4
\$\begingroup\$

Link1: http://ww1.microchip.com/downloads/en/appnotes/00954A.pdf
Link2: http://arduino.cc/playground/Code/ACPhaseControl

Simple version: I want to use a picMicro 5v microcontroller combined with the figure 10 circuit from Link1 (Shown below) and a circuit similar to Link2 for motor control. It has to control 2 AC motors at 4 amps each (but the programming will make sure only one motor can turn on at a time). I can't afford SSR's to handle that (according to digikey) so I'll go with triacs instead. What parts do I need?

schematic

simulate this circuit – Schematic created using CircuitLab

Long version: I'm building a new control board for a pop-up range hood. I plan on combining the figure 10 circuit from Link1 (transformerless power supply with safety considerations) with a phase control circuit similar to Link2. The actual parts from the phase control circuit won't work, as my range hood states a usage of 4 amps. I don't see how the blower can use that much, so I suspect it is a rating if both the blower and the lift are running at the same time, but I can't say for sure, so each of my triacs have to be rated for 4A and I'll just make sure by programming that they aren't driven at the same time (and it will be fuse protected as well). One of the triacs (for the lift) will only be driven full-phase, while the blower will be driven at 3 speeds.

I've done a lot of electrical work, so that's not an issue, and the programming won't be an issue, but I haven't done much electronics work, so reading the numbers are actually giving me the most problems. I'm going to use one of the picMicro chips running at 5v. That means the output pins will be at 5v, right? But the max gate voltage for any of the triacs on digikey are 2.5v. Do I need a voltage dropper to get the output pin voltage to 2.5v? One of the triacs I'm considering is part L6004L3-ND at digikey. Digikey's chart says it uses 3ma gate current, but am I reading the data sheet right that it could draw up to 1.2A at the gate? Also, that triac isn't opto-isolated, but still appears to be isolated to 2500v? If that's the case, do I need an opto-isolator? Basically, I need to know which parts will go together, but I'd also like to know how I'm supposed to read these datasheets and such so that I can figure it out for myself next time.

\$\endgroup\$
1
\$\begingroup\$

I don't think I'm going to answer all your questions, but ...

First, you are talking about unisolated circuits connected to mains. You should assume that any part of the circuit can carry lethal voltages at any time power is available. You say you have done electrical work, so you should already know something about this, but most electrical doesn't require the kind of debugging this circuit might/will require. Be sure you really understand the safety issues before proceeding with this project.

picMicro chips running at 5v. That means the output pins will be at 5v, right? But the max gate voltage for any of the triacs on digikey are 2.5v.

A current-limiting resistor would probably be enough to reduce the drive to something the triac could withstand. But you'll probably want to add opto-isolators anyway (see below), which changes the whole problem.

... it [L6004L3 triac] uses 3ma gate current, but am I reading the data sheet right that it could draw up to 1.2A at the gate?

IGT (gate trigger current) and IH (hold current) are the key parameters. IGT tells you the minimum current needed to trigger the triac into conduction. IH tells you the minimum current needed to keep it in conduction. Also note figure 4, which shows how the trigger current varies with temperature.

The 1.2 A figure is the abosolute maximum peak gate trigger current. It means that if you provide more than 1.2 A, even momentarily, you could damage the device. It's up to you, as the circuit designer, to ensure that you never provide more than 1.2 A. Even better, design in a safety margin and never provide more than, say, 100 mA.

Also, that triac isn't opto-isolated, but still appears to be isolated to 2500v? If that's the case, do I need an opto-isolator?

From my reading of the datasheet, the key indication is the note on pg 57 (the drawing of the "L" package). This says that the package has an "isolated mounting tab". A different package has the tab tied to the center pin (MT2, which would be a hot wire).

I think that means that what is isolated in the L package is the heat sink tab on the back of the package. This would allow you to bolt the part directly to your case and not electrocute anyone who touches the outside of the box. (But I'm just reading between the lines of the datasheet --- do you really want to trust your life to some guy on the internet?)

It doesn't mean there's any isolation between the gate and main terminals and it doesn't mean there's any protection for your digital circuit or for anyone debugging the digital circuit.

My advice is, use an opto-isolator between the digital circuit and the triac.

\$\endgroup\$
0
\$\begingroup\$

Sharp has a range of SSRs that do 6A or 12A, and cost around $10 in singles. I used to get them from Digi-Key. Is that really too much cost? Also, why is a regular, good-old relay not appropriate? High-quality isolated relays with 60 mA coils and 10A/250V rating are $2-$3 in singles. If you're driving that from a microcontroller, add an optocoupler or small MOSFET to drive the coil.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.