Confirming PSU spec for steppers.

[JAZZCNC]

From reading around a bit more I think I'm sold on the AM882 drivers. Currently got 3 of them (+ a lead) sitting in my basket at Zapp. Wisdom dictates that I don't pull the trigger until the end of the month, but my trigger finger is getting fairly itchy at this stage

Do it because sure has Egg's is egg's they'll be out of stock when you want them.! . . PUSH THE BUTTON!!

[Rogue]

Do it because sure has Egg's is egg's they'll be out of stock when you want them.! . . PUSH THE BUTTON!!

Don't tempt me! I really, really shouldn't. If the drivers are out of stock then I can always move the ballscrew purchase forward by a month.

[Swarfing]

House prices are rising, everything else will follow?

[Rogue]

House prices are rising, everything else will follow?

That is very possible but if an unexpected expense crops up and I've already cut into my "emergency petrol money", then I can't travel. If I can't travel, I can't work and can't invoice clients. That makes my bank manager sad and my wife furious

[JAZZCNC]

That makes my bank manager sad and my wife furious

That's easy sorted.?? . . . .change both.!!

Now on another serious note.? if you haven't bought the screws yet then you don't want to be buying the electrics. Only buy the electrics when you need to fit them has just having them to spin on the bench is wasting the warranty and you won't learn anything from it.!!

[Swarfing]

Rouge the only thing i can say to that is what is more important you? On another site i saws omething that i liked, just came out on the market. I decided then and there to buy it. 6 hours later they doubled the price? and that was from China, go figure?

I know what you mean about work though it has to come first so it sounds like you have answered your own question and it will have to stay on hold. Hope you get there soon, bank managers are a PITA to please.

[Rogue]

Now on another serious note.? if you haven't bought the screws yet then you don't want to be buying the electrics. Only buy the electrics when you need to fit them has just having them to spin on the bench is wasting the warranty and you won't learn anything from it.!!

Wise words indeed, though (i) the steppers are already outside of a year old and (ii) in any event I've got 6 years to bring a claim for breach of SoGA; warranties do not do what most people think they do. Entirely academic as I'd not pursue that route for a hundred quid's worth of motors, and probably not even for three hundred quid's worth of drivers. Point taken, though, hassle is worth avoiding where possible.

In fact, taking that on board, I might shift my plans around a bit. I'll still finish up the PSU first because it's nearly finished anyway!

[Robin Hewitt]

In fact, taking that on board, I might shift my plans around a bit. I'll still finish up the PSU first because it's nearly finished anyway!

If you have already bought the toroidal I say go for it, something else could crock the project and fixing the PSU would only waste money.
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The problem with transformers is that when you rectify them to DC the voltage drops when you draw power. You can stuff capacitors in until the cows come home but if you don't rectify all 3 phases, the volts will sag.
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Similarly the volts will soar if you get a mains surge so you really need to protect your drivers if they don't protect themselves. Fortunately protection is cheap and easy if you use a crowbar circuit and wire a diac to blow a fuse on the DC side if you go over volts. A crowbar circuit is cheap and simple. Leave yourself a note by the fuse because it is very easy to forget it is there even after it just saved you hundreds of pounds

[Rogue]

Quick question in relation to a soft start circuit on the 70V system.

In all the various circuits I've seen, there is some form of resistance inline before the transformer which, after a very short delay, is then bypassed. Many circuits also then isolate the resistors, though there seems to be various thoughts on whether this is really necessary.

My question relates to using a DPDT relay for this: when the relay activates, the first pole (NO) is closed, creating the bypass. The second pole (NC) which leads to the resistors is opened, taking them out of the circuit. In this arrangement, there is presumably the possibility that the second pole could open before the first pole has closed, so there is no power flowing at all for a very brief time. If this is correct, is this likely to have a significant impact given the timescales involved? Is any switching delay likely to be long enough to negate the effect of the soft start circuit?

If not practical then fair enough, but it would be neater to use one relay to do both functions!

[Jonathan]

If the rating of your transformer is less than around 600VA, I wouldn't bother with any soft start circuit.
Thermistor is an easy option, as it's resistance decreases with temperature so no real need to take it out of the circuit.
You can use a relay to short circuit the fixed resistor. Essentially you want to switch the power on, wait about a second, then switch the relay on. You've reminded me of a circuit I came up with when I was about 10 - put a largeish capacitor in parallel with the relay coil and charge it via a resistor, then wire the relay output to break the capacitor charging current and you end up with a simple oscillator! In your case you would just use the capacitor and resistor to make a 1 second delay, so calculate it such that it takes 1 second for the capacitor voltage to reach the threshold voltage of the relay coil. Remember to include the resistance of the relay coil in parrallel with the capacitor in your calculation. You can power the relay coil from the 70V output via a potential divider. It's a bit of a crude way to do it, but easy to understand. Bonus points for using a solid state relay.