Important Information About Power

Can devices share a common power source?

This question has a technical answer, and also has a "legal" answer.

The technical answer is that you will damage devices if you mix half wave and full wave rectifiers on the same AC source. Use extreme caution when sharing a common AC source. Sharing a common DC source is less problematic.

The "legal" answer, strictly speaking, is "no". Meeting the UL definition of a Class 2 device means providing a class 2 power supply or transformer for each Class 2 device powered. At most, only a small number of Class 2 devices may be powered from a single Class 2 power source. The power must be limited to 50 watts to meet Class 2. This means that if you do your math, the answer is not a straight "no", but "yes" with strict limitations.

What is a Class 2 Device?

You need to consult UL standards for a full definition of Class 2 since it addresses this on a circuit by circuit basis. For purposes of discussing power supplies, it means power is limited to 50 watts. Everything on the secondary side of a given Class 2 power source is considered part of that Class 2 circuit. UL standards also state that Class 2 circuits should not be intermixed in a manner that makes the circuits electrically common.

What should I watch out for using AC and a full wave rectifier?

DO NOT ground either side of AC when using a full wave bridge rectifier. Doing so will short out one side of the bridge and destroy it. Do not simply observe whether the power source itself is grounded. Very often failures due to AC grounding are introduced by the devices later connected to the powered device in question. Avoid sharing AC power sources if you do not know exactly what type of power input is found in each device to be connected.

What should I watch out for using AC and a half wave rectifier?

One side of the AC will be connected to ground in these devices. Make sure the same side of the AC source is connected to ground in all devices that share the AC source (if you must share at all). Treat AC like DC for purposes of watching polarity in this case.

What should I watch out for using DC?

Make sure you connect positive to the terminal marked POWER and connect negative to common or ground usually marked GND. Note that terminals marked GND and COM are electrically common. The terminal marked GND is intended for carrying the negative side of power input. The common terminals marked COM are there for use as signal common.

Why do some products list 10-30VDC while others are 18-30VDC input?

Devices having analog outputs need a minimum of 18VDC input to drive a 20mA output. If the device does not have analog outputs, or you will not be using them, they will function down to 10VDC input. (Testing shows that except for analog outputs, most devices actually function down to 8VDC. We recommend using a minimum of 12VDC to allow some margin for line fluctuation.) 

When is 24VAC power required?

The only device requiring 24VAC power input is the AddMe II, and this applies only when using the 24VDC excitation output. If not using the excitation output, then the 18-30VDC rating applies.

Which devices are half wave, full wave, or DC only?

Full wave bridge	AddMe II (AM2-MX-30) LonWorks I/O Babel Buster 485 LonWorks to Modbus Gateway Babel Buster 232 LonWorks to Modbus Gateway Babel Buster 10/100 LonWorks to Modbus Gateway BAS-704 LonWorks Processor Module BAS-7050 Ethernet Processor Module
Half wave rectifier	AddMe III - All variations including AM3-SM Six Pak LonWorks I/O AddMe Jr. - All variations including AddMe Lite Babel Buster 485XM BACnet to Modbus Gateway Babel Buster 485XL BACnet to LonWorks Gateway i.Report GSM/GPRS Remote Data Monitor
Half wave retrofit	Babel Buster X2 Polled LonWorks to Ethernet Gateway Babel Buster 485 LonWorks to York Modbus
DC Only	i.Board Embedded Web Server Babel Buster SP

The notation "half wave retrofit" means these products use hardware originally designed for full wave, but have been modified to operate as half wave rectifiers instead.

What are the advantages or disadvantages of half versus full wave rectifiers?

A full wave bridge provides more DC power given the same input voltage. The AddMe II includes a 24VDC excitation output, which is essentially a DC power supply. The AddMe III family does not include this output and cannot support such an output because these products use half wave rectifiers. Under load, the unregulated DC from the half wave rectifier falls too low to sustain a 24VDC output without very large filter capacitors. The bulk of filter capacitors necessary to sustain 24VDC output from 24VAC input using a half wave rectifier would occupy nearly half the available circuit board space in the product. It's not impossible, just impractical.

The disadvantage of the full wave bridge is that neither side of the AC coming into the rectifier can be grounded. Since grounded AC is common in HVAC applications, most of the newer Control Solutions products are using half wave retifiers to avoid the grounding problem. The cost of doing so is giving up the 24VDC excitation output.