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Toolman321
20-09-2017, 05:48 PM
Hello
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
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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
Lee

Doddy
20-09-2017, 06:40 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.

Toolman321
20-09-2017, 07:06 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

Doddy
20-09-2017, 07:39 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!

Toolman321
20-09-2017, 07:57 PM
All of the wires look the same to me.
Thanks lee

Doddy
20-09-2017, 08:22 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)

Toolman321
20-09-2017, 09:30 PM
I will try that and report back
Many thanks lee