Supplying the Power
 
 

Mains
Planning for power on mains fed sites is nothing more than ensuring there are enough sockets for all the pieces of telemetry equipment and 2 pieces of test equipment. It is nothing short of extreme annoyance when, after being asked to help repair a site, to arrive and there is nowhere to plug in a soldering iron! Remember to also include the sockets to cater for expansion.

The telemetry mains supply should also be fed off its own circuit breaker, including earth leakage, so that any plant faults do not deprive the radio system of power as well as allowing the radio system to isolated during maintenance.

Many telemetry modules allow for powering either via 24 volts (a standard in process control systems) or an AC supply of about 16 to 30 volts. If using a mains transformer plan the current size, both in voltage and current (i.e. VA). Using smaller than specified transformers could result in reduced charging currents as well as burned out mains supplies (fire hazard). Plan for transformers of twice the required current ratings and opt, if possible, for magnetic transformers as opposed to switch mode power supplies. Magnetic transformers are more resilient to sharp surges and transients (lightning strikes) than are SMPS units.

Wind, Solar & Wind-Solar
Wind, solar, or combination wind-solar power systems need to be very carefully planned. The requirements should already be known from the an earlier section, these figures need to be brought into this calculation for finally specifying the system for ordering.

Two complete sections "Solar Calculator" and "Wind-Solar Systems" have been dedicated to this alternative form of energy supply.

Powering Sensors
This requires careful planning if the reliability of readings is to be maintained. Many modern sensors have relatively low voltage requirements i.e. voltage available on a loop at full current may be fairly low, usually under 9V. This voltage, although apparently low, is the minimum required after all voltage drops on a loop are taken into account e.g. instrument inputs, wiring, etc. Many inputs exhibit as high as 250 ohms resistance which translates into 5 volts drop at 20mA thus requiring a minimum of 13V drive voltage for the loop which.

Deriving the loop drive from the backup battery may appear ideal as this would ensure power-fail redundancy as the voltage across this is around 13.85V. However, this voltage is not available off the battery during times of no charge be it a mains fail, dark hours (solar), or calm (wind charger). During these times the voltage will be less than 12.8 and fall to as low as 12.4 resulting in insufficient drive for the loop.

Some manufacturers of telemetry devices recognize this requirement for a higher loop drive voltage but there is one pitfall that must be known when calculating the autonomy of a backup battery and that is to generate 20mA at 24V will take a minimum of 40mA at 12V, assuming a 100% efficiency. Some modules use a "pump" circuit to generate the loop excitation voltage and, although the voltage is just short of twice the input voltage, the current drawn on the supply is twice the current supplied to the output.

At first this does not sound like much but take a module with two analogue inputs and two analogue outputs, all supplied by the loop supply. When calculating standby battery systems one always assumes the worst i.e. the loops will be at full scale of 20mA. This totals up to 80mA which means 160mA is required from the battery. Over 24 hours this adds up to 3.84Ah which means on a solar system 1.5A from the panels for 2.5 hours is used just to keep the sensors going.

When you see these sorts of calculations it is obvious why many try to get away with powering the sensors from the battery (for starters you halve the daily Ah required for the sensors) but many forget just how far battery voltages fall when not being charged. Be careful of the pitfall of using low voltage sensors at installation, these may not be available should one fail and have to be replaced. Rewiring the sensor to the higher voltage may suddenly push the Ah requirements over the limit and the site starts to fail under the increased load.

Know the full current requirements before calculating the standby/autonomy of a site.

Backup Battery Autonomy
Calculating the back-up battery in either a mains powered or solar/wind site requires careful thought. Using manufacturers 'recommendations' can lead to trouble, especially as the designer may only be interested in keeping the unit alive for a predetermined period but you've connected other devices to the same battery. The first part of the Solar Calculator works superbly in determining the battery capacity required, please do remember to enter all the figures.

Laying Cables and Catenaries
Although laying cables, especially power cables, is best left to those trained to do it (there are many who are not trained and their work methods may give you some maintenance in the future), there may be occasions that the cable laying will fall within the job requirements and a little help will not go to waste.

Planning Healthy Comms  >>