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Thread: Divide voltage

  1. Divide voltage

    I'd like to cut the 0-12 volt output from the .NET Serializer motor drivers in half to signal my Sabertooth motor controller which requires a 0-5 (or, 0-6) volt signal. Any ideas on how to divide the voltage output?Thanks, in advance!

  2. #2
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    Re: Divide voltage

    With a voltage divider...? Excuse the ASCII art.
    Code:
    +12V -----/\/\/\-----V_div-----/\/\/\-----GND
                    R1                    R2
    V_div = (+12V)(R2/(R1+R2))

    Choose R2 and R1 such that you achieve the desired V_div and minimize the wasted current. Choosing from among the commonly available values R2=6.8kOhm and R1=10.0kOhm, would give a V_div of approximately +4.86V which should be well above the minimum voltage for the Sabertooth to read the input as HIGH. At +12V and 16.8kOhm total resistance, the wasted power would be ~8.5 mW.

    Although I think an important question to ask is: "should you be using the output of the actual motor drivers on the Serializer to control the Sabertooth, or should you be using the pins that control the drivers on the Serializer?" I seem to remember there being a way to directly connect the control pins to an external driver via a pin header. It should be explained in some detail in the Serializer manual.
    Last edited by tician; 10-18-2011 at 08:09 PM. Reason: grr. annoying white space remover borked my ASCII drawing
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
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    [git][mech][hack]
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  3. Re: Divide voltage

    Thanks for the advice -

    I liked your second suggestion of reviewing the pins on the Serializer. I think you were referring to the external h-drive pins. I looked in the documentation but couldn't find what the output voltage range on those pins are. Would you happen to know what the output voltage range is? To use those external pins, the documentation recommends cutting the wire trace under the card, but I don't want to permanently disable the internal pins....do you think it would be okay to simply jumper the internals to the externals, and then use the external?

    From your first suggestion, I tried the circuit you recommended (this circuit made sense and my searching on the internet seemed to agree with it), but the resistors immediately started smoking and it didn't look so good after all. I did: +12V -----/\/\/\-----/\/\/\-----GND . Any ideas?

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    Re: Divide voltage

    If you can post with pictures of what you were hooking up to what, that'll help us tell you went wrong.

    We really need to know what (1) Sabertooth Motor controller you're using, and (2) what resistors you used (how many ohms)

    Also doing a voltage divider with regular resistors is usually a bad idea for high current stuff... Basically it sounds like you sent a lot of current through the resistors, and into the Sabertooth. I've not used a sabertooth, so I won't say more for now.

  5. #5
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    Re: Divide voltage

    Page 11 of the "Serializer WL User Guide v2.1" explains it all quite well with a labeled photo of the board. Basically the older version of the board had no traces to cut (only jumpers on the top connecting the main controller to the H-bridges), while the newer version has traces on the bottom of the board with the option of using jumpers on the top of the board to reconnect the control pins to the onboard drivers.
    If you cut the traces and then decide to go back to using the onboard H-bridges, simply make sure all jumpers are installed across the five H-Bridge pin (as shown above).

    Definitely need to know what size resistors you were using. Most of the what you can find in radio shack or similar will be 1/4 watt, and should not smoke at 12V with a 10,000 Ohm and 6800 Ohm resistor in series.

    Regarding the usage of voltage dividers for controlling high current devices, most drivers I know of use high impedance inputs (MOSFETs are Voltage-Controlled-Current-Sources where their input (the Gate of the MOSFET) resistance is so large as to allow only a negligible amount of current into/out of that pin - usually on the scale of nanoamps). In this case, there would be no problem using a voltage divider to bring the 12V output down to the 5V maximum of the input. If the driver is current controlled or BJT based (not common for most recent low/medium power devices), then using the voltage divider would be a bit more complicated. After a quick scanning of the manual for the Sabertooth2x25, I assume it uses a microcontroller (the vast majority of modern designs being based on some type of MOSFET) to control the MOSFET H-bridges. So, I don't really see a problem using a properly sized voltage divider to drop the 12V output down to ~5V for the driver input. Then again, it is not my $125 motor driver board at risk.

    The high logic level is 4.5 to 7 Volts (with 5V typical) for the L298P H-bridge ICs used on the Serializer, meaning that one way or another the Serializer board produces a ~5V logic output for it. I do not have a board to examine and the manual does not include any schematics, so I cannot confirm that the pin headers connect the output pins of the Serializer's controller directly to the input pins of the L298P ICs.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    [git][mech][hack]
    gives free advice only on public threads

  6. Re: Divide voltage

    Good news on two fronts -

    (1) The 10k/6.8k resistor circuit works. I had two problems initially: a) was using the wrong multiplier band (third band) on the resistors, and b) was using a newly purchased multimeter that was defective. After getting my old, trustworthy multimeter and the correct resistors, it worked just as you said it should.

    (2) Armed with a correctly-functioning multimeter...the h-bridge pins put out 0-5 volts, which is exactly what I was shooting for with the resistor circuit, too, in the first place.

    So, thanks to your suggestions, I now have two solid options for signaling the Sabertooth motor controller (I'll probably use #2). I really appreciate it.

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