Power Supply Project

Hello,

so, this is perhaps one of my biggest problems yet. It's going to be a power supply with the following specs: * Outputs: o +5V o +12V o +1.2V to +37V o -1.2V to -37V * Relays for switching off the outputs individually. (6x relays) * LED 7-Segment display for displaying the output on the variable outputs. (2x 3digits) * LEDs for showing the states of the relays. (5x 5mm Red LEDs) * Buttons for switching the outputs on and off. (6 buttons) * Potentiometers for setting the value of the variable outputs. (2x 10k pots) * Serial connection with the possibility of controlling the supply from the computer. * Arduino for connecting... o 1 MAX7219 (display controller) o 2 MCP4131s (digital pots for the variable outputs) witch o 4 Relays (for switching the outputs) o 6 Buttons (for controlling the relays) o 1 FT232 (USB-to-Serial converter) * Easy to build, without to much SMD parts and with easy to get parts.

And now I have a first schematic done, and as my perhaps weakest point in electronics is the calculation of values and similar things I now want to know if the part values and the rest is done in a at least working way from someone who knows better than I do.

I have attached the schematic as eagle .sch and as .png image if you don't have eagle.

JanD

P.S. What does "the upload folder is full" mean???? I'll upload the files to my webpage instead and post a link... Image Eagle schematic

I checked on schematic briefly. There is a question: Both (7812 and 7805) has a limits 35V on input.

"Table 1: Absolute Maximum Ratings DC Input Voltage "Value for VO= (5 to 18V) = 35"

Power supply up to 37V,

And thermal , have you calculate power dissipation? 6 leds 7 digit = 42 + 6(comma) = 48 leds I = 48 * 20 mA = (almost) 1A.

I checked on schematic briefly. There is a question: Both (7812 and 7805) has a limits 35V on input.

"Table 1: Absolute Maximum Ratings DC Input Voltage "Value for VO= (5 to 18V) = 35"

So I would need three power inputs? +40V, -40V and +30V?

And thermal , have you calculate power dissipation? 6 leds 7 digit = 42 + 6(comma) = 48 leds I = 48 * 20 mA = (almost) 1A.

The LED driver (MAX7219) multiplexes the LEDs, thus a maximum of 8 LEDs are lit at the same time. Thats 8 * 20mA = 160mA = 0.16A. So on that my calculation should be right.

Anything else? I will change the supply thing later.

JanD

I would recommend to use a switching voltage regulators. 78xx IC is linear, it's not impossible, but it difficult to design something with wide range of voltage, like 1.2 - 37 with linears.

I was wonder, if I=20 mA for leds to provide specific level of brightness, and you set 6 digit display, current will flow only 1/6 period of time. so 20/6 = 3.3 mA on average . What about brightness? It will decrease. How to keep it the same level? Drive a current up proportionally to number of digit, in this case supply 120 mA in 1/6 time frame. Max7219 data doesn't say anything. It's obvious , 1 digit or 8 digit display have a huge difference, and current must to be different, 20 mA for 1 digit, and 160 mA for 8. "Commercial secret" - this is why no explanation in their data sheet ?

I would recommend to use a switching voltage regulators. 78xx IC is linear, it's not impossible, but it difficult to design something with wide range of voltage, like 1.2 - 37 with linears.

For the 1.2V to 37V I use LM317 and LM337, not any of the 78-range.

I was wonder, if I=20 mA for leds to provide specific level of brightness, and you set 6 digit display, current will flow only 1/6 period of time. so 20/6 = 3.3 mA on average . What about brightness? It will decrease. How to keep it the same level? Drive a current up proportionally to number of digit, in this case supply 120 mA in 1/6 time frame. Max7219 data doesn't say anything. It's obvious , 1 digit or 8 digit display have a huge difference, and current must to be different, 20 mA for 1 digit, and 160 mA for 8. "Commercial secret" - this is why no explanation in their data sheet ?

On what your right... But if the calculation here isn't wrong I only need 170mA (at 20mA/LED) included the supply for the MAX7219. I think you can be totally right, but I have never read anything about what. Perhaps the MAX7219 switches the LEDs so fast what it doesn't matter...??? Does anybody know anything about what?

JanD

LM317 is excellent, but it is a linear. Power dissipation would be potential issue: P = (V(input) - V(output)) * I (current); Approximately: P = (40 - 12 ) * 100 mA? = 2.8 W, that is overlimit

And thanks for the link. There is some "typo error" : This really depends on the number of Leds which are lit in a row of the matrix at the same time.

First, matrix is design in a way only one led lights at a moment, not a whole row. And second, not only quantity of rows in a matrix's affect current, columns * rows. In all other meaning page is really informative, I will read it again when I save a money (12.95$!!!) for max7221 :)

LM317 is excellent, but it is a linear.
Power dissipation would be potential issue:
P = (V(input) - V(output)) * I (current);
Approximately:
P = (40 - 12 ) * 100 mA? = 2.8 W, that is overlimit

Witch IC would you recommended?

And thanks for the link.
There is some “typo error”
:
This really depends on the number of Leds which are lit in a row of the matrix at the same time.

First, matrix is design in a way only one led lights at a moment, not a whole row.
And second, not only quantity of rows in a matrix’s affect current, columns * rows.
In all other meaning page is really informative,
I will read it again when I save a money (12.95$!!!) for max7221 smiley

I think I go for the thing what stands in the link for the beginning. As my design (I posted an updated design on my site, just follow the link in the first post) uses a 5pin connector to connect the main parts with the display part I could easily add a separate supply for the display part.

JanD

Loot at : http://www.linear.com/products/high_input_voltage_buck

There are others chip makers, maxim-ic, motorola etc. Just Linear Technology was the easiest to navigate, and find this product search page.

Magician:
Loot at :
http://www.linear.com/products/high_input_voltage_buck

There are others chip makers, maxim-ic, motorola etc.
Just Linear Technology was the easiest to navigate, and find this product search page.

The only switching voltage regs I can find at my hardware store are either to expansive, are unvariable or have a ampere limit below 1.5A.
Is there perhaps any other “normal” voltage regulator what has better power dissipation?

JanD

Have you considered LM2576/LM2576HV? You would require a new circuit but they are pretty good. Also, cheap.

www.national.com/ds/LM/LM2576.pdf

I've made a new schematic and uploaded it to my homepage (follow link in first post) to use the LM2576. Now everything is split up onto 5 modules; 1. Main (with ATmega) 2. Display (with MAX7219 and 7-segment) 3. Adjustable power (with one LM2576 and MCP4131) 4. 5V power (with another LM2576) 5. 12V power (with another LM2576)

Could someone please take a look again?

JanD

P.S. I took away the negativ power part.