PDA

View Full Version : PSU question



Dwayne Dibbley
31-12-2012, 09:02 PM
I have four 42BYGHM809 http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Robotics/42BYGHM809.PDF stepper motors all driven with four A4988's.

How do you calculate what PSU i should get? the A4988 can take 2A and 35V but i have not a clue how this translates to the 1.7A/phase and 3.06v for the spec of the steppers?

based on the driver i would have comeup with a 35V 8A PSU? but im sure its not that easy?

Thanks for looking

John S
31-12-2012, 09:41 PM
24 volts max and 5 to 6 amps

Dwayne Dibbley
31-12-2012, 09:59 PM
24 volts max and 5 to 6 amps

Thanks John, is there a formula on how you reached that figure?

JAZZCNC
01-01-2013, 03:35 PM
Think you'll find the formula is 20yrs experience and several dozen blown Allegro(A4988) chips.?

The Amp's is simple multiply the number of steppers Amp's plus a bit spare capacity.

The lower voltage is safety margin due to the allegro chip being notoriously bad at letting out the Magic smoke any higher. Coupled with the fact the next standard PSU will be 36V.

Robin Hewitt
01-01-2013, 04:34 PM
There is a wonderful gizmo called the "crowbar circuit". You put a fat Diac across the PSU output and a zenner diode to trigger it should the volts rise above your max threshold.

When you trigger a Diac it stays triggered, hard on, until the Volts go away. A cheap Diac can handle lots of Amps.

In this case the Volts go away because the fuse blows. You then replace a 10p fuse rather than expensive hardware.
It's called a crowbar because the effect is like dropping a crowbar across the power input terminals :beer:

Dwayne Dibbley
01-01-2013, 06:24 PM
The Amp's is simple multiply the number of steppers Amp's plus a bit spare capacity.

in that case as i have four steppers i should look for a 7-8A ( 1.7x4 + a bit ) ideally?

Thanks

John S
01-01-2013, 06:31 PM
No you want about 60% of the total because not all 4 motors are working under max load at any one time and even though your motorsare 1.7 amps they will never pull that current.
You can go larger but it will not do anything.

Voltage as Jazz says is based on what the chips will take before they catch fire, stated voltages is always max and no room for any errors.

Dwayne Dibbley
01-01-2013, 11:10 PM
Thanks for the pointers, what do you need to look for in a PSU, some are 20 quid and others 100 quid does the wattage come in to play?

John S
01-01-2013, 11:21 PM
Wattage is amerage, it's Watts Law, so say 24v 200W = 200 / 24 = 8.3333 amp

cropwell
02-01-2013, 12:04 AM
Wattage is amerage, it's Watts Law, so say 24v 200W = 200 / 24 = 8.3333 amp

Two simple formulae for DC electricity in resistive circuits -


Ohms Law is Volts = Current x Resistance (V=I x R) and there are two transpositions of that:- V/I = R, V/R = I.
Power (Watts) Law is W = V x I and you can transpose that in the same way. W/V = I, W/I = V


But when you get to AC or with capacitors or inductors in the circuit then it gets complicated, and I never did that at school.

Cheers,

Rob-T

Robin Hewitt
02-01-2013, 06:05 AM
Wattage is amerage, it's Watts Law, so say 24v 200W = 200 / 24 = 8.3333 amp

What's Watts Law? What!

I don't think he had a law, but he did have a fundamental unit named after him which is better.
I am so pleased we don't measure anything in Eddisons :beer:

cropwell
02-01-2013, 10:35 AM
He had both a law and a unit. That was status in the days before personalised number plates !

How many dynes per square fortnight in an Eddison ?
Is the unit measure of vacuum a Hoover ?

Cheers !!

Rob

Dwayne Dibbley
02-01-2013, 10:39 AM
Something along these lines what i should be looking for? free shipping CE approved 24V6A switching mode power supply(S 145 24)-in Switching Power Supply from Electrical Equipment & Supplies on Aliexpress.com (http://www.aliexpress.com/item/free-shipping-CE-approved-24V6A-switching-mode-power-supply-S-145-24/599043508.html)

Thanks again

Dwayne Dibbley
13-02-2013, 08:12 PM
OK now have my PSU 24V 6A and my stepper drivers 2.5A, what wire is recommended for the stepper motors SY or CY? 0.75mm2 4 core? and do you just add this wire to the existing stepper tails or solder direct into the stepper?

Thanks again

JAZZCNC
13-02-2013, 08:27 PM
Connect to existing wires DONT take the steppers apart you'll wreck them.!

Dwayne Dibbley
13-02-2013, 08:48 PM
Ok wire to existing, what about the cable type for the stepper extension

thanks

JAZZCNC
13-02-2013, 09:15 PM
Ok wire to existing, what about the cable type for the stepper extension

thanks

Sorry didn't see that bit.! . . . CY cable is better has it's more flexible and better shielded and 0.75mm2 will be ok for your needs.