Connect with us

turn off circuit at low voltage

Discussion in 'General Electronics Discussion' started by jayusa123, Aug 20, 2013.

Scroll to continue with content
  1. jayusa123

    jayusa123

    9
    0
    Aug 20, 2013
    Hi all,

    I'm making an iPhone charger for my bike out of an old 6V dynamo I found in my basement. (It actually outputs more like 8V). I have the circuit built which uses a rectifier to convert the AC to DC, a capacitor to smooth things out, a 7805 regulator to give my 5V, and two voltage dividers to provide 2V to the USB data pins. (This is because the iPhone expects to see this voltage in addition to 5V on the power lines. Without it, the iPhone won't recognize the presence of a charger--If the voltage is too low on those data lines, the iPhone will see the charger, but warn that charging isn't supported by the accessory.)

    That's where I run into my problem. Since it takes a couple seconds to get the dynamo up to speed, the iPhone senses the power on the line too early and gives a warning that charging isn't supported. Then, no matter how fast you pedal, it won't charge. If I pick the wheel up off the ground and kick the pedal really fast, the iPhone senses the correct voltages and starts charging. Additionally, if I start riding, get up to speed, then connect the phone, it charges without a problem.

    I'd like to add something to my circuit that will delay the supply of power to the charging cable until after there's enough voltage to support it and fool the iPhone. I know a transistor can help me here, but I'm not sure how to design a circuit around it to get this behavior.

    Any thoughts?

    Thanks!
    Jay
     
  2. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    648
    May 8, 2012
    Since we're going to be modifying your existing circuit I think you should post it.

    Chris
     
  3. jayusa123

    jayusa123

    9
    0
    Aug 20, 2013
    Agreed. Here it is!

    Thanks,
    Jay
     

    Attached Files:

  4. jayusa123

    jayusa123

    9
    0
    Aug 20, 2013
    Whoa, that's small. Let me retry that again.

    Oh, and IC1 is an LM7805C. It's what I had lying around. I'll probably go with a switching regulator for the additional efficiency, since I am pedaling and would like to maximize my reward!
     

    Attached Files:

    Last edited: Aug 20, 2013
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    10,025
    2,138
    Nov 17, 2011
    Your circuit could look something like this:
    [​IMG]
    Put an NPN Transistor as a series switch between the 5V regulator and the output. R1 would normally bias the NPN such that it is open. The voltage drop across C-C of the transistor is approx. 0.1V...0.2V and should be acceptable for a USB charger.
    A 5V microcontroller voltage supervisor (preferably with open collector or open drain output) monitors the regulated 5V. If the 5V is below the threshold (e.g. 4.65V), the output of the monitor goes low, disabling the NPN. Only if the 5V line's voltage is higher than the threshold, the output of the monitor goes high, enabling Q1.
    Note that if you use a monitor with open collector output, R2 can be omitted.

    Select Q1 and R1 such that your power requirements (aka charge current) can be fulfilled.
     

    Attached Files:

  6. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    648
    May 8, 2012
    Harald, just a thought... Shouldn't that read "If you use a monitor with open open collector / drain output, R2 should be replaced with a jumper"?
    jayusa123 seems like a bright fellow but a nube stumbling into this thread could interpret "omitted' as no connection.

    Chris
     
  7. BlinkingLeds

    BlinkingLeds

    180
    0
    Feb 23, 2013
    can't he just use a zener and a 555 schmitt trig instead of the complex microcontroller ?
    Or something without hysteresis
     
    Last edited: Aug 20, 2013
  8. Harald Kapp

    Harald Kapp Moderator Moderator

    10,025
    2,138
    Nov 17, 2011
    Chris, you're on the spot right. That's what I meant - it just was so obvious to me that I didn't see the ambiguity of my write up.

    Well, that is no microcontroller, it is a very simple voltage supervisor used to reset microcontrollers when the supply voltage is too low. See here for examples.
     
  9. BlinkingLeds

    BlinkingLeds

    180
    0
    Feb 23, 2013
    Oops sorry misunderstood that nevermind .
     
  10. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    648
    May 8, 2012
    Ha, that's because you understand the circuit. The more I deal with nubes and students the more I hang on every word I type. I do a lot of editing but it's still no guarantee that it won't be misconstrued! :D

    Chris
     
  11. jayusa123

    jayusa123

    9
    0
    Aug 20, 2013
    Thanks for the replies!

    Harald,

    I have a couple questions on your suggestion:

    1. What do you mean when you say 0.1V...0.2V voltage drop across C-C of the transistor? Do you mean C-E?

    2. I understand that an open collector output externalizes the collector of an internal NPN transistor (and an open drain output externalizes the drain of an internal MOSFET) but I'm a little fuzzy why it would allow for the omission of the resistor. Is it because an open collector output won't go high, meaning that a current limiting resistor is not needed to protect the (external) transistor?

    3. Are C1 and C2 just for smoothing as described in the 7805 data sheet or do they contribute anything to the monitoring portion of the circuit?

    4. I've never used a voltage monitor am a little overwhelmed by the huge number of options. Is there a popular one that that I can look at as a starting point? (Kind of like what 7805 is to voltage regulators.)

    Thanks again for your insight!
    Jay
     
    Last edited: Aug 21, 2013
  12. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    648
    May 8, 2012
    Can't answer any of these questions right now. It's Wednesday! If Harald hasn't replied by the time I get back I will.

    Chris
     
  13. jayusa123

    jayusa123

    9
    0
    Aug 20, 2013
    Good man. Thanks.

    I've taken a look and found a voltage monitor at Mouser. S-80850CNY-B2-U

    Is this what I'd be looking for?
     
  14. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,268
    Nov 28, 2011
    Yes that's what he meant.
    Harald's circuit uses the series transistor as an emitter follower. To properly saturate the transistor, the base needs to be brought around 0.7V more positive than the emitter; since the collector-emitter voltage will be around 0.2V in saturation, this means that the base must be brought at least ~0.5V above the collector, which means ~0.5V above the positive supply of the reset generator. This is the reason why R1 is connected to the INPUT of the 7805 (which will be a higher DC voltage).
    If the reset generator has a push-pull output, it will not allow its output to be pulled up above its VCC supply; this is the reason for R2. R2 should be higher than 47 ohms, but I would use a diode instead of a resistor, with its cathode connected to the reset generator. This will allow R1 to pull the base up to ~0.7V higher than the collector. When the reset generator's output is low, the base will not be pulled all the way down to 0V, but with the base-emitter voltage drop in the transistor, the emitter will still be a pretty low voltage (around 0.2V maximum).
    If the reset generator has an open collector output, it won't mind its output being pulled to ~0.5V above VCC, so no R2 is needed. This is a slightly better option, because when the output is low, the transistor's base will be at ~0V and there will not be ANY voltage on the emitter.
    They're just for stability.
    Yes there's a huge range available.
    The Seiko S808xxCN devices (N for open-drain output) that you linked to in post #13 look good for this application. The threshold voltage should be lower than 5V though; I'd suggest 4.5V or so.

    I don't necessarily recommend this circuit; I'm just answering those questions. The idea of using a reset generator is a good one, since it combines several needed features - accurate voltage detection, hysteresis, and a short delay (to allow voltages to stabilise before the transistor is turned on).
     
  15. Harald Kapp

    Harald Kapp Moderator Moderator

    10,025
    2,138
    Nov 17, 2011
    Kris, as usual a crsip explanation, thanks.
     
  16. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    648
    May 8, 2012
    Yeah, a short bike ride and two measly beers. I come back and it's all sorted out by our resident tech writer!

    Stellar job Kris.

    Chris
     
  17. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,268
    Nov 28, 2011
    Thanks guys :)
     
  18. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,268
    Nov 28, 2011
    Oh dear. I was too hasty in recommending that Seiko part. It DOESN'T have any time delay!

    (I didn't consider that would be possible, because every voltage monitor I've seen for microcontroller resetting DOES include a delay. But I was re-reading the data sheet and there is NO mention of any time delay.)

    I think it would be better to use a device with a time delay. This will ensure that the 7805 output has reached 5.0V and stabilised, before the transistor is turned on. The Diodes Inc APX803-46, which is available from Digikey for about USD 0.50, looks suitable. It's in a SOT-23 package (surface-mount). If you want something in through-hole, the Microchip MCP121-75 is ideal - the open-drain output is designed to withstand up to 13.5V (because Microchip's PIC microcontrollers use a high voltage on the -RESET signal for high-voltage device programming).

    For that reason, the SMT version of the MCP121-75 might be better than the APX803 in any case.

    If you want to search for other options, Digikey have devices from many manufacturers: Analog Devices, Diodes Inc, Intersil, Linear Technology, Maxim, Micrel, Microchip, ON Semi, Rohm, STMicroelectronics and Texas Instruments. You can either look on the Digikey site or go to the sites of those companies.
     
  19. jayusa123

    jayusa123

    9
    0
    Aug 20, 2013
    Thanks so much for all the help, guys.

    Sorry I've taken a bit to reply, been caught up in other things for a few days. This was tremendously helpful. I'm going to work on picking out the right transistor and resistor value for the switch and will probably try one of the voltage monitors you recommended, Kris.

    I'll update this thread when I have some progress.

    Thanks again!
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-