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AC Power filter and phone line filter (homemade circuit breaker)

power line filter - surge protection

AC-Safe can protect or filter the phone line and protect or filter the ac power supply (mains), from lightnings (or surges) and extra currents. Toss in a few modifications, and also noise coming from power lines can be easily filtered.
A classic example is noise on the TV, when an hair dryer is being used . Well, I field tested Ac-safe and, interposing it between the TV and the dryer, the disturbs on the TV were removed!
Moreover I have tested it in a CE laboratory, using Schaffner instrumentation, against unexpected overloads (surges) (EN61000-4-5 tested with impulses up to 2.2KV) and the behavior has been excellent!

Documentation regarding protections circuits for phones lines or data networks, is huge. This is because noise characteristics are not known "a priori", so we work with presumed models of noise. So while designing a PCB I take in consideration many different possible noise sources.



The schematic MUST be used only as a reference, and can be used as a starting point for several experiments:
PHONE LINE protected from over current and/or over voltage.
230V ELECTRICAL LINE EMI protection and/or extra tensions protection.

So, components to be assembled on board depend from the experiment you want to run

Download the complete PDF documentation --->> download

Buy the file GERBER to build the Printed Circuit (PCB) --->> ac-safe_gerber

Components list

AC Protection AC_SAFE Revision: 0
Bill Of Materials
Item Qty Ref Part Note
2 1 GT1 GAS_TUBE3 T23-A350X(EPCOS) NO radioactive
3 2 JP1,JP2 HEADER 3 MT_2103
4 3 RV1,RV2,RV3 V250LA V250LA20
5 1 TR1 1.5KE440CA 1.5KE440CA

Considerations on the Part List:

This component list is valid for a 230Vac power line protection from overvoltage (by means of the Varistors), lightninigs (thanks to the gas discharger and the transil), and for a network filter (that's the purpose of the DELTA Filter).

Here are the possible solutions/experimentations


Item Qty Ref Part Note
2 1 GT1 DO NOT mount
3 2 JP1,JP2 HEADER 3 MT_2103
4 3 RV1,RV2,RV3 V250LA V250LA20 (DO NOT CONNECT RV1 and RV3 to EARTH, but on a GAS DISCHARGER that connects to EARTH)
5 1 TR1 1.5KE440CA 1.5KE440CA


Item Qty Ref Part Note
3 2 JP1,JP2 HEADER 3 MT_2103
4 3 RV1,RV2,RV3 V250LA V250LA20
5 1 TR1 1.5KE440CA 1.5KE440CA

Printed circuit PCB

Component side (TOP)

Solder Side(BOT)

Please note the EARTH layer and the use of sharp points used to "discharge" further extra tensions.


Side Component Serigraphy (SST)
ac-safe_sst.pngDownload the project PDF documentation --->> download

Buy the file GERBER to build the Printed Circuit (PCB) --->> ac-safe_gerber

Realize a board protection

With AC-Safe it is possible to implement 3 different models





This circuit typology usually protects the device from over voltage, and its working parameters are conditioned by the current flow, usually limited by a series protection or more banally a fuse placed upstream.
The parallel protections can be divided in two subcategories, Primary and Secondary

Primary protections are the very first dam against spikes, thus absorbing a great amount of energy, but usually powerful doesn't mean fast. This means that the first part (and we are talking about microseconds ) of the disturb is not properly filtered reaching thousands of volts. That's where secondary protections come handy.


This Application Note from ST (AN579) describes very well the concept of primary and secondary protection.

The following picture shows the REAL answer of one primary protection:


As you can see, in the first 80uS the trace exceeds the 230V threshold, thus making necessary a secondary protection for sensitive circuits.

Primary protection

The primary protections can be realized in several ways, sharp points on the printed circuit that discharge to earth, GAS discharger and Varistor.

Their main purpose is to reduce as much as possible the spike and their restore time is essential to filter the harmful event.

Points on the PCB:
When designing a printed circuit board, it is very important to provide means to physically discharge the spike energy thus avoiding an excess of energy that would make other protections explode.

Parameters of a electricity discharge in air on a circuit without solder protection and at an altitude lower than 3000 m, must be kept in mind to design this PCB - Spark Gap.

The following formula can be applied


where p=1 (atmospheric pressure) and d = distance in millimeters keeping in mind the following norms IPC about isolation (minimal isolation of 2.5mm @500V), and vias designed with sharp poinys to discharge energy.

GAS discharger: These components become active when voltage raises over a certain threshold, discharging spike energy to earth. Though bipolar devices are available (working on two phases), using tripolar ones guarantees the same threshold voltage. When igniting the gas discharger emits light. More recent devices are compliant to more restrictive norms that bans use of radioactive materials from gas dischargers, making them safer.
Their best use is in protections using (series) varistors as well as clearly explained by this "Surge Arrester" EPCOS Application Note


By providing an upstream protection with a series Varistor, we avoid the G.D. self destruction caused by the overcurrent flowing in the protected circuit.

The Varistor: Varistors (aka MOV) principle of operation is quite simple. When voltage exceeds their nominal threshold value, they divert energy from the protected circuit shorting to ground, acting as a "shunt", thus forcing an upstream fuse to blow.
They are enough fast and powerful to act as a fundamental primary protection.
The following Littlefuse-Harris Application Note shows protections examples using MOVs, providing selection criteria to select the proper device given a threshold voltage and power.

protez4.png _ protez3.png _ protez5.png

Finally, note how the Varistor can be both a valid parallel protection and a series device to be coupled to the GAS discharger. It is with no doubt a keystone for circuit protection.

Secondary protection

This kind of protection acts at the very beginning of the spike (nS), clipping the over voltage to the maximum allowed voltage for the circuit under protection. Fast enough devices for this purpose are : Transil (TVS), Trisil, Zener.

Transil (TVS): Transil is a transient suppressor available in both unidirectional and bidirectional versions. It clips voltage when it exceeds its nominal threshold, dissipating energy in excess. Its behaviour is similar to the zener diode but transil is much faster and stronger, specifically designed to protect from over voltages. Bidirectional ones are usually used as protection on a transformer secondary, or on a telephone line (take a look at SINCLAIR) or as secondary protections in power lines. They are used in unidirectional version for transmission devices or lines protection. Following we show a practical case occurred when designing a remote controlled board using a relay to switch on/off a neon lamp:
PROBLEM : When the lamp is switched on it creates noise on the power line. This noise enters the Transformerless power supply (a directly connected to the power line series resistor, capacitor and regulating zener )causing the onboard microcontroller to reset.

SOLUTION : Using a monodirectional 5V transil, placed very near the Vcc Microcontroller pin solved the problem.
(reset had been previously connected internally to Vcc via software)

Trisil: Trisil can be considered the Gas Discharger younger brother, with similar behaviour (with the due physical differences).


Interesting ST Application Note (AN574) on the differences between Transil and Trisil


This ST Application Note (AN1826) can be also very useful on the differences between Transil and Varistor.

Probably in most of the cases it will be convenient to use, in combination, all the presented devices.


Series protections protect circuits from over currents, when the nominal max allowed current is exceeded.
Fuses, power resistors, Circuit Breaker and PTC thermistors(Positive Temperature Coefficient) are used as series protections.

Usually consumer devices protections do not use fuse being irreparable. leaving to upstream devices such as thermal switches this duty. On the other hand applications for avionics, expressly demand a non restorable
fuse. In these cases the fuse is replaced by a power resistor, to limit current.

PTC: These devices vary their resistance according to the temperature, the greater it is the current that flows, the greater will be the temperature and therefor greater will be the resistance, protecting the downstream device. They are frequently used in protections, both for power and phone lines, because they are restorable (even if slowly).

EPCOS e VISHAY are without a doubt the most important producers of PTC

protez20.jpg _ protez21.jpg

EMI Protections

They protect the devices from frequencies due to power line noise.They are particularly necessary (for norms compliance) in DIMMERS, in SWITCHING power supplies, etc, all considered potential noise source for the power lines. Moreover they can also be used for protecting a device from potential incoming disturbs.

They can be self made using resistors and capacitors and particular coils (generally in the locals power supply these are made for having the possibility to design a specific filter and to intervene in the phase of certification) or they can be already found out of the shelf.

The connection of various parts, in the different levels of protection, will protect your circuit from a several kind of disturbs.

LINKS to download Application Note, Data Sheet and Risources

ST Transil producer, Trisil and interesting Application Note
ARCOTRONICS Net filters and specify anti-disturbance capacitors
LITTELFUSE (HARRIS) Varistors producer, GAS discharger, Circuit breaker and interesting Application Note
EPCOS Varistori producer, PTC, GAS discharger and interesting Application Note
ONSEMI (MOTOROLA) Transil (TVS) producer and Circuits Protection EMI-RFI-ESD-Telecom
SCHAFFNER Net filters and bobbin of suppression disturbs authoritative producer
VISHAY Varistors producer, PTC, Transil and interesting Application Note

Download the PDF project documentation --->> download

Buy the file GERBER to build the Printed Circuit (PCB) --->> ac-safe_gerber

If a lightning hits the

If a lightning hits the circuit, there is nothing you can do. There are no "circuit protection" or "surge protector" to take. Your circuit board will burn!

If a lightning hits....

You are bloody right, so why don't you provide us an automatic fire exstinguisher for our circuit ? ;)

Dear Sir, lightning is not a joke

Dear Sir, lightning is not a joke. You are not playing with lightnings. There's a lot of documentation and thesis about this. The lightning-arrester is a complicated thing, a serious one. Btw I appreciate this project 'cause it's a good diy project to begin studing surge protections.

Maybe you can use this to

Maybe you can use this to extinguish your bum's fire. The perfect breaker!

Mateev, please post comments

Mateev, please post comments related to the content. Lightning protections and noise filter.

Sure, if a lightning hits

Sure, if a lightning hits you, never you can do... but if a lightning dropped near you (or your home), this board can safe you PC or TV or DVD Player.....

You know what the circuit

You know what the circuit breakers are.
You know what a power line filter is.
You know how design a noise filter.
You know how to build a homemade surge protection?

WooW Ionela (are you a girl or is it a nick?)

Homemade circuit breaker

I'll surelly try to do this circuit breaker.

Waiting for the lightning

Sure, but in a dark and stormy night

Great idea the "double

Great idea the "double protection". Cool stuff guys

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