Triac vs. FET

[Ken H.]

Wanted to ask if a small triac can be used as a bilateral switch replacement. Correct me if I am wrong, but a FET would not work because it is not bilateral.

Would 4 small triacs make up a CD4016 (in effect)?

[Carl]

Re: Triac vs. FET

No, triacs have a dead zone. When turned on, they behave like 2 parallel diodes, one forward & one reverse.

Yes, you can use a single FET as a switch, as long as the voltages between the gate & drain/source are such that it stays on. A bilateral switch (4016 or 4066) is nothing but 2 FETs in parallel, one NMOS and one PMOS. One of them is always "on" regardless of where the I/O voltages are.

- Carl

[Ken H.]

Re: Triac vs. FET

Thanks for the info Carl. I had tried two FETs (one with the S-D reversed) and that did not work....did not think of using the N and P channel together.

This was for the Garrett TR I am working on. Finally tracked down the audio problem to a missing signal voltage from the Rx coil to a dual op amp, through the 4016 switch. After posting, I see that all 4 sections of the 4016 are out also, so will just replace it.

Hated to order just a 4016, but then though it would be a good time to order the IC's for your Hammerhead plans.

[Carl]

Re: Triac vs. FET

The 4000 series chips, and any other CMOS chips (74Cxx, 74ACxx, 74HCxx, etc), are all static sensitive, and can be zapped relatively easily, esp. when not soldered down. So handle with care, esp. in winter with the drier air.

It is rather unusual for them to die once they're soldered onto a circuit board. If you don't have a desoldering tool, gently clip all the leads of the bad part, remove the plastic body, then desolder each pin. Solder braid is wonderful stuff for this.

- Carl