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24 Volt to 5 Volt converters — Parallax Forums

24 Volt to 5 Volt converters

Alan BradfordAlan Bradford Posts: 172
edited 2010-02-05 08:27 in General Discussion
Hi,
Does anyone know of a 24 (or 12) volt dc to 5 volt dc converter.
Kind of like a reverse ULN2803 chip.
I use an opto isolator on each input but the parts count gets high when you have 32-48 inputs.
I have searched Maxim, AD, Microchip, and Phillips to no avail.

Thanks in advance if such a beastie exists,


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Alan Bradford ·N1YMQ

Plasma Technologies
Canaan NH 03741
www.plasmatechnologies.com

Comments

  • Alan BradfordAlan Bradford Posts: 172
    edited 2010-02-05 02:31
    PS....
    I do not need opto isolated inputs, and I have used transistors but that is still 3 components.
    A nice IC chip to plop into the Pc board would make life Oh So Grand.

    Thanks,

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    Alan Bradford ·N1YMQ

    Plasma Technologies
    Canaan NH 03741
    www.plasmatechnologies.com
  • Roy ElthamRoy Eltham Posts: 3,000
    edited 2010-02-05 02:52
    A 7805 voltage regulator with take in up to 12v at least (the ones I have can take up to 18) and output 5v.

    Maybe some other 5v regulators can take in 24v?

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    Check out the Propeller Wiki·and contribute if you can.
  • Mike GreenMike Green Posts: 23,101
    edited 2010-02-05 03:03
    How about a voltage divider? If the 24V source is clean, no significant spikes, no significant noise, just 24V or 0V with maybe 10% variation, a simple voltage divider will work fine. Use a 20K resistor in series with the 24V source and a 4.7K resistor in parallel with the 5V input (and ground). Those values will give you roughly 4.5V at the logic input and a current of roughly 1mA. If something happens to the 4.7K resistor, the 20K resistor will limit the current to a little over 1mA which should be ok for most IC protective diodes. You can get multiple resistor packs like 8 in a SIP package, so two SIP packages would handle 8 signals.

    If you might have spikes or significant noise, I'd use the optoisolators.

    Post Edited (Mike Green) : 2/5/2010 3:08:51 AM GMT
  • Alan BradfordAlan Bradford Posts: 172
    edited 2010-02-05 03:11
    Hi,
    Thanks for the ideas.
    What I was actually looking for was if anyone makes an IC chip like the 2803 but works backwards.
    I currently use an optoisolator (4 per pkg) and it takes 2 resistors per Opto.
    Some of my Plasma Machines have 30-40 plus 24 VDC inputs and that equals 90 to 120 parts to insert and solder.
    A nice IC chip with 8 or more (I can hope) would make life simpler.
    Maybe a 3082NLU would shift 24 Volt signals to 5 volts to feed my Stamp Inputs!!!

    Thanks Guys,

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    Alan Bradford ·N1YMQ

    Plasma Technologies
    Canaan NH 03741
    www.plasmatechnologies.com
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2010-02-05 08:27
    Alan, you can use a 2803 back to front if you like but maybe with an extra input resistor. But what I do is simply have a 100K series input resistor straight to the I/O pin, nothing else (maybe). You didn't say what it's connected to but I'm guessing a normal I/O pin. The input current is limited to around 200ua at 24V which is easily handled by CMOS built-in "clamp" diodes. Alternatively you could use the voltage divider as mentioned but just use much higher values as the I/O pin for all practical purposes doesn't require any current to switch, just voltage.

    In some old PLC designs I have done I have had a 20-pin socket for the 2803 so that you can reverse the chip to make it an input buffer instead.

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    *Peter*
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