USB supply for breadboard.......

Discussion in 'General Electronics Chat' started by easilyconfused, Apr 3, 2015.

  1. easilyconfused

    Thread Starter New Member

    Mar 25, 2015
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    I'm in the process of setting up a breadboard with power from a USB.

    Will I be able to get a Vout of up to 5V from a LM358 op amp with a gain of 2 ?

    I tried reading the spec sheet and all I could figure out was Vin had to be -1.5V of supply voltage.
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    That's correct. Look at the schematic diagram and you'll see why. There are op amps that will go to the positive rail.

    Do you know what voltage and current a USB port is spec'ed to provide?

    HTH
    Dennis
     
  3. #12

    Expert

    Nov 30, 2010
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    No, the LM358 will not peg to the positive rail until your load is down in the microamp range.
    Always check the datasheet!
     
  4. easilyconfused

    Thread Starter New Member

    Mar 25, 2015
    16
    1
    I always assumed usb was 5V ????

    I looked at the data sheet and it did a good job of confusing me (it was like reading a japanese motorcycle manual :eek: )

    Okay, if I run parity gain on the LM358 with a Vin of up to 2.5V on a usb supply I should be alright ...... correct ?

    Then if I want to go higher I'll have to go for maybe a 9 or 12V supply ?

    Thanks for the help. The more I learn here, the less things I melt on my breadboard.
     
  5. #12

    Expert

    Nov 30, 2010
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    I think the USB supply voltage is 5V.
    The LM358 can supply about 40 ma at 2.5 volts (from a 5 volt power supply).
     
  6. dl324

    Distinguished Member

    Mar 30, 2015
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    It's 5V nominal. Spec is 4.5-5.5V. If you're connecting to a USB port, you're only guaranteed 100mA. To draw more, the circuit needs to negotiate with the host for up to 500mA (USB 2.0 spec); and stay in low power mode if denied. Dumb USB peripherals are only allowed 100mA max.

    Learn to read data sheets; they're your best source for information. It only took me 10-15 seconds to determine max output voltage and why.

    HTH
    Dennis
     
  7. dl324

    Distinguished Member

    Mar 30, 2015
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    Hi easilyconfused,

    Since it appears that you're in the learning phase, when you design anything you should use conservative parameters; unless you cherry pick your parts by screening.

    USB is only guaranteed to provide 4.5V @100mA (USB 2.0 spec, I haven't studied 3.0).

    LM358 is guaranteed to provide 20mA with 40mA being typical. Have any ideas why that's the case?

    BR
    Dennis
     
  8. wayneh

    Expert

    Sep 9, 2010
    12,123
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    One way to get closer to the rail voltage, and with more current, is to drive a transistor with the op-amp. Saying anything more requires a schematic and/or a description of what you want to do.
     
  9. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,694
    904
    You have not said how much current you need. An alternative to a rail to rail op-amp would be to use a voltage converter, like the xxx7660 (xxx=several brands, ICL7660 is common). That converter is often used to generate a negative voltage, but it can easily be configured to generate twice the input voltage or a negative and positive voltage, which might be an advantage in your application, whatever that is. It just uses a few capacitors (up to 3), maybe some diodes (2) for that. No inductors are required.

    John
     
  10. easilyconfused

    Thread Starter New Member

    Mar 25, 2015
    16
    1
    Thanks for your input guys, this is all new to me so I appreciate your help.

    I'm trying to set up a basic workbench so I can start playing with a breadboard and make a few circuits. I bought one of those cheap 15Vdc adjustable power supplies off ebay to use as Vin, and I thought I might get away with using the usb to power my LM358 (3-32V). It looks like I might also have to buy one of those wall wart things to provide the 12V I'll need at the breadboard. (I'm sticking with 12v so I can't hurt myself)

    There will be NO load on the circuits, I'll just be making circuits (to see if they work) and taking measurements with a multimeter (I have to learn somehow)
     
  11. atferrari

    AAC Fanatic!

    Jan 6, 2004
    2,648
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    Everything at the output is a load :eek:, let me tell you; not always impressive but a load anyway.

    If you look closely, even the multimeter is/will always be. :eek: :eek:
     
  12. wayneh

    Expert

    Sep 9, 2010
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    I recommend keeping your board voltage at 9V or below unless you have a specific need for more. The lower the voltage, the less likely a mis-wiring on your board will destroy a part. Murphy's law is real, and you will make mistakes. Everyone does. The penalty for a mistake goes up in proportion to voltage.
     
  13. dl324

    Distinguished Member

    Mar 30, 2015
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    I'd recommend getting a good power supply, surplus is fine regardless of your budget. If you'll be using any 5V logic, a triple supply would be a good option. When I started my electronics hobby (a long time ago), one of my first projects was an adjustable power supply.

    BR
    Dennis
     
  14. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Yes, no matter the gain, the output cannot go to more than (typical value) Vcc-1.5 which, with a 5V supply, amounts to 3.5V
    In other words, to get 5V on the output, you'll need a supply of 6.5V or higher.

    If you make a switch-selectable supply with eg. 3V, 5V, 6V, 9V and 12V for your breadboard, you should be golden for lots of tinkering :)
     
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