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My Introduction and a Question — Parallax Forums

My Introduction and a Question

johnnycarlosjohnnycarlos Posts: 44
edited 2010-06-16 00:33 in Propeller 1
Hi Everyone,

I have decided to get my electronics skills to the next level and have settled on the Propeller. After reading about the chip and seeing what it can do, my excitement level has gone to the moon! I have a degree in software engineering, but haven't used it much as my career has lead me down the System Administration path. But I've always wanted to do more with my knowledge and think I have found the perfect platform.

My question is regarding the assembly of the 40-pin DIP version from the kit. On the final verification step of connecting the battery, I am seeing 6V AC across the breadboard power rails, where there should only be 50mV AC. All of the DC tests check out fine however. My understanding is that capacitors short circuit AC, and I assume that's why the AC voltage should be so low. Do I have a bad capacitor? Any suggestions appreciated.

I look forward to sharing my future projects and participating in these forums. I will also be attending UPEW and will hopefully meet some of you there.

Thanks,

John

Comments

  • BeanBean Posts: 8,129
    edited 2010-06-09 13:56
    John,
    It sounds like your supply is faulty.
    You should have nowhere near 6VAC on the power rails.

    Bean.

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    Use BASIC on the Propeller with the speed of assembly language.
    PropBASIC thread http://forums.parallax.com/showthread.php?p=867134

    March 2010 Nuts and Volts article·http://www.parallax.com/Portals/0/Downloads/docs/cols/nv/prop/col/nvp5.pdf
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    If you choose not to decide, you still have made a choice. [noparse][[/noparse]RUSH - Freewill]
  • RaymanRayman Posts: 14,888
    edited 2010-06-09 14:01
    I can't imagine how you're getting 6V AC from a battery connected to the Prop!

    I'd guess you're just seeing noise.

    You do want to have a big capacitor in your setup though from 3.3V Vdd to ground to get rid of any noise or AC ripple.

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  • SapiehaSapieha Posts: 2,964
    edited 2010-06-09 16:04
    Hi johnnycarlos.

    Most people forget THAT - BATTERY powered systems need one small /100nF and one bigger /33uF Tantalum else even bigger Electrolytic CAPACITOR.

    BATTERY'S are very sensitive to Back noise transients from electronic systems.


    Regards

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    For every stupid question there is at least one intelligent answer.
    Don't guess - ask instead.
    If you don't ask you won't know.
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    Sapieha
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2010-06-09 17:03
    It's more likely an oscillating DC voltage, which can happen if the output capacitance on the regulator is not met.

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    Parallax Engineering
    ·
  • ElectricAyeElectricAye Posts: 4,561
    edited 2010-06-09 17:34
    johnnycarlos said...
    ....On the final verification step of connecting the battery, I am seeing 6V AC across the breadboard power rails, where there should only be 50mV AC. All of the DC tests check out fine however. ..

    Are you powering your system with an actual battery or with a 120 volt AC transformer that is supposed to provide 9 volts and has a connector that matches a 9 volt battery?
  • johnnycarlosjohnnycarlos Posts: 44
    edited 2010-06-09 20:38
    Thanks for the suggestions everyone, here is where I'm at:

    I am using a 9V battery.

    @Chris - what would cause a mismatch in the output capacitance of the regulator? I have everything wired according to the PEK manual.


    I have verified the result on two multimeters, one digital and one analog. I get 3.3V AC when plugging in the USB socket and 6V AC when connecting the battery.

    I went ahead and pinged to the chip via Identify Hardware and it connected successfully.

    Do folks think this something I need to concern myself with, or can I move ahead and immerse myself in Propeller goodness?

    Thanks!

    John
  • LeonLeon Posts: 7,620
    edited 2010-06-09 21:39
    How do you know those are AC voltages? Have you checked them with a scope?

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  • johnnycarlosjohnnycarlos Posts: 44
    edited 2010-06-09 21:56
    Leon said...
    How do you know those are AC voltages? Have you checked them with a scope?

    Hi Leon, no, I do not have a scope(yet). I am using the AC setting on the multimeter.
  • RaymanRayman Posts: 14,888
    edited 2010-06-09 21:57
    I think you should go ahead and see if you can get an LED to blink...
    If that works, I'd say something is wrong with your AC voltage measurement...

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  • Miner_with_a_PICMiner_with_a_PIC Posts: 123
    edited 2010-06-09 22:06
    John, this sounds very peculiar especially if confirmed by two separate multimeters. I have seen this type of signal on an oscilloscope if the scope ground isn't connected to system you are testing. The AC signal picked up is due to coupling of the system to the 60Hz radiating from wires in your home.

    If you connect both multimeter probes to the system under test then this AC component should be zero as you will be measuring common mode voltages. Where exactly are you placing the probes? Can you try moving the propeller board outside and retaking the voltage measurements to see if the AC you are seeing disappears/changes in magnitude?
  • HarleyHarley Posts: 997
    edited 2010-06-09 22:57
    @ johnnycarlos,

    Why use an AC selection? Try using the DC voltage setting instead. That might be the 'problem'.
    johnnycarlos said...
    Hi Leon, no, I do not have a scope(yet). I am using the AC setting on the multimeter.
    yeah.gif

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  • johnnycarlosjohnnycarlos Posts: 44
    edited 2010-06-10 01:30
    Hi Everyone,

    I pressed ahead with the PEK lab to test all the the pin I/O's and everything checks out. I don't know what the problem is, but I bet I'll figure it out later someday. Everything seems fine so far.

    @Harley - the manual specifically says to test for AC
    @Miner - I like this theory, I'll check it out.

    Thanks to all for your help. I know enough now to be dangerous [noparse]:)[/noparse]

    John
  • yarisboyyarisboy Posts: 245
    edited 2010-06-10 02:03
    For some reason the regulator needs a tantalium capacitor right where your battery voltage goes into it. The capacitor on the output side is also very important. I suffered through these problems before I read the specification sheets for the regulators. With boards from the same run at express, some would run with out the extra cap and some wouldn't. Too high an output voltage is one of this problems symptoms. Key words to search are "minimum capacitance" and "equivalent series resistance"and "stability". When a regulator is unstable oscillation sets in hence an AC reading. I actually had one case where the oscillation phase locked with a separate propeller operating elsewhere on the bench. Weird. I can vouch for the value of a dual channel scope for finding this type of sneek circuit or RF leakage. Can drive you nuts. Its weird to see channel 1 and channel 2 differ only by amplitude when everything is on batteries.

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    Post Edited (yarisboy) : 6/10/2010 2:23:49 AM GMT
  • johnnycarlosjohnnycarlos Posts: 44
    edited 2010-06-10 02:14
    @yarisboy Oh, ok. That concurs with what Sapieha was saying. Makes sense now.

    Thanks!

    John
  • grouchygrouchy Posts: 16
    edited 2010-06-10 02:59
    check your meter.
    disconnect the battery from your circuit and check the battery voltage directly with the meter set on ac volts.
    Do you still get a reading? (It should be zero after it settles)



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  • TubularTubular Posts: 4,717
    edited 2010-06-10 04:04
    Are you powering it via usb from a laptop? Sometimes the laptop chassis can have a few volts of AC with respect to true ground/earth, due to the isolation effect provided by the laptop PSU. That would also carry though to the "Vss" rail via the usb connection

    I agree with others though, regulators including linear regulators can certainly oscillate especially at low/zero load and when they don't have the recommended minimum capacitor at their output
  • Andy Lindsay (Parallax)Andy Lindsay (Parallax) Posts: 1,919
    edited 2010-06-16 00:33
    John,

    There are supposed to be 1000 uF capacitors across both voltage regulator outputs. The kind of AC oscillations you are measuring indicate that the capacitors might look like they are properly connected, but one or both capacitors are not really connected. The first thing I would do is recheck for wiring errors. The Propeller Education Kit Labs: Fundamentals book lists the breadboard coordinates for each capacitor lead. Double check them. Also check to make sure the that the clip inside the breadboard is gripping·each capacitor lead tightly.·Check to make sure the LM2940-5.0 and LM2937-3.3 are in the correct locations and orientations. If your multimeter can measure capacitance, probe the capacitor leads to verify that they are 1000 uF. This would be just to make sure the lead is making a connection to the metal inside the capacitor's canister.

    Andy

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