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  1. #11
    Clive S's Avatar
    Lives in Marple Stockport, United Kingdom. Last Activity: 19 Hours Ago Forum Superstar, has done so much to help others, they deserve a medal. Has been a member for 9-10 years. Has a total post count of 3,333. Received thanks 618 times, giving thanks to others 78 times. Made a monetary donation to the upkeep of the community. Is a beta tester for Machinists Network features.
    My both 420VA & 720VA has 30VAC and gave 41.5VDC (calibrated Fluke 87V and 1587)
    Primary coil was ~32VAC
    Tom you are reading too much into all this. It has been done to death many many times on the forum.

    Have a look at the quote and notice you are saying primary coil is 32 AC. The secondary would be the the 32 volt coil.
    It is facts like these that can mess people up.

    Also you are stating a 1.57 A load is that on the primary or secondary as they would be different.

    All this can be very confusing for people seeing the thread title thinking this is the right way to pick a Tx.
    Last edited by Clive S; 05-01-2017 at 03:50 PM.
    ..Clive
    The more you know, The better you know, How little you know

  2. #12
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 3 Days Ago Forum Superstar, has done so much to help others, they deserve a medal. Has been a member for 9-10 years. Has a total post count of 2,908. Received thanks 360 times, giving thanks to others 8 times.
    You need to allow for mains voltage tolerance (230V -6/+10% or 216 to 253VAC), transformer load regulation (the toroid spec I just looked at was 5%), and a lower volt drop over the bridge rectifier when zero load.

    So taking a 33VAC toroid, applying 10% more power (mains voltage here is at the higher end of tolerance) gives 36VAC (we'll round for simplicity) at rated load. Add 5% load regulation, which means we bump our basic unloaded voltage to 37.8AC.

    Multiply by root 2 (or 1.41), gives us 53.3VDC. Take away a little bit volt drop for the bridge rectifier, and we have in the region of 52VDC.
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  3. #13
    Quote Originally Posted by Clive S View Post
    Tom you are reading too much into all this. It has been done to death many many times on the forum.

    Have a look at the quote and notice you are saying primary coil 32 AC the secondary would be the the 32 volt coil.
    It is facts like these that can mess people up.

    Also you are stating a 1.57 A load is that on the primary or secondary as they would be different.

    All this can be very confusing for people seeing the thread title thinking this is the right way to pick a Tx.
    yes , sorry corrected to "secondary coil was ~32VAC"

    1.57A has nothing to do with transformer, just a picture. VOLT/AMP meter red/blue LCD shows the same voltage as calibrated Fluke - that was my point.
    I had only picture with Amp displaying on Fluke which also was calibrating. I know I confused enough but some guys say that my meter might be off.

  4. #14
    This F@C£ING STUPID and Dangerous.! . . . LEE STOP It PLEASE. . . . Far too many errors and misunderstandings.

    Tom I'm not having a go at you personaly here and see your just trying to elp and give back but this is potentialy dangerous to others and shouldn't be put over has " How To Guide".
    I'm helping and dealing with people young and old all the time who never post or ask questions, which they should but don't for what ever reason. Some do have the good sense to ask privately and this is my concern.

    I'm for ever helping people, esp the young who haven't got the patence to read past first few posts and when the First post comes over in manner where looks like OP knows what doing they take it has Gospel.
    They then proceed to go out and buy everything mentioned without any regards to if correct or not and follow to the letter whats wrote.
    This isn't so much problem if it's frame related or Even some components like ballscrews rails etc because worst case they end up with under performing or over expensive machine.
    But when comes to Mains electrics like this then it's Bloody dangerous in the wrong hands so unless the OP is Expert which clearly isn't the case here then shouldn't be allowed to proceed any further.

    Anyone who's been on here long enough should know how much I hate Moderation and Censorship, Bloody hell I've come close to being banned in protest to it so it should show you how strongly I feel about this because this NEEDS STOPPING or at least Moderating closely.!

  5. #15
    Hi guys,

    After receiving a couple of reports on the original thread (it needing attention), I decided to split the discussion, allowing it to continue regarding the selection process, with a new discussion for the PCB side of things, as both topics deserve the proper observations.

    You can find the links to each discussion below.

    Tom J
    Thank you for your efforts and suggestion, however due to errors and inconsistencies in your original approach to this, I feel it is in the best interest if we strip this thread of any kind of guide/tutorial type wording or suggestion it may have, as it's just not ready for that yet.

    Please don't be discouraged in your efforts, the original idea was a good one and would be an asset to the forum, however due to the severity, that kind of article/document needs to be correct in its absolute entirety before it can boast as being the definitive. Please try again once you have everything worked out.

    Thank you to everyone involved, feel free to continue both discussions:

    Link 1: Help please with toroidal transformers selection

    Link 2: Please don-t push 14A through that PCB !!!

    .
    Last edited by Lee Roberts; 05-01-2017 at 04:46 PM.
    .Me

  6. The Following 2 Users Say Thank You to Lee Roberts For This Useful Post:


  7. #16
    I don't want to get hung up on this transformer voltage business (except that I was surprised by the toroidal transformer I bought recently that measured about 5% above nominal voltage when off-load - the "regulation" factor that's mentioned above which I hadn't expected) but I am interested in the practical choice of smoothing capacitors. I am assuming here that the linear PSU is feeding only the stepper drivers and nothing else, which I think is generally the case.

    Stepper drivers can cope with fairly wide voltage ranges. From memory, the EM806 which I am using is rated at 24-80V absolute min/max, for example, although the Leadshine recommendation is to stay at least 10% away from either extreme. We are not building audio amplifiers here, so we're not interested in hum levels on the output. The capacitors are being charged at 100Hz (assuming the usual bridge rectifier) but the steppers are pulsed at a few KHz, so any given pulse will see only the variation in voltage during that pulse - small compared with the variation across a half-mains cycle? On top of all that, one of the stepper driver's main jobs is to control and limit current through the stepper coils, so one thing it can do very well is cope with a range of input voltages while trying to maintain the target current through the stepper coils. So do I really care if the DC output voltage sags by a few volts, across a half mains cycle? In practice, then, just what size capacitor will do an adequate job without going OTT? What is the effect of a too-small capacitor? Don't know the answer myself, but once my machine is back running I'll stick an oscilloscope on the PSU output and see what's happening, just in the interests of science.

  8. #17
    Clive S's Avatar
    Lives in Marple Stockport, United Kingdom. Last Activity: 19 Hours Ago Forum Superstar, has done so much to help others, they deserve a medal. Has been a member for 9-10 years. Has a total post count of 3,333. Received thanks 618 times, giving thanks to others 78 times. Made a monetary donation to the upkeep of the community. Is a beta tester for Machinists Network features.
    Very well put Neale some sense is prevailing at last.
    ..Clive
    The more you know, The better you know, How little you know

  9. #18
    Quote Originally Posted by Neale View Post
    In practice, then, just what size capacitor will do an adequate job without going OTT? What is the effect of a too-small capacitor? Don't know the answer myself, but once my machine is back running I'll stick an oscilloscope on the PSU output and see what's happening, just in the interests of science.
    1000uF for 1A, voltage is important (20% more than operating voltage and you are safe, min 100V caps for 80V system.)

    Small capacitor won't take enough power from stepper when slow down or change direction (EMF). I am scared to write anything now
    Last edited by Tom J; 06-01-2017 at 12:28 AM.

  10. #19
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 3 Days Ago Forum Superstar, has done so much to help others, they deserve a medal. Has been a member for 9-10 years. Has a total post count of 2,908. Received thanks 360 times, giving thanks to others 8 times.
    I can't remember the specifics, but ultimately capacitor sizing comes down to load, and how quickly it charges. I'm hoping somebody more knowledgeable will be along with the more exact details, but I'll give a basic view.

    As you know the capacitor has to provide energy during the period between charging cycles. The more load for any given capacitance, the more the capacitor voltage will drop between cycles.
    Now if you choose too low a capacitance, you get a high voltage ripple on the output.
    Most stepper drives will probably handle this, however by having the capacitor voltage fluctuating too much, you risk overheating the capacitor. As with nearly everything, nothing is 100% efficient, so every time you charge/discharge a capacitor, energy is lost to heat.
    This is why large electrolytic caps normally have a vent, so if they do heat up and pressure builds up inside them, they can vent in a safe manner rather than exploding. For completeness, if you look at the end of small electrolytic capacitors, you'll see a scored line, which is a built in weak point, for the same reason.
    Another thing is during motor deceleration, a smaller capacitor will not absorb as much energy, so over-voltage becomes a risk.

    In terms of a powered circuit, other than cost, there's not really such a thing as too much capacitance, but you will reach a point where extra capacitance won't provide any tangible benefit.
    With a large capacitance, you have the issue of start up surges, as a fully discharged capacitor will appear as a dead short at initial power up gradually increasing in resistance until fully charged. The more capacitance, the longer the initial charge will take during power on.
    Then you need have to deal with discharging the capacitor after power has been removed. As Jonathan mentioned somewhere already, you could end up with quite a lot of heat being generated by a discharge resistor in order to discharge the capacitor(s) in a reasonable time frame once power is removed.

    There are calculations that will work all this out, but it's not something I've done for a while. However I'm sure a google search for capacitor ripple voltage will turn up some detailed results.
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  11. #20
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 3 Days Ago Forum Superstar, has done so much to help others, they deserve a medal. Has been a member for 9-10 years. Has a total post count of 2,908. Received thanks 360 times, giving thanks to others 8 times.
    Quote Originally Posted by Tom J View Post
    1000uF for 1A, voltage is important (20% more than operating voltage and you are safe, min 100V caps for 80V system.)

    Small capacitor won't take enough power from stepper when slow down or change direction (EMF). I am scared to write anything now
    The thing is, that 1000uF per A is an arbitrary figure provided as a guideline.
    I just had a quick scan through the Gecko Drive guide, and they list the following formula for minimum capacitance in uF-
    (80'000 * I)/V

    I'd imagine they've done some calculations and research to reach that figure, and it'll involve the maximum voltage ripple their drive internal capacitors can handle safely under various conditions.
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

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