Very weird problem

Discussion in 'General Electronics Chat' started by dannycrafts, Aug 22, 2013.

  1. dannycrafts

    Thread Starter New Member

    Aug 22, 2013
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    Hey all,

    I'm still just learning a lot about electronics, but I wanted to build something already, so I am wanting to try some simple circuits in the hope I'll be able to build something useful.

    I started with a simple voltage divider circuit, but I did not get the results I was expecting, so I narrowed the circuit down to 2 resistors I had (both 325 Ohm), and connected them in series, to a source of 5V. Then I got my multimeter, and touched one probe to the positive side of the source, and the other probe to the negative side of the source.
    What I don't understand is that the multimeter reads about 0.8V, (and 0.4V when touching one probe in between te resistors, which is to be expected when the voltage (drop) across both resistors is 0.8V) but I thought that the voltage drop across the whole circuit (in this case consisting out of only 2 resistors) would be 5V.

    When I touch my probes to the voltage source, and this source is not connected to anything at all, the multimeter reads out about 4.9V (which I guess is either because the multimeter is not calibrated accurately enough, or the power supply may just be 100mV less than stated. Anyway, the multimeter seems to work correctly when 'connecting' the meter itself in series with the voltage source, BUT, not when connecting the meter in parallel with some resistor components.

    Can anybody guide me into the right direction, or explain whats going on and why it is not what I am expected to see? Or is it maybe possible that something is wrong with either my multimeter or power supply. If you want to know, I took an USB-B receptor, and soldered two wires to the +5V and the ground pins, and every time I want to test a circuit I connect it to an USB port on my computer. Anyway, I think it is rather the lack of my knowledge in electronics than some faulty hardware that may be the cause of problems here.

    Thanks for reading this shitload of text. ^^
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Yes, it appears that the load of those two resistors in series is pulling down the supply voltage as you've observed. That's if you are using the meter properly, which you seem to be since you get 5V on a USB port. Using the meter in it's current-measuring mode would be like shorting your power supply and would definitely cause a problem.

    5V across 650Ω should be less than 100mA of current, which your USB port should be able to handle if there are not other demands on it. So that's a bit mysterious.

    How did you determine these are 325Ω resistors? That is a non-standard value. Is it Orange-Red-Green, perhaps? That would be 3.2MΩ. That would explain your observation.
     
  3. tshuck

    Well-Known Member

    Oct 18, 2012
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    Try measuring the resistance of the resistors without anything attached... 325Ω is an odd value...

    Edit: darn last minute edits... ;)
     
  4. dannycrafts

    Thread Starter New Member

    Aug 22, 2013
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    Thanks for the reply, I was kind of frustated because I didn't think anything was wrong with my theory of voltage divider circuits.

    I think I will be trying to tap some 5V from my PSU itself, or just find a 5V battery and work from there.

    I'm not familair with the USB specification at all, so I don't know if it may do anything weird, but I thought it wouldn't.
     
  5. Austin Clark

    Member

    Dec 28, 2011
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    Where is the USB port your using located? Is it a dedicated charging port, or connected to a PC, or something else?

    From what I understand, USB Ports connected to a PC, or used mainly for data transfer, need to communicate to the computer and basically "ask" for more power before it is supplied. Otherwise, if memory serves, they're capped at at something like 100mA (Which seems like a good ballpark range given that you have a 4.2V drop drawing just under that). Try the same experiment with higher-value resistors, or with a different USB Port. That should fix the problem. Let us know how it turns out! :)


    Edit: You could also try using a 9V Battery.
    Just note the internal resistance:

    Battery - Internal Resistance
    9V zinc carbon - 35Ω
    9V lithium - 16Ω to 18Ω
    9V alkaline - 1Ω to 2Ω
     
    Last edited: Aug 22, 2013
  6. dannycrafts

    Thread Starter New Member

    Aug 22, 2013
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    Uhm yeah, so I don't really read color codes because I haven't really put much time into remembering the function of each color and position. But what I'm seeing here is orange-orange-brown-gold. I've got a strip of resistors here and I don't really know where it came from and so I just measured it with the multimeter, which reads about 325 Ohm. However, now that I think about it, I've taken another resistor, 7.5K Ohm, and measured it, but it read 7.45k Ohm so I wouldn't be suprised if that actually where 330 Ohm resistors I guess.
     
    Last edited: Aug 22, 2013
  7. dannycrafts

    Thread Starter New Member

    Aug 22, 2013
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    Yeah seems fair I guess, but I was still thinking that I'd get at least a maximum of 100mA from the USB port and it had to be enough I guessed.

    Does the resistance of the power supply has to be taken in account too? (50 Ohm I guess, taken that I probably am only receiving 100mA)

    Anyway, I will be experimenting again tomorrow when all daily activities have taken place. :p
     
  8. wayneh

    Expert

    Sep 9, 2010
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    Try making a divider with your 7.5k and one of your 330Ω resistors. This will reduce the load on the supply and should work as calculated.

    It's possible your USB supply will not supply anything more than device-detection current unless it negotiates with device. This would also explain why you can get a voltage on your meter alone but not when the port is loaded. I don't think that's normal USB behavior, though.
     
  9. tshuck

    Well-Known Member

    Oct 18, 2012
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    ...are you sure you have tapped the supply, as opposed to the data pins. Tapping the data pins would probably result in what you've described...

    By the way, orange-orange-brown is, indeed, a 330Ω resistor...the gold band denotes a 5% tolerance, so the value is 330Ω ± 16.5Ω...which includes your measured 325Ω value.
     
  10. Austin Clark

    Member

    Dec 28, 2011
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    The USB port won't act like a perfect voltage source, but it also won't act exactly like a battery. It can't be thought of as a voltage source with series resistance, because that's not the full story, particularly with regulated power supplies.

    The amount of current you draw won't change the way the voltage divider works, you could technically use 1M ohm resistors. HOWEVER, your meter will have an influence of it's own (it must draw a little bit of current on it's own.) and on top of that, with really really high resistor values, you will eventually have to consider thermally-induced noise :p

    Like I mentioned before, you could use higher-value resistors and run the experiment again. Put less of a load on the power supply.
     
  11. WBahn

    Moderator

    Mar 31, 2012
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    I would recommend NOT using your USB ports as a power source for circuit experimentation, especially when you are brand new. First, it is not a simple power supply, as others have noted, and so it will be difficult for you to understand what effects you are seeing are due to you possibly not understanding the circuit you are trying to build and what effects are due to the idiosyncracies of the USB source. Second, you are going to be making mistakes as you learn; it would be best if one of those mistakes didn't damage your computer. So keep things simple and use a 9V battery while you tinker. One of your first projects can be building a power supply. Again, keep it simply and first build one that produces a single, fixed output voltage (pick whichever voltage seems most useful to you at the time). Then make one that has multiple outputs and also variable outputs. Then make one that has meters of some kind to let you monitor the voltage and current. Then make one that lets you set limits on the voltage and current that will be supplied. You will learn a LOT along the way and end up with a small collection of very useful bench supplies in the process -- and don't worry about making multiple supplies as you go, you will quickly find yourself needing multiple supplies in your various projects. Also, having multiple supplies that range from simple to fancy let you pick the right tool for the job; you will probably find that your original fixed-output supply is the one you use most often because it is the simplest to use.
     
  12. dannycrafts

    Thread Starter New Member

    Aug 22, 2013
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    Ok, so now I feel stupid.

    I've unplugged all usb cables from my computer (except for my CPU fan, which is connected to a USB header on the motherboard), but no improvement.

    Then I used a 300K Ohm resistor, and read 4.5V, which was an huge improvement over 0.8V I guess.

    Then I investigated if I was using the correct pins, and I found out I was using one of the data pins as ground...
     
  13. dannycrafts

    Thread Starter New Member

    Aug 22, 2013
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    Hey,

    Acutally, thanks for the advise. I was thinking about building a power supply before, but never thought I should do it as one of my first projects because I assumed I had to be working with mains voltage and that scared me off. Maybe I should make a 'power supply' that takes a battery to supply 5V.
     
  14. Austin Clark

    Member

    Dec 28, 2011
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    Store-bought wall-warts, or those black box AC to DC adapters that you plug into your wall, are great for beginners, so long as you stay well under their rated maximum. They're pretty cheap too. If you're expecting to stick with electronics for awhile however, I'd recommend getting a true desktop power supply. Something like this: http://www.thelashop.com/30v-dc-con...m_medium=cpc&gclid=CPXiyZecmbkCFeRj7Aod5loAUQ
     
  15. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,514
    2,369
    The USB port is generally fed from the +5v P.C. power supply but is fed through the M.B. so if you need a high current +5v source and also a +12v source, you can obtain this from one a spare P.C. P.S. 4 pin accessory connectors, yellow is +12v both blacks are common (earth) ground, the red is +5v at around 35amps rated.
    If you use this it is advisable to use fusing protection, and also realize that all PC port commons and Pwr.S. common is at earth ground potential.
    Max.
     
  16. adam555

    Active Member

    Aug 17, 2013
    858
    39
    When I began I used those 9V batteries for my experiments, as they said above. Or better still, if you want around 5V use one of these as a power supply:

    [​IMG]
     
  17. tshuck

    Well-Known Member

    Oct 18, 2012
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    Better still - use a regulated supply...
     
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