What do ya'll think of my AC and DC power supplies? - Page 1
Forum section: electronics / projects

author: Jewbaccah
author_rank: Newbie
timestamp: December 08, 2017, 06:09:57 pm
content: Hi everyone, hope you are doing well!This is my first post on the forums, I just wanted some feedback on my power supply section of a project I am working on! I'm also going to post some of my project builds on a new thread soon.Sorry about all the questions but if you feel like helping out that be awesome.Here's my schematic. I have a big transformer sold for stepper motors connected up to the AC lines. Both rectified to 12vdc for a small stepper and 80vdc for the big motor. I'm basically looking for some feedback on whether or not this supply is protected enough from EMI and spikes like that. I have SCR crowbar, well at least the components, on the schematic. Should I use one? I also have a overvoltage clamp set up on the 15vdc line. Should that be on the 80 line? What about the fuse? Two or one?I'm also concerned on placement of my microcontrollers and wireless, since I have both bluetooth and wireless on the same PCB. I'm planning on doing as much as I can to isolate everything. Any tips on that for PCB construction?Thanks guys!

author: duak
author_rank: Super Contributor
timestamp: December 08, 2017, 06:46:25 pm
content: Welcome to the forum!Please show the wiring of the 80 V transformer secondary windings to the board. It's easy to get this wrong and I think I see an error in connecting GND to both the transformer and the output.What is the actual AC voltage of the transformer secondary for the 80 VDC supply? If it's 80 VAC, you will get something like 110-120 VDC on the +80V output.For an actual 80 VDC output, C1 & C2 should be rated for 100 VDC, not 80 VDC.What is the total current draw on the +12V line? If the transformer secondary voltage is 15 VAC you will see 20-22 VDC into the regulator. With a 1 A current draw, the regulator IC2 would have to dissipate about 8 - 10 W.Cheers,

author: Jewbaccah
author_rank: Newbie
timestamp: December 08, 2017, 07:15:23 pm
content: Thanks for the reply!I already have this exact transformer.http://www.antekinc.com/as-4450-400va-50v-transformer/The secondary coils are only 50v and 15v, so I think I made a mistake in the schematic and the 80 VAC IN should be labeled 120 VAC. This would be correct, right? Or the 50V up to it? I need to get this right so I don't kill myselfThe total current on the +12v is going be more than 1A, probably 1.5A. There's a stepper that might draw 650ma, a 4.3" LCD, two microcontrollers.I'm not really clear on power supplies.. why would the IC2 regulate it to 20-22 vdc instead of 12?

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 08, 2017, 07:28:20 pm
content: Several things look wrong or at least questionable.In particular the 80V input shows two of the terminals grounded (center-tap???)But then the output shows one side also grounded. I can't see how this could work?Labeling a fuse with its voltage rating but not its current rating seems weird.The current rating is REQUIRED while the voltage rating is secondary and often just assumed.You MUST show the COMPLETE schematic INCLUDING the mains AC input AND the transformer.Showing only generic connections to the transformer leaves too much to be assumed.It would help to use more conventional schematic drawing conventions.Feel free to put one on each page instead of trying to crowd them altogether.All those diagonal lines are rather unconventional and somewhat distracting.

author: Audioguru
author_rank: Super Contributor
timestamp: December 08, 2017, 07:41:08 pm
content: 15VAC is 15V RMS. Its peak voltage is 1.414 times higher at 21.2V and the bridge rectifier reduces the filtered DC to about 19VDC. That isthe input intothe 12V regulator that might not produce 1.5A at 12V because it might get too hot dissipating 10.5W and shut down.

author: Jewbaccah
author_rank: Newbie
timestamp: December 08, 2017, 08:28:24 pm
content: Thanks guys.Richard,The only reason I have two terminals grounded there is because I didn't have a three terminal library file in EAGLE. It will be three terminal eventually.Yeh, those diagonal lines I was fond of for some reason, but since I didn't really keep the convention all the way through, I'll probably change those back.Audioguru,What would you recommend I change to account for that?

author: james_s
author_rank: Super Contributor
timestamp: December 09, 2017, 12:43:05 am
content: Look at existing schematics for ideas, generally you should never have diagonal lines except for a few cases where it can make sense like when you need to make a jog in several parallel wires where 90 degree bends can be harder to follow. Here's a random page I found with lost of examples, pay close attention to how the schematics are drawn and try to emulate the style of some of the more readable ones. Most of the wires should be straight vertical or straight horizontal.Then when you go to route the board you should avoid 90 degree bends whenever possible, a board full of sharp 90 degree bends is an instant indication that it was laid out by an amateur. Likewise never use odd angles unless you *really* have to, I realize this is mostly cosmetic but neatness counts, stick with 0, 45 and 90 degree traces.Also I would encourage you to forget about most of the built in schematic symbols and layout modules and get in the habit of creating your own. I see a lot of people shoehorn in approximate parts they find in the default libraries rather than learn to create new ones and it's a bad habit to get in. If you take an hour now to learn how to create new components it will save you so much time down the road and give you cleaner results.Lastly I encourage anyone still using Eagle to consider transitioning to KiCAD now that Autodesk has raped Eagle and gone rental-only just weeks after publicly stating they had no plans to do so. IMHO they cannot be trusted.

author: duak
author_rank: Super Contributor
timestamp: December 09, 2017, 11:03:37 pm
content: Thx for the clarifications.Assuming the transformer's 50 VAC windings are connected to OXI-1 & OXI-4 and OXI-2 & 3 are not connected, you'll get about 70 - 75 VDC on the +80V output.The NCV7812abtg regulator is rated for a guaranteed 1.0 A. Some of the device's curves show a higher current limit but it will require a good heat sink and there's a small chance that some devices under some circumstances may not. (Your mileage may vary!) I would consider a Linear Technology LT1085 3A unit instead, however, it's an adjustable regulator and needs a few more parts.A linear regulator has to dissipate the power as a consequence of the voltage drop between its input and output. In this case, it's about (22 V - 12 V) * 1.5 A = 15 W. A very rough rule of thumb says you'll need at least 1 sq in of surface area for every Watt dissipated so you'll need at least 15 sq in heat sink. Note that most factors increase the area needed. Assuming the 1.5 A will be drawn for minutes at a time, It seems that only forced air allows using a smaller unit.I haven't gone thru all of the circuitry and so can't say if there aren't any other gotchas in there.Cheers,