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PROP chip setup — Parallax Forums

PROP chip setup

morris4019morris4019 Posts: 145
edited 2009-05-23 06:35 in Propeller 1
Just got my Propeller setup today. I am currently wiring it up to my breadboard using the schematic out of PELabsFunBook.pdf. Just have a couple questions:

1) Looking at the schematic power is run through a 5v voltage regulator and then through a 3.3v voltage regulator. The 3.3 volts is then wired to everything on the board. (according to the schematic) Why the use of the 5v regulator, why not just the 3.3v?

2) I have everything needed, except the 3.3v voltage regulator, but i have a few brand new LM317T's. would this work for the 3.3? the only thing i worry about is the fact that as voltage drops, the voltage divider is going to be different.. e.g. I can make 3.3 v starting with 7.5 volts, but when voltage drops so does the output voltage of the LM317T. I thought I had a 3.3v regulator but turns out i do not. Fry's is a long trip (30+ mi) for a $1 part. And gas is currently 2.50/gal.

-Mike

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Comments

  • Greg LaPollaGreg LaPolla Posts: 324
    edited 2009-05-22 01:52
    You could order from Digikey and you will have it in a few days.
  • morris4019morris4019 Posts: 145
    edited 2009-05-22 02:02
    Same deal, and ordering it would probably cost more than me driving 30 miles to get it. Is there no substitute? I am searching through my pile of salvage and typing in part numbers to alldatasheet.com to try and find a 3v regulator.

    -Mike

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  • kwinnkwinn Posts: 8,697
    edited 2009-05-22 02:21
    Yes, you can use the LM317T to go directly from 7.5 to 3.3V ... BUT ... it is an adjustable regulator, the pinout is different from the standard 78xx series, and you need to set the output voltage to 3.3V BEFORE you connect it to the prop. It may also need a heat sink since it is dissipating more power.

    You can download the data sheet from National Semiconductor. It has a schematic for a typical circuit connection.

    btw - The main reasons for having a 5V and a 3.3V regulator is to have 5V available for circuits that require it (servos, driver IC's etc.) and reduce the power dissipation of the 3.3V regulator, as well as providing it with a pre-regulated voltage.
  • morris4019morris4019 Posts: 145
    edited 2009-05-22 02:30
    ok kwinn awesome, now here is the problem with the 317t. I've used them before, and i have a circuit setup on a breadboard using the schematic off of the datasheet, the problem is, as the voltage drops from the voltage that i set it up at, the output voltage (ie the 3.3 ) starts to go down in unison with the input voltage. To test it i setup the circuit using 7.5v (5aa cells), perfect 3.1-3.3v output. But i tested it again with 5v and i only get about 2.3-2.5v. Therein lies the problem.

    -mike

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  • Beau SchwabeBeau Schwabe Posts: 6,572
    edited 2009-05-22 03:12
    morris4019,

    The voltage output on the 317 should hold much better than that, what value resistors are you using? ..and what does your connection look like?

    A 330 Ohm resistor from Vadj to GND, and a 220 Ohm resistor from Vout to Vadj, should produce about 3.2V ... As long as the input voltage remains at 1.2V above what your are trying to regulate at you shouldn't see much if any drop as the input voltage changes. Don't forget the caps... .1uf between the input and GND, and a 1uf between the output and GND.

    Vout = 1.25V ( 1 + R2 / R1) + Iadj * R2

    R2 = 330
    R1 = 220
    Iadj is controlled to less than 100uA from within the regulator.

    Vout = 1.25V ( 1 + 330 / 220) + Iadj * 330
    Vout = 1.25V ( 2.5) + .033
    Vout = 3.125 + .033
    Vout = 3.158 <-- I've done this setup a few times and have always measured a little high





    Another reason for having two cascode voltage stages is not only to provide a 5V supply, but to also dissipate the HEAT (wasted power) across both regulators as opposed to just one regulator.

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • morris4019morris4019 Posts: 145
    edited 2009-05-22 03:33
    I used about 470 and 200 as my resistors, to give me aprox 3.27v output, with aprox 7.25v input. I have a breadboard voltage regulator that i built that gives me either no-regulation, 6v, or 5v. I have connected the VR to 7.5v (now about 7.25v) and tried un-regulated and works fine. But when i switch over to 6v or 5v (and i did verify the voltage at the input pin of LM317T as well as accross the pos and neg terminals of board) , I get about 1.5v (sorry typo obove when i said 2.5) output. And yes I did remember the caps (although most of the time i forget, so thanks for the heads up).

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  • kwinnkwinn Posts: 8,697
    edited 2009-05-22 03:45
    That's because the 317 needs the input voltage to be about 2.5V higher than the output voltage. To make this work you need to connect the input of the 317 to the 7.5V not the 5V.
  • Beau SchwabeBeau Schwabe Posts: 6,572
    edited 2009-05-22 05:41
    morris4019,

    Still curious what your circuit looks like, the LM317 should hold a tighter regulation.

    470 and 200 would produce about 4.23V ... A 7.25V input minus the 4.23V would give about 3V and you would "see" this difference much more as the voltage dropped.

    likewise an input of 6V minus the 4.23V would give about 1.8V and an input of 5V would give about 0.8V






    As mentioned earlier, the pin out for the LM317 is NOT the same as a standard regulator.

    With the correct LM317 wiring I measure 4.3V using a 470 ohm and a 200 ohm resistor with an input voltage ranging from 5.6V to 11.8V

    With an incorrect wiring to the LM317 resembling how you would connect a typical 78xx regulator I saw results as you are describing. ... 5.6V input I see 1.8V on the output ... 7.3V input I see 3.12V output ... 11.8V input I see 6.4V

    Using the correct wiring and the 330 Ohm and the 220 Ohm resistors I see a steady 3.2V on the output with an input voltage ranging from 5.6V to 11.8V

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
    LM317.JPG
    845 x 639 - 139K
  • morris4019morris4019 Posts: 145
    edited 2009-05-22 06:55
    Yes i am aware, like i said, about the wiring difference. LM317T from left to right (with inscription facing toward you) Pin 1: Vin, Pin 2: J-Adj, Pin 3: Vout. Whereas Pin 2 on the 78xx would be a common ground. I did not do the math correctly when i was working before. Actually i had to run and pick up my fiance and that gave me a good while to unjar my brain. I ran the recommended values to produce a 3.3v using 7.25-7.5v input (i did not realize that i needed approx 2-2.5v over the expected output) and it seemed that i got a .5v drop through my indicator LED. It made the outer pos run at about 2.8 which i thought was a little strage so i evened it out on the lm317T to bump up the output voltage to about 3.6-3.7 giving me an even 3.3v to the PROP and EEPROM along with the pos rails on the board. (which is completely within the 2.8-3.6v min/max of the prop)

    I ran all the multi-meter tests and everything checked out perfect. Hooked the prop plug on and away we went with no problems. YAY!

    Thank you everyone for your help. And I called the nearest Frys (which is the only store in washington state that i can find that sells individual components) and they did not even carry the 3.3v regulator, so this setup will have to do for prototyping until i order it online.

    Again thank you everyone!
    -Mike

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  • Mike HuseltonMike Huselton Posts: 746
    edited 2009-05-22 14:19
    I'll send you 3.3v regulators. PM me.

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    JMH
  • Beau SchwabeBeau Schwabe Posts: 6,572
    edited 2009-05-22 14:52
    morris4019,

    "...LM317T from left to right (with inscription facing toward you) Pin 1: Vin, Pin 2: J-Adj, Pin 3: Vout." <- This is my point, this is incorrect, and this is how I wired it to get the same results that you are describing. From left to right with inscription facing toward you on the LM317T, the wiring should be: Pin 1: Adj , Pin 2: Vout, Pin 3: Vin

    I don't mean to sound harsh, I just don't want anyone to damage their Propeller due to excessive voltage if they decide to go this route with a regulator. The LM317 will work fine if it is wired properly.

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
    LM317.JPG
    295 x 419 - 16K
  • kwinnkwinn Posts: 8,697
    edited 2009-05-22 15:41
    Beau is right, you have wired the regulator incorrectly. The fixed resistor (240 ohms) should go between pins 1 and 2, the adjustment pot between pin 1 and ground. A 500 ohm pot will allow you to adjust the output from 1.2V to 3.7V.

    I have attached a diagram with the pinout of the 3 most common regulators.
  • morris4019morris4019 Posts: 145
    edited 2009-05-22 16:57
    Oh, ok, thank you for pointing that out, i must have been looking at the wrong datasheet or something. I could have sworn i was on the LM317T datasheet that said the pinout was how i described. As i tried to verify this morning, all the datasheets i found confirm what you both said. My apologies.

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  • morris4019morris4019 Posts: 145
    edited 2009-05-22 17:37
    I rewired the VR setup with the LM317, with it outputing 3.2v (with no effect now if the LED is attached or not, thanks everyone for the tip on the pinout). Before i changed the wiring my indicator LED glowed dimmley while battery is disconnected, and prop plug inserted and connected to pc. But now it does not. It turns on when the battery is connected, and i've gone through all multi-meter test with perfect results. Any ideas? it's wierd.

    -mike

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  • kwinnkwinn Posts: 8,697
    edited 2009-05-23 04:00
    The led/prop was probably drawing a little current from the USB connection.
  • PhilldapillPhilldapill Posts: 1,283
    edited 2009-05-23 06:04
    Mike, I'd like to introduce you to TI Samples Program. TI, this is Mike.

    If you are designing a commercial product, I highly recommend ordering some free samples from TI, then use their products in your own design. I am in no way affilliated with TI, other than living 30 minutes from their Dallas Campus. [noparse]:)[/noparse]

    My suggestion is to order some of the uA78M33 voltage regulators. They come in a surface mount D2PAK, and a run-of-the-mill TO220 case. They can handle 500mA, and have a pretty good voltage tolerance.

    If you are expirementing with different parts, many companies offer a free sample program to their customers. This is usually a good way for you to find the right part for your application, and for the semiconductor companies to gain new customers.

    A smidge off topic, but I hope that helps in the future.
  • Mike HuseltonMike Huselton Posts: 746
    edited 2009-05-23 06:35
    Spot on, Philldapill. I also would suggest Maxim. They Fedex sample quantities from 1 to 8 chips to you absolutely free, as in free parts and free Fedex. I should have thought of that while pitying the poor fellow yeah.gif

    Just pick the part you want, click on Sample Quantity, enter quantity. Rinse and repeat smilewinkgrin.gif
    I save a fortune in parts costs that way.

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    JMH
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