# What happens if you put a BJT in a 12V AC circuit?

#### supak111

##### ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★
Apr 29, 2012
341
What would happen if I were to wire a BJT or some other type of transistor into a circuit which is 12v AC? If the AC is connected between Collector / Emitter?

Wouldn't it just act as a rectified when the BJT is activated?

Oct 5, 2014
6,705

#### Ylli

Jun 19, 2018
399
The collector - base junction would rectify and a half wave rectified voltage would be seen at the base. The polarity of this rectified voltage would be such that the base - emitter junction would remain reverse biased. THis would assume the applied AC voltage did not exceed the Vcbo of the transistor.

#### supak111

##### ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★
Apr 29, 2012
341
Perhaps I should explain more why I ask. I have a 12v AC (low current 50ma) circuit which I'm hoping to be able to control / switch ON and OFF with say an Arduino and BJT.

Don't want to use a relay because it wastes a lot of power. This circuit even though its AC still turns ON if I just short the 2 wires with a simple diode depending on which way I turn the diode. So if I put a BJT in this circuit Collector and Emitter on the 2 wires, and Arduino between Base and Emitter wouldn't this turn ON just the same?

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,470
A photomos "relay" e.g. here is ideal for this application.

#### supak111

##### ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★
Apr 29, 2012
341
Just found some cheap AQH2223, should that work?

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,470
These could work. But: they are thyristor (scr) based and the voltage drop on the output side is >1 V. A MOSFET based semiconductor relay could operate at a much lower voltage drop (considering your expected current).

#### supak111

##### ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★
Apr 29, 2012
341
Ok, Ill give these a shot just because of the price and if not I'll order the other ones

#### bertus

Moderator
Nov 8, 2019
3,123
Hello,

You could try a solid state relays.
On page 8 of the attached datasheet there are small pcb mount versions.

Bertus

#### Attachments

• 0859_solid_state_relays_data_sheet.pdf
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#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,470
You could try a solid state relays.
Slightly overdimensioned, aren't they?
The photomos relays I recommended are far less expensive and smaller.

#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,865
I haven't read any of the datasheets yet on the photomos relays cited by @Harald Kapp, but just the name leads me to believe these are "ideal" interfaces between the real world and any electronic processor. As for the AQH2223, as Harold mentioned these are photo-coupled, zero-crossing, triac switches. If you don't need that functionality, why specify this component? Choosing which component to use based on price instead of functionality is not a rational way to design anything, and it seldom works with electronics. Used auto parts, maybe; electronics, no. This is not to say I haven't found some rare, exotic, or spiffy electronic component and attempted to design some neat-o impressive circuit around it... Burroughs beam-switching electronic-counter tubes come to mind... but almost invariably I discover there is a more appropriate component for the task and only nostalgia leads me on.

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