1. Two last questions on my power supply project. Is it worth putting an inrush current limiter http://www.produktinfo.conrad.com/da...B_250V_16A.pdf in with the power supply. The transformer will be 500va 2x18v 35amp 600v bridge rectifier 3 x 4700uf 100v caps. Is a heat sink needed for the rectifier or is it enough to bolt the rectifier to the metal case ?

Andrew

2. Originally Posted by the great waldo
Two last questions on my power supply project. Is it worth putting an inrush current limiter http://www.produktinfo.conrad.com/da...B_250V_16A.pdf in with the power supply.
Your call. Personally I've used an NTC 10D-15 thermistor in series with the transformer primary (mine is an 800VA toroidal transformer). I've not put the PSU under significant load (yet!) but it's working fine. If I have problems with the NTC heating then I can replace at a later date with a similar limiter to what you linked, or place a relay across it. But I'm going to test it before spending £40.

Originally Posted by the great waldo
The transformer will be 500va 2x18v 35amp 600v bridge rectifier 3 x 4700uf 100v caps. Is a heat sink needed for the rectifier or is it enough to bolt the rectifier to the metal case ?
You need the data sheet for the rectifier to answer that one. And your case size

I'm going to use some wet-fingered maths. Say you're dropping 1V across each diode element in the rectifier at full load, and two diodes in conduction at any time, so you're dropping 2V (out of 36) at a full-load current of (500/36) about 14A. That's 2x14 (=28, let's call it 30) watts dissipated in the bridge. Use https://www.heatsinkcalculator.com/f...alculator.html - you may need to guess at some of the numbers. Using 1.2mm mild steel and a 100mm x 100mm plate, that gives a junction temperature of 450 degrees C. You're aiming for a junction temperature below 150 degrees (better below 100 degrees). So - a 1.2mm 100x100 plate is clearly insufficient under full load. But you need to run the numbers for your enclosure. Based on a wet-finger keep-it-below-100-with-25-C-ambient, i.e. 75 degrees head-room, you need a thermal resistance of 75/30 = 2.5 degrees/Watt, which is no small heatsink.

One question. Why 2x18V secondaries in series?, why not 2x36V secondaries with independent rectifiers and smoothing capacitors. That way the thermal load through e.g. the BR is halved, for the cost of essentially 1 additional BR. Copper losses in the transformer and wiring are reduced. Use one half to drive 2 motors, the other half to drive the other motor (plus growth for a fourth). Common the 0V lines if you want.
Last edited by Doddy; 10-05-2018 at 07:29 AM.

3. Hi Doddy

I'll be wiring the transformer for 36 volts, I hadn't thought about the upgrade to a 4th axis. I'll use a heatsink (they aren't so expensive and fix it to the case ) I'll have a look at the thermistor. I'll be using a couple of fans in the case for good cooling. My old control box has a switching power supply and for some reason the inbuilt fan in the power supply really gets on my nerves with the noise it makes.

Cheers

Andrew

4. Hello Doddy
Hmmm!!

Youv'e got me thinking about the 2x36 transformers. Do you think 2x 4700uf on each output would be enough. The Mx3660 has a wiring diagram to connect a 4th stepper. The question is would I ever need it and or would I be forever building new control boxes to upgrade. It's a holiday here in Austria which gives me some time to check this out. I'm definitely warming to the 2x36 v transformer excuse the pun.
Cheers

Andrew

5. Originally Posted by the great waldo
Hello Doddy
Hmmm!!

Youv'e got me thinking about the 2x36 transformers. Do you think 2x 4700uf on each output would be enough. The Mx3660 has a wiring diagram to connect a 4th stepper. The question is would I ever need it and or would I be forever building new control boxes to upgrade. It's a holiday here in Austria which gives me some time to check this out. I'm definitely warming to the 2x36 v transformer excuse the pun.
Cheers

Andrew
Do I think?, put it this way - I've used 10,000uF on each of my 2x50V supplies - 2x4700uF is pretty much the same, so I think the answer is a yes.

Cheers
Andrew

7. Hi Doddy

I was having a beer last night and a thought came to me that I can't use the double rectifier supply as the mx3660 has only one power supply input connector. I should have noticed that before.

Cheers

Andrew

8. Originally Posted by the great waldo
Hi Doddy

I was having a beer last night and a thought came to me that I can't use the double rectifier supply as the mx3660 has only one power supply input connector. I should have noticed that before.

Cheers

Andrew
Aha, an all-in-one?, yup, fair cop.

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