Connect with us

Short burst voltage problem

Discussion in 'General Electronics Discussion' started by Lord_grezington, Aug 2, 2013.

Scroll to continue with content
  1. Lord_grezington

    Lord_grezington

    117
    2
    May 3, 2013
    Evening all

    I have a problem where I need to get a potential difference between 0-1.6Vdc to source an input to a drive chip.
    My source voltage is 3.3V.
    I want to use a trim pot as a potential divider to produce the 0-1.6V. (a user interface)
    I want to, for a short period of time use a digital signal (3.3V) to change the voltage from whatever the existing voltage is from the trim pot to jump to 1.6V. (the trip pot can be anywhere between 0 and 1.6V)

    Is there a simple low cost way to do this?

    The only way I have thought of doing it is by using a input on a PIC for the trim pot, then outputting a pwm via a buck converter to create the analogue voltage. This way I can change the voltage through the PIC for the increase that I need for when I need it. This though needs an analogue input to the PIC, and a PWM output.

    I would like to (if its possible) use the Pot as a direct 0.1-6V directly to the chip, and only use a digital IO to increase the voltage to 1.6V.

    I have looked at using a Zener Diode, but these are only limited to 1.7V which is too high and can destroy the drive chip.

    Is this possible to do without using 2 IO from the chip?
     
  2. dh390

    dh390

    33
    0
    Jul 30, 2013
    Have you thought of using a voltage regulator either fixed or adjustable & the pot for your 0.1 to 1.6V source?

    And/or you can use the pot with some resistors in a simple voltage divider circuit so you don't exceed your 1.6V max limit. So when the pot is full you have 1.6V & you can turn it down to 0 or 0.1V depending on how you set up the divider circuit.
     
    Last edited: Aug 3, 2013
  3. Lord_grezington

    Lord_grezington

    117
    2
    May 3, 2013
    I have not thought about using a LDO regulator, It will add cost where in this project every penny counts. I cant yet see how it will work, but I will look into it in a bit more detail.
     
  4. BobK

    BobK

    7,680
    1,685
    Jan 5, 2010
    If I understand what you want correctly, this should work.

    Use a MOSFET between the center of the trim pot and the 1.6V side. When the MOSFET is on, the output becomes 1.6V, when off it is whatever the trim pot is set to.

    Bob
     
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,267
    Nov 28, 2011
    Let's see if I can make sense out of your original post.

    You have a circuit that runs at 3.3V and includes a PIC. You need to generate a voltage signal with a range of 0~1.6V, to be used by a "drive chip".

    You have a trimpot and you want to set the normal (default) level of the 0~1.6V output according to the position of the trimpot.

    You also have a need to briefly force the "drive chip" control voltage to 1.6V (maximum), regardless of the voltage coming from the trimpot.

    That's a lovely abstract description of the way you want to address your requirements, but there may be a better way; there may also be several alternatives, and the more we know about your project, the better we can advise you.

    To start with, you could answer a few questions.

    Would I be right to assume that the value you are controlling is brightness?

    Can you describe the characteristics of the "drive chip"'s control input? We know the voltage range is 0~1.6V but that's not enough detail, really. Can you provide a part number for this chip or a link to its data sheet?

    What kind of timing requirements apply? When generating voltages using PWM and an R-C filter arrangement, there is always a lag before the smoothed output has settled to the correct value. What resolution would be sufficient? Would eight bits (0.4% resolution) be precise enough?

    What is the current function of the PIC MCU? Can you add functionality? What is the part number of the PIC device?

    If I understand your post properly, I think BobK has the right idea. There are several reasonably simple ways to do what you want. If the device you're using has an ADC and a DAC, or a PWM output, with sufficient resolution, your problem is solved. But your requirements can also be met without passing the analogue signal through the PIC at all.

    Please make a GOOD attempt at answering ALL of the questions I've raised in this post, if you want the best quality answers from us.
     
  6. Lord_grezington

    Lord_grezington

    117
    2
    May 3, 2013
    Hi Kris

    Yes, I am looking to produce 0-1.6V via a pot for an input into a drive chip (A4988 Vref input with 100mOhm sense resistors).

    No, it sets phase current limits for a stepper driver
    I can use an acceleration to rise up to the required value, I can also not enable the chip until I know the voltage has settled.
    PIC Part number is pic16f1828, my original idea was to use ADC and DAC, but I am trying to save a pin if possible.

    I have looked at Bobs Idea, not tested though. I was looking at using a very cheap and small NPN.
     
  7. BobK

    BobK

    7,680
    1,685
    Jan 5, 2010
    A bipolar transistor would not work well, since it will always have a voltage drop from C to E of a couple of hundred millivolts. A MOSFET would not have this problem.

    Bob
     
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,267
    Nov 28, 2011
    You haven't answered very many of my questions.

    Can you also post a schematic of what you have so far? I would like to see the voltage reference, the trimpot, and the driver IC.

    It would also be useful to us (as well as common courtesy) to tell us more about the overall project you are working on.
     
  9. Lord_grezington

    Lord_grezington

    117
    2
    May 3, 2013
    Bob - ok, I will look at the Mosfets.

    Kris - Sorry, I thought I got all your questions.

    I have posted the schematic, as you can see I am not at the point yet where I have used this "over drive". Please note that the pot looks to be upside down.

    I am not too sure on the trimming requirements, I cant see it being massively important so a resolution of 0.4% will be more than good enough.

    control features of the drive chip. As you can see on the schematic I have allocated a PWM output pin (incase it was needed as PWM) in anticipation for the "Over Drive".

    Here is the link to the Drive Chip Data sheet http://www.farnell.com/datasheets/1481258.pdf

    Has this answered all your questions?
     

    Attached Files:

  10. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,267
    Nov 28, 2011
    OK thanks. That's good information.

    Are there any possible current or future advantages to controlling the voltage on this pin from firmware? Would you want to override the trimpot value to any value other than 100%? Because if you don't, then Bob's suggestion is much simpler than digitising the signal, applying the override, and regenerating it using PWM. There are several ways to do what you want; it might be as simple as a resistor and a diode.

    I assume you are using a resistor in series with the trimpot to drop the 3.3V rail down to 1.6V so that the trimpot's wiper voltage range will be 0~1.6V. Is that right? The end-to-end resistance of trimpots and potentiometers is not well controlled; using it in a voltage divider with a fixed resistor will not be very accurate. Also, how accurate is the 3.3V rail? You might be better to use a shunt regulator such as a TLV431 (http://www.digikey.com/product-detail/en/TLV431BSN1T1G/TLV431BSN1T1GOSCT-ND/2122150).

    If you use a voltage reference, you could add a tiny MOSFET in series with the ground return of the trimpot. If you turn that MOSFET OFF, the bottom end of the trimpot will jump up to 1.6V and the motor control chip will see 1.6V on VREF regardless of the position of the trimpot. A suitable MOSFET for this would be a BSS138 (http://www.digikey.com/product-detail/en/BSS138LT1G/BSS138LT1GOSCT-ND/917858).

    Edit: That link is actually to the BSS138L which has a low gate voltage (also called "logic level gate") which is designed to work with gate voltages as low as 2.75V. The standard BSS138 (no L) does not have this feature and is not suitable. At 3.3V the BSS138L will be conducting pretty hard and will have an ON-resistance of about three ohms. With a 10k trimpot, the presence of the MOSFET will increase the fully anticlockwise trimpot voltage from 0.00% of scale to 0.03% of scale, which is negligible.

    That motor controller seems pretty complicated. The "translator" affects the threshold current, in combination with the VREF voltage. Perhaps there is a way to get the behaviour you want through the translators, instead of changing the VREF voltage?
     
    Last edited: Aug 6, 2013
  11. Lord_grezington

    Lord_grezington

    117
    2
    May 3, 2013
    Hi Kris

    There may be some advantages of controlling through the firmware to improve the flexibility and add features, however it has been decided that this in not the way we want to go. I have tried looking at a resistor and diode, but it wont work at it will only increase a voltage to the trim pot.

    Yes, there is a resistor in series with the trim pot to drop the 3.3 down to 1.6. the 3.3 does not need to be too accurate as the Vref is stated with respect to the 3.3v (I think). But if it overdrives 1.61V for a very short overdrive period its very unlikely any damage will occur

    Thanks for your help Kris, I am going to run some tests on the Mosfets .in the next day or 2.
     
    Last edited: Aug 6, 2013
  12. Lord_grezington

    Lord_grezington

    117
    2
    May 3, 2013
    Hi Kris/Bob

    I have just simulated the Mosfet Idea, Its working very well. Thanks for your help
     
  13. BobK

    BobK

    7,680
    1,685
    Jan 5, 2010
    Great, always good to hear about a success.

    Bob
     
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

-