Isolation, Electrical

• Introduction
• Laptop Isolation
• USB Isolators
  • Single-Channel USB Isolator
  • 4-Channel USB Isolator

Introduction:

WHENEVER ELECTRICAL EQUIPMENT IS CONNECTED TO A LIVING SUBJECT THERE IS A DANGER OF ELECTROCUTION due to malfunction. To minimize this risk, all connections to the subject should be electrically isolated from the main power lines, including the ground or neutral line. This means that there must be no electrical path between the subject and the line.

For use on human subjects, always use properly designed and isolated equipment certified for the intended purpose. For small lab animals like rats, mice, and guinea pigs, however, the danger is often considered minimal enough to use certain equipment without isolation. Consult the animal use committee at your institution for details.

Laptop Isolation:

An excellent isolation method for non-human subjects or self-experimentation is to use a laptop computer that is running entirely from its internal battery; it must be completely disconnected from mains power, and from any other equipment (such as external disk drives or power amplifiers) that may be connected to mains power. The laptop is thus completely isolated from the power lines. You can then connect USB- or battery-powered preamplifiers or stimulators directly to the laptop, and to your subject. Although providing excellent isolation, this method can suffer from limited battery life of the laptop.

USB Isolators:

One way around the battery life problem is to use a commercial USB isolator. This is a device that you connect between a USB sound card and a laptop or desktop computer running on the normal AC mains. The USB sound card accepts digital stimulating signals from the computer, and returns digitized reponse signals from the sound card.

The digitized signals are high-frequency pulse trains (rectangular waves) that are made to pass through insulating circuits in the isolator. In the products tested here, tiny transformers are used with high-voltage insulation between input and output windings. Other digital isolators use tiny capacitors with high-voltage dielectrics to provide the isolation. Some specialized isolators use optical couplers, where the digital signal drives an LED whose light passes through a transparent insulator and is reconverted to electrical pulses by a photo-transistor.

Both of the isolators below use the Analog Devices ADuM3160 USB chip, which limits them to 12 Mbps maximum data transfer rate (USB 2.0 "Full Speed").

Single-Channel USB Isolator:

This device is under US $25, and works well with simple devices such as the CM6206 or Behringer UCA202 sound cards or Arduino or Numato devices. However, it can't supply enough power for more-advanced sound cards like the Creative X-Fi or the ASUS Xonar.

The actual unit may be covered with clear heavy plastic shrink tubing, with an opening cut away to allow the switch to operate. The switch may be either blue or red, and should be in the position towards the computer, as shown in the image, for the fastest USB speed (12 Mbps). A blue LED near the switch will be lit. If you accidentally switch it the other way (1.5 Mbps) a yellow LED will be lit instead.

Search for "ADUM3160 Isolator Module".

4-Channel USB Isolator:

This device is also under US $25, and works with all the devices that the above single-channel isolator does. However, it also includes a connector for an external power supply which allows it to also support the Creative X-Fi at 48 kHz and 96 kHz sample rates, and the ASUS Xonar at 48 kHz only. This makes it a better investment than the above unit. Search for "4-channel ADUM3160 Isolator".

The power supply used in testing was an inexpensive "wall wart" type rated at 5V and 1 Amp. Such supplies are readily available for about US $10 or less.

PLEASE NOTE that the supply itself must be an isolated type, since the isolator external power ground is connected directly to the ground of the output USB connectors. These cheap wall wart supplies are usually isolated simply by use of a transformer; equally important, they use switchmode regulation which employs a small, lightweight transformer.

Don't be tempted to try an old, heavy, cube-shaped wall wart that you happen to have on hand. These often have no voltage regulation at all; they may only put out the rated voltage at the rated current, while the voltage may be much higher at lower currents.

Switchmode supplies are much better for isolation and voltage regulation than the older style. The smaller transformers have less inter-winding capacitance, and thus are less able to pass a transient powerline fault. (They still don't provide as much isolation as the USB isolator itself, however.)

Note that these isolators typically come covered with clear heavy plastic shrink tubing, which you will need to cut away from the power connector terminal block. This plastic is very tough; use caution and a sharp X-Acto knife or similar.

You can cut off the connector on the power supply cable and identify which conductor is positive and which is negative with a DC voltmeter. The negative connector on the isolator is the one nearest the 4-USB output, as shown in the image. Expose enough of each lead to insert it into the terminal block, and tighten the screw on top.

This is the simplest approach, as long as you can leave the cable connected to the isolator. Otherwise, you will need to guard against the exposed ends shorting together if power is applied accidentally. An alternative is to leave the power cable intact and obtain the mating connector, and wire it to the isolator with short "pigtail" leads covered in shrink tubing.

Important: You must apply external power to the isolator before it sees power from the computer USB port, not the other way around. The isolator apparently decides which source of power to use based on the order of application, then maintains that source. It's OK to keep everything plugged in, with the external supply and the computer powered by the same mains power strip, so that switching the power strip on will start the external supply before the computer boots.