What is a sidactor, what does it do, and does this circuit need it?

This circuit is used to connect a BASIC STAMP to the telephone line. I know what almost every part does, and where to find almost all the parts, exception the "Sidactor". Could anyone tell me what it does, if the circuit really needs it, and where i can find one or how I can substitute.

UPDATE: Thanks, all who answered.

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Its a clever little device that can very, very quickly clamp excessive voltages present on the phone line, perhaps from nearby lightning strikes or whatever.

Connecting uncertified equipment to the phone line is illegal in most countries, and is often detected by the exchange's line monitoring equipment. ]


If you want to experiment with telephone-connected devices, there are interface boxes specifically made for the purpose -- presumably containing the SIDACtor and possibly other safety features. Buying and using one of those provides you, and the folks maintaining the phone lines, with both physical and legal protection. HIGHLY recommended.
jasen4 years ago
You've got a transformer in the middle of the circuit there and then you have a switch that interrupts the current through the transformer.

A transformer is a type of inductor and when the switch interrupts the current you'll get inductive kick this will produce a voltage spike that could be thousands of volts.
The Zeners on the left of the transformer will eat most of the energy in this spike, but the leakage inductance of the right side of the transformer will still retain some energy, this could damage the insulation in the transformer, and the switch contacts.

If possible I'd be looking to redesign that circuit and use an optocoupler instead of a transformer
kevinhannan6 years ago
here is what you want:


for those too busy to click a link, it says a sidactor is this:

DO-214AA SIDACtor solid state protection devices protect telecommunications equipment
such as modems, line cards, fax machines, and other CPE.
SIDACtor devices are used to enable equipment to meet various regulatory requirements
including GR 1089, ITU K.20, K.21 and K.45, IEC 60950, UL 60950, and TIA-968-A
(formerly known as FCC Part 68).

you can google for other explanatations.

So the short answer is, you might get away with not using one, but the risk is yours as it provides (regulatory) protection.

good luck!
What happens if equipment gets hit by lightning with these devices? Can you check them with an ohmmeter to see if they are good?

Super_Nerd (author)  kevinhannan6 years ago
Two followup questions. Do you know where to buy them?


Will the circuit operate differently with a higher voltage SIDACtor than 270v, I'm assuming no.

I think sidactor is another name for SIDAC, a type of transient voltage suppression device, kind of like a DIAC.

Sidactor(r) is also a brand name of Littlefuse(r). I think this data sheet includes the one you want:

The device itself is a kind of transient voltage suppression device. Such devices have a very high impedance under normal circumstances, but temporarily go into a low impedance break down mode whenever a voltage greater than whatever the rating for that device is.  These things are intended for absorbing high voltage spikes, and the low impedance, conducting, mode only lasts long enough to absorb the energy in the spike.  When current stops flowing, the device returns to its original high impedeance, non-conducting, state.

There are other devices that do the same, or a similar, thing.  e.g. MOV's, gas tubes, etc.