Synchronous voltage limiter (ONS)

Development overview

The synchronous voltage limiter model is designed and assembled just for low-power equipment that requires automatic power recovery without much delay. ExperimentalI'm working out and its main tests have just been completed (thermal tests are ahead). ONS (see. in the photo below) can be connected to the break of the existing power line, or directly to the outlet with the load connected through the sockets.

The voltage limiter is designed for power up to 250 W. It is assembled on the basis of a standard distributionnd boxes from Tyco electronics, - 75x75 mm. It should be noted that the ballast control circuit is the same for all power levels, only the ballast itself (classic) changes - the bridge, transistor and cooling radiator. There is no way to talk about circuit solutions here, since the device is an object of "know-how" and expects a serious business check within the framework of contractual work. We can only say that the circuit is analog and only elements of widespread use are used in it. ONS is designed for the mode of regular limitation of the input voltage up to 255-260 V, - the most probable level, and short-term - up to 275 V, with a load current of up to 1A. To protect against overheating, a miniature thermo-automatic device is fixed on the radiator. The following functional properties of the synchronous limiter have been achieved:

1. The possibility of a permanent connection in the power circuit, that is, switching on the power supply of the load with an inrush current (for switching power supplies) and limiting the voltage, or turning off the power when excessive overvoltage in networks.
2. Instant response of the limiter in a wide range of surges and surges, dependent only from the frequency properties of controls and ballast (up to about 3 MHz - for conventional elements of wide applications).
3. Possibility of testing in operating mode for maximum voltage limitation (ballast test) and load disconnection (using micro buttons).
4. Instantaneous disconnection of the load, depending only on the response time of the relay (several ms).
5. Automatic recovery of the power circuit with a delay of several seconds, provided that the voltage drops to an acceptable level (less than 250 V).

It should be noted that in connection with the characteristics of the load, its purpose, two modifications of the ONS are advisable - with automatic power recovery and only manual recovery. The device of the second modification of the limiter is much simpler, since instead of the relay and related elements, a typical, widespread thermo-automatic machine (breaker) is used, modernizedby the designer to provide automatic reset from the protection circuit (see. previous article - new overvoltage protection device). This machine retains the property of protection and overload.

In the minimal design, the ballast radiator is cooled convectively through the holes in the box (protected by a mesh). To provide more power protection (heat dissipation), you can use an additional box in which you should place a cooler with a current transformer and a thermo-breakertomato. It is convenient to connect the boxes with the lower planes, having previously made windows or holes for blowing the radiator (this principle is convenient to use for other modules placed in similar boxes and requiring cooling).

What is the significant advantage of ONS?

In a previous article, the developer has already noted that all consumers in a 230 V, 50/60 Hz network (rated voltage of a single-phase network according to the new GOST, with a tolerance of +/- 10%), which have switching power supplies (with their own stabilization) require a special approach to protection against increased voltage. All of them need not just over-level protection, but protection against a wide range of impulse and surge voltages. The modern market is extremely saturated with filters and automatic voltage switches (voltage relays), which include protection elements against impulse noise in the microsecond range. As for longer pulses and surges, surges, it should be noted that these devices have a certain smoothing (filtering) in front of the sensitive element of the machine (so as not to annoy the owners of frequent triggering). That is, they pass some part of the impulses. As for the pickup setting, it should not be higher than 250 volts. Many "voltage relays" have external set point adjustment, but this should be considered a disadvantage rather than an advantage. It was introduced just in order not to annoy with frequent disconnections. But, voltage over 250 volts is very dangerous for any electronic equipment.

As mentioned in the previous article, it is not profitable for all Manufacturers to provide a large "margin of safety" voltage for their products. Thus, the entire mass of passive filtering and relay protection devices is suitable only for level-stable voltage and disturbances of networks, that is, it is designed for rare, accidental overvoltage (during a thunderstorm or accidents in network). Many of them will still "bring" the owners to "white heat", to a decisive replacement with a stabilizer. However, modern stabilizers, although they look like perfect devices (including advertising characteristics, especially for the common buyer), but they still have a number of significant drawbacks that can only be identified by appropriate engineering testing in a special laboratory. There are very few articles on this topic on the Internet, and they only contain verification of the content and limit stationary modes.

What is the main, fundamental difference of the new approach? It consists of the following:

• a synchronous limiter (ONS) monitors each voltage half-wave and synchronously "cuts" its amplitude to an acceptable level, based on the allowable effective voltage less than 250 volts;
• the size of the cut-off part is determined only by the boundary voltage of the ballast transistor and the expedient limitation of heat release, - for stable network, it can be extremely large, for example, up to 100 volts (then the ballast will cut off pulses of this magnitude without disconnecting load);
• the entire spectrum of pulses is cut, depending only on the frequency properties of the ballast and its controls;
• the disadvantage of heat dissipation on the ballast is not as great as it seems due to the fact that impulses, the duty cycle of which proportionally reduces the allocated power, - for example, in the range of 245 - 250 volts of the output voltage at the input - 245 - 275 V the maximum heat release is about six times less than with a continuous voltage (the duty cycle is calculated from the sine angles at the cutoff boundary sinusoids).

For loads over 0.5 kW in a network with frequent overvoltage, it is necessary to equip a synchronous limiter fan (cooler), which is advisable to be powered from a miniature current transformer (based on a step-down transformer). Starting with a power of 1-2 kW, it is advisable to use a tandem - "STAB - ONS" - effective combination of the properties of these devices. The stabilizer provides a static mode, and the ONS provides a dynamic and active filteringshy, with minimization of heat release.

It should be noted that the use of modern autotransformatorsrn stabilizers for low power is irrational in principle, since the transformer itself has significant consumption. These stabilizers are designed for a group of consumers and for their total power, close to the nominal, for continuous operation without a significant reduction in power consumption. Only in this case is a satisfactoryth efficiency. Thus, the proposed ONS is seen as a practically necessary and successful addition to modern stabilizers and effective replacing them for low-power equipment, which is becoming more and more (while maintaining and increasing its cost, and value for owner).

Developer tip

In conclusion, A. Vasiliev gave advice to those who wish to test their purchased or homemade protection devices. As a source of increased voltage, not LATR should be used, but a conventional step-down transformer with several secondary windings and leads from the primary, so so that with the phase connection of the secondary windings with the primary and using certain terminals of the primary, a high voltage could be obtained, for example, up to 270-275 volt. This voltage must be applied to the control electronic part of the protection device through a variable resistor of 10-20 kOhm. The consumption of the control electronics is usually (and should be) no more than 10-15 mA. And the power section must be connected directly to the network, taking into account the phase. With such a power scheme, you can more smoothly and accurately set the voltage and form an ideal jump by closing the entire variable resistor or an additional one.

If you are interested in the synchronous voltage limiter, which was mentioned in this article, please contact for additional information by e-mail [email protected] (Alexander Vasiliev, development engineer).

It will be interesting to read:

• Instructions for connecting voltage monitoring relays
• How to protect home wiring with a voltage stabilizer?
• Why does the RCD work and what to do in this case?