Dallas 1 wire isolation

Hi All,

I am considering using a Dallas DS18S20 temp sensor for a remote ( outside air temp ) sensor. Sensor will be at most a few feet from the processor doing the measurement but on a 2 wire ( ground and signal ) cable plugged into the instrument. I would like some way of protecting the processor in case someone connects the 1-wire line either to ground, or worse, +12 volt main supply. I cant see any nice easy way of doing this given the drive constraints of the 1-wire system ( 1K5 pullup to +5 volt supply, processor pin pulls low to communicate on bus ).

Any ideas welcome.

--
Regards, 

Adrian Jansen           adrianjansen at internode dot on dot net 
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Reply to
Adrian Jansen
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Use some zener and normal diodes to protect.

Reply to
pbdelete

See Figure 4 on the datasheet? Right where it says "1-Wire Bus" insert a 120 ohm, 0.5 W resistor. Then, add a 5.1 V, 0.5 W zener with anode to ground and cathode to the left side of the resistor you just inserted (so, drain of the PMOS). The connector (processor-cable) is to the right of your 120 ohm resistor. That will allow you to connect the 1-wire line to 12 V, without blowing anything (not even the zener). To tell you whether your PMOS will die when you connect the

1-wire line to ground, you need to tell us which PMOS you have. It needs to stand Ids=-42 mA and P=0.21 W (in the worst case).

Best,

Reply to
Mochuelo

Or better yet, a transzorb -- which is electrically roughly the same as a Zener diode, but specifically designed to generally fail short-circuit, whereas Zeners typically fail open-circuit.

Reply to
Joel Kolstad

Wonder if there's any list with available semiconductor types somewhere.. (makes me wonder what more types I might have missed :)

Reply to
pbdelete

That does not protect against connecting the 1-wire line to ground. You would blow up the PMOS. Same for a transzorb.

Reply to
Mochuelo

Hmm,

The short to ground is ok, the processor pin is internally current limited to about 20 mA when driven into a short. It will withstand that without any further protection.

The short to 12 volt is much harder.

5 volt zeners are a bit 'soft' for this. Typical for a 500mW zener would be about 0.5 volt rise above the nominal zener voltage at 50 mA load ( assuming 7 volts drop across the 120 ohm resistor ). Given the zener tolerance of 4.8 - 5.4 volts, that will pull the processor pin up to around 5.6 - 5.9 volts worst case, well beyond its Vcc + 0.3 volts max rating. Tranzorbs are good for fast spikes, but even 'softer'.

Putting a schottky diode cathode to power is probably better in principle, except that it feeds the possible 12 volt fault input onto the 5 volt rail. I doubt the voltage regulators will like this much.

Its not as simple a problem as it seems. But there must be a reasonable solution, things like Dallas I-buttons have to work in all sorts of hostile places with direct connections to processor pins.

--
Regards,

Adrian Jansen           adrianjansen at internode dot on dot net
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Reply to
Adrian Jansen

You can use a programmable zener (such as a TL431). It will give you a much more abrupt transition (it includes an opamp) and also the chance to fine-tune the trip-point.

Regarding short to ground, I thought you were using parasitic supply (you mentioned a 2-wire cable), in which case it would mainly be the PMOS who would be at danger.

Reply to
Mochuelo

Thanks, I hadnt thought of an active zener.

Yes I will be using parasitic supply, but from what I see, a 1K passive pullup will work, and the processor pin has enough pull-down to override that. So I dont need an active pullup. Correct me if I am wrong.

--
Regards,

Adrian Jansen           adrianjansen at internode dot on dot net
 Click to see the full signature
Reply to
Adrian Jansen

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