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Gerrit
Joined: 15 Sep 2003
Posts: 58
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| Off topic : 24 volt level shifter |
 Posted: Sun Oct 12, 2003 11:46 am |
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What is the simpelest and cheapest way of getting 32 digital channels of (8-32)volt input circuit to 5 volt digital output.
gerrit |
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Steve H.
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ajt
Joined: 07 Sep 2003
Posts: 110
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| Posted: Sun Oct 12, 2003 9:15 pm |
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| Steve H. wrote: | | I like the Maxim MAX6818 Octal debouncers. OVP/ESD protected and easy to make into an 8 bit bus arrangement |
These are good parts but in the application requested there would need to be a level shifter on each input since the input is 8-32 volts and the threshold of the Max6818 is 2.4V (5V Vcc) and the absolute max input range is +/-30V. Also there is a long delay for the debounce function, typically 40ms, that needs to be accounted for.
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Al Testani |
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John P
Joined: 17 Sep 2003
Posts: 331
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| Posted: Sun Oct 12, 2003 11:58 pm |
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| Quick and dirty method would be to set up a 10:1 voltage divider with resistors, then straight to a CMOS logic input. You'd get 2.9v as a high, and .73v as a low, which should work. It would be easiest with resistor packs. |
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Haplo
Joined: 06 Sep 2003
Posts: 659
Location: Sydney, Australia
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| Posted: Mon Oct 13, 2003 12:47 am |
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| Or a 6:1 voltage divider and a 5.1v zener. |
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Pete Smith
Joined: 17 Sep 2003
Posts: 55
Location: Chester, UK
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| Re: Off topic : 24 volt level shifter |
| Posted: Mon Oct 13, 2003 12:54 am |
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| Gerrit wrote: | What is the simpelest and cheapest way of getting 32 digital channels of (8-32)volt input circuit to 5 volt digital output.
gerrit |
I'd use transistors myself. Certainly cheap, very simple (even if you don't want the level to be inverted), but could get a bit unwieldy with 32 channels 
You could even use "digital transistors". which don't need input resistors. That would cut the components you need down a little.
Pete. |
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Kenny
Joined: 07 Sep 2003
Posts: 173
Location: Australia
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| Posted: Mon Oct 13, 2003 1:55 am |
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Another possibility is a single clamp circuit for all channels using a shunt regulator, the TL431, and a resistor and diode for each channel.
http://www.onsemi.com/pub/Collateral/TL431-D.PDF
Fig. 19 shows the configuration for higher current. A TIP32A is a suitable PNP transistor. The input resistor is connected to a higher voltage to keep some current going through the clamp if all the inputs are low.
Each channel has a resistor and diode in series to the clamp circuit (set to about 4.4V). The inputs to the PIC come from between the resistors and diodes.
The circuit is also suitable for a/d input limiting. This is the application that I saw on a Perkin-Elmer minicomputer years ago.
HTH |
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Gerrit
Joined: 15 Sep 2003
Posts: 58
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| Posted: Tue Oct 14, 2003 4:10 am |
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Thank you all for sharing your idea.
I will go for a solution where the input clamp diode of an HC
is used with a high input resistor.
Kind regard
Gerrit |
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SherpaDoug
Joined: 07 Sep 2003
Posts: 1640
Location: Cape Cod Mass USA
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| Posted: Tue Oct 14, 2003 6:47 am |
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Look at Microchip's app note AN521. It describes just what you can reliably get away with in using input clamp diodes. Interesting reading!
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The search for better is endless. Instead simply find very good and get the job done. |
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