Triac - am I doing this right.

GICU812

I need to switch 24V AC (furnace control)

I have a opto coupler - 4N35 -·(AKA-SPX7005) http://docs.bgmicro.com/pdf/ics4N35.pdf
I have a Triac - Q6008RH4 -(AKA-Q1257) http://www.teccor.com/data/en/Data_Sheets/E4AlternistorTriac.pdf


Ive looked over the datasheets, checked some examples, but I wanted one last check before I sent 24VAC coursing through my stamp or something.

So please let me know if you see any problems. This will be very slow switching, and on for a long time, if that matters.

Here is my proposed schematic for one circuit

GICU812

My triac seems to be bleeding voltage. I get about 12Mohms across it when it is supposed to be off, 500 ohms when it is supposed to be on. Ive unhooked it and tested resistance across it to eliminate the optocoupler. When in circuit, Im getting about 18volts out of it when its supposed to be off.

KatyBri

Check out http://www.signindustry.com/electric/articles/2000-11_ALDORtriacexplainted.php3·about three-quarters the way down (the opto triac driver circuit).

KatyBri

Here is some more really good basic information-
http://www.epanorama.net/circuits/semiconductor_relays.html#triacuse

GICU812

Well maybe I got the wrong parts, I dont know more than the gritty basics about triacs, but mine show anywhere between 15 an 7 Mohms from M1 to M2, and they are leaking quite a bit of power, something like 13+ volts when connected to 28 volts AC, and the G pin is not activated.

These are actually called alteristors. I dont know, they were 33 cents each, I grabed them to learn with.

LoopyByteloose

It could be the optocoupler or the triac, but not likely both.

Some optocouplers and phototransistors are two wires on the output side and others are three output. The third lead is to allow a negative bias. In that way, the extra lead allows you to fine tune the levels of on and off.

If the triac is not connected to the optocoupler and still leaks, that would be the source of your problem.

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Post Edited (Kramer) : 9/11/2008 3:43:38 PM GMT

PJAllen

1) Incorrect circuit -- won't work.

2) A triac has two states: conducting and not conducting.· When conducting --·it's a low resistance.· When not conducting --·it's a high resistance, but it's not an open circuit.

skylight

They are actually alternistors not alteristors basically a triac, we use them in our dimmer racks at work, yes they do leak like triacs do but if you put enough load on them this will drag the voltage down to zero until it is switched on.

GICU812

What do I· need to change, or what is the problem you see? Thanks



Skylight, what do you mean put enough load, like a big motor? See I'm basically logic switching here, the 24VAC signal just tells the furnace to turn on the fan\cool\heat. So will a triac not work in this case? Should I put a pulldown resistor of some sort on the fan\cool\heat signal lines?



I guess I could just go with relays, I only got the Triacs because they were cheap and I wanted to learn how to use them for the future.

PJ Allen said...

1) Incorrect circuit -- won't work.

2) A triac has two states: conducting and not conducting.· When conducting --·it's a low resistance.· When not conducting --·it's a high resistance, but it's not an open circuit.

LoopyByteloose

Enough load? Would a 120VAC source be better than a 24VAC one? Or is this by power rating of amps times volts?

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It's sunny and warm here. It is always sunny and warm here.... (unless a typhoon blows through).

Tropically, G. Herzog [noparse][[/noparse] 黃鶴 ] in Taiwan

GICU812

I dont follow you, but I have to work with 24vac, its little diffrent that the stamp working with 5v DC, im just logic switching here.

Id still like to know what is wrong with the circuit.

Ive got some realys I guess I could just use them.

skylight

What i mean is you need to draw a minimum current or threshold current in order for the device to switch off cleanly on each zero crossing of the AC wave, the specs will tell you what this current is.

PJAllen

GICU812: "Id still like to know what is wrong with the circuit."

You have a phototransistor on the triac gate.· You are not triggering both alternations ("half-cycles.").

·

KatyBri

Use a traic driver with built in opto-coupling, not an optocoupler. See the liks in my preveious posts. There are examples there to show you how to do it.

LoopyByteloose

KatyBri brings up a good point. There are even devices called SSRs or Solid State Relays that might be better suited for low power 24VAC.

If not, the best choice might be a low voltage relay without any leakage. If the relay requires less than 20ma, if may be driven by the BasicStamp directly. Otherwise, you need to insert a 2n2222 to drive the relay and it may require 6 volts to actually supply 5 volts to the relay.

Apparently Triacs prefer a heavy load to better perform. Since this is not a heavy load or a high voltage, a relay should last a long time. Relay contacts actually last longer with AC than DC.

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It's sunny and warm here. It is always sunny and warm here.... (unless a typhoon blows through).

Tropically, G. Herzog [noparse][[/noparse] 黃鶴 ] in Taiwan