How to Supply Enough Power?
Hello All,
As·you will notice I am new to this forum as a poster but have been keeping up with it the past couple of weeks. Anyway..
I am building a small project that has one SX28AC/DP as the main controller that will run two 7 segment displays. First display will run off of Pins 0b - 7b, and display number two is on pins 0c-7c. 0a-1a pins will act as my control lines from the computer. When the computer sends the signals the SX decodes the message and displays the number, pretty simple.
Each one of these display boxs has its own address and the computer will send out a signal with the correct address and number to display. So when the computer sends out a message each box will be listening and if the address matchs the address of the box then it will display the number.
Well all of this works great but my only problem is that I want to be able to only run two wires for the power so that all of the boxs are daisey chained together.
What my plan was to have 2 wires running at 12 volts and each unit will be hooked up in parallel to these wires. Also each unit will have a regulator for the sx. I am guessing that the only thing i need to worry about is getting enough amperage through the wires.
I already took care of the signal lines, i just to power up all of these units as easily as possible. I can't have a hundred wires running around. I plan on having anywhere between 2 - 50 display boxes. Is it possible with only two wires.
Jesse
PS sorry about the long post i wanted to be as clear as possible.
As·you will notice I am new to this forum as a poster but have been keeping up with it the past couple of weeks. Anyway..
I am building a small project that has one SX28AC/DP as the main controller that will run two 7 segment displays. First display will run off of Pins 0b - 7b, and display number two is on pins 0c-7c. 0a-1a pins will act as my control lines from the computer. When the computer sends the signals the SX decodes the message and displays the number, pretty simple.
Each one of these display boxs has its own address and the computer will send out a signal with the correct address and number to display. So when the computer sends out a message each box will be listening and if the address matchs the address of the box then it will display the number.
Well all of this works great but my only problem is that I want to be able to only run two wires for the power so that all of the boxs are daisey chained together.
What my plan was to have 2 wires running at 12 volts and each unit will be hooked up in parallel to these wires. Also each unit will have a regulator for the sx. I am guessing that the only thing i need to worry about is getting enough amperage through the wires.
I already took care of the signal lines, i just to power up all of these units as easily as possible. I can't have a hundred wires running around. I plan on having anywhere between 2 - 50 display boxes. Is it possible with only two wires.
Jesse
PS sorry about the long post i wanted to be as clear as possible.
Comments
That said, you could have one less voltage regulator if you daisey-chained 3 wires (12v, 5v, Gnd). Another idea would be to always have a "First" box in chain that has the voltage regulator that has only the two power outputs, but then daisey chains the 5v to the rest.
Jim
But running on two lines sounds even better... i need to look into this.
Thanks for the replies.
Jesse
i might be on the wrong page but why dont you get a 2pair shielded cable(jameco #232451cr, #234179cr) and run it to each box individually. if all 4 wires are common throughout the system then run all the cables into a junction box in a central location. a setup as such would be easier to troubleshoot. but thats just my opinion.
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League Bowling.... it's not a sport, it's a way of life
The reason I don't want to run a seperate set of wires to each unit is because I want to be able to add more display boxes easily. If they are daisey changed together then I should be able to just plug in another display box very easily.
Is it that obvious that I am still a novice when it comes to electronics[noparse]:)[/noparse]. It is true I know the basics of electronics but I figured that I am doing pretty good since I have only been working with it for a month now. The programming is the easy part, right now I am using SourceBoost C++ IDE to write the code and am loving it. I have been programming VC++ for a few years so naturely I did not want to have to learn assembly for the chip, even though I have taken classes for assembly of the 8086/88 processors, I just don't have the time.
When I finish the project and get it running for this company I am working for then I will show you guys the finished project if you are interested.
Anyways so far you guys are great.
Jesse
I just finished testing one of the display box and the most current it used was 97.8mA. So if I hooked up 255 display boxes in parallel connection, the wires would have to handle atleast 25.5 amps(255units * .1 amps = 25.5 amps).
Thanks
Jesse
I don't know what speed you're running the SX at, but lower clock speeds will definitely lower your power consumption. Also, as you've already figured out, daisy chaining the full 255 will result in some massive power requirements. I think you'll find you need to daisy chain some limited number, and then feed those into some sort of concentrator hub.
Thanks, PeterM
Right now I am running off the internal 4mhz. I could run it a little lower without any problems I believe. It is just hard to believe that it will take that much current to run them.
I originaly thougt that I could run everything through cat 5 cable, but it looks like I will have to change my plans a little.
Thanks,
Jesse
Post Edited (Paul Baker) : 3/2/2005 4:17:48 PM GMT
I can't lower the brightness of the display, it needs to be as bright as possible.
Jesse
Just thinking out loud here....
Can you supply your "display boxes" from the relative center of all your units rather than on an end?
....in a sort of T configuration, where the vertical component would be supplying 25.5 amps, but each horizontal component
would only need to supply half of that or 12.75 amps.
....Thinking further if your power supply was AC, and you divided it in a way that each unit had a half wave rectifier (a single diode
with capacitor filter) where HALF of the units were powered from the High side of the AC signal and HALF of the units were powered
from the Low side of the AC signal, you would still need to supply 25.5 amps on the vertical component, but each horizontal
component would only need to supply 1/4 of that, so now your down to a little over 6 amps.
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Beau Schwabe - Mask Designer III
National Semiconductor Corporation
(Communication Interface Division)
500 Pinnacle Court, Suite 525
Mail Stop GA1
Norcross,GA 30071
Could you draw that out in a schematic, maybe then I could understand what you are saying.
Thanks,
Jesse
····························64 daisy chained units
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64 daisy chained units
Central controller
64 daisy chained units
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····························64 daisy chained units
instead of:
controller
256 daisy chained units
it still keeps the wires fairly minimal and cuts the·current requirement of any wire by 1/4 or to 6.375 Amps taking you to a maximum gague of 18 instead of 10 guage, this still isn't low enough for cat 5 which I believe is 22 gauge.
Here is a calculator for 12V, providing proper gauge, given length and current: http://www.engineeringtoolbox.com/33_730.html
I don't think youll be able to use cat 5 in any daisy chaining scheme, the max current for 22 gauge is .95 Amps.
wait re-reading Beau's post, I realise I don't diagram it correctly, I'll leave it to him to explain.
I think he may be refering to this:
································Central controller
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64 daisy chained units
+
64 daisy chained units
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64 daisy chained units
|
64 daisy chained units
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Post Edited (Paul Baker) : 3/2/2005 6:43:22 PM GMT
in this case the widest point at the bottom, your current demand will be halved.
By supplying your power from a central point, you only need to supply half the current out to each "leg".
With AC, if you turn half of your devices on with one polarity, and half of your devices on with the opposite
polarity, you can again decrease your overall current demand by a factor of 2 because only HALF of your
devices are on (being supplied) at any time.
During the "OFF time" each unit would have a simple diode cap filter to sustain power within each unit.
Doing this for 100mA is not to difficult.
In reference to the E-Mail above:
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Beau Schwabe - Mask Designer III
National Semiconductor Corporation
(Communication Interface Division)
500 Pinnacle Court, Suite 525
Mail Stop GA1
Norcross,GA 30071
Post Edited (Beau Schwabe) : 3/2/2005 7:27:27 PM GMT
The AC approach sounds a little like the "free lunch" problem - e.g. there is no free lunch.
If the circuit needs 100 ma to run, and you power it with a pulsed current where the cap provides power during the off cycle, doesn't that mean that during the on cycle, you need power the circuit and recharge the cap, thus doubling the current input during the on cycle and bringing the peak power cunsumption back to 100%?
Thanks, PeterM
This is why I said the above.... I see what you are saying, and you may be right. I usually design with
DC, so my mind does funny things when AC gets involved.
Ok forget AC for the moment ..... At least we can agree that this will work for DC by reducing the
current in each leg to 12.75 amps.
In reference to AC What if this is pulsed? Not a sine wave... Hard polarity swap... What is the value for a 50% duty cycle?
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Beau Schwabe - Mask Designer III
National Semiconductor Corporation
(Communication Interface Division)
500 Pinnacle Court, Suite 525
Mail Stop GA1
Norcross,GA 30071
To reduce costs, you may even be able to use the SX28 as the switcher controller as well by bootstrapping the initial supply to the SX to get iy running. After the SX starts running, it would just pulse the pass element using the comparator as feedback to adjust the pulse width.
However, switcher controllers are not that expensive and will make the design easier and quicker.
The limitation on the distributed supply voltage would be UL limits and also low enough to prevent sparking during hot connect/disconnect.
Another consideration is that the bootstraping may require larger startup currents than normal running, you would therefore like to provide some form of power sequencing to limit the draw from the upconverter at the master converter.
You may get some better ideas from researching POE (Power Over Ethernet). CAT5 consists of 4 twisted pairs of which only two pares are used for signaling. With POE, the other two pairs are used to distribute power. However, 100BaseT is a star network and power is then distributed in the same way. You want a bus network.
If I have not explained myself well enough or if you have more questions, feel free to ask.
Doug
My suggestion considering your experience is getting heavy duty power cord and CAT5 and split looming or taping the two together, its the simplest and sure fire way to get the system working.
You do have a point, first we need to know if he took the current measurement on the power side of the regulator or the supply side of the regulator. Assuming it was computed on the supply side, its still 25.5 Amps but 127.5 Watts which drops him to 13 gauge, assuming the current tree configuration thats 31.875 Watts per branch and 19 gauge. Ive tried to find what would work with cat5 but there is no solution, the resistance inherent to 22 gauge wire make carrying any appreciable current beyond a few feet impossible (6V, 4A, 50ft = 7.8 Voltage drop!)
RE: black68cougar
Like I stated in response to Beau, I really think given his knowledge level he shouldn't try to attempt a switched power scheme.
Post Edited (Paul Baker) : 3/2/2005 9:21:24 PM GMT
www.currentsolutions.com/knowledge/vdrop.htm
Thanks, PeterM
ok I think I have it all planned out on how I am going to wire this stuff together.
Thanks for all of the help.
Jesse
This is basically known as IR drop. (Current drop due to Resistance of wire length).
Our formulas for Electromigration rules also include temperature in the equation
Iavg <= Dt*A(W-B)
Irms <= 5*A(W-B)
IPeak <= 20*A(W-B)
IESD <= 40*A(W-B) only for ESD pulse < 0.15uS
Where:
Dt = Sqrt[noparse][[/noparse] 1.408X10-10 * e^(8121/273+Tj) ] = Temperature derating factor
Tj = the junction temperature in degrees Celcius.
A = mA/um
B = um
W= width of the metal line (in um)
Note:
A and B are coefficients related to the cross section
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Beau Schwabe - Mask Designer III
National Semiconductor Corporation
(Communication Interface Division)
500 Pinnacle Court, Suite 525
Mail Stop GA1
Norcross,GA 30071
Post Edited (Beau Schwabe) : 3/2/2005 11:48:40 PM GMT
I just finished one for finding the common and phases for unipolar stepper motors based on resistance readings... Not sure it works correctly, but:
http://www.sxlist.com/techref/io/stepper/wires
On the subject of problems with long wires, keep in mind that AC issues (EMI, noise, etc...) will be much more of an issue if AC power is used to supply the remote units. That AC power will cause problems with the signal lines. In fact, the signal lines may have more than enough trouble given some length and the right phase of the moon, etc... I would really suggest twisted pair.
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What you do is have a 12-volt wall-wart power 10 nodes. This is around 1 amp power, which should not require huge conductors. Build 25 of those, and you are good to go.
Your signal wire should be 4 conductor (phone wire) or CAT-5 (8-wire, more expensive, but better noise isolation). Run +12 on one wire, signal on another, ground on a third. This gives you one spare wire to use as a second signal wire, a power-supply ground wire, or for future expansion.
You CAN do this on two wires, but the complexity of putting the signal on the power wire and getting it off again without blowing up your circuits rises a lot. 4-wire cable is cheap enough you should run separate signal wires.
Why do you need 255 nodes anyway?
I beleive that this is how I am going to end up doing this. To answer your question about running 255 displays is the company that wants this setup needs a lot of these displays. But they also want this system where they can scale it to any number between 1 and 255. So basically this system has to be where I can plug in any number of displays(up to 255) and only have to change the program on the server and the address on the SX chip.
Thanks,
Jesse
Note each '485 chip is limited in how many drops it can have -- I think each driver can drive 32 recievers. This means you'll also need 're-driver' boxes to get to the 255 node limit -- which is another good place to put in a power-port. A 're-driver' box can be relatively 'dumb' -- Black Box makes them, if you want to see what is commercially available.
For the 'address' it occurs to me you have a couple of approaches.· The hard one is to hard-code into each SX what address it is.· This means your code is slightly different for each node, and a node which dies requires reprogramming an SX to replace it.
Another approach is to add an 8-pin DIP switch to each unit which holds the 'slave' byte address, read once on power-up (connected through a 74HC164 to limit I/O pin use).
One last approach is to assign each unit you build a unique 16 to 32 bit number.· You then design some nice protocol to poll every box on the party-line, get its number, and have the master assign a unique 'listen' byte address to that 'slave'.·· This takes the least hardware, but is the most difficult to get right.· I'd go with the 'brute force' DIP switch solution, myself.
Post Edited (allanlane5) : 3/8/2005 3:14:26 PM GMT
I don't know how up to speed you are on electronics, so what I am about to suggest may seem foolish.
If I had to tackle this project, and based on your requirements as I've gleaned them from following this post, I would approach it from the perspective of using a single pair of wires, regular 18 Ga lampcord in fact, and use that to power the nodes as well as simultaneously communicate over the same wires.
How can this be done?
Firstly,·as it appears to be a commercial application, I'm assuming there is not an extreme price sensitivity, I bet that a few extra components per node will not be a problem.
You need to find a way to reduce the current consumption while still keeping the LEDs as bright as possible. This can easily be done by driving each of the seven segments of each display with its own transistor/flyback inductor or transformer. This implies using one dedicated port bit for each segment (that is, non-multiplexed) and pulsing power into ech inductor from a relatively high voltage, say 30 VDC. When the processor turns the power (ground) to the inductor off, the built-up field can collapse into the diode, lighting it in a realatively constant-current basis while the field collapses. When the field is depleted, the cycle is repeated. This way the wattage that·each LED segment needs (say 10 or 20 mA times a forward drop of·2.5 Volts = approx 50 mW) can be scaled to the higher 30 V supply voltage, and hence each consume 2 to 3 mA.
Next, feed 24 VAC down the wire, and·in each node rectify and filter that somewhat to provide the 30 odd volts DC. Insert blocking inductors in series with the rectifier bridge to prevent the data signal (yet to be added) from being swamped in the low impedance of the filter cap. Add a small low voltage regulator to run the SX at 4 Mhz.,drawing only·a few mA.,·and use some appropriately small coupling capacitors to pick off the data signal from the line yet block the AC sinewave. Feed that signal to the micro for software processing and ulimately display generation.
At the "head-end" you simply have a similarly connected·node as well as the transformer. It will place the desired data onto the line, again coupling the data through yet blocking the slow 60 Hz AC. There is no requirement for·RS485; the processor can readily deal with the bit-banging directly from its port bits.
While I have not yet fully developed·a similar·sytem for my own applications, I have gone along quite a ways, and determined (in fact proven) that each of these concepts is viable, and hence the whole package is possible. That said however, it is not for the faint-of-heart, and considerable experience is an enormous asset.
If you wish to follow this approach further, I'd be happy to give pointers. In order to do that well, however you will need to spend some time to very precisely deatil ouline EXACTLY what you are needing to accomplish.
Sounds like a fun project.
Peter (pjv)
·
So, with one LED per segment, seven per display, this is 140 mA per display. Not counting decimal points and/or colons. With 255 units (boxes?), this is 35.7 Amps! And we haven't factored in consumption of the processors and any support circuitry, such as the serial level-translator chips for RS232 communications to the PC.
Or did I miss something?
kenjj
All my pencils *used* to have erasers!