC from the green. (The colors are irrelevant other than my personal reference). Any ideas? Or a breakdown of transformers that I haven't read yet that explains it to me more thoroughly.

Also, I'm not hooking up my capacitors yet because I don't have a way to drain them down. Any suggestions on that front? I'd like to see what is coming out of the capacitors, but it'll be a few days before my BOB and Geckos get here. I have 2 10,000uF Caps @ 80V. And no, I'm not going to put a screwdriver across the terminals. :-P

bradm Wed 20 June 2012 14:24

Eric, a household light bulb makes a good cap drain for voltages of 120v and below. It's also a visual indicator.

The two windings of the transformer give those different readings under exactly the same conditions - nothing connected to either one? Sounds like there was an error made when it was wound (or somebody shipped the wrong transformer).

domino11 Wed 20 June 2012 14:32

Kind of hard to say whats going on without a proper diagram. Put up a wiring diagram and show where you are putting BOTH probes from the MM and we might be able to shed some light. Dont be so afraid of the caps, if you have an old incandescent light bulb, you could use that to slowly drain your bank. Monitor the voltage on the caps before working on the circuit after draining to be sure.
Yeah dont use the screwdriver trick, you will have a chunk out of the driver and probably a damaged capacitor screw. I have seen a capacitor bank take a quarter inch round chunk out of a wrench that slipped on a tech. :)

Zouave Wed 20 June 2012 15:23

1 Attachment(s)
Here is my current setup, please ignore most of the markings, those are references I jotted down when I was first getting things organized. Routing everything through the DIN rail at the moment for ease of organization, and to keep loose ends contained at the moment.

So, 110V coming into a jumpered pair of DIN blocks, the red rectangle on the left. The two red leads from the transformer go into there, 1 for each. Two black leads go to the ground block adjacent to it (black rectangle) and to the ground of the cord.
Coming out of the transformer, Blue + green is supposed to output 45VAC, Don't worry about orange+orange and brown+brown.

Both blues feed into a pair of jumpered DIN blocks (for a single out to the Rectifier)
Both greens feed into a pair of jumpered DIN Blocks, for reason above.

Readings, and these are all taken from the DIN blocks.

Blue + Ground (black) = 80.0 VAC
Green + Ground (black) = 43.9 VAC
Blue + Green = 48.6 VAC
Across the two terminals of the Rectifier = 44.7 VDC
Red + Ground (black) = 120.0 VAC

Also, sorry for the poor picture quality.

Zouave Wed 20 June 2012 15:33

My suspicion is that there is something different in how the transformer induces the current that I just don't understand, as the actual output is where it should be, its just the readings off the blue wires to the ground that I don't understand. IE, because I don't understand exactly what is happening, I'm not understanding what I'm seeing, even if I am doing everything correctly.

Anyone have any insight into it?

KenC Wed 20 June 2012 20:08

The red & black are the Primary coil terminals; The green & blue are the secondary coil terminals.
Why would you want to measure across primary & secondary??? (ie green-black, blue-black) They serve no purpose.
The important thing is do you get a DC voltage across the + & - terminals of the rectifier?

Zouave Wed 20 June 2012 20:17

The red and black are the incoming wires, carrying 110vac. The blue/green are primary, double orange and double browns are secondary windings.

Zouave Wed 20 June 2012 20:19

I was measuring across to the black because I know that is my ground.

domino11 Wed 20 June 2012 21:29

The red and black are the primary windings. These carry the input voltage, in your case the 120VAC. The black is not actually a ground it should be the neutral. The input ground is usually tied to case, just like in an electrical outlet box. The Blue and Green ,orange and brown are all secondary windings. Your secondary ground will be the minus terminal on your bridge rectifier. Primary and secondary grounds are not tied together. It looks like you have it hooked up right as far as I can tell. Reread the beginning of this thread again. The 45volts you are reading at the bridge is the rectified ac not dc, once the caps are in you will read 63Vdc. See Mike R's more detailed description in post #5 of this thread.

Zouave Thu 21 June 2012 03:16

Sorry, I guess I should get my terminology correct! So the primary windings are the input, and the secondary are the outs? I guess my understanding of the transformers is incorrect, as I thought it was the induced magnetic fields of the core that generated the output voltages in the wires wrapped around the core. So, as I understand it, and please, do correct me if I'm wrong.... The primary coils induce the field, and the secondary coils, based on their wrapping, produce an output voltage of the appropriate amount (based on the number of wraps?). Is that roughly how things work?

Sorry for the confusion in terminology, I was assuming that the primary coils were referring to the primary voltage that was being output, and any additional were secondary outputs. Obviously I was rather confused as to what the terminology meant.

So, yes, red and black are primary windings, and black is neutral/ground. I linked my neutral and ground together when I tied my green and white into the same DIN ground block. I'd seen this done on numerous other setups, am I incorrect to do so? Would I be better off to tie my ground (green) to the case and have only the white be interfacing with the transformer?

Also, another terminology clarification... Coming out of the rectifier, it was my understanding that it was DC, just with a rather accentuated ripple, thus the need for the caps. The Capacitors aren't going to change AC to DC, are they? The capacitors makes everything show its peak voltage, but otherwise we're always dealing with DC current, aren't we? Coming out of the rectifier, the current should show up as DC, just very rippled, right? Or wrong?
Am very curious to learn more about this.

danilom Thu 21 June 2012 03:35

Yes Eric thats the way it works, current flows trough the primary winding and you have output on the secondary winding prortional to number of windings turns.
Output is a steped down AC, after you run it trough the rectifier (diode bridge) you get pulsing DC equal the look of two sinusoid periods from AC they are just on the side from zero up, not alternating around it. Capacitors serve the purpose to smooth those spikes as they charge and discharge so you get a nice flat DC.

For specifics you can watch this video

Richards Thu 21 June 2012 07:04


Everyone is confused about electricity, including me.

I always treat the AC Line and AC Neutral separate from the DC + and the DC ground lines. I never tie the AC Neutral to the DC ground. It is often done, but I tied the AC GROUND line to the DC GROUND line. On the boxes that I build the AC Neutral is NEVER switched except at the Disconnect switch. Common practice is to switch all AC lines at each contactor. and relay, but I like to only switch the HOT or LINE side of AC.

Think of a transformer as a gear box. Think of it as having a ratio. Pretend that it has 100 windings on its coil and that the output side has a different number of windings, for instance 50. The ratio between 100 and 50 tells us that we will have half the voltage coming out as we have going in.

So far everything is simple. But now we get to the dreaded RMS factor. Most voltage meters read AC using RMS (Root Mean Square). All that means is that the AC reading is the Average (Mean) of the AC sine wave. If you looked at a 120 Volt AC sine wave, you would see that it is really 169 Volts on the plus side and 169 Volts on the negative side, or really 338 Volts peak to peak. The power company tells us that we have 120 Volts AC, but we really have 338 Volts when we measure the voltage's highest point and it's lowest point.

Keep in mind that we have a sine wave. In North America, that sine wave rolls through at 60 times (cycles) every second.

Now we introduce the bridge rectifier on the output (secondary) side of the transformer. Let's pretend that the input side and the output side have equal voltages for this example. After the bridge rectifier (4 diodes) do their job, some people say that we have DC. That's not perfectly true. What we really have is AC voltage that has half the amplitude and twice the frequency, using the voltages from above, we have an AC cycle that goes from 0 volts to 169 volts 120 times a second. All the bridge rectifier did was change the negative portion into a positive portion of the sine wave. That's why we get half the voltage but twice the frequency.

When we add capacitors to the output, we store that voltage for future use. The large capacitors that we use store at least 95% of the voltage, so we can say that we have a reservoir full of electricity. Using the example voltage, we would say that the reservoir holds 169 volts with just little waves rippling across the top of our reservoir.

That's what we want in a power supply that powers stepper motors. We want lots of electricity in our reservoir with just a little ripple (AC waves).

KenC Thu 21 June 2012 21:12

Just an added note on Richard's explanation.
Alternating Current A.K.A. AC, is one that it goes from +ve yo -ve (current & voltage) periodically.

Direct Current DC is one which always stay on the +ve or -ve.

The current that comes out of a rectifier is considered a rippled DC. i.e it is not smooth as it is riddle with ripple pulses. Passing it through a filtering capacitor will reduce the magnitude of ripple but it is never free from ripple. in most cases, 3~5% ripple is acceptable as "pure" DC.

On the earth connection.
Earth, Ground, Common may have the same symbol, but they might not be the same thing...
Earth is "mother Earth"
Ground is for small system ground or isolated group of systems...
Common is for descrete common points...
Confusing? you tell me LOL!
It is best to treat each & every electricity loop separately.
I reckon what Eric saw all the ground together is the earthing technique call Star-ground connection which is a mysterious thingy.

For simplicity, this is my logic approach to star-ground connection.
1) wire up individual electricity loop as an stand a lone system. eg, the primary coil is wired as Eric did, eg2 secondary coil connect to rectifier, the capacitor.
2) wire the -ve terminal of the Filter cap to the earth point with a thick fat dedicated wire to the Earth screw which is also has another dedicated tick fact earth wire connected to the mains' earth line (the green/yellow wire)。
CAUTION you do not connect the earth directly to separate electrical loop system. that may give potentially dangerous outcome if there is a potential difference between individual electricity loops.

Zouave Fri 22 June 2012 01:18

Thank you for the information, everyone. It has helped tremendously for me to wrap my head around what is going on. I did some additional research as well, and even came across a write-up on how to calculate how many windings you need to achieve a specific output voltage. Very interesting.

How does it work to not connect grounds? At some point, aren't they all connected? At least at the point where they are grounded to the box, why should they be kept separate within the system?

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