View Full Version : Please help me build a power supply safely

20-09-2017, 06:48 PM
I would like to build a power supply for 3 x dm542t stepper drives they require 50v max and I've learned that I will need to set the current for each drive at 2 amps.
I have this transformer left over from my last build, I replaced this with a regulated power supply because I didn't know how to wire it up.
However somebody recently told me that with some simple electronics i.e. A bridge rectifier/capacitor/fuses, i would be able to re-use it.2282722828

I'm unable to find any information about this transformer on the internet and so I'd like to ask for your help please!

If anyone could explain to me the taps and which ones to connect to, and what size capacitor I will need that would be great
Many thanks

20-09-2017, 07:40 PM
The primary (the bit that you plug into the mains) will be the series-wound 10-100-0-100-10-20 windings (think of each pin relative to the 0v point, with the pins on the left being a notional -, and the pins on the right being a notional +, then the left-most '10' is -10-100 (=-110) wrt to the 0v line, and the right most '20' is 100+10+20 = 130V, and the difference between these is 130 - (-110) = 240V. If that makes sense?

Then the secondaries, you have a 20-0-20 winding (essentially, a 40V, centre-tapped winding) and two individual 50V windings, which I guess is what you're looking to use. The problem is that this 50V is the RMS value of the AC output, whereas your DM542t - if these need a maximum of 50VDC, then the peak voltage from the transformer under no/low load is effectively 50 x 1.414 = ~70V (drop a volt over the bridge rectifier) and the smoothing capacitor would rise to this. This is likely enough to damage the drivers.

The problem is how to lower the voltage to a safe level for the drivers - which is a bit more complex than the bridge/capacitor solution proposed. Plus, if you're trying to drop say 25V from this you're essentially trying to dissipate a third of the power of the transformer somehow (i.e. big heatsinks = ).

Do you already own the drivers?, if not it might be better looking for drivers that support 80V.

20-09-2017, 08:06 PM
Do you already own the drivers?, if not it might be better looking for drivers that support 80V.

Yeah I have the drives already, given that I'm probably better of getting a toroidal type transformer with a more desirable output?
Could you tell me if I can use the 40v center tapped output please?

Thanks lee

20-09-2017, 08:39 PM
The 40V, centre tapped output would still be a little on the high-side (voltage-wise) - around 57V... you'll drop a volt under no-load across the bridge (closer to 2V under load), so call that 55VDC, and you're still above the rated supply of the 542s. It's only 10%, but I'd guess the drivers wouldn't last as long as you might otherwise expect (unlikely to go Bang, but more likely to fail after months/years of being stressed beyond the design limit).

The main problem is for me to understand the label for the 20-0-20 winding. I'm inclined to think the "2,5/8" refers to an asymetrical current capacity - unusual, but not unknown, of 2.5A on one tapping and 8A on the other. This would limit the current draw for a 40V RMS supply to the lower 2.5A, which feels a bit on the low side for what you're trying to use it for.

Can you see the copper winding leading to the terminals?, if there's an 8A and a 2.5A tapping then the 8A should be evident with a heavier gauge wire. There's then the prospect of creating two 20 V supplies - one at 2.5A and a second at 8A, which you could balance across the steppers.

But, you're throwing 80% of the transformer capacity away, it feels like a bit of a waste!

20-09-2017, 08:57 PM
All of the wires look the same to me.
Thanks lee

20-09-2017, 09:22 PM
Random experiment - get a standard mains incandescent bulb, attach 240VAC to the primary, connect three wires to the 20,0,20 secondaries, then dab each 20-0 pair across the bulb terminals. Hopefully one pair will be brighter - that'll be the 8A secondary (I have seen an advert online that supports the asymmetrical current rating)

20-09-2017, 10:30 PM
I will try that and report back
Many thanks lee