[sdeering]

I’ve been kicking the idea around for about 3 years.

I got a few of quick questions.

1976 Boss5

I'm using the original steppers. Drivers? Larken or Gecko?
I have herd of some geckos burning up because of table inertia. I’m kind of on the fence hear.

What breakout board there are a few different ones? ( I will be using Mach 2/3). Ease of installation, compatibility with mach, ability to add another axis would be good. I'm not looking for any warp drive speed I'm just a hobbiest.

I have my power supply theoretically made to get 70V DC out. With a toroid about 8" across my guess 1000 or 1500 VA. The trans puts out 50V AC. I have 2 caps rated at 27000uF -10 +75%
Rated 75V surge 90V

Should I use 1 or 2 caps? If 2 will I have current inrush problems, is there a way around it?

Any other advice would greatly be appreciated.
Stephen

[jdelaney44]

So you are in the 15 - 20 amps DC range? The rule of thumb is 1000uf - 2000uf / amp. I've seen guys using 1000uf with no issue. I am using a little less than 2000 uf. So you could be OK with one cap. If you want to make sure the ripple is really low go with two. I come from the better safe than sorry camp on this one.

Yes, there is math to figure out the ripple under a specific load. Problem with this is that your load will vary dynamically under real conditions. So the saged wisdom seems to be build for the worst case you can afford to build for.

27000uf looks like a dead short for a fraction of a second and tapers off quickly. 18,000 will incinerate a 22 ga test lead. Yes, it was spectacular! I suspect it will blow a 20A fuse.

I had my 56,000uf worth blow 30A slow blow fuses on both legs of my 220.

In rush can be regulated with what are called negative temperature coefficient (NTC) thermistors.

Or you can build an active circuit that detects the voltage coming back up as the caps charge up and then cut out an inrush limiting resistor. My first supply uses and active circuit. Kinda complex and expensive.

I am building one for someone now using the NTC Thermistors. They are about $6.00 each. These come highly recommended from several sources.

Also....your cap voltage is really close to your supply voltage. I would spend some time assessing this for safety. If you can return them, I would get something closer to 100V. Either that or knock off the trans voltage by 5V to give yourself some room. My personal, non-engineer, thoughts.

Hope that helps.

Best,
-jd

[UKRobotics]

I was discussing my own CNC power supply reqirements with a dc motor/electronics specalist at a local company and he seemed quite certain that inrush from the capacitor would be the least of my problems. He went on to explain that large transformers draw massive surge currents when first switched on regardless of any load further down the line. The solution was to use special 'D' type breakers which are designed to handle such loads.

[sdeering]

Thanks guys I will try one and see if the breaker flips.
Stephen