Texecom Digi Ouput with XBee in Voltage Divider Circuit

[ilnli]

Hi,

I have a XBee which I want to use to read the value of Digi output from a Texecom Premiere 24 Alarm Panel, the XBee has:

VIH : 0.8 * VCC
VIL : 0.2 * VCC

(Where VCC is be 3.2 V)

The XBee IO Input has an impedance of 10k, so since output voltage from Texecom output pin will be 12V, so I want to use a Voltage Divider to convert that voltage to 0.8 * VCC so that XBee can then read that, my question is what correct values of R1 and R2 should I use so that the power dissipation from the resister is within a reasonable range when they convert 12v to 3.2v? I know there could be many combination of resisters in Voltage Divider circuit which could bring the voltage down to 3.2v but I need the best possible values of R1 and R2, could you please tell me what resistors values should I use while considering the power dissipation and XBee's pin impedance?

Here are the HW characteristics of XBee:

XBeeHW

• ilnli
• 18 Nov 2014
• 1

Thanks in advance

 

[Deryck Tintagel]

Assuming 12V, then 1K and 2k7 will be OK and will give 3.2V at the input. However, you will need to measure the voltage at the panel to confirm the actual voltage and you may need to change one or both values.

You could use a higher value for the upper resistor and add a resistor in parallel with it to trim the voltage (Select On Test) to the Xbee input. You could add an inverse diode between the input and VCC to protect the input

 

[ilnli]

Hi

I am fairly new to electronics, could you please explain a bit more about inverse diode to protect the input? At which end would I put it, do you know where can I learn more about that?

 

[secureiam]

thought bernardgreen

and already gone through this in another post?

 

[ilnli]

Hi,

I'm looking for the exact values that I should use, I know how to find them but still I can't figure out which values will be the best which is what I want to get here.

Thanks

 

[Deryck Tintagel]

Have a look here

http://www.digikey.com/en/articles/techzone/2012/apr/protecting-inputs-in-digital-electronics

or here

http://www.kevinmfodor.com/home/My-Blog/microcontrollerinputprotectiontechniques

for ways to protect the input.

The potential divider is connected to the 12V input from the panel and the mid-point is connected to the Xbee input. The protection diode(s) are then connected to at the mid-point connecting the the Xbee - you may need a series resistor and pull-up as shown in the links.

Looking again at the potential divider resistors I suggest a 3k3 and 1k, as using 2k7 will give a voltage that is slightly too high. will give the required division based on exactly 12V. However, is the voltage from the panel exactly 12V, or could it be more at, say, 13.8V? If it is the latter then the values will need to be something like 1k with 3k6 or 3k9 to give at least 2.6V required at the input.

600mW metal film resistors from Maplin would be fine.

 

[ilnli]

Hi Deryck Tintagel

Thanks for referring to the resistors from Maplin, that is something I was worried about as I wanted put the resister in the enclosure of the Texecom, so power dissipation is very important to make sure that resistor don't burn out in the divider circuit.

Thanks

 

[ilnli]

Hi Deryck Tintagel

I would really appreciate if you could explain the formula based on which you suggested 600mW resistors to used, and what considerations are normally to be taken when coming up with the power dissipation? This is so that I can experiment with different resistors values for some of my other projects as well.

Thanks.

 

[bernardgreen]

The formulas are

V = R * I Volts = Resistance times Current { Volts Ohms Amps }

re-arranging

I = V / R Current = Volts divided by Resistance


W = V * I Watts = Voltage times Current { Watts Volts Amps }

substituting for V

W = ( R * I ) * I

therefore W = R * I²

substituting for I

W =V * ( V / R )

therefore W = V² / R