Electrical Question from Mechanical Engineer 3

I suspect you are looking for the wrong type of supply. The fact that you found rack mount supplies and such, adds to my suspicions.

A 30 Volt 7A supply puts you in the 250Watt range, which isn't terribly big by today's standards. As a matter of fact, most PC supplies are around 250 Watts but distributed over several diffeerent DC voltages.

Check out power supply manufacturers, such as Meanwell. You should be able to get a power supply in your wattage range for about $100. What I am not certain of is you can get one that has an adjustable output. At that price, it may be feasible to get a couple of supplies. They come in standards, such as 12V, 24V, 48V, etc.

These supplies would all be single phase 120/240Vac in with a DC output, so you can simply plug them into a wall socket or use your 220V single phase line to reduce the current draw.

Three phase power supplies with a transformer input can not be used on a single phase line. You could dump them on ebay, but there is little interest in 7.5V supplies except to gut them for parts. Sometimes a gift horse should be looked in the mouth.

One possibility is to hunt down a 1:3 phase converter, which will synthesize the remaining two phases. But, it'll probably be expensive and gigantic.

TTFN

FAQ731-376

Hello all,

Well, thanks for the responses. I was afraid my three phase supplies might not be of any use to me. I see these things opened up and note their high quality and it makes me wish for the best but I was prepared for what you have told me.

I will use the two single phase supplies and be happy that they work. Obviously, the big 7.5 volt, 300 amp, three phase supplies are too heavy to ship anywhere so maybe I will offer them at "hamfests" for parting out.

Thanks again for your input.

Regards, John.

Oh, actually, depending on how the 3-phase is configured, you might get away with running one or all phases from a single phase. Sometimes, bear in mind the "sometimes," all phases are run directly into full-wave rectifiers and the rectified voltage is used as the input to a voltage converter. Other that possibly higher ripple, the power supply might not care.

We had a computer chassis thus configured, so we were perplexed as to why we were getting proper power to the boards, but the cooling fans weren't working. Turned out the fans' AC motor was supplied by the miswired 3-phase to the box, but the power supply didn't care about the phasing error.

Anyways, you might want to peruse: it seems to imply that the "S" designation refers to single phase, and that a 3-phase should be marked 7.5T300

TTFN

FAQ731-376

IRstuff

Thanks for that observation!!

I have looked at that data sheet many times and had not noticed the "S" verses "T" designation before. One of the guys in our radio club stopped by and upon examining the heap of power supplies, told me that the triple toggle switches (on/off) on the face plate indicated that the supplies were three phase (?). So, I accepted this pronouncement as my "everybody knows more about this electricity stuff than I do" mentality kicked in.

I looked at the rear of the PS and the "AC power in" strip has "(1) AC, (2) AC and (3) Ground" marked on it and I take that to mean you are correct (?).

I am now willing to "plug one into" the 220 line and give it the old "Westinghouse smoke test". I assume the main breaker panel will keep me from burning down the house. If the fans run and I get an output voltage of 7.5 at the DC out strip (and there is no overwhelming cloud of smoke) I will begin to again count my blessings.

Hey, maybe this was the correct forum in which to ask my question <grin>. Thanks IRstuff and thanks to all for your patience with an electron challenged ME!!

Regards, John.

"...two 7.5 volt supplies in parallel will give me 15 volt capability..."

In addition to all the other points...

"Parallel" would double the current, not the voltage.

They'd have to be in series to double the voltage.

But even then you'd need to be very careful to make sure that the supplies are isolated from ground so that they can be put in series. If the negative terminal is grounded (not isolated), then you wouldn't want to connect it to the positive on the other.

You're probably okay, but just triple check first.

Be careful with a heavy current supply like that - turn the current limiter down! I have a big HP 8V 580A supply and it sent a carelessly discarded test lead into the next world in a shower of sparks and smoke. It could have been a nasty accident had someone been in the immediate area - a lesson learned.


----------------------------------

If we learn from our mistakes I'm getting a great education!

VE1BLL

"Parallel" would double the current, not the voltage.

They'd have to be in series to double the voltage.

Yes, thanks for that - I was so wrapped up in the phase issue" that I was not thinking (just like two back-to-back 1.5 volt dry cells in a flashlight provide 3 volts to the bulb).

If you have a moment, could you please have a look at the Internet site address given above for the data sheet?

There are grounding taps on the AC inlet strips of both PS's (each PS's "AC power in" strip has - "(1) AC, (2) AC and (3) Ground") and I thought that both of these "ground" lugs should be grounded directly (and the rack would also be grounded directly). I was thinking of connecting the positive DC lug from one PS to the negative DC lug on the other PS (10 gage wire?). Then wiring the two other DC output lugs (positive from one PS and negative from the other PS) to the appropriate well insulated DC busses in the rack (these would be tapped for DC power to the rack mounted radios). The radios (each is fused) would be also grounded to the rack.

ScottyUK

The current (amperage) is adjustable from 0 to 300 amps so I would want to keep it down to about 30 amps on each PS (I guess there would be more energy used running fans and generating heat than powering radios).

The AC inlet lugs have a cover to protect against inadvertent contact and the DC outlet lugs have similar covers.

If anybody sees any problems with this please post. I have a healthy respect for electricity - not a lot of hair left to curl (I hate it when that happens <Grin>).

Regards, John.

I can't see a data sheet as such, just an ad.

(I hope you didn't pay $1200 each.)

You should certainly ground the units' AC input section ground terminal to the normal commercial power ground. Not doing so might be dangerous, for example if an internal fault means the chassis becomes hot with AC. Ouch.

Lacking any further information about the outputs, connect a suitable light bulb from the positive of one supply to the negative of the other. If it lights up, then there must be a return path and thus the negative output is probably not isolated from ground.

It probably is isolated, but with 300 amps on tap and $2400 on the line (not to mention you) you want to be very careful.

Have a close look at the output terminals. It is common to have a removable strap so you ground either + or - outputs. If so, then that's good news and we'll have to discuss that next go-around.

-.-

VE1BLL,

John was refering to the link I posted for the datasheet from Lambda.

TTFN

FAQ731-376

Hello All,

Yes, that is the data sheet that I was referring to. VE1BLL please DO have a look at that sheet as it has much of the information you are asking about. The figures and photos shown on the data sheet will give you more information.


Actually I got the power supplies as a gift from an old buddy in Texas. They even wrapped them and put them on their truck and hauled them up to Pittsburgh for me (10 power supplies was all they could carry with other cargo). My buddy bought 12 racks (each 7 feet tall) that were each populated with six power supplies (various models) and a PC to control them (so long ago that the PC's in the racks were 486's with a 66MHz speed). I believe they were originally used in an electroplating operation but I am not sure. My buddy picked them up because he wanted to make welding machines for his grandsons - he ran out of grandsons before he ran out of power supplies. I have an 11 meter radio station north of Austin Texas at my buddy's ranch (7700 acres) and he has been using the radio equipment in my absence (he is planning to take his ham license examination soon and get on the "ham bands"). I guess playing on 11 meters has given him "the itch" to "get real" before the next sun spot cycle kicks in and he can legally talk to people around the world. We use one of the 20S30 supplies to power the radio equipment in Texas.

As you can see these power supplies are still rather pricey on the used equipment market and this is (I suppose) because of their quality components. I have a user's manual on the way from EMI-Lambda. The rear panel has a contact strip with "jumpers" connecting some of the contact on the strip - this to "program" the units (one PS in a group can control and the other PS's in the group act as slaves. The manual will give me information on the "programming".

You fellows have been of great help to me and I would again like to say thanks (and keep up the good work <grin>).

Regards, John.

Ahem, anything over 4 or 5 watts on "11 meters" is not kosher. Since I'm a licensed ham I can't really encourage such activities. Tsk-tsk-tsk. It's best to get the ham license ASAP because if you get caught, you might not be eligible after that.

The spec doesn't clearly say anything about the isolation of the outputs. Normally the spec sheet would say something like, "Isolated output, or can be strapped for either polarity relative to ground." But this one doesn't say.

So use the light bulb test (as I described earlier) to confirm if there is a common ground.

VE1BLL

Ahem, anything over 4 or 5 watts on "11 meters" is not kosher.

Hey! Don't look at me. I got my General ticket back in 1958. I had my CW speed up to 36 wpm (with a "bug") back in the day. However I am glad to see the resurgence in license applications since CW requirements were dropped (does that make me a "bad guy"?). I guess I am now better described as an "appliance operator" with an array of Kenwood transceivers (and a Yaesu HT dualbander for two meters). Nope, I stay strictly legal. Also, I have just received my third WiNRADIO, a new WR-G315e.

There is a story behind the acquisition of that "rig" in Texas from the estate of a departed Air Force General (I guess I "own it" but I have never "operated" it). The Russian final tubes in the linear are bigger than beer cans and it can can go to 800 watts on TX (albeit he usually never exceeds 300 watts to keep the bird body-count down)!!! The exciter ranges from 12 meters to 6 meters and he uses it mostly in the "unused portion" of the 25.5 through 26.5 MHz neighborhood (the international "11 meter" call frequency is 26.300 MHz). Most of the ranchers in central Texas (illegally) use marine radios (156.0 to 162.4 MHz) to coordinate their day to day work. It is a different world down there. He also has three tricycle landing gear ultralights to play with down there but I won't get into that story <grin>.

So use the light bulb test (as I described earlier) to confirm if there is a common ground.

I can do that, I seriously dislike electrical shocks - thanks for the tip. Seriously though, I am trying to reorganize the old radio room/office and rack mount much of the radio equipment to better use space. If I can replace a bunch of old "foot warmer" power supplies by placing these in the bottom of the rack it will go a long way toward making more space (for more radios?).

Thanks again and regards, John.

Hey Y'all,

I have been talking to another of my ham radio buddies (his PhD is in chemistry and he works for the local electric companies) regarding these power supplies and he made the following observations that I want to fly by y'all for additional comment:

"I can't imagine that you can hook these up in series or parallel without some kind of feedback loop. Have you ever heard of "current hogging"? I seem to remember tales of "matched" power supplies not being perfectly matched so the more sensitive one will "hog" current and the less sensitive one will back off. I'm unsure of how regulation fits in there. All this stuff wasn't in my chemistry classes and I neglected to become an electronics engineer due to time and money constraints.

On second thought, after reading in the data sheet about inrush current limiting, it doesn't sound too oppressive for the power mains on startup. And my bench is in the basement right next to the power center panel. 220vac isn't any burden at all! Three feet of 10-gage is all I'll need! Now if I can figure out how to equalize the voltage and current between the two supplies, I can weld with 15 volts! Or supply a linear that will guarantee to have my neighbors mad!"

Any thoughts?

Regards, John.

Current hogging applies only to paralleled output configurations.

TTFN

FAQ731-376

Look at the block diagram in tha page titled "Theory of Operation". The regulation works by PWM of three sets of SCRs and then a three-phase transformer and rectifier bridge. Those three phases share the load. If you try to run this on one phase what will happen is (1) ripple will be much larger than it should be, (2) the elements (SCR, transformer winding, rectifiers) of that phase will be overloaded as they try to supply three times their designed current. If you are lucky some protection will shut the PS down. If you are not so lucky you may lose some magic smoke.(

So you would want to limit the output power accordingly.

Keith Cress
Flamin Systems, Inc.-

GR3,

Thanks for your response.

I am getting confused again. I think that we have established that my power supplies are all single phase as they are all models with numbers of the form: XXSYY where

XX is the adjustable voltage
S indicates that the unit is single phase
YY is the adjustable current (watts)

".....and then a three-phase transformer and rectifier bridge"

Would I have a three phase transformer in a single phase Power Supply?

".....If you try to run this on one phase what will happen is (1) ripple will be much larger than it should be, (2) the elements (SCR, transformer winding, rectifiers) of that phase will be overloaded as they try to supply three times their designed current...."

But since it is a single phase model I will be supplying single phase.

Would you expand upon your thoughts please?

I have a WIFE and "that L'il Blonde Texan" will be on me hard if I burn down the new curtains on my radio room windows. I do not think I want to lose any smoke, magic or otherwise.

Keith,

I certainly do not need the entire 300 amps, just the 15 volts - 30 amps would do nicely thankyewverymuch <grin>.

Regards, John