Power Supplies, multiple output, negative included

[skimask]

I got another project, searched Digikey and Mouser, nothing turned up yet...

I need a multiple output power supply to test some aircraft equipment. Needs:
-28v @ about an amp
-6v @ less than an amp
+5v @ less than an amp
variable 0-5v, both at less than an amp.

I could easily build one (maybe for use at the house on my own workbench), but trying to get one approved by the FAA (or whoever) using a homebuilt PCB would be practically impossible.
Heck, if I could just find a PSU with -28v out, I could probably build up something to regulate the -6 from that and use a seperate supply for the +5v. That might fly...
Worse comes to worse, I'm going to end up with a 33 cell NiMH pack with taps for the various voltages running to regulators...which wouldn't be so bad, at least it would be halfway portable.

Anyone got any good ideas?
Any bad ones?

[Melanie]

You're losing it ski...

Most good PSU's have floating outputs, so until you link them together they are all independant. Lots of PSU's out there with single, dual and tripple variable outputs. All those outputs are completely independant... nothing to say you've got to strap all the Black terminals together.

So, just connect the Red +ve Terminal of one output to your Common Ground (0v) point and the -ve Terminal then becomes your -ve rail (with respect to the +ve terminal). So, just dial-in 28v and you've got your -28v Supply. Ditto for all your other rail requirements, positive or negative... Black or Red, all depends on which one you connect to 0v.

[Acetronics2]

Hi, Skimask

There are TONS of those supplies ... manufactured in the far east ... you'll find one with a different sticker in the US !!!

http://www.selectronic.fr/article.as...r=80.4677-9999

for the price, just multiply by .7 and you'll get an US $ price ...

in english here : ( with the scheme !!! )


http://www1.produktinfo.conrad.com/c...ins=62&lang=FR

Alain

[skimask]

You're losing it ski...
Most good PSU's have floating outputs, so until you link them together they are all independant.

I had that same DUHHHHH moment this morning at about 3am.
Vout,,,+Vout,,, -Vout, it's all relative...
33v difference between -28 and +5, make a +33v and tap out of that as required, of course the 'ground' at +28v will have to handle ALL of the 'return' current.
No problems now...

Hi, Skimask
There are TONS of those supplies ... manufactured in the far east ... you'll find one with a different sticker in the US !!!
Alain

I guess I forgot to mention one key aspect of this 'project'.
I can't use a 'power supply' like we (we as in tinkerers, hobby type folks) think of power supplies.
What I really need is a 'brick' or at least an open frame PSU...but, as you can see from the comments above, I got this one licked....I hope...

Thanks for the pointers...

[T.Jackson]

I could easily build one (maybe for use at the house on my own workbench), but trying to get one approved by the FAA (or whoever) using a homebuilt PCB would be practically impossible.

You're an aviation technician Ski?

That sentence has good reasoning (explanation) a long with a conclusion, and implies that you are (at least your readers will think so)

[mackrackit]

Hey Trent
Have you read skimask's public profile, Yep, I would say he is an AV TECH.

[T.Jackson]

Yep, I would say he is an AV TECH.

Gee whiz aye -- that must be a considerably high paying job, I'd also imagine that you'd need to have some serious credentials (especially in America) Unlike a motor vehicle, there's no second chances with an aircraft if a fault occurs due to malpractice on the technician's part (the thing's just going to crash, you can't exactly just pull over to the side of the road) This is my reasoning for saying that it's a high paying job that would require a certain type of person with the appropriate credentials.

[mackrackit]

I have a good friend that is what I call a Harrier mechanic. 20 some years in the Marine Corps.

Yes, he has credentials out the you know what. As far as the type of person to work on these things...Attention to detail, enjoys reading data sheets and manuals, has a protocol for every thing and expects everyone else to also. So much so that he can be a real pain at times but you over look this. They know all to well that there is not a second chance.

The guys and gals that keep these birds in the air for us are are perfectionist, could not be any other way.

[T.Jackson]

The guys and gals that keep these birds in the air for us are are perfectionist, could not be any other way.

Exactly. Someone doesn't fit the profile for it.

[mackrackit]

Exactly. Someone doesn't fit the profile for it.

If I get your meaning and from what I have seen, I have to disagree.

[mister_e]

Don't worry with Trent.. i think he have a way too high self-esteem...maybe he think he's just over everybody... common attitude with some existing, newbs or wannabe IEEE.

Oh well at least there's a cure for it... euthanasia.. but it seems to be illegal in most country

I'M JUST KIDDING..

[T.Jackson]

maybe he think he's just over everybody...

Not at all. There's people on here much smarter than me.

[mackrackit]

Don't worry with Trent.. i think he have a way too high self-esteem...maybe he think he's just over everybody... common attitude with some existing, newbs or wannabe IEEE.

Oh well at least there's a cure for it... euthanasia.. but it seems to be illegal in most country

I'M JUST KIDDING..

Yeah, you are correct, its just that where I come from we are taught to respect our servicemen and women.

[T.Jackson]

Don't worry with Trent.. i think he have a way too high self-esteem...maybe he think he's just over everybody... common attitude with some existing, newbs or wannabe IEEE.

Oh well at least there's a cure for it... euthanasia.. but it seems to be illegal in most country

I'M JUST KIDDING..

You know what -- I'd love to have your level of expertise with PBP, but I certainly wouldn't want your level of maturity.

[skimask]

Don't worry with Trent.. i think he have a way too high self-esteem...maybe he think he's just over everybody... common attitude with some existing, newbs or wannabe IEEE.
Oh well at least there's a cure for it... euthanasia.. but it seems to be illegal in most country
I'M JUST KIDDING..

Do you still have that graphic from awhile back by any chance? I think you know which one I'm talking about...
It was either you or DT that had it...

[mackrackit]

You know what -- I'd love to have your level of expertise with PBP, but I certainly wouldn't want your level of maturity.

You fall off the wrong side of the bed?

[skimask]

I feel like B.S.'ing...

You're an aviation technician Ski? That sentence has good reasoning (explanation) a long with a conclusion, and implies that you are (at least your readers will think so)

You figure that one out by yourself did ya?

Hey Trent
Have you read skimask's public profile, Yep, I would say he is an AV TECH.

Pretty close. Especially since all my profile has in it is all acronyms and a picture

Gee whiz aye -- that must be a considerably high paying job, I'd also imagine that you'd need to have some serious credentials (especially in America) Unlike a motor vehicle, there's no second chances with an aircraft if a fault occurs due to malpractice on the technician's part (the thing's just going to crash, you can't exactly just pull over to the side of the road) This is my reasoning for saying that it's a high paying job that would require a certain type of person with the appropriate credentials.

Credentials don't mean squat without integrity.

I have a good friend that is what I call a Harrier mechanic. 20 some years in the Marine Corps.
Yes, he has credentials out the you know what. As far as the type of person to work on these things...Attention to detail, enjoys reading data sheets and manuals, has a protocol for every thing and expects everyone else to also. So much so that he can be a real pain at times but you over look this. They know all to well that there is not a second chance.
The guys and gals that keep these birds in the air for us are are perfectionist, could not be any other way.

And I get double the pain out of it since I'm a private pilot myself, whereas the overwhelming majority of the people I work with aren't. A lot of those people think 'good enough' is 'good enough', especially the young kids I'm training. When something like that comes along, I ask them if they want to fly with the bird and see if the fix is good enough, or if I get to write a letter to their parents and explain why this kid is dead. Morbid...but fact...

Exactly. Someone doesn't fit the profile for it.

Profile this...

If I get your meaning and from what I have seen, I have to disagree.

Thanks for the vote...not sure if I deserve it, but thanks.

Don't worry with Trent.. i think he have a way too high self-esteem...maybe he think he's just over everybody... common attitude with some existing, newbs or wannabe IEEE.
Oh well at least there's a cure for it... euthanasia.. but it seems to be illegal in most country
I'M JUST KIDDING..

I know that, most of us know that, somebody doesn't know that.

Not at all. There's people on here much smarter than me.

In the words of Dr.Evil... Riiiiiiiight...

Yeah, you are correct, its just that where I come from we are taught to respect our servicemen and women.

Don't know if I (me in particular) deserve any special accolades, haven't done anything really neato-important-yadda-yadda. I got a handful of last-minute quick fixes, saved a load of $$$ a few times by discovering this, that or the other thing (got one really good story that involves about 3 pages of math in small print to discover 2 swapped wires that saved 2 engine changes in the middle of an important thingy), other than that, quite frankly it's been a relative 9-5 job for 20+ years now. Like they say, 99.9% boredom with a sprinkling of .1% excitement.

You know what -- I'd love to have your level of expertise with PBP, but I certainly wouldn't want your level of maturity.

Don't worry, you're still in school, right? I'm sure they teach it.

Now then, where's my can-coozy? 15F outside, cold enough for a bonfire and a choice beverage...and think about how to keep that 33 cell NiMH pack charged up reliably...

[mackrackit]

Pretty close. Especially since all my profile has in it is all acronyms and a picture

Well, the USAF is pretty easy to figure out and the two acronyms I know of that go with the first stand for. First guess...
Instruments and Flight Controls
From post about gyros and other things.

Second guess for IFC would fit also. The "F" is for fusion.

[T.Jackson]

If I get your meaning and from what I have seen, I have to disagree.

Yeah, I think you might be right.

[mackrackit]

Yeah, I think you might be right.

Just call'em like I see'um

[Melanie]

Just a quick thought for you ski... properly charging that Battery-Pack is going to be more tricky than you think because the level of discharge across the entire pack isn't going to be constant... because of those taps you're going to be discharging some cells at a greater rate than others. So, if you're going to be charging the entire pack connected in situ, a lot of cells will reach full charge perhaps a very long time before others.

[skimask]

Well, the USAF is pretty easy to figure out and the two acronyms I know of that go with the first stand for. First guess...
Instruments and Flight Controls
From post about gyros and other things.

Second guess for IFC would fit also. The "F" is for fusion.

I've changed jobs 6 times, but haven't changed 'JOBS' yet...
Started off as Instruments...moved to Instruments/Autopilot...moved to Instrument/Autopilot/INS, then changed to Guidance and Control (GAC), then to Instruments and Flight Controls...
All the same name for different stuff.

[skimask]

Just a quick thought for you ski... properly charging that Battery-Pack is going to be more tricky than you think because the level of discharge across the entire pack isn't going to be constant... because of those taps you're going to be discharging some cells at a greater rate than others. So, if you're going to be charging the entire pack connected in situ, a lot of cells will reach full charge perhaps a very long time before others.

That's what I'm talking about. But as you implied back in post #2, ground is ground, wherever it is, it's ground, or at least it can be. So, I whip up a 36v battery pack, 40 cell to get 36v at end-of-charge .9v/cell, 36v total, with 4 regulators:
-28v -> 0v (i.e. ground), -6v -> +22v, 0v -> +28v, +5v -> 33v.

The unit, during testing, isn't connected to anything else besides the tester while be tested, so it shouldn't matter where ground is, as long as the right potentials are there to drive things, even the chassis is completely electrically isolated from the circuitry. Overall current draw is going to be far less than 3 amps, once in a great while peaking near 3 amps.
So using linear regulators to keep parts count and cost down, I regulate out the 33v direct from the batt pack, regulate 28v off the 33v, regulate 22v off the 28v...instead of putting all of the regulators in parallel (40v regulated to 22v using a linear is a lot of wasted heat), use pass transistors on all of them and I should still have enough overhead V+ to run them all, although the 33v might be pushing it at full load with a near end-of-charge batt pack.

Charging however, as you eluded to, will be another story. 40 cells would max out at around 58v, probably need something like 64v to push any current. Some of the cells will undoubtedly charge before others. So, either I go with a very slow charge (less than 1/10C) to keep out-gassing down, or I split the pack into smaller segments during charge. I've charged a 16 cell pack successfully before using a simple 'linear' (not sure what you'd call it) charger that charged the whole pack at once, started off with higher current, as the pack charged up, the current dropped to something like 1/50C. Probably not healthy for the battery pack, but it's still alive and kicking after 3 years.

Or I just skip the battery pack, and go with a unit that runs off the mains.

More thinking required...


PS.: Mel, and BTW, that fix involving the 2 swapped wires, was done while I was at Fairford, practically in your 'backyard'. And I couldn't have done it without a book from Mike M. at nikamelectronics, and PIC guy.

[T.Jackson]

Here I was thinking that Melanie was merely referring to a balance type charging arrangement -- whereas each cell is individually charged to ensure optimal life expectancy, by eliminating the possibility of overcharging. But don't take my word for it, I'm just a pathological liar.

[skimask]

Here I was thinking that Melanie was merely referring to a balance type charging arrangement

Yes, I know, charge balancing, always the best way to go, with an unlimited budget, loads of time, a nice looking PCB, and so on and so on...(i.e. educational environment)
Then there's the practical way of doing things (i.e. welcome to the real world)...

I'm not planning on using rechargeable li-ion batt's (which would be the obvious way to go unless I want to start semi-contained chemical reactions, i.e. fires). I've also got to think about the (and I use this term loosely) morons (read that as uneducated persons who don't know the ins and outs of various battery types) who are going to be using this tester long after I'm gone. Therefore, Ni-Cad's are out due to the memory effect (these guys won't know how to do the charge/discharge thing for max life, even if it's specifically stated in the manual).
So, I'm basically left with NiMH if I want a rechargeable setup. Memory effect is practically nonexistent, low temp performance isn't the best, negative delta-V during charge is small, offset that with a low charge rate and a timer.

I took a good hard look at the box I'm testing. There's only one component in there that's polarity sensitive...and that's dump diode across a single relay coil. That one thing ruins everything for me. Without that one diode, I could easily use N-ch MOSFETs under PIC control to self-test the whole thing for me and tell me where the fault is, if there is one. And it turns out I only need 28v. The extra 5v section is completely separate from the rest of the box. Figure I only need 33v (28v + 4v of regulator overhead with pass transistors plus some fudge factor), brings me down to a nominal 32 cell pack, 4 packs of 8 cells. The circuit can tolerate a couple volt drop at end-of-charge on the pack.

So, it's time to figure out how to get rid of all those rotary switches and replace them with banks of P- & N- channel MOSFETs and automate something...

P.S.

I'm just a pathological liar.

Don't be putting words in my mouth there sparkles.