Unregulated power suply

[zeed]

My second cnc build is coming to its end and I ordered some 80V 6A drivers from ebay.
My steppers are 270oz-in 6 wire,1.6mH wich if my calculations are correct work best at 40V (Bipolar Half Coil) or 80V if connected (Bipolar Series)

I have transformer that is 220/380V 400VA with multiple outputs from 4V to 113V.
I ordered some bridge rectifier rated 50A 1000V and 2 electrolytic capacitors 100V 10000uf.
I am planing to run the motors at 40 or at 70V,witch ever shows best performance for my needs.

The question is can I use this bridge rectifier circuit at both voltages and what do you recommend for a bleed resistor or some protective circuit to protect drivers from power surge...
What is the best place for a switch,at AC primary,AC secundary od after the bridge rectifier?

[Al_The_Man]

The bridge will work on all the secondary's.
Switch and fuse the primary of the transformer.
Al.

[zeed]

Ok,thank you for the info.
Any suggestions on the protective circuit?

[vanjast]

Some quick calcs..
- 80Vx6A drivers = 480W (VA)
- 3x drivers = 1440VA
A good estimate, say your motors consume at least half of this ~ 770VA

When they talk about VA ratings, take the real power available at approx 80%, so most probably you have 320 Watts available on the transformer.

If you drive all 3 motors at the same time there will be a dip in the power availability, and you mentioned that this is an unregulated PSU, which might result in inaccuracies in the movements as the voltage and available power takes a momentary dive.

This, of course, all depends on your motors power requirements.


Place the switch on the AC power input Transformer primary, and a fuse before the switch. Sometimes it's advisable to fuse both Live and Neutral in case the L and N are swopped.

[zeed]

Ok,I see your point...
So this 400VA transformer would be ok only for up to 45V with 3A motors?

[vanjast]

That looks OK! assuming you're using only 3 motors - Is the transformer only powering the motors?

Forgot to mention that you'll have to experiment with the smoothing capacitors, that will absorb the surge when turning on the motors. 10K uF looks like a good place to start.

It might be a good idea to place a NTC power resistor (1 ohm - 10 Ohm) in parallel with a power relay, between the transformer secondary output(or rectifier if it's placed here) and the capacitors. Having the relay kick in a second after power-on. This will prevent surge conditions on the primary side which might set the 'alarms' off.

This happens on power-on (you hear a big dull 'thud' from the transformer) as the uncharged capacitors present a momentary short circuit. Also it'll protect the rectifier(s) somewhat.


Edt: If you have an storage/digital oscilloscope available (or ask a local techie to help), you can use it to measure the power rails (lines) when driving the motors. Usually any dips in the voltage can be elliminated with larger capacitors, but there's a limit to this, where you must move to a higher power rating transformer.

[mike_Kilroy]

nice comments vanjast. fyi, some drive mfgrs are now using NTCs only (in parallel if necessary) in series with the ps before caps to limit inrush - without the time delay short em out relay.....also I have added extra caps to old DC power supplies for a tad more decel capability - with series NTCs to limit the inrush on power on...... so customer did not have to increase his input contactor size......

[zeed]

Well,simple power supply just got more complicated...
This power supply will only power the motors (3x3A) as I am planing to use pc power supply to power 5V for drivers and 12 for and other stuff like fans etc.

Was planing to use 2x10k uf 100V capacitors,now just have to get some NTC resistors and this should be it...
I dont have any 40V relays so will have to go without them.
Is this schematic ok

[mike_Kilroy]

I dont think you need NTC current limiter for such a small power supply...

[zeed]

Just wanted to share my supply so far:


Edit:The 2 transformers on schematic are actualy only one transformer with multiple outputs so I just took 9V ac output to power second bridge to get 12Vdc for powering the 12V relay.Also planing to put led diode on the relay so i can see that all is ok
Also missing here is the fuse on the 220V line..