Complementary device to Thyristor

A long intro:
I want to make an on-off timer for Christmas lights. To keep it small, cheap and simple I plan to use ATTiny13A, use mains frequency as the time source and use resistive transformerless power supply (should be safe - no accessible parts, just 6 hours on and 18 hours off, just as some Chinese lights do). There comes the problem: How to switch the load (up to 100 mA)?
With 220 V of RMS mains voltage 1 mA of average current in the low voltage circuit means about 1/4 W of power dissipation in the current limiting resistor(s). That excludes relays with continuous tens of miliamps needed.
MOSFETs are also out of option because all in our local shop able to withstand 400 V need 10 V gate voltage - it would make the circuit much more complicated.
BJTs need too much base current.
Triacs look better but the smallest one still need 3 mA of Gate current to fire. Since I want to control also capacitive load (LED bulbs) which have nearly unpredictable current draw it is not as simple as one pulse just after zero crossing. Ideally I would apply the Gate current for the first half of each half-cycle (from ZC to maximum) to fire reliably. That is too much current.
There comes what I believe is a trap: a triac equivalent schematic is presented like this (this picture is from here):
power-power19.gif
Firing current for small Thyristors is an order of magnitude smaller - 200 uA maximum. Perfect! Except for the equivalent schematic is far from equivalent IMHO. I don't see any way how to control Gates of Thyristors like this. I think they must be driven with separate circuits and the voltage on the Gates must be full mains voltage apart when they are both off.

And the question:
Is there a part complementary to the Thyristor - that may be switched with low voltage with respect to the Anode instead of Cathode? I would expect such device would be useful.
If not is there some simple circuit to drive two antiparallel Thyristors? Or is it better to use a Triac after all?

power-power19.gif

Smajdalf:
...and use resistive transformerless power supply (should be safe...

Make that a capacitive power supply.
Then 5volt/~10mA is not a problem.

G3MB-202P solid state relays are cheap, and can do 0.1-2Amp.
Most of them are zero-crossing (no mains noise injection), and require about 5mA LED drive current.
Leo..

Smajdalf:
Is there a part complementary to the Thyristor - that may be switched with low voltage with respect to the Anode instead of Cathode? I would expect such device would be useful.

It is called - a Triac!

A Triac has no anode or cathode, they are interchangeable. The gate is driven with respect to MT1. Note that there are three and four-quadrant Triacs.

Note also that the second "equivalent" in that illustration is complete nonsense.

Anode-gated thyristors do exist, AKA "PUT"s but are generally low-voltage devices.

Use a triac, you trigger them with a short pulse of current into the gate to MT1, so the common 0V of your circuit is connected to MT1. Load on MT2. If it's mains then DO NOT connect 0V to protective ground.

Once triggered you can remove the current from the gate until the next half cycle when you want to trigger again.

What's wrong with a regular light timer?

and use resistive transformerless power supply (should be safe - no accessible parts, just 6 hours on and 18 hours off, just as some Chinese lights do.

The danger is that you'll probably be touching the microcontroller and connecting it to your computer during testing & development. The low & high voltage electronics should be isolated.

...The first time I built a TRIAC dimmer it tested it with a 12V transformer and 12V bulb before connecting the power line, even though I was using an opto-isolator,

With this SSR, you’ll need only 0.8mA typical to activate, 5mA to 1A load @ 260Vrms max.

I'd go with Dlloyd - for safety just use an SSR

Hi,

use resistive transformerless power supply

Why when you can use a Mains USB power pack, then your control circuit power requirements will be irrelevant and safe.

Tom... :slight_smile: