# Longest Usable Wires for Low Voltage DC

I have a question that I am curious about. I am designing an IoT device from my microcontroller. My question is how far can jumper wire (standard solid-core copper wire) go before it loses significant signal or power? The purpose is to drive a relay about half-way around my house. It isn't high frequency on/off, at most once per minute, so I'm thinking it won't pickup interference. The signal is +5V. Does thin jumper wire make the cut?

Cheers

Edit: by popular demand, the current draw is 71.4 mA and the coil resistance is about 70 Ohms.

More Edit: Significant voltage drop would be probably more than 1.0 V, and ringing is acceptable, so long as it is not outside of this range.

• You need to define "standard" and "significant" before this can be answered. – Ignacio Vazquez-Abrams Jul 24 '18 at 2:05
• As is this is unanswerable, you need to figure out how much current the relay draws, or what its coil resistance is. – Chris Stratton Jul 24 '18 at 2:05
• Define acceptable voltage drop , ringing , current and assuming AWG wire gauge inductance and nearby stray noise ( dI/dt, dVdt ) and circuit impedance then you can get an answer , maybe – Tony Stewart Sunnyskyguy EE75 Jul 24 '18 at 2:10
• I bet the OP can either measure current draw on the relay or check the specifications and then using standard tables for per-unit resistance of the wires to be used, the voltage drop can be computed. Question answered. – K7PEH Jul 24 '18 at 2:19
• To get a good answer, you need numerical values for everything. There is no such thing as "thin jumper wire" or "standard solid-core copper wire." Something like door-bell wire is solid wire. It can be obtained in American Wire Gauge (AWG) sizes like 24, 20 and 18. The smallest of those (24 AWG) would drop about 0.2 volts in a 50 ft. run with 70 mA. An online voltage drop calculator will do the calculation or you can use a table of wire resistance per foot vs. wire size. Tables and calculators are available for metric units and both AC & DC etc. – Charles Cowie Jul 24 '18 at 3:00

Given your parameters of 71.4 mA and 70 ohms it appears you are driving a 5 V relay coil.

You have a choice:

1. Use a current source to drive the relay
2. Use a voltage source to drive the relay

My preference would be to use a current source, since this makes it easy to make the solution short circuit proof.

Perhaps something like this:

simulate this circuit – Schematic created using CircuitLab

You can select your cable from sites like this, but ordinary telephone cable may suit your purpose, it is typically 20 - 23 AWG and about 1 - 2 ohm each way per 100 ft. Anything below 20 ohm total resistance is unlikely to cause any problems.

The diode (D1) at the remote end is not required, but some might suggest it.

The IRF530 should work fine with a 5 V MCU, but if you are using a 3.3 V MCU you would need to select a FET with lower VGS(on) threshold.