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  1. #41
    Quote Originally Posted by JAZZCNC View Post
    SO I completely agree with Irving and always build them into my PSU's..!! . . Now the only thing I don't like about Irvings way is that the resistors are permanently draining while the machines running, this creates heat while running so warms the control box and therefore the drives etc.
    I'm a bit Anal with my control box and build them with relays and full safety using 24V and control lots of things with relays, one of these is a NC relay which kicks in and out a resistor so it only drains when off.!! . . No such thing has being too safe IMO.
    I've read elsewhere about people using lamps rather than resistors as a "visual" indicator, is this the same kind of thing? I'm intrigued if a lamp connected to a normally closed relay would work, though I have no intention of trying it out...

  2. #42
    Quote Originally Posted by Rogue View Post
    I've read elsewhere about people using lamps rather than resistors as a "visual" indicator, is this the same kind of thing? I'm intrigued if a lamp connected to a normally closed relay would work, though I have no intention of trying it out...
    Don't see why not, but bear in mind a lamp can blow/fail - so even if it's not illuminated you don't know for sure that there's no voltage across it.
    Last edited by Jonathan; 12-09-2012 at 11:30 PM. Reason: Grammar
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  3. #43
    Quote Originally Posted by Jonathan View Post
    Don't see why not, but bear in mind a lamp can blow/fail - so even if it's not illuminate you don't know for sure that there's no voltage across it.
    Took the words right out my finger tips.! . . Resistor is a safer bet, could always use a resistor and a bulb but why complicate things.

  4. Again, its good practice to have 'voltage present' indicators, especially if you have a secondary fuse. (plus I like lots of lights :) ).

    For the primary side a panel mounted neon indicator wired directly across the transformer primary.

    For the secondary side a 5mm LED mounted in a panel mount with a 3k9 2W resistor wired across the output.

  5. #45
    Quote Originally Posted by irving2008 View Post
    You'll also need a bleed resistor across each capacitor to discharge it when the power is turned off (for safety). The energy stored is 0.5CV^2 joules. For each capacitor spec'd above, the energy is 0.5 * .005 * 70^2 = 6joules. A watt is a joule per second, so for a 5 second discharge you need to dissipate 6/5 = 1.2W so we're going to need 2W rated resistors. Assuming the resistor dissipates the same 1.2W when the power supply is turned on the value will be R = v^2/W = 70^2/1.2 = 4000ohm. I'd use 3900ohm (3k9) @ 2W wire ended and solder them directly to the ring tags on the capacitor: Buy Through Hole Fixed Resistors Carbon Resistor, 2W ,5%, 3k9 RS RS-Carbon-3k9-5%-2W online from RS for next day delivery. (pack of 10 @ 72p)
    After a cup of tea and a bit of a read, I have a quick question in relation to 0.5CV^2. Where you give 0.5 * 0.005 * 70^2 = 6, I end up with 12.2. In fact using the time honoured technique of "randomly changing numbers until they worked", I only got 6j by changing the 70v to 16v?

    Trying to follow your calculation and sticking with the 5 second discharge, I ended up with this:
    0.5 * 0.005 * 70^2 = 12.25j
    12.25 / 5 = 2.45W
    70^2/2.45 = 2000ohm, so I would need a 2k ohm @ 3W resistor to discharge over 5 seconds.

    I'm rather hoping you made a typo because the alternative is that I'm more stupid than I thought!

  6. Quote Originally Posted by Rogue View Post
    After a cup of tea and a bit of a read, I have a quick question in relation to 0.5CV^2. Where you give 0.5 * 0.005 * 70^2 = 6, I end up with 12.2. In fact using the time honoured technique of "randomly changing numbers until they worked", I only got 6j by changing the 70v to 16v?

    Trying to follow your calculation and sticking with the 5 second discharge, I ended up with this:
    0.5 * 0.005 * 70^2 = 12.25j
    12.25 / 5 = 2.45W
    70^2/2.45 = 2000ohm, so I would need a 2k ohm @ 3W resistor to discharge over 5 seconds.

    I'm rather hoping you made a typo because the alternative is that I'm more stupid than I thought!

    Errrmmm... looks like I made a boo boo... might have multiplied by 0.5 twice in error... your calculation is correct... now I'll go and edit my post..

    BTW 2k2 @ 3w will be fine
    Last edited by irving2008; 13-09-2012 at 09:36 PM.

  7. #47
    Then I shall take off my dunce's cap and come out of the corner. For now, at least

  8. Okay,

    I have been reading and talking to folks (Thanks Jazz) on the power supply issue. This is what I understand and what I want to make sure of.


    1. Mains power (fused at plug) To
    2. Fused Switch (main Switch as will wire in a secondary for the motors) This is feed to a + rail - rail and GND
    3. Positive rail Feeds:
      1. A Circuit Breaker going to the Switch for the Soft start for the transformer (will be asking on how to build one of those)
      2. The PMDX-126 BOB (Which has built in Circuit Breaker with auto reset)
      3. The on board relay (on the PMDX-126) for the spindle (30A relay) Currently this feeds a Kress 1050 FME. Does this need a Circuit Breaker as well?

    4. Negative Rail Feeds same as listed above.
    5. GND includes all of above as well as grounding the shielding on the stepper cables, the computer frames (control box and computer), Machine frame, and the unshielded side of the PMDX board.


    Okay that is so far, now onto the power supply that is going to be going in shortly. The way I am looking at setting it up is as follows:


    1. From Circuit breaker in number 3.1 to Switch for motor power (both +/- plus GND if that seems best)
    2. From there Negative and GND go to the Soft start for the transformer (need help on designing one of these so that I can build it in)
    3. Positive goes to a Relay (12V) attached to the PMDX in the set up in NO position so that the motors cannot be powered unless the board has power.
    4. From Soft start to 750VA 50+50 Standard Range Toroidal Transformers: CM0750250: 750VA 230v to 2x50v This is to power four 4.2A peak current Nema23 steppers and by what I have read, should meet even heavy running requirements.
      1. I am going with the 7.5A 70V in Parallel as that gives my 15A output in theory minus any loss and should more then cover the 12.3A needs to drive the four stepper motors (I recall from Physics that current is on demand and Voltage you get what is there, a 3A item only draws 3A but will take all the Voltage that comes with those 3A).

    5. This is where I am a bit confused. should I put another fuse here between the Transformer and the Rectifier or should I run directly to them?
    6. The Rectifier is a 35A 200V Metal casing rectifier and will be mount to a ali' plate to help with heat transfer and I have thermal paste (does the bolt/post need to be isolated?).
    7. Next is The Capacitors and I am looking at 5 4700 uF units (23.5 uF to meet a need for 21.4uF)
    8. Okay I am looking at this and trying to figure out how to wire in the the resistor to drain off power when the system shuts off. The idea I have is to put a relay in place that is in the NC position with the resistor (50W as they are not expensive and give some added safety margin) set to drain off the power when the switch closes. Now I could power this relay from the PMDX as with the other relay in place the motors should not normally be able to be powered if the board power is off. Other then that I am at a dead end on this one. Also the resistor should go to ground when it is activated correct? Also could use a double check on the Ohms I need for that Wattage.
    9. After the Capacitors the power is then run to a common rail from which the fuses (5A) for each Driver is in line with the + and the _ runs back to a common.


    If that all makes sense and anyone wishes to help with the Soft start modules and other questions I have posted here in, please do. Also hopefully this will help some other folks with design and structure building. I plan on putting this all on a board and Then mounting on standoffs in the control box. Yes it is bit of over kill but then again I make a part of my living from this machine and it needs to be safe and run well.

    For those who want to get a bigger picture of the electronics of this System I am using a AMD based PC running XP Pro SP3, using Mach3, That goes to a Smooth Steeper Ethernet card and then to the PMDX-126. Which then passes the data onto AM882's Digital drivers. This drives 2 motors on X, and one each on Y and Z axis. Spindle is currently a Kress 1050 FME. I have Jazz, James, and a few others to thank for aid in the development of this machine and in the new thread it passed the test you can see a picture of the detail it can do. The reason for the new power supply is that I owe the one I am using back to the person who has so kindly let me use it this long.

    Michael

    edited to correct a typo

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  10. #49
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Current Activity: Viewing Has been a member for 9-10 years. Has a total post count of 1,838. Received thanks 192 times, giving thanks to others 5 times.
    Soft start for that size of transformer isn't really needed. Just use a zero volt switching SSR (most AC SSR's are), and you shouldn't have any issues.
    One thing to note though, is if the SSR fails, it'll likely fail short circuit, so oversize and fuse it to minimise the risk. If you're really concerned, put a mechanical relay in circuit before it, incase anything does happen.

    Generally you only fuse the transformer primary side. Any overload on the secondary side should blow the fuse on the primary side. If you oversize the rectifier enough, then any overload on the DC side should also blow the primary side fuse.
    Fusing individual drives isn't usually recommended, as if it fails for any reason, then you risk the drive being fried due it having no where to dump power from a moving motor to.

    The drain resistor should simply connect across the capacitor terminals. Unless you have some reason to quickly dump power, you don't need to have it switched. The motor drivers themselves will quickly drain the capacitors with power removed.

  11. Mike,

    Can I suggest you get some terminology correct, it might save you some pain (physically and metaphorically) later

    When referring to AC (mains) its Live, Neutral, Ground The colours being Blue, Brown, Yellow/Green

    When referring to AC (transformer secondary) its AC Secondary and general practice to use Brown

    When referring to DC then its +/positive, -/negative, chassis ground and colours are generally Red, Black, Green


    Here's a sample circuit with estop and interlocks. Turning on the main switch powers up the +24 volt safety circuit. Push to start closes relay 1 (AC side) and powers up the +70v and aux supplies. Relay 2 (LV side) also closes holding in the relays unless the eStop is triggered. As an example I've shown one set of Realy 2 contacts switching the bleed resisors though I personally don't bother and have them permanently connected.. Other arrangements are possible...

    Click image for larger version. 

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    Last edited by irving2008; 15-09-2012 at 05:43 PM.

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