Confirming PSU spec for steppers.

[m.marino]

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

[m_c]

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.

[irving2008]

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...

[Rogue]

Just out of interest, if you wanted to get 5V out of that system as well, would you run a 7805 off the +24v circuit, or should that circuit be kept purely for the relays/e-stop?

[irving2008]

Just out of interest, if you wanted to get 5V out of that system as well, would you run a 7805 off the +24v circuit, or should that circuit be kept purely for the relays/e-stop?

Depends. The 24v is present as soon as the primary switch is turned on. If its OK for the 5v to be present too, and the current drawn is small, say < 0.1A, then yes. Otherwise I'd run a small 5v supply off the aux AC.

Incidentally +24 is nominal. A 15v transformer would give 21v, an 18v transformer would give 25v. A 3VA transformer is more than enough (200mA output). The smoothing capacitor need only be 470uF @ 40v rating and needs no bleed resistor. The relays are 24v, 600ohm coil, needing 40mA each, with 7A 4PDT contacts.

[Rogue]

Intriguing, and hopefully stripboard friendly! I think I will start here before attacking the bigger stuff.

Here we are talking about mA: would the same 35A bridge resistor still work? Or would it be too "robust" (for lack of knowing what the appropriate term may be) in this situation? Does there come a stage when the safety margin becomes so excessive that it degrades performance at the lower end of the operating range?

[irving2008]

Intriguing, and hopefully stripboard friendly! I think I will start here before attacking the bigger stuff.

Here we are talking about mA: would the same 35A bridge resistor still work? Or would it be too "robust" (for lack of knowing what the appropriate term may be) in this situation? Does there come a stage when the safety margin becomes so excessive that it degrades performance at the lower end of the operating range?

You mean 35A bridge rectifier. It would but one that size is needless. This one is more than sufficient and cheaper.

[m.marino]

Irving my apologies for not using the correct terms. I know the AC usage and colors (Though I still prefer the older colors as they made much more sense to me).

Michael

[Rogue]

Here's the plan - listing is indicative of order, should roughly correlate to this:



It's a little rough, I rushed it for the post. You might need to pretend there is a relay-controlled bleeder. And you might need to pretend there are any outputs to the rest of the world, but hey, that's the power of i-m-a-g-i-n-a-t-i-o-n!

Fused IEC Chassis Plug (23-0360) with 5A slow-blow fuse (20-1072)
High Current Toggle Switch DPST (on/off) rated 250VAC@10A/12VDC@20A (75-0253)
Splits out to
- 12mm 240V Green Neon indicator (42-0330)
- 24v system
- 70V system (via relay)
- potentially other systems, ie BoB etc.

24VDC System (well, 23-ishV)

Transformer: PCB transformer 230V 12VA 2x9V (88-3877)
Bridge Rectifier: Woo5 1.5A 50V Bridge Rectifier (47-3190)
Indicator: LED to be determined (probably have something suitable upstairs)
Smoothing Cap(s): 2000uF according to my magic spreadsheet, so 2 x 1000uf 63v Caps
Bleed Resistor: Not needed, but otherwise I worked it out as 3.5kOhms 1W
Toggle switch 24V for turning on 70V system

70VDC System

Relay: 24VDC 4PDT, rated 240VC@7A (60-1310)
Transformer: Well this is annoying, see below.
Fuse: Slowblow 15A in fuse holder
Bridge Rectifier: 35A 600V Bridge Rectifier (42-3228)
Indicator: LED to be determined (probably have something suitable upstairs)
Smoothing Cap(s): 4 x 4,700uF 100V caps (11-2912)
Bleed Resistor: I work this out as a 5 second bleed = 2 kOhm 3W, controlled by a relay wired NC.


Transformer for 70V system - Rapid has 500VA 2x50V transformers which should give me 50VDC@10A in parallel. I'd prefer 625VA but they don't carry 2x25 and the 2x50 has a 1-2 month wait from order. As I'll be sourcing this component last I have time to hunt around. A few other links to sources were posted in the thread; I'll check up on them later.

The wires to the relay that turns on the 70V will pass through another, normally open relay. This second relay is powered

Various crimps, wire and connectors should be easily sourceable at the local shop.

If I don't post for a month, my widow will probably be selling my stuff on ebay so keep an eye out for some bargains - she might be selling it for what I told her it cost before I got turned into crispy bacon...

[JAZZCNC]

Using my imagination I've imagined some of that same 24v running thru E-stop going thru limit switch's as well and the RL2 relay having another contact for the 5v signal from BOB.!!

Edit: Oh and you'll need an limit override momentry button in there as well to allow reversing off the switch's.