Need help reading a schematic.

[Spork Schivago]

Originally Posted by budm

So You are getting 2.116V between the two legs of the 150 Ohms resistor using 15V power supply, then 2.116V/150 Ohms = 14.1mA on each string of the LED, so 8 string = 14.1 x 8 =112.8mA for TOTAL current draw, so your power supply must be able to supply at least that much of current.

'2.116VDC * 8 = 16.928VDC, ' No, You are still confused about the circuit (sorry I was not clear about calculating the current through the resistor), you forget to calculate the current value after you know the Voltage drops across the 150 Ohms resistor.

We was cross posting I think. I got different readings now that I had a good power source going to the board. Yeah, I measured the voltage between the resistor's two ends.

I think I got it figured out. The current should be the same for each row, right? So the current for the resistor, being 14.7 milliamps, should also be the current that is going to the LEDs as well, right? Because the current in a series circuit is everywhere the same. And the current at the bottom of the board layout, the parallel circuit part, the current there would be 14.7 * 8 = 117.6 milliamp, the same number that the whole circuit uses. Do I have this right?

[Spork Schivago]

Originally Posted by budm

Hey, you got it now. So the power supply he sold you is on the over rated side as far as the current capacity is concerned. 15V 1/4A power supply will be good enough.

But I have 11 more boards to hook up. Wouldn't each board require 117.6 milliamp? So 117.6 * 12 = 1,411.2 milliamps or 1.4112 amps?

[budm]

That is correct based on 12 boards, and then plus the control circuit and the display, so regulated 15V supply with 2A current capacity should be good.

[Spork Schivago]

Originally Posted by budm

That is correct based on 12 boards, and then plus the control circuit and the display, so regulated 15V supply with 2A current capacity should be good.

Oh, that's great! I think I understand this! Here's some pictures of the timer board and the UV LED PCB, what do you guys think? Thanks for all the help! Really appreciate it. This stuff is really fun when I'm understanding it and each day, I seem to understand a bit more!

I know the resistors I could of done a better job with. They were really really tiny though and it was really hard with no one helping. I was thinking of maybe trying to use my hot air reflow station to add some hot are and try to get the solder to melt a little bit so it looks not so mountainy like. You know, so the solder on each resistor kind of levels out a little bit and looks more uniformed.

[Spork Schivago]

Got the second board done as well! Looks a bit nicer with the SMD resistors. I think I'm getting better at soldering those dang little things! Lost one on the carpet and took us like 30 minutes to find!

I wanted to find flat pieces of metal to solder to the VCC / GND pads there to connect the boards. Anyone has any suggestions for that? Hopefully, once I get this built, I can use it to make two giant LED boards instead of having to use 12 smaller ones.

[Spork Schivago]

Hey Budm, one last question here. I think I know the answer though. But each board will be provided 15 VDC, right? Because of that trace that connects the various boards together, the big fat one, is part of a parallel circuit and it's connecting to another parallel circuit, the voltage going to every board shouldn't be affected by how many boards I add, right? The only thing that would change is the amount of current that the boards are using, right? Ie, more boards, more power required, but not higher voltage.

[budm]

That is correct, the supply Voltage stays the same feeding the parallel loads, only the amount of current draw will change, you got the idea now.

[keeney123]

You can use like a Blunted fiberglass screwdriver to hold down the resister. First tin your iron then hold the resistor down with the instrument and tack one side of the resistor. After that is done you will have two free hands to solder the other side of the resistor. Wait till that side cools and finish up soldering the side you tack down. So, you could go down tacking all the resistors and once finished start at the first one as that should be cool and start soldering the other side.

[Spork Schivago]

Okay! Thanks guys. What I've been doing is putting solder to one pad, then using tweezers, I've been putting the resistor over the pad that I soldered, then I heat that solder a little bit, move the resistor in place. It's hard because they're so small and even a little movement throws it out of alignment. I will try your suggestion though Keeney123 and see how it works.

Any idea how I can figure out what package these SMD resistors are? They're really small. About 2mm long by 1mm wide. Would that make them 0805's? I see 0805's are 2mm by 1.25mm. I see DigiKey have a few different things that have 0805 in them.

They have a category called Package / Case which has Wide 0805 (2012 Metric), 0508 then they have 0302 (0805 metric) then 0805 (2012 metric). They also have a section called Supplier Device Package. For that section they have 0805 and then 0805 (2012 metric). I don't know the difference between Package / Case and Supplier Device Package. I'd also like to know what's the difference between the various 0805 stuff so when I go to order SMD components for the count down timer PCB, I can try to get the same package for all of them. Thanks and Happy Thanksgiving everyone!

[Spork Schivago]

Would this be one of those fiberglass screw drivers?

http://www.hardwarestore.com/fibergl...er-283754.aspx

[keeney123]

Originally Posted by Spork Schivago

Would this be one of those fiberglass screw drivers?

http://www.hardwarestore.com/fibergl...er-283754.aspx

No it would be for tuning coils etc. in radios, like this

http://www.rf-microwave.com/en/shop/...1-CT-4192.html

It can be anything that is small enough to put pressure down on the resistor without getting in your way of tacking down the resistor with the iron. Preferably something a flat surface that contact the resistor with a straight handle. I picked fiberglass because you can put a look of pressure on it without damaging the resistor. I assume you are using flux core solder? If you are not then you will need to put a little flux down before you tack it down and a little flux on the tip before you tin the tip.

[keeney123]

Here is another company that carry the fiberglass TV alignment tools.
http://www.minute-man.com/acatalog/T...ent_Tools.html

One more company look under soldering tools
http://www.gcelectronics.com/order/c...y.asp?CatID=16

[Spork Schivago]

Originally Posted by keeney123

No it would be for tuning coils etc. in radios, like this

http://www.rf-microwave.com/en/shop/...1-CT-4192.html

It can be anything that is small enough to put pressure down on the resistor without getting in your way of tacking down the resistor with the iron. Preferably something a flat surface that contact the resistor with a straight handle. I picked fiberglass because you can put a look of pressure on it without damaging the resistor. I assume you are using flux core solder? If you are not then you will need to put a little flux down before you tack it down and a little flux on the tip before you tin the tip.

Thanks. I haven't been using a flux core solder. I've been using a silver bearing solder and I have various types of flux. What seems to work okay is this liquid no-clean flux. I wasn't putting any on the actual tip though. Haven't really had any trouble with the solder sticking. Only problem seems to be getting those little buggers lined up just right.

[stj]

if you flux them and heat them with hot air they will float and re-align with the pads.

[Spork Schivago]

Originally Posted by stj

if you flux them and heat them with hot air they will float and re-align with the pads.

OKay. I'm not very good with the hot air. I'm afraid I'm going to fry one of the boards. I usually use the hot air rework station for removing components but perhaps I should give it a shot. Perhaps I'll try throwing some flux and solder paste on the pads and see what happens.

[stj]

practice on something scrap.

btw, you dont use flux with solder-paste.
solder paste is flux already with powdered solder in it.

[Spork Schivago]

Originally Posted by stj

practice on something scrap.

btw, you dont use flux with solder-paste.
solder paste is flux already with powdered solder in it.

You sure about that Stj? Is the flux just for desoldering and I mistakenly thought it was for soldering as well?

One product is ChipQuik Lead-Free SMD291 Tack Flux N/C for rework & reflow. The other product is ChipQuik Lead-Free SMD291SNL10 N/C Paste SAC305 L/F for rework & reflow.

What's usually a good temperature / air flow to get the stuff working? I tried, unsuccessfully, to replace a really small IC. It had a lot of pins, like 48 or something. Really small. I ended up using wwwwayyyy to much paste. But I tried putting just a dab. I found that if the tip of the syringe was a bit warm, the paste would come out really smooth like and I could just use a dab. I couldn't get the paste to melt without using a higher air speed though. It ended up pushing the paste out of the way. I took the temp and increased it, trying not to push the paste. In the end, I burned the board and a trace lifted up and broke. I suck at this hot air stuff, unless it's removing components. I'd love to nail down using it though.

What's the best way to get the paste on? I know it's probably using a stencil or something but I don't got one of those and I have no way to make one. I tried using some stencil cutter thing but it couldn't go small enough (we borrowed it from a friend). I could probably get a custom one built, but I'm sure there's an easier way.

[stj]

why would you need flux to remove something???

say i want to remove a 144pin chip - because i do them by hand.
(yes, i do!)

i heat the chip with the handheld air unit, till it lifts off the board.
no flux used.

then i paint the pads with flux and re-heat them to make the solder flat and smooth

then i paint the pads wih flux again,
and the pins on the new chip

then i place the new chip and re-heat the pads/pins till the solder flows.

then i inspect it and if it's all aligned and o.k. i clean off the flux with ipa and get a beer.

btw, when i refer to "painting" the pads/pins, my flux is water-thin liquid and i'm using a brush.

[Spork Schivago]

Originally Posted by stj

why would you need flux to remove something???

It was something that was suggested on a forum. I didn't actually look into the reasoning behind it. This was the post:

I am working with SMD for a while and I suggest you use leaded solder to treat the oxidized solder joints with flux before you get to the hot air stage. After solder mixture process, preheat the board close to 200C and continue with hot air station. I use analog hot air stations because I don't want to turn around and look at the temperature. I know where I have to set the hand dial without looking and I think that is between 7-8 on Hakko 852 station (around 420C). Preheat the chip from distance and can feel when it is enough to start. I count to 5 and viola pull the chip out without any problem. The hard ones are BGA chips and need precision timing and good BGA preheater. Average ICs are rated to 380C/10Sec I think but I am not sure.

Also, I remember when I was looking at videos on how to remove the 360 GPU's / CPU's, for some reason, they were squirting a lot of flux under the BGA type components. I just thought it helped the solder flow a little more easier and that it'd help it melt all at once, instead of maybe a little of it melting here, a little still solder there.

Was this incorrect and the flux isn't actually needed for desoldering?
As for soldering with the hot air, I think I remember now. The flux was to help hold the component into place. I watched many videos on how to use the hot air station when I first got it. Some people would use flux to hold it in place and then put the solder paste on top of the pins, not on the pads. Others would put solder paste on the pads and then put the chip on. I wish I could find the video but I can't. Maybe it's my memory just remembering it wrong?

Originally Posted by stj

say i want to remove a 144pin chip - because i do them by hand.
(yes, i do!)

i heat the chip with the handheld air unit, till it lifts off the board.
no flux used.

then i paint the pads with flux and re-heat them to make the solder flat and smooth

then i paint the pads wih flux again,
and the pins on the new chip

then i place the new chip and re-heat the pads/pins till the solder flows.

then i inspect it and if it's all aligned and o.k. i clean off the flux with ipa and get a beer.

btw, when i refer to "painting" the pads/pins, my flux is water-thin liquid and i'm using a brush.

Okay, thanks. My flux is a bit thick. The paste is really thick too unless I warm it up a little (like preheating the board and then touching the tip to the warm board. That seems to make it liquid like).

[keeney123]

The flux is to transfer the heat from the iron to the solder joint. It allow a even flow through the joint and helps to get rid of oxygen in the joint. If you have no flux in your solder, flux will be needed to unsolder a joint with a solder iron. With a hot air gun, I do not know. I would think the flux would aid in a uniform heat. You do not need a lot of flux. With Surface Mounted Components they usually use a small flux bottle that has needle is attached and use just enough to wet the pads. This would be a liquid not a paste. I would think paste would be messy and harder to control.