Control Engineering

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How do I transmit voltage signals?
March 10, 2008

The full text of this question is: I need to transmit voltage from fusebox to a receiver 5 m away. What kind of transmitter and receiver do I need?

Good news is that 5 m is actually not a very long way to carry a voltage signal, so you may not need any transmitter at all, and you probably already have the receiver. 

Bad news is that, depending on the signal you’re sending and the use you plan to put it to, you may have a difficult measurement problem, anyway. Let’s start with the best of all possible worlds, then see what we have to modify when things start to go bad.

The fact that you want to pull your voltage off a fuse box indicates that you may be living in an “order 100” alternating current (ac) world. Order 100 voltages (those between 30 and 300 V) are very easy to send 5 m (about 15 feet). To get the signal from A to B, all you need is a pair of wires bridging the gap C between them.

At the receiver end, you are probably making your measurements with a high-impedance measurement front end (oscilloscope, digital voltmeter, or data acquisition system), in which case your receiver is already there. That is, assuming your front end has a dynamic range greater than your peak voltage. If not, you will need a simple 10X voltage divider consisting of a 100 kilohm resistor across the measurement inputs and a 900 kilohm resistor in series with the “hot” input wire. The “hot” wire is the one connected to the fuse or breaker. 

A simple voltage divider can bring high voltages into the measuring instrument’s dynamic range. The values here provide 10X division.

These resistor values assume that the measuring system’s input impedance is >10 megohm, which is typical of most measurement front ends. If it is lower, reduce the resistor values to make the higher value resistor one tenth of the input impedance. That will keep the loading error below 1%.

As described so far, this measurement system will do an adequate job in an electromagnetic interference (EMI) environment. Most industrial – even most office – settings are, however, very definitely not low-EMI environments. Typically, you will find lots of glitches, hums, and harmonics inductively coupled into your measurement lines because they form a great big single-turn transformer secondary. 

Long unprotected leads are guaranteed to pick up electromagnetic interference (EMI). Twisting the leads drastically reduces inductive EMI. Shielding the twisted pair eliminates capacitively coupled EMI.

The simplest way to drastically reduce inductive coupling of EMI into your measurement system is to twist the measurement leads together. Twisting the leads does two things: it ensures that the distance between the leads stays as small as possible, thereby minimizing the inductive-loop area; it also reduces the length of the inductive loop to no more than the pitch of one twist. That is, if you put 50 twists into your 15 m twisted pair, the pitch is 5 m / 50 = 10 cm/turn.

Faraday’s law says that the EMI field will induce a voltage in the first half of each twist that will just counterbalance the voltage induced in the second half, leaving a net induced voltage for each full turn of zero. If you end up with an odd number of half turns, the net voltage will be proportional to the EMI field times the wire separation (which should be just twice the insulation thickness for a tightly twisted pair) times the length of one half turn. That’s a whole lot smaller than what you get with a couple of wires flopping around across a 5 m distance.

Not all EMI, however, couples inductively. If the twisted pair drapes over or near any energized electrical equipment, you can get capacitive coupling as well. Capacitive coupling happens when electric fields surrounding any voltage-carrying metal item. For example, a poorly grounded motor housing may carry an ac voltage without carrying any current. Similarly the “hot” conductors and terminals in all supply wiring surround themselves with such fields. The important thing to remember is that it makes no difference whether there is current through the conductor or not, the electric field is still there.

The way to get rid of capacitively coupled EMI is with a Faraday shield, which is any grounded conductor that completely surrounds the wiring you’re trying to protect. First, it is important that the shield completely surrounds what you’re trying to protect with no gaps.

Second, it is important that the shield be continuous. Taking another piece of wire, for example, and wrapping it around your twisted pair won’t do the trick. The best shield is a tube braided from bare wire with the twisted pair running through the opening. The best strategy is to run down to your nearest cable supplier and buy a roll of shielded twisted pair already made up. It’ll be cheaper in the long run.
Third, ground the shield only at one end. Grounding at both ends will only set up a giant ground loop that will defeat the entire exercise. It makes little difference which end of the shield you ground, just ground one and only one end.

For more about making accurate measurements, consult the Keithley Instruments Low-Level Measurements Handbook, available for free from the Keithley Instruments website. 

Posted by Charlie Masi on March 10, 2008 | Comments (0)