Isolated RS-232 Transceiver

When there are high voltage circuits, such as in a high voltage power supply, and you need to control them via an RS-232 interface, it is usually a good idea to isolate the computer from the analog electronics. Here is information about a new isolated RS-232 transceiver that may be of interest.

Fully Isolated RS-232 Transceiver Readies For Harsh Environments
Claiming title as the industry’s first fully isolated single-package surface mount RS-232 transceiver with integrated dc/dc converter to supply isolated power, the ADM3251E transceiver integrates a line driver, line receiver, oscillator, rectifier, regulator, voltage doubler, voltage inverter, and transformers into a single chip that isolates both the data and power lines on chip. The transceiver incorporates the company’s iCoupler and isoPower isolation technologies to enable a 2.5 kV isolation rating and it complies with industry-standard isolation regulations including UL1577 and DIN VDE 0884-10. Additionally, the ADM3251E comes in a lead-free, 13 mm × 10.56 mm 20-lead WSOIC surface-mount package. Price is $2.99 each/1,000. Analog Devices, Inc., Norwood, MA. (800) 426-2564.

Welcome Package for Opto Isolators

New high voltage opto isolators are always a welcome addition to a designers. Optek has a new configuration that you may find handy.

They have axial fiber optic isolators offered in isolation distances from 18mm to 80mm. Providing power electronics engineers with a means to isolate high voltage input/output circuits over varying distances, TT electronics OPTEK Technology has developed a series of axial optical isolators using plastic optical fiber. Designated the OPI1270 Series axial fiber optic isolators, the devices are constructed using a visible red LED emitter and a phototransistor detector in separate opaque molded plastic modules, linked by a shrouded optical cable that shields the optical signal from dust and other contaminations.

Great Circuit for Highly Isolated Floating Power Supply

Every so often you have an application where you need to provide power to a circuit that is floating at high voltage. In general, isolated power supplies can be an expensive, complex endeavor. Sometimes undesirable effects, such as substantial ripple injected into the powered circuit can cause problems. In EDN Design Ideas, November 22, 2007 issue, there is a circuit for a floating power supply that can withstand a separation of 10cm between the ground section and the floating section. This flexibility can come in very handy. I might consider a changes with the bridge rectifier diodes, and I might consider some protective components if the output section is floating at a very high voltage.

For details on this excellent idea, please see EDN Design Ideas: Wireless “battery” energizes low-power devices

Isolated signal measurement

It's often useful to measure voltages that are floating at a high voltage. One technique is to use analog to digital converters, feed the digital signals into fiber optic links, then digital to analog converters (or just use the digital signals) at ground level. This can also be done with voltage-to-frequency converters and frequency-to-voltage converters. Other techniques include capacitive pick up, transformers, etc. Linear optoisolators can be used in some applications. There are a few manufacturers that can make them, or you can assemble your own. There is an application note from Agilent that can provide additional information.

For less voltage isolation, other techniques can be used, such as isolation amplifiers.

There are a few commercially available devices provide isolated voltage measurement.