Issue 142 May2002
You're Not Alone
Dealing with Isolation

Power Isolation

Isolation can’t be achieved using a common power supply. This need creates a requirement for additional circuitry beyond the signal isolation. There must be a separate power supply for the circuitry on each side of the isolation barrier. Often this will be more expensive than the signal isolation.

The simplest solution might be to purchase a DC-DC converter. In this case the converter is used as an isolator instead of converting from one voltage to another. Take your time choosing a converter; if you aren’t careful you can find yourself being locked into a single-source manufacturer, especially if you choose a converter with a weird pinout or physical size. An alternative might be to build one directly on-board.

Both Linear Technologies and Maxim specialize in power products. Linear’s LT1424-5 is good to about 2 W. For lower currents up to a 1-W converter, take a look at Maxim’s MAX253. Few external components are required to use the MAX253 as a transformer driver for an isolated power supply circuit (see Figure 7). C&D Technologies makes an isolation transformer specially designed for the MAX253. The 78250 series transformer is available in 1500-V and 4000-V isolation from Mouser.

I made prototypes of the circuits in Figures 6 and 7 to create an isolated RS-485 transceiver for use with a microcontroller. Notice the surface mount IL-485 mounted on a dip header in Photo 1. Pins 4, 5, 12, and 13 of the IL-485 are soldered to the header and the other pins are wired to the appropriate dip header pins. Using a dip header for surface mount parts makes them easier to handle and reuse elsewhere (after you get past the surgical wiring).

Although the IL-485 is only available in the surface mount variety, the other devices are through-hole components. I measured typical required currents in the isoloop IL-485 circuit in Figure 6 and found the isolated RS-485 side required considerably less than 50 mA without any load on the twisted pair. Using the isolated voltage to supply the termination load adds 50 mA to the load requirements of the MAX253 isolated supply. The MAX253 circuit easily supplied this current at just over 5 V after the Schottky rectifier drop, thanks to the 1:1.31 winding of the transformer. No regulator is needed because this is within the recommended range for the IL-485 (4.5–5.5 VDC) and well below the 7-VDC absolute maximum rating.

The 78253 transformer from C&D Technologies is rated at 200 mA, so there is plenty of overhead even with a heavy RS-485 termination. Large spikes were seen on the isolated side of the MAX253’s circuitry, but a series choke tamed them nicely (see Photo 2).

Are isolation techniques necessary? In many situations, they aren’t necessary. Overlooking the more obvious safety issues (such as distances between devices) in some applications increases the potential for ground loop problems.