Resistor Size

[ibwev]

My supply voltage is 5 volts. The attached file is the static characteristic of TRIAC 3-QUADRANT SOT-223 (**broken link removed**

Ohm's Law is R=V/I

1) Does this mean I can solve for resistor size by
Needed Resistor Size = Voltage Supply / holding current
= 5V / 0.01A
= 500 Ω

2) If I used a 500Ω resistor, would the Gate Trigger Voltage be in an acceptable range (typically 0.9V)?

 

[Mike odom]

I believe you are confusing gate current with hold current... hold current is the current going through the triac (T1-T2) ie, your load current, which keeps it on once triggered until the load current drops below the hold current. Look at the figures referenced by the specifications (I can't because you didn't include them, or give the triac part number).

 

[Mike odom]

2) If I used a 500Ω resistor, would the Gate Trigger Voltage be in an acceptable range (typically 0.9V)?

The max is listed as 2V. You have to look at not just the typical, but design for ALL conditions, that's why they list the max and min.

 

[Mike odom]

The gate current would be 5v - Vswitch (.7v for transistor) - max gate voltage, then divided by the gate resistor, 5 - .7 - 2 = 2.3V. Set gate current for 10mA (twice the max listed), 2.3V/.01 = 230 ohm. If the triac falls in the "typical" range (Vgate = .9V), then the gate current = 5-.9-.7 / 230 = 3.4/230 = 14mA... just make sure this does not exceed max gate current (I use a triac rated at 60mA max gate current, and drive 7mA into it).

Or Voh (for micro Voutput high @ 10mA) - Vgate max = 3.3V - 2.0V = 1.3V. 1.3V / .01 = 130 ohms. If Voh approaches 5V, and Vgate = typical, then 5-.9 = 4.1V.... 4.1V / 130 ohm = 31.5mA. Make sure this does not exceed max gate current, which is not listed in the data sheet you provided. (do not confuse with max gate trigger current). NOTE: ATMEGA8 controller will source 40mA, most PICs will only source 20mA, except on the GPIO on the PIC12F683 which will source 90mA. This resistor may need to be adjusted so max output port pin SOURCE current is not exceeded. Read your microcontroller electrical specifications CAREFULLY.

 

[ibwev]

I'd like to send you a Christmas card. Thanks again for all the help.

 

[dr pepper]

I just looked at a project I built way back with 4 tic226 triacs that are fired from the 5v supply, they use 220r resistors, so that would concur with mikes calcs.
I probably didnt calculate the values, I'dve just nicked part of the schematic from someone else's project.

 

[ibwev]

Or Voh (for micro Voutput high @ 10mA) - Vgate max = 3.3V - 2.0V = 1.3V. 1.3V / .01 = 130 ohms. ........... most PICs will only source 20mA, except on the GPIO on the PIC12F683 which will source 90mA. This resistor may need to be adjusted so max output port pin SOURCE current is not exceeded. Read your microcontroller electrical specifications CAREFULLY.

I am using PIC16F690 (https://www.electro-tech-online.com/custompdfs/2012/12/41262c.pdf).

How do I calculate Voh?

 

[Mike odom]

I am using PIC16F690
How do I calculate Voh?

Even though the absolute max ratings say the part will source 20mA, they spec it at just 3mA and Vdd at 4.5V, and it would be Vdd - .7V. Any time you go over the spec'd value, the level is not guaranteed, but it should be close. I would use 4.0V.



4.0V - 2V = 2.0V. 2.0V / .01 = 200 Ohm. 2.0V / 220 ohm = 8.18mA. 4.0V - .9V = 3.1V. 3.1V / 220 ohm = 14.1mA.
Since the Voh will go down as Ioh goes up, your current will be between 8mA and 14mA, so the 220 ohm would be a very good value to choose.

 

[Mike odom]

I probably didnt calculate the values, I'dve just nicked part of the schematic from someone else's project.

A very sound design methodology.

 

[dr pepper]

Yes I think so why mess about when someone else has allready done it.
The only thing is you need to know what your copying isnt pants.