Power Supply Build for Router CNC

[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.

[Wobblycogs]

Yeah, I've got that page open and had a bit of a read of it.

I've looked at quite a few designs now and there's almost an even split between the resistor camp and the thermistor camp. There are certainly benefits to using resistors but I don't like the failure modes much. With a thermistor if the relay fails to close at least the resistance should drop to a fairly low level, with the resistor you're always pumping out the full power until, hopefully, the resistor goes open circuit. That strikes me as a bad design. The thermal fuse isn't really a solution either as it probably won't operate fast enough to prevent serious overheating.

This is essentially why I'm seriously considering just using a thermistor and living with the waste heat. All the disconnecting designs just move the potential fault somewhere else and if you want the design to fail safe you have to design for the possibility the resistors / thermistor will be taking the full load.

[Wobblycogs]

Well I've finished the power supply and most of the rest of the wiring for the electrical cabinet, there's just a few wires left to run once I get some connectors for the steppers.

I was just double checking my earthing plan and I came across something I'd not considered before. Should the neutral (DC common) of the secondary be bound to earth at a single point (perhaps with a 1000 ohm resistor in series) to prevent it floating to a high potential above earth?

If you Google for "linear power supply" there seems to be about an even split between circuit diagrams that show neutral bound to earth and those that don't. The only reason I have seen for not doing it is to prevent noise but this is a power supply for steppers there's noise all over the place anyway.

I think I can see where a problem would occur. Without the earth bond on neutral what's to stop the neutral rising to, say, +100V relative to earth as long as the positive is always 70V higher? Having said that my natural aversion to letting the blue smoke out makes me hesitant to connect a cable from the secondary side of the coil to the same earth the primary side is using - I don't properly understand why this can't act as a short.

[EddyCurrent]

You shouldn't use a resistor between earth and neutral.
If it's a linear power supply this implies a transformer, rectifier, capacitor, setup and there is no problem connecting both primary and secondary to earth because they are not electrically connected, they are connected magnetically through the transformer metal core.
You are correct about some people connecting the -ve to earth and some don't, it's the same with 110v control transformers in larger installations.
One thing about having one pole to earth is that if there is an earth fault on the other pole it will blow the fuse but if one pole is not earthed the fuse will not blow because there is no circuit. In the second instance two earth faults have to be present, one on each pole before the fuse blows, it's then the same as shorting the poles together.