Power Supply Build for Router CNC

[Wobblycogs]

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.

[EddyCurrent]

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

[Wobblycogs]

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.

[EddyCurrent]

You didn't mention trying a time delay fuse

[Wobblycogs]

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.

[EddyCurrent]

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.
http://mingrong1981.en.made-in-china...5-10-3-38.html

These may be better

Your Search Results | Farnell UK | Results

[Boscoe]

You utterly need soft start for a 1000VA transformer. You will pull in the order of hundreds of amps when it's first turned on. Remember when the field hasn't established in the transformer you only have the DCR of the primary to limit current. This DCR may be anything from 2 to 10R so you may well pull ~160Apk when first turned on!

[Rogue]

I'll be interested to see how you tackle this, Wobbly, as I want to incorporate a similar thing.

Your build thread is excellent, by the way!

[Wobblycogs]

I'll probably end up building a little circuit something like this. Initially I'll just wire a thermistor in series and see how much heat it dumps into the case, I suspect it will be an unacceptable amount. The thermistor I've got in mind is this. My concern is that if the relay fails to close then power will continue to go through the thermistor and I wouldn't be aware of it slowly trying to burn the house down. The alternative situation where the relay fails to open should blow a fuse / trip a breaker on start up so I should spot the problem fairly quickly.

I'll keep this thread up to date either way though.

[Rogue]

I was wondering if you had seen this article: link to article. It seems to warn away from using thermistors and recommends using resistors (along with appropriate calculations).

I can't comment on the accuracy of the article but it made for an interesting read.