# Thread: Power Supply Build for Router CNC

1. Originally Posted by m_c
If you want to be such a pedant, a sine wave doesn't necessarily have an average of zero. It can be offset from zero. You've just assumed that all sine waves are drawn with zero as the base line.
That's just changing your reference point - if it's got a DC offset then it's a sine wave with an offset, not a sine wave. I thought it was misleading to just say 'squrt(2) is essentially just the average value of a sine wave'.

Originally Posted by m_c
As I'm sure you're aware, I meant that the average value for 90degree either side of a sine wave peak or trough i.e a 180degree section, is sqrt(2).
No that's a new one to me, as it's incorrect!
As I'm sure you're aware, to find the average value of a function you integrate it over the section in question, and divide by the 'length' of that section. So lets do it:
Let y=sin(x)
We want '90degree[sic] either side of a sine wave peak', so in radians the limits of out integral are 0 and pi and we integrate over pi:
Average=1/pi*Integral(sin(x)dx) from 0 to pi.
Average=1/pi*(-cos(pi)--cos(0))
Average=2/pi

2/pi is not equal to sqrt(2).

To get the correct RMS value of a function, you first square the function, then find the mean of the function (using the above method) then find the square root of it. Hence the name - root mean square.

2. Attachment 10254

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3. I can't believe my last post here was in September and it's November already!

I've been away and done some recalculations based on your advice, before I rush out and buy parts though it'd be great if someone could cast their eye over the spec.

The transformer is still 1000VA but has been dropped down to 24+24 wired in series. That should give me a maximum voltage at the drivers of 73V assuming the mains voltage is at it's upper limit of 253V.

The relay for the bleed capacitors has been ditched in favour of always connected resistors. I've gone for 6k, 1W resistors, one per capacitor. The calculation shows a bleed time of 30 seconds with 0.88W being dissipated per resistor. The capacitors are the same 10mF one's I linked to earlier.

The neon on the primary side of the transformer has gone and I've got a blue LED with the 15k, 0.5W resistor on the secondary side.

What should I attach all the components to? I've got some 4mm aluminium plate, wood, plastic, something else e.g. thick PCB blank.

Thanks

4. I tried a bleed resistor and it's surprising how much heat 1watt gives off but in reality it's not required because the stepper drives drain the capacitor quite quickly.

Traditionally it's mounted on an aluminium plate about 3mm thick which allows it to be bent quite easily and obviously with enough space to fit everything else on. The vertical part protects the transformer from mechanical knocks which could easily damage it. The capacitor I used was mounted vertically as it had a one bolt fixing at the bottom, the rectifier is bolted to the aluminium with a little heatsink compound added, terminals and fuses are whatever you decide but I found a 4 way one of these to be useful and compact, it's 12volt rated but I decided it would be fine for my 60v supply, you need to decide yourself. 2/4/6/8/10/12 WAY + HEAVY DUTY AUTO FUSE BOX/HOLDER 12V VOLT STANDARD BLADE | eBay
Last edited by EddyCurrent; 15-11-2013 at 12:57 PM.

5. Thanks Eddy,

Just about the only other electronics project I've done was build a 50 LED lamp (a bit like this). IIRC that required a 2W resistor and I was very surprised how hot it got and was why I was looking at using a relay to control the bleed resistors. I realize the steppers will normally dissipate the power in the capacitors but I want to make sure the power supply reverts to a safe state even if the steppers aren't connected.

The aluminium plate looks like a great way to mount everything.

6. That's a nice hypnotic kind of lamp

You can see how my power supply turned out here
http://www.mycncuk.com/forums/electr...html#post51392

7. Over Christmas I've got the power supply built and most of the electrical cabinet wired up but I've got a little bit of a problem which has left me scratching my head.

As far as I can tell the power supply is correctly wired but I'm getting an annoying number of nuisance trips from the breaker it's running off. The transformer I'm using is this 1000VA 24+24V part and at the moment the only load is the bleed resistors.

I initially wired it up with an 8A regular fuse but after blowing a couple of fuses I switched it over to a 6A Type C MCB which I had and this stayed in about 70% of the time on power up. Since the Type C was almost enough I figured a 6A Type D would do the trick so off to the shops, one quick swap over later and.... trip. The Type D stays in 90 to 95% of the time but I'm still getting some trips on power up.

I suppose it's possible there's a wiring fault but I've had the power supply powered up for 40 minutes and the blue smoke didn't escape so I think the problem is just the initial inrush current. I found somewhere that gave a rule of thumb for the inrush as 8* the nominal current for 0.1 seconds for transformers under 2500VA. If I'm understanding it correctly new for this coil that would be 4.3 * 8 = 35A which is well inside the 60A my Type D breaker should handle as an instantaneous load. This page gives a maximum inrush of 104A if the AC voltage is at 0V at power on, that seems more believable considering the trips I'm getting.

Right now I'm kicking myself for getting such a big transformer, I think a more moderately sized one would probably have run fine on the D6 but it's too late to change that now. I think the solution is probably to fit an NTC thermistor in series with the primary supply to provide a soft start. I'm hesitant to do that as I've not seen any other power supply builds like that so I'm concerned I'm doing something wrong.

8. You didn't mention trying a time delay fuse

9. No, I haven't (intentionally) tried a slow blow fuse yet.

I initially tried these RT19-8 8A fuses. I think they are Type T but as you know Chalon aren't exactly big on describing the products they have on offer.

10. This link shows they come in 3 time delays and looking at the Chalon pictures those are gG or Gg and that looks to be 'Normal' type.

These may be better

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Last edited by EddyCurrent; 02-01-2014 at 08:49 PM.

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