Power supply questions

1. There should be no problem using a suitably rated switching supply vs. a linear. xVDC out at xA is xVDC out at xA reguardless of the supply type.

1. There should be a current rating for the coils, or at least a watt rating that you can derive the current from. Total the amps of the valves and add 10% for a safety margin.

2. MPJA, All Electronics, Jameco, Surplus Center, etc.

Pete C.

Tom Gardner wrote:

Shouldn't be a problem. I've heard of extra noise from switching supplies causing problems with sensitive digital circuits but solenoids aren't sensitive.

Depends on the rating for the valves. Do you have specs for them? Wattage?

Best Regards, Keith Marshall snipped-for-privacy@progressivelogic.com

"I'm not grown up enough to be so old!"

Probably. Some switchers have a minimum current draw.

ummm. As many as they need?

Marlin P. Jones, MECI, maybe Electronic Goldmine

But they only need to sell _one_....

Kevin Gallimore

Since they are DC, just measure resistance with any DVM. I= E/R (current in amps = voltage / resistance in Ohms)

Jon

Yes, you can use a switcher on valves, with a "but": some switchers expect a minimum current to maintain regulation. If there might be times when no valves are actuated, you might need to put a parasitic (resistive) load to maintain minimum current draw. Ditto on the servomotors if there will be times when they're drawing no current.

For 48 volt power supplies, check with phone people. There are a zillion old PBXs and key systems out there, virtually all of them with hefty 24 volt power supplies (mostly analog). Hell; if you happen to live is S. Florida, check with me.

Vaughn

I am using an autotransformer with bridge rectifier and capacitor for this purpose. Great price/performance. Google my earlier threads on "autotransformer".

Herbach and Rademan are in Philly. A lot closer to you than they are to me. But I just looked at their 9 pages of power supplies and did not find a single 48 volt supply.

Dan

Tom Gardner (nospam) wrote:

Two 24's in series should work ok but MPJA has 3 48's to offer:

Best Regards, Keith Marshall snipped-for-privacy@progressivelogic.com

"I'm not grown up enough to be so old!"

Tom

I dont suppose you are interested in making your own supplies. But, I'd give you a 17 amp 48 volt battery charger and a sorensen QSA 18 - 3.2 22VDC

2 1/2 amp supply.

I'm in the Los Angeles area.

Jerry

A little something the other posters haven't mentioned...

When you shut the current off to a solenoid, the magnetic field in it collapses and induces a voltage across the coil which can be pretty significant. This can produce a substantial arc at the switch contacts and a spike that can damage a switching supply (for technical reasons that would take a somewhat long-winded discussion to explain, switchers tend to be more susceptible to damage from transients...).

To prevent this problem, put a diode across the solenoid in the reverse direction of the normal current flow to damp the spike (some solenoids have diodes built into them...).

Also, it is a good idea to de-rate the supply about one-third. In other words, if you need a maximum of five amps, use at least a 7 1/2 amp supply. They last a LOT longer that way.

Jerry

1) Yes, assuming it has sufficient current. If you have large capacitors to store the current, you can make the smaller supply act like it's larger. The higher the duty cycle, the more capacity you need. 2) You need to check the ratings on the coils to find out the current. 3) Plenty of suppliers have been mentioned to get power supplies.

Got a warning that hasn't been mentioned: When DC coils are turned off, there's inductive kickback from the collapsing magnetic field, usually a higher voltage than you fed it, but in a negative direction. Decent current kick, too. You need to make sure the coils either have diodes in the coil (you'll usually see a (+) and (-) on the wires or package if that's the case) or that you provide them somehow. The closer to the coil the better. These diodes are heavy duty diodes, hooked up in the reverse direction. Won't pass current while you've got them in reverse polarity, but will dampen the kick back and save your power supply and control circuits.

Only if you can trust *everybody* there to not fiddle with the inviting big knob (or even small "steering wheel" if a big one).

I was in a sound lab in a Latin American country (with all US-made sound equipment. Since the power line was rather soft, they had a 240V Variac driven by its center tap to allow boosting the voltage as necessary. A small kid (one of the kids of the fellow who had the rights to go there and use the equipment) was with us, and while we were firing up an old Magnecorder, he gravitated towards the big Variac with the "steering wheel" knob, and spun it full CW. The magic smoke escaped from the selenium rectifier in the Magnecorder, and I wound up replacing it with a silicon rectifier. We were lucky that nothing else was on at the time.

So -- perhaps adjust it, remove the knob, and fit a clamp to the shaft.

And of course, you don't have isolation from the power line, so be careful of that.

Enjoy, DoN.

I think you're confusing autotransformer and variac, they are not the same thing. Autotransformers do not have adjustment knobs (taps perhaps), variacs do.

Pete C.

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Variacs are adjustable autotransformers, and are the most common kind of autotransformer generally found. And finding a tapped autotransformer which is right for 48 VDC output is a bit more tricky, so a variable autotransformer (e.g. Variac or Powerstat) is more likely.

I agree that if Richard Kinch was writing about fixed tap autotransformers, yes, the knob twiddling problem does not exist -- but the lack of isolation from the power lines still does with *any* autotransformer.

Enjoy, DoN.

I am using variable autotransformers to run Geckodrives and PMDC motors. Variac is a brand name (and even defunct?), so I don't like to refer to that term.(mine are mostly Powerstat brand).

We discussed the isolation issue already, a non-issue in my opinion, so I won't repeat my analysis.

Yes, you want to lock or limit the adjustment range somehow to avoid overvoltage. I would assume that anyone building a CNC system could figure out how to improvise a simple limit mechanism. I'll admit that so far I have just been careful, but $400 worth of Geckos *are* at risk. But it sure is a cheap way to get kilowatts of DC, less than $100 in surplus parts.

By the way, if the isolation issue is that critical to you, there are lots of surplus gutsy transformers that are cheap because they are some random ratio that doesn't have much application, but with a variable autotransformer following, you would have whatever isolated voltage you like.

The "buck-boost" type of autotransformers are quite common and do not have adjustment knobs. Granted they are not typically used a a standalone power supply...

Pete C.

Richard J K>

Keith! I went to goldmine and they had 24v 2a supplies for $1.99!!! I got

5...want one? Thanks!

5...want one? Thanks!