The Tech Blog

I think the 4–20mA current loop is one of the simplest and most reliable ways to send analog sensor data over long distances. I’ll share a bit of history, why those spans exist, and some practical tips from my experience.

The 4–20mA loop dates back to 1950s process industries. The idea of a current loop really took hold when plants had to move from using pneumatic control (3–15 psi, purely mechanical) to electronic signals. Engineers needed a signal that could travel long over noisy wires without degrading. It was expensive to detect signals below 3psi hence the lower range of 3psi got stuck. By driving a constant current rather than a voltage, you avoid voltage‐drop issues in the field wiring: no matter what resistance the wires add, the receiver always sees the same current. BTW, lower limit is 4mA and not 0mA because of “live zero” measurement. It means if we use 0mA we can’t detect if the value is actually meant to zero or the loop is broken because of power loss or wire-break in circuit.

Technically, a 4–20mA loop is just that: the sensor or transmitter adjusts its internal resistance so that it draws between 4mA (minimum) and 20mA (maximum) from a fixed 24V DC supply. The controller reads that current across a shunt resistor (commonly 250Ω, so 4mA→1V, 20mA→5V) into an ADC(If its Vref is in that range). Why 20mA at the top end? It’s a practical limit. Early transistors and relays tolerated up to around 20mA without burning out or dropping too much voltage.

In practice, you’ll see 4–20mA used for pressure transducers, temperature transmitters, flow meters, level sensors, pH probes, and almost any industrial instrument.  When practically implementing circuits for 4-20mA, make sure to take care of ground loops and to have isolation circuit in the front end of your receiver, purely as you are working in industrial environments. There are a lot of ready-made ICs from major suppliers to help build the 4-20mA loops. Search online and pick from them.

Fun fact: you can layer HART digital communication onto a 4–20mA loop without disrupting the analog measurement, allowing for diagnostics.

I came across these Dehumidifiers only recently and they seem to be the real deal. I was always on the lookout for using tech to remove water from inside smaller products, and most of the time I was stuck with peltier style ones or the silica gel units. The solid state membrane ones (called Rosahl) are from Mitsubishi Electric and seems to have around for a while.

Rosahl operates using a solid polymer electrolyte (SPE) membrane. When a small 3V DC voltage is applied, it electrolyses moisture in the air. This process breaks down water molecules into hydrogen ions and oxygen(Check Images). The hydrogen ions migrate through the membrane and recombine with oxygen on the other side, releasing water vapour outside the enclosure. I find this method efficient because it doesn’t involve liquid water, making the system maintenance-free and eliminating the need for water drainage, which is a big deal for consumer products.

It’s designed for enclosures ranging from as small as 250 millilitres to up to 8 cubic meters and comes in various sizes. The absence of moving parts like the fan means it’s silent and vibration-free and can be used in any small product for water removal. The device uses relatively low power(60mW – 5W), and the power consumption decreases further as the relative humidity within the enclosure drops.

However, one significant drawback I’ve noticed is cost. Even the smallest modules start around $25, with larger ones exceeding $500. Also, it isn’t practical for larger spaces, like entire rooms or bigger areas. I assume scaling the membrane might reduce its efficiency and significantly drive up manufacturing costs.

I’d have loved to incorporate this tech into some of my client products if it were more affordable. Interesting tech nevertheless.