# A basic solid state relay circuit

I have this solid state relay basic circuit: However there are some details I don't understand. What is the reason of putting R3? And if the gate of the BTA06 needs only 50mA as a maximum triggering current, then how could the value of 180 ohm resistor be Ok? It seems calculated as if the maximum current of the MOC3020 optotriac would be 1A and the supply voltage is 120v rms but has it to be the maximum value ? And wouldn't that current destroy the BTA06?

• THe TRIAC operating reduces the voltage on the gate so that it never sees high voltage - and R2 also never dissipates high wattage. R3 prevents false triggering - a much higher value may be OK. – Russell McMahon Nov 28 '19 at 15:24
• but isn't false triggering prevented by an inductor in series with the triac ? – Gh-B Nov 28 '19 at 17:35
• Since the Triac input is like Vbe , isolator leakage must be shunted<1V and R2 is more for the random turn on at peak voltage to absorb a few tens? of microseconds of power at 160Vp until the Triac conducts to clamp the voltage. – Tony Stewart Sunnyskyguy EE75 Nov 28 '19 at 20:38

I answer to your questions following their ordering: let's start from the first.

What is the reason of putting R3?

As stated by Russel McMahon, the resistor $$\R_3\$$ is used to avoid the unwanted turn-on of the TRIAC when the gate is left floating due to stray capacitance coupling between the anodes $$\MT_1\$$ and $$\MT_2\$$ and the gate $$\G\$$. This in turn is due to normal variation of the AC voltage $$\V_{MT_1MT_2}=V_D\$$: if the value of anode voltage variation rises above the limits stated in the BTA06 datasheet, the device may be turned on by a capacitively injected gate current $$I_G\approx C_{jDG}\frac{\mathrm{d}V_D}{\mathrm{d}t}$$ The $$\R_3\$$ resistor shunts away from the gate $$\G\$$ a large part of this spurious gate current, thus preventing unwanted operation: for this reason, I would not rise too much its value, because this may rise the gate sensitivity to anode voltage variations. If you need to avoid this dynamic power dissipation, there are some smart gate triggering circuits involving depletion MOSFETs which may be very useful from this point of view, as the ones described in this TEMIC application note AN901, pp. 4-5. As remarked in the comments, these design choices can improve the performance only for resistive loads as the shown heater, since they can only improve the performance respect to anode voltage variation, as remarked also in the application note.

And if the gate of the BTA06 needs only 50mA as a maximum triggering current, then how could the value of 180 ohm resistor be Ok? It seems calculated as if the maximum current of the MOC3020 optotriac would be 1A and the supply voltage is 120v rms but has it to be the maximum value ? And wouldn't that current destroy the BTA06?

This group of questions requires a joint answer since they are all tied together and tied to the first question. Now, keeping in mind the considerations in the answer to the first question, the low value ($$\180\Omega\$$) of the $$\R_2\$$ resistor is required by the presence of the shunting resistor $$\R_3\$$: in order to get a wanted $$\I_G\$$ of at least $$\50\mathrm{mA}\$$ the optotriac should inject a larger current to the gate node. And this will not damage the MOC3020 nor the BTA06, since $$I_{TS_\mathrm{max}}\simeq\frac{V_{AC_\mathrm{max}}-V_{GT}}{R_2}=\frac{120\cdot\sqrt{2}-1.3}{180}\simeq 0.94\mathrm{A}< I_{TSM}$$ (according to the value of $$\I_{TSM}\$$ stated in the MOC3020 datasheet), and this implies that even the value of the gate current is well below its absolute maximum rating $$I_{G_\mathrm{max}}=I_{TS_\mathrm{max}}-\frac{V_{GT}}{R_3}\approx 0.94 \mathrm{A}- \frac{1.3\mathrm{V}}{56\Omega}\simeq 0.917\mathrm{A}< I_{GM}\:(=4\mathrm{A}).$$

• i understand now everything thank you but still isn't false triggereing prevented by an inductor or by a snubber circuit ? – Gh-B Nov 28 '19 at 17:45
• @Gh-B: I have some references to excellent documentation from Littlefuse and ON Semiconductor at the bottom of an article I wrote on Opto-triacs, solid-state relays (SSR), zero-cross and how they work which are well worth a read. – Transistor Nov 28 '19 at 18:07
• @Gh-B this is only true in case of inductive loads, where the problem is due to the anode current variation and the associated voltage spike. For that loads, a properly designed anode RC circuit prevents spurious firing of the device. However, when the load is an heater as in your circuit and this it is practically resistive, a simple resistor is used for this purpose (see my addition to the answer). – Daniele Tampieri Nov 28 '19 at 18:57
• @Gh-B Why do you keep thinking an inductor would prevent false triggering? It seems like whatever is driving this thought should be cleared up. – DKNguyen Nov 28 '19 at 19:45
• thank you Daniele Tampieri it is clear now i first thought that we use snubber circuits or inductors not resistors – Gh-B Nov 29 '19 at 8:47