12. Electric protection, UPS, inverter...

12.1. Introduction.

The European electrical supply network is fed in 240 V alternate. On the other hand, the equipment (PC and computing peripherals) are supplied with D.C. current according to various values, but of low voltage (generally included/understood between + 12 and -12V). To transform voltage of the electrical supply network into acceptable voltage by the electronic instruments, a power supply is used.

In first years, we saw an assembly power supply by rectifying bridge . The efficiency (the relationship between the consumption and the power returned uninterrupted) of this power supply per rectifying bridge (4 diodes) after passage by a transformer is too weak.

The power supply used in data processing is of type "switching" . This principle is not only adapted to the power supply, but also in inverter. The conventional power supply generally has an efficiency close to 50 %, until 80 % for the switching power supply.

12.2. Operation of a switching power supply.

2. The parts are difficult to find. The technique of repair is more specific to truths electronics specialists than with the data-processing technicians. So possible repairs are difficult to realize (if not impossible). Moreover, the precautions by opening these apparatuses is types "if you the finger puts there, it is of 230 V alternate, there it is of 380 continuous..." In short, if you do not have serious knowledge in analogical electronics (transistors, high voltage) ATTENTION

We start from an alternative voltage and let us rectify it by a bridge of diodes without intermediate transformer. At exit, the continues voltage is about 230 V (330 V at a peak). The bridge is followed of a condenser to smooth the voltage (230V continuous). The following component is a transformer: a transformer traversed by a D.C. current with the primary education does not produce any signal with the secondary. On the other hand, if you make pass an alternating voltage to the primary education of a transformer, it arises from it with the secondary an of the same voltage forms but different value (a division according to the report/ratio numbers reels inputs/outputs). With what can thus serve this transformer well?

Just on the outlet side of the transformer, a transistor is placed. This one will chop the voltage, inducing a discontinuous voltage in the transformer and thus will make pass from the current. Chopping is controlled by the control circuit which is present at the secondary. Contrary to the circuit above, the base of the transistor is generally connected to the control circuit by a second transformer which completely isolates the exit (connected on the electrical appliance) from the entry (electrical supply network). This avoids problems in the event of overpressures on the electrical supply network.

Let us see operation. With the starting of the power supply, an impulse starts a certain discontinuous voltage. This voltage will initially induce to feed the control circuit which will start to make chop the continues voltage at the boundaries of the transformer.

The more the proportion of chopping will be large on the trigger of the transistor (or bases in the case of a bipolar transistor), the more the output voltage will be large. The control circuit will vary this cutting according to the output voltage of the power supply and thus to control this voltage.

A small remark before continuing. As the bridge is directly on 230 V alternate, consider that half of the assembly is under 230 V.

As the departure of the assembly is a rectified voltage (continues), this assembly also makes it possible to start directly of a continuous voltage (batteries).

12.3. Disturbances of the electrical supply network.

1. Complete supply breakdown : generally comes from a breakdown of the electric installation of supplier or from a circuit breaker on your installation

2. Over voltage , the network voltage is higher than the voltage for which the power supply is conceived (think of the diodes of entries). This is specific to the installations close to the electric cabins "high voltage". Even if an overpressure is not by inevitably dangerous for the data-processing installations (in reasonable values), this disturbance causes constraints of the components of the power supply which, in the long term, cause the breakdowns.

3. Under voltage, lower voltage than that for which the power supply is conceived and the power supply cannot provide one sufficient power. In the case of the power supply for PC, it go down at least until 180 V under voltage is generally caused by a sudden increase in electric consumption on the network by the starting of heavy electric devices: engines, compressors, elevators... but also by a too significant distance compared to the high-voltage cabin.

4. Transients . Interfering signals forwarding at the same time as the basic electric signal, these transients can reach until 4000 V but are generally definitely weaker

6. Peaks: overpressures of very weak duration (less than 1/120 second), but of intensity being able to reach 4000 V and more. They are caused by the stop of various machines of strong powers (air-conditioners, electric household appliances...) which dissipate the surplus voltage on the network. Here also, one attends a wear of the components.

7. The lightning . An abrupt and significant overpressure. The lightning comes from weather phenomena (storms), on the electrical supply network and the telephone network. The third source of the lightning goes up ground and there, you can almost nothing make, no effective protection does not exist really even if that represents less than 1% of the cases. These increase of voltages of the ground are often localized in the same zone (a district for example).

In the event of complete voltage cut, the switching power supply cannot supply the PC In overpressure any more, cutting will control the output voltage (with a voltage of entry until 280 V for the PC), as long as that does not last too a long time. In transients, after a first rectification, one can hope that it is reduced. The transformer will completely let it pass and the second condenser (often coupled to a coil of smoothing) should remove it (would have). In practice, part of these transients pass PC power supply ATX. It is the same for the peaks for voltage. In the case of the micro cuts, the condensers act as shield partly. For the lightning, directly think of changing and the following power supply and charts. In short, a part is protected, but not all.

12.4. Surge outlets

These apparatuses protect from over surge and the lightning. In the event of over surge, they disconnect equipment from the electrical supply network. Often, after only one overpressures, they can be replaced. Only some apparatuses use a restoring which makes it possible this protection to continue. Worse, certain apparatuses having taken the lightning continue to function. without protection.

12.5. UPS

A UPS (Uninterruptible Power Supply or Inverter) includes batteries which supply the processing equipments connected at the time of an interruption of current, fact office of circuit breakers and on the whole regularize the network. One distinguishes three types of inverters:

In the three cases, they can be provided with a connection (series or USB) which, via a software installed on computer PC, the computer at the time of an interruption of current stops properly. An inverter does not have an inexhaustible resource and generally stops after 10 minutes. In the case of a server, connection towards the PC makes it possible to correctly stop the applications before extinguishing the server. The inverter stops the power supply of the server when it is extinct. By an option in certain BIOS, one can ask to start again the PC when the power supply network returns.

The duration of the safeguard depends on the power of the UPS, expressed VA from there (and not in Watt). Consider that for a given installation, the power of the UPS for a 10 minutes safeguard must be of P installation (Watt) X 1,6. For an installation of 350 Watt (server + monitor), the UPS must thus make minimum 350 X 1,6 = 560 VA.

If the power used by the installation is higher than VA from the UPS, the inverter risk either to be destroyed, or to put itself in safety. In the same way, avoid connecting printers laser, the current consumption all at the beginning of the impression is very significant and is likely to damage the UPS.

Remain the batteries. It are generally with lead in 12 V, settings sometimes in series to reach 24V and parallel to allow one duration of longer safeguard. From their design, the batteries must regularly be completely discharged to avoid a ratchet effect which makes them unusable, on average every 6 months.

Some inverter includes protection for the of cables network and of telephones. APC even left the redundantly UPS (duplicated) for the of server, one is never too careful. The OTHER large one of market MGE should emergency long in doing it.

You cannot in no case to use an inverter to protect a laser printer. This is related to the excessive consumption of these printers at the time of impression starting.

12.5.1 Off-line UPS

Off-line technology is the least expensive and thus most current. The electric voltage passes by a relay. The output voltage is then filtered to accept certain variations of voltages and to remove a part of the parasites. At the same time, the batteries 12 V are reloaded via the converter. When the voltage on the network disappears (or decreases below 176 V or higher than 280 V), the relay opens and the alternative output voltage 220 V - 240 V east recreate starting from batteries 12 or 24 V. The reaction time is relatively high, considering the time of closing or opening of the relays. These electric apparatuses of regulation do not control the micro cuts of the network.

12.5.2. Line Interactive UPS.

These inverters of average range are similar with the UPS Off-line. The difference comes from management of the cuts by a microprocessor which monitors the quantity of voltage and reacts to the variations. A booster rocket, compensating network of voltage is activated in the event of fall of prolonged voltage. The constraints of the power supply are thus less important in this case.

The electrical supply network passes initially by a filtering of the transients. If the network supply is sufficient (above 176 V), the network passes by the booster rocket which does not intervene. When the voltage remains sufficiently a long time in under voltage, the booster rocket will inject a voltage via the continuous/alternate converter for "booster rocket" the network fed for a short period. When the voltage of the electrical supply network passes below 176 V, the inverter starts completely by opening the relay of entry (more power provided towards outside).

In the case of the regulation of voltage, in the event of lower then 205 V, the booster rocket sends a voltage on the whole of the assembly for a short period (same if this voltage can be repetitive). In the event of is fall of voltage under 176 V, the voltage supply of the PC done only via the batteries.

12.5.3. On-Line UPS

The operation of an On-Line UPS is definitely different. The voltage of entry is systematically rectified and permanently supplies the batteries. This voltage recreates then a output voltage 240 V In the event of cut or of fall of voltage, the batteries ensure the power supply of the continuous/alternate converter. In the event of under voltage, the output voltage is at the same time created starting from the batteries and of the network, which is not the case of the UPS Off-line. Unfortunately, the batteries are practically requested all the time. They thus are more often changed. As the batteries are worth easily 2/3 of a price of a new equipment ... These equipments use also 2 circuit of by-pass. The first circuit of bypass makes it possible to pass in addition to inverter. This manual possibility makes it possible to supply the equipments without passing by the UPS (UPS breakdown for example). The second by-pass functions a little like the off-line UPS and makes it possible to save the lifespan of the batteries.

12.5. Protections comparisons

Electric protection remains a compromise between the price of the protection equipment and the importance of the material to be protected. The safety of a processing server of company requires an inverter online at least, whereas a micro-computer is probably satisfied with a circuit breaker. Lost one hour of production costs definitely more than the price of an inverter. In order to to ensure longer batteries protections, you cannot plug laser printers in an inverter. In this case, the only possibility of electric protection is a surge only outlets. UPS protect only computer and its screen.

Next of Hardware 2 course > Chapter 13: corporate network structure

Course: Introduction to electricity and electronics Bases in electricity and digital electronics and computer/network technicians

Course: AT and ATX Power supply Types of power supply, cases and format motherboard

Course: Ethernet by electrical supply network Use of the electrical supply network for the network transfer

entry	Surge outlets	Off-line UPS	On-line UPS
< 180V	Equipment is not supplied any more	opened relay, power supply only by the batteries	Power Supply by batteries and network
180 - 220 V	Normal operation	According to the model, simple voltage regulation or complete intervention of the batteries the values is decided with the design	Power supply by the network and the batteries so necessary
220 - 240 V	Normal operation
240 - 280 V	Normal operation	According to the model, simple voltage regulation or complete intervention of the batteries the values is decided with the design	Power Supply by electric network
> 280 V	Abrupt cut	Operation by batteries.
High-voltage, the lightning	Abrupt cut	Operation by batteries, attention with the times of opening of the relay.	Operation by batteries and cut of the circuit breaker (Reset obligatory)
Peak of short duration	No detection generally	Partial filtering	completely removed
Micro cuts	No detection generally	Partial filtering	completely removed


Diagrams of an inverter line-Interactive	Voltage regulation