1. Here is the link from UK supplier I found on this forum (no relation to merchant)

1. Common design with 50VDC Drivers - lets focus on voltage first

50V - (10%50V)=50V-5V=45V safe voltage

10% is deducted -EMF(slowing down motors, change direction)

Now we need calculate correct voltage of secondary coil of the transformer

24V/1.41=31.92V

From above link we can get 2x15VAC (in series 30V), 2x30AC in parallel or closest or 2x33VAC n parallel too.

I have similar one 420VA 2x30V AC
This gives me 41.5V DC after being rectified on idle
I loose about 3.5-4V, would like to have max 45-46VDC

Now how to choose power? (current figure, as voltage is known)
Power source can have lower current than current of those motors (usually 50-70% of nominal motor current) - why?
Driver takes current from psu condenserwhile PSW cycle is on, so avarage current consumption by motor is lower than motor current.
Lets say we have 3x3A motors, so no need to have 9A as 6.3A is enough
(3x3A)x70%=6.3A + 10% spare power
6.3Ax1.1=6.93A

so we choose 7A

32Vx7A=224VA
here is exact tranformer:

Does somebody used 33VAC or 36VAC?
I would like to know what DC voltage will be after Diode Bridge Rectifier and big capacitors like 2x10.000uF
and same for popular 70VDC drivers please. 70-10%=63VDC
48VDC what ?VDC
50VAC what ?VDC
55VAC what ?VDC- probably more than 63V - guessing here
Last edited by Lee Roberts; 05-01-2017 at 02:57 PM. Reason: moderation

2. AC is forever bobbing up and down either side of zero volts.

Feed it through a transformer and you change that voltage in the exact proportion to the number of turns in the primary and secondary coils. It is still bobbing up and down.

Bridge rectify it and you mirror the volts below zero volts so they show above zero volts. It is still bobbing up and down but only one way.

Add a capacitor, any capacitor and the volts will stabilise at peak volts because there is no load to pull it down.

Add a load and discover what the problem with transformers is... to sustain the voltage at something useable you need a snogging girt capacitor. Any load will reveal the bobbing, the capacitor merely determines the degree.

1 Farad will give 1 Amp for 1 second. Unfortunately capacitors are usually rated in micro Farads, millionths of a Farad.

You can get 1F capacitors but they are either for memory back up purposes of they are the size of a house.

If your drivers don't care about the Volts bobbing up and down when you apply a load then transformers are for you

3. Thanks Rob I know all that from college & university.
My power supply for drivers includes capacitors 2x10.000uF so can deal with back EMF well, that is why I am so concern about leaving spare 5V before max operating voltage.
I know some guys go for max like 1-2 volts off.
My 1st panel 3 years ago had 720VA trafo with soft start and stabilized 12v for BOB lpt1.

I am building another box 300x300 which is only High Voltage PSU and 24vDC

here are industrial PSU 24Vdc all stabilized

this is my pcb with rectifier and 2 capacitors.

I need trafo to my 70VDC Leadshine EM705
60VAC after bridge and capacitors will be above 70VDC so have to get another one

I was asking for final dc voltage after rectifier and capacitors?
48VDC what ?VDC
50VAC what ?VDC
55VAC what ?VDC

Send the question to provider but no reply so far
Thanks
Last edited by Lee Roberts; 05-01-2017 at 12:36 PM.

4. I was asking for final dc voltage after rectifier and capacitors?
48VDC what ?VDC
50VAC what ?VDC
55VAC what ?VDC
Are you asking for the dc after caps if so its about AC V x 1.4

5. Originally Posted by Clive S
Are you asking for the dc after caps if so its about AC V x 1.4
...yes Clive DC after cups but the real reading tbh.
my 30V trafo has 32VAC and after bridge and 2x10.000uF reads 41.5VDC unloaded
calculation gives 32V*1.4=44.8V 3.3V more than real test.

6. Originally Posted by Tom J
...yes Clive DC after cups but the real reading tbh.
my 30V trafo has 32VAC and after bridge and 2x10.000uF reads 41.5VDC unloaded
calculation gives 32V*1.4=44.8V 3.3V more than real test.
The rectifier will drop a bit of volts.

But remember the AC mains fluctuates all the time
Last edited by Clive S; 04-01-2017 at 08:15 PM.

7. Originally Posted by Tom J
Jonathan this pcb is for 3x3A steppers which takes no more than 7A from my calculation, but real test by clamp meter revealed that was no more than 0.86A therefore even perforated prototype board worked fine (but should not)
You're probably mixing up the supply current with the motor phase current - the two are related, but the supply current will always be less.

Originally Posted by Tom J
For my 70VDC 7A motors I use the same layout but wider path on pcb.
You can avoid cutting the ground plane with one of the tracks by routing it another way ... not that it really matters for what you're doing.

Originally Posted by Tom J
Just checked current-carring capacity and 1mm2 will handle 11A.
For 16.5A i need exactly 1.5mm2 conductor, so 1mm pcb has 0.035mm of copper.
Path width=1.5/0.035=42.86mm - my entire PCB is 50mm wide, according to this calculation we both wrong
It's a thermal limit and a cable is somewhat different to a PCB - try this:

http://www.4pcb.com/trace-width-calculator.html

Originally Posted by Tom J
I like that drain resistor - how many ohms?
3.3kOhms. But that's not the right question. You size it based on the energy stored in the capacitors, time in which you want the capacitors to discharge, and acceptable power through the resistor.

Crudely, the capacitors are discharged in t=5*R*C and the resistor power dissipation is P=V^2/R. I used a 3W resistor, but to get good lifetime from the resistor and not set fire to things it's a good idea to use less than the rating, say 2W. R=V^2/P=80^2/2=3.2kOhm. t=5*3200*0.02 = about 5 minutes.

Originally Posted by Tom J
Where do you buy components if you don't mind me to ask - your price are better than mine:(
Generally Onecall (farnell,CPC) or Rapid Electronics. Also Mouser and Digikey have started doing free postage on smallish orders, so they're good bets.

Be extremely wary of components from eBay in China - rife with fakes, especially capacitors. I treated myself to an LCR meter so I can at least check the capacitors.

8. Originally Posted by Tom J
...yes Clive DC after cups but the real reading tbh.
my 30V trafo has 32VAC and after bridge and 2x10.000uF reads 41.5VDC unloaded
calculation gives 32V*1.4=44.8V 3.3V more than real test.
Your off-load DC voltage is largely irrelevant, other than to consider any maximum voltage for component selection. The RMS voltage (i.e. 32V) represents the effective average voltage under the specified rating of the transformer. The problem you will have with an unregulated supply is exactly that - it's unregulated and with large value smoothing capacitors you can expect that off full-load that the capacitors can hold up the DC level as the AC supply transits through the zero-crossing point, and the effective average DC voltage will increase above the full-load value (but below the peak value).

Expect to lose 1.2-1.6V (typically) across a bridge rectifier. This affects the peak DC voltage directly.

When looking for variance between your measured vs theoretical, as already mentioned in the replies - consider the actual AC line voltage at the time of test, and the ratio of the primary and secondary windings (many transformers are rated with primaries at 220VAC - if fed with 240VAC then expect the secondaries to read 240/220 x the rated voltage).k

9. For a read of the how/why behind choosing a toroid based on steppers, have a look at my post in my Triac retrofit thread - http://www.mycncuk.com/threads/10344...5703#post85703

Although having just skimmed over the post, I've just realised I never mentioned about sizing the capacitor. It's a 100V 22'000uF from a reputable source.

10. ## The Following 2 Users Say Thank You to m_c For This Useful Post:

11. Originally Posted by m_c
For a read of the how/why behind choosing a toroid based on steppers, have a look at my post in my Triac retrofit thread - http://www.mycncuk.com/threads/10344...5703#post85703

Although having just skimmed over the post, I've just realised I never mentioned about sizing the capacitor. It's a 100V 22'000uF from a reputable source.
That kind of answer I was looking for.
33VAC - 52VDC, big boost over calculation, any idea why was like that?

My both 420VA & 720VA has 30VAC and gave 41.5VDC (calibrated Fluke 87V and 1587)
Secondary coil was ~32VAC

picture shows Amp meter after calibration, voltage was set too
Last edited by Tom J; 05-01-2017 at 11:09 AM.

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