DS18B20 – on to external power December 7, 2011

When you want to learn something new in electronics, you have to start at the bottom.  In my mind a circuit layout like this is the bottom for me and it is hard for me to follow.  I will be the first to admit that I am not an electrical engineer and don’t want to be one either.  I just want a simple way to hook things up and have them work when I need them too.  So this layout just gives me a general idea of where to start on hooking up a external power source for my probes.  I was at this same point when I started to learn about DS18B20’s and use with an arduino.   The good news is that I have a more clear understanding how the DS18B20 functions and how it operates than when I first started on this project.

WHY DO I NEED EXTERNAL POWER FOR MY DS18B20’S  ——-

Well that is a very good questions.  In most applications and small projects you do not need to know anything about this aspect of the DS18B20’s.  In my case as I have been working on this blog several questions have been posed as to power drains on arduino’s if you request to many sensors being driven by the unit and you have long runs to your sensors.  So I need to do some research as to how to use a external power source.  A lot of places on the internet talk about it, but I have yet to find any source that shows a person how to actually make this work.  As for the research here is the best I could find.  If you know of others please let me know and I will add it to my library and give you credit for it here.

HERE IS THE BIBLE ON THIS SUBJECT  —

http://www.arduino.cc/playground/Learning/OneWire

Normal (external supply) mode
With an external supply, three wires are required: the bus wire, ground, and power. The 4.7k pull-up resistor is still required on the bus wire. As the bus is free for data transfer, the microcontroller can continually poll the state of a device doing a conversion. This way, a conversion request can finish as soon as the device reports being done, as opposed to having to wait 750ms in “parasite” power mode.

Note on resistors: For larger networks, you can try something smaller. The ATmega328/168 datasheet indicates starting at 1k6 and a number of users have found smaller to work better on larger networks.

THAT IS ALL THERE IS — in this bible.  Not much of a help in my mind.

HERE IS THE OTHER THING I FOUND:

http://sheepdogguides.com/dst9parasitic.htm

Choosing your 1-Wire chip power mode

Many of the chips in the 1-Wire family from Dallas Semiconductor can be powered “parasitically”… that’s Dallas’s (apt) name for the system used.

The advantage of parasitic powering is that it means you can have a two wire MicroLan. (“MicroLan” is just Dallas’s trademarked name for a bunch of 1-Wire chips “playing nicely” with each other and a “master” device, e.g. a PC or simple microcontroller, e.g. Arduino, in charge of the network of 1-Wire sensors, actuators, storage units, etc.) If you don’t use parasitic powering, then you need to run three wires from chip to chip to chip. (An aside: You should be aware that MicroLans don’t like star topographies. If you need a star, you need a hub to split it into separate, non- star, legs.)

I’ve often used parasitic powering entirely successfully. However, if you have a lot of chips on the MicroLan, or high poll rates, the time may come when supplying the chips which can be powered parasitically makes sense. It isn’t difficult… especially if you provided for the possibility that you’d like to make the change when you first set up your MicroLan.

For example…

Just as an example, let’s consider a MicroLan that is reading temperatures in various parts of a large greenhouse.

We’ll assume you’re using chips from the DS182x family, in a TO92 package, the little blob of plastic with three “legs”.

To use those chips parasitically, you just connect two of the legs to one another, and that leaves you with two connections which go to the two wires of the basic MicroLan.

Now… and this is what I meant about giving yourself options for the future… rather than doing that the “obvious” way, using either a small PCB, or simple discrete wires, connect each of the three pins to a separate track or wire, and THEN make your connections to the MicroLan. If later, you want to use non parasitic power, then all you have to do is change a link, or disconnect one of the wires, and you can then supply the chip with its independent V,_cc easily, without disturbing what would be fiddly connections to the package’s pins.

Here are the details of which pin is which on the DS1820…

The DS1820’s middle leg goes to the 1-Wire data line.

The DS1820’s leg marked “See text” should be attached to the 5v, or, for parasitic power, connect this pin to ground (zero volts))

And the DS1820’s “1-Wire 0v” should be attached ground,i.e. to zero volts.

(If the above seems over-explained, please remember that this page may be accessed by people who want more than just an idea of what this “parasitic power” thing is all about.)

AND THE LAST ONE I FOUND:

http://forums.parallax.com/showthread.php?135523-ds18b20-sensor-fails-at-high-temperature