phantomcow2

My DC power supply died this morning. I dont understand why, it was a 24vDC 30Amp power supply. But today the voltage dropped in the middle of an operation to 5v. Well considering the things it drives are more than that...obviously this did not work.
The people who i bought my driver board offer a schematic for a power supply which many users seem to build with good results. I am somewhat of a newb when it comes to the electronics dept. So is this a pheasable project? And if so, where do i get all of these things.
Here is a list of the stuff:
1. 35,000uf MINIMUM 50vDC capacitor
2. Bleed Resistor @2k 1W
3. 35A 600V Bridge Rectifier
4. 20-28VAC 10A transformer
5. DPST switch (is this a regular switch like you have at radioshack? I have plenty of these i have installed).
6. Fuse 4Amp (familiar with these, and have plenty.)
7. Small Prong on Plug, Large Prong on plug (is this just the plug in cord).

Thankyou to all who give helpful replies

catatonic

So long as you take precautions aginst capacitor dischage, you should be OK. Cap discharge on one of those can hurt really bad. Only thing nastier than electrical burns are radiation burns.

Go check out www.mcmelectonics.com to get those...the site is hard to search, but they have everything electronic, literally.

catatonic

BTW good precaution against cap discharge is to use hot glue to insulate the solder joints aroud the cap leads. This will minimize the chances of having any "bad touch" experiences. Do this before ever charging the caps.

Once you get better at navigatinng your way around a PCB, you will eventually not worry about cap discharge...but for now, it would be a good thing to err on caution.

phantomcow2

hot glue around the solder joints near cap leads, and i assume this only insulates, does not in any way interfere with anything. I dont see how it would, but I feel like a plague right now i so i dont trust anyhting i think.

phantomcow2

HOly crap that online store is a nightmare. The most specific i could get for a 50v capacitor was 138 pages!!

catatonic

haha, That's actually one of the largest component suppliers in the nation, if not the world.

It takes patience to find what you want, but they literally have about anything you will ever need.

phantomcow2

Well i got a part # from allied electronics.
The transformer is 40 bucks by itself! I can buy a premade for less than that

lilHinault

Where's your regulation? You need a voltage regulator in there.

Serpico

BUMP to yer thread phantom

don't know how, but you always find an answer to these obscure engineer/electrical/machining threads

good luck

RedHairedScot

I'm assuming this is a power driver for the CNC stuff you're doing, so I can understand the lack of 7805. I don't play with power tron; I need my day job as a hand model too badly. I can see building a rough power regulator out of that for motor control; it's simple.

My best advice is to realize that nobody will be watching you so it doesn't matter how much of a dork you look. Make sure the caps are discharged; through your 2k resistor at first [1], then through your screwdriver.

[1] 50V peak voltage across your 2k resistor will dissipate 1.25W which is technically out of spec for your resistor, but it's better than discharging your cap with your finger. And it's not sustained; the cap will drop quickly. And, honestly, if you've managed to peak your cap without knowing it and when about to handle it you're horsing around, you're Dr. Fester from the Addams Family, or you have an inordinate number of silk scarves, glass rods, and cats in your basement.

phantomcow2

WEll a schematic is here:
https://home.comcast.net/~phantomcow3/POWERSUPPLY.pdf

The price of the transformer alone is nearly the same as if i was to buy a power supply already made.
https://www.mpja.com/productview.asp?product=16056+PS

jur

I'm an EE, and NONE of this makes any sense to me. Cap discharge dangerous? At 24V? I must have been sailing the day they taught that stuff at uni.

phantomcow2

I keep thinking im crazy, but this got moved to bicycle mechanics. Sydney, can you help with this?

KingTermite 

Moved to bicycle mechanics....now THAT is appropriate! lol

The Speaker Guy

OK, IMHO opnion this is a "safe" circuit on which to work, if you stay on the secondary side. 50V is a rule of thumb for beyond safe for casual contact. That's one reason why the new voltage tossed around for car battery is 42V. I can't wait, high power amps without power supplies required.

Anyway, back on topic. The 28VAC recitifies out to 39.6Vdc. Two diode drops across the bridge gives about 38Vdc. The bleed resistor is 2k, it will draw 19mA current, with a power dissipation of 0.019*38 = 0.722W. OK there.

The capacitor is a bit more difficult to qualify. It must be rated for the load current. Given the 10A transformer, and a 10A fuse, let's assume you are shooting for 5A nominal load current.

For the capacitor 35,000uF is recommended. At digikey I see a 50V, 18,000uF device with a 6A ripple current rating. ESMH500VSN183MA50T on this page https://dkc3.digikey.com/PDF/T053/1027-1029.pdf

Two of these should work great, connect them in parallel.

If your circuit is working correctly, the bleed resistor should be a bit warm.

I would change the input fuse to 2A slow blow.

nodnerb

And the real danger with capacitors is that they can explode if hooked up to the wrong polarity. Some can be hooked up either way but the ones you'll need have to be hooked up correctly. The negative side is usually marked on the case.

jab

Hmm, the schematic you linked to shows the transformer rated at 10A and a fuse limiting the output to 10A. How were you planning to get 30A out of this?

Anyhow, the schematic looks like a basic, unregulated DC power supply. If it will be used to drive inductive loads such as relays and motors, you may want to put a (big) diode backwards across the output of this or whatever regulator you end up using, in order to protect the capacitor/regulator against reverse voltage spikes when the inductive load is switched off. If you don't add a regulator, beware that the output voltage may vary substantially with the load as well as with fluctuations in your AC supply. Unless you're sure that the load doesn't require further regulation, I wouldn't use the supply as-is.

The "DPST switch" bit is just referring to an on/off switch -- "Dual Pole, Single Throw" -- which has two contacts that operate at the same time (so, four terminals). You can also use a DPDT switch (DT: double-throw) which has six contacts; just ignore the extra two. You may also use an SPST (single-pole) switch, though if you do that you'll want to make sure it's located on the "hot" side of the input, which is the small prong.

As far as the capacitor value itself, 35000uF is pretty big; beware that it can remain charged after the supply is switched off, and be wary of shorting that beast out. Whether that is an appropriate value depends on the load: how much current it pulls, and how much voltage ripple it can tolerate on its input. As far as safety goes, I personally wouldn't think twice about touching a cap charged at 28V with my hands, but if you accidentally short the contacts with a screwdriver, you'll get a fat spark and may damage the capacitor or whatever is between it and the short.

Finally, if I needed 24V at 10A, I would consider buying a switched-mode power supply module which is rated for that current. They tend to be a lot smaller, lighter, and can be far more efficient that the traditional style. I imagine a 120V -> 24V 10A transformer would weigh a ton.

My local electronics store (Fry's) carries one brand of switching power supply modules, Meanwell, for $20-30 per supply for the 25-60W supplies. A web search turned up https://www.trcelectronics.com/Meanwell/ , and the Meanwell S-240-24 looks like it's right up your alley; it's adjustable for 20-28V output, is rated for 0-10A output, boasts 84% overall efficiency, and has self-resetting overload protection. They offer other models with higher power ratings, too. I don't know what the S-240-24 costs.

Best of luck (and watch your polarities),

JAB

phantomcow2

All that is why i wont make the circuit.
I dont need 30 Amps, thats just the power supply i found. But i have decided to go for this one:
https://www.mpja.com/productview.asp?product=16056+PS

askrom

I don't get it. What does this have to do with bikes?

Brian McDonough

undefinedHello phanton I did some work with electronics and the bicycle generator using a very simple circuit that I use to supply small current to my garmin GPS and also to charge my cell phone while riding my bicycle and its nice not to mess around with changing batteries all the time.

phantomcow2

HOld on i just want to make sure im going in the right direction. I ordered a power supply but i just wnat to make 110% sure that it is the culprit. So heres what i did...
Remember this is a 24vDC and 30AMP (supposedly) power supply.....
1. Tried to run ONLY a computer fan with power supply, works great.
2. connected power supply to driver board which is hooked up to 3 stepper motors and computer fan. Also, there is a switch to shut off power reaching the board.
3. Put test leads of voltmeter directly on outputs of power supply. With switch NOT on, it reads slightly under 24vDC.
4. Test leads still on....flip switched on. Voltage drops immediately to just under 5vDC. Also, high pitch screaming type noise is heard. Also, the LED which indicates power supply is on goes almost faint.
Im 99% certain this is a power supply problem, but i just want to make sure.

genec

Not much of a power supply... the DC will have lots of ripple in it and there is no regulation. You basically have a full wave recifier with a capacitor as your filter.

Any large current draw will make your DC very "noisy."

BostonFixed

Why the hell is this in bicycle mechanics?

phantomcow2

TO irritate you.

phantomcow2

Actually its because this is related to the mechanics of something which may very well produce cycling type products.