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Voltage Drop question

assafelbaz

Nov 1, 2011
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Nov 1, 2011
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Hi.

I have the MRF24J40MB transceiver. I want to solder it on a PCB and attach a data terminal to connect a data wire (such as Cat5). The PCB will be placed in a waterproof box and i will place it outside.

Now, the wire will be 50-100 ft. long. I assume that i will have problem with the data in/out and also the power supply to the box. The wires will carry the 5V from inside the house together with the IN/OUT data.

How can i fix this, or how can i make it stable?

Thank you.
 

davenn

Moderator
Sep 5, 2009
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why do you want to put the transceiver module so far from the data source instead of using its radio abilities to cover the distance ?
seems to be a bit self defeating ;)

Dave
 

ohfour

May 27, 2012
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May 27, 2012
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Voltage drop is dependent on the current flowing in the conductor, and secondarily to the conductors size and length.

For a low-current applications (such as data transmission) you will have practically no current flow, and so practically no voltage drop. You will however have a significant capacitance between the different conductors, and that will interfere with the data signal. As far as I know, this parasitic capacitance is the primary limiting factor in the length of cat5 cabling.

For the transceiver power supply, the current draw could be quite high (how powerful is the transmitter?) and so you'll need relatively large cables to minimize voltage drop. I would recommend that you supply 24V or 12V out to the radio box, and use a 5V regulator to step it down. This reduces current and therefore volt drop in the supply cable and gives you better voltage regulation at the radio

If you know how much current the radio will draw, and the resistivity of the cable you intend to use, you can calculate voltage drop: http://en.wikipedia.org/wiki/Voltage_drop#How_to_calculate_voltage_drop
 

ohfour

May 27, 2012
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May 27, 2012
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Based on the datasheet it appears to draw <50mA, which means volt drop won't be a major issue for you. I would still strongly recommend that you run 12V out to the box and regulate it to 5V close to the radio, though
 

assafelbaz

Nov 1, 2011
5
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Nov 1, 2011
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Thank you all for the reply.

This transceiver used as a receiver, and i have transmitters placed outside.
I want to have it far from the main box because if not i am losing reception.
The main box located at a basement... It is a strong transceiver, but in the conditions i have here i better place it outside.

If you are saying that Cat5 has an issues with capacitance, what type of cable will be the best to use? (can i try this Wire?)

Thanks again.
 

ohfour

May 27, 2012
12
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May 27, 2012
Messages
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Cat5 will probably be fine for your data. All multicore cables have issues with capacitance between conductors, but for a 30m cable run I don't think you need to worry about it.

Cat5 can't carry much current but if the receiver is drawing <50mA @ 5V then you should have no trouble powering it off a 5V reg powered off a 12V supply through the cat5
 
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