I need a triac but all I have in my junk box is a pile of these:
formatting link
I've read in various books that a triac is just two of these devices wired back to back, but is that strictly true? Is it possible to make up a triac from two SCRs that will perform as well as a discrete triac?
I'm really short of time on this project and don't want to have to order a triac if I can simply nail two of these other things together and achieve the same result.
It's a light load, btw (a 4k ohm exterior telephone bell which requires
Your problem is that the two SCR's back-to-back don't have a common-potential GATE, each will have to be triggered relative to its own cathode.
You can probably avoid that with transformers for each gate, but you probably don't have transformers on hand, do you ?>:-} ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I'm looking for work... see my website.
Got *hundreds* of them! Particularly if you mean the little audio matching transformers that come less than one cubic inch in overall size
- the ones you used to find in old transistor radios. Would they suffice?
Still not sure what you mean, though. Are you saying the fact that the proper triacs are 'carved' out of a single chunk of multi-layer semiconductor means they don't suffer from the problem identified?
Maybe. Probably won't support the proper risetime.
Yep. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I'm looking for work... see my website.
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I'm looking for work... see my website.
Connect the AC leads of the bridge in the AC line as you would the TRIAC. Connect the SCR across the DC lines of the bride in the appropriate polarity to conduct the current when triggered.
On each phase of the line the bridge will steer the current through the SCR in the correct direction for it to work.
Not STRICTLY I believe. I mean, that is not what is in the package, however the operation of the device is the same or similar.
I think to emulate it properly you would need one anode fried and one catho de fired. Damn this stuff is old to me, I don't remember for sure but I thi nk the cathode fired is more common and easier to make, like an NPN regular transistor.
The transformers will solve that problem as someone already said I think. B ut that adds complexity to the whole thing you don't really need.
I almost wanted to say like, don't you have a junk microwave oven around th at might have one, but now that I think of it I would be hard pressed to fi nd one. And I don't want to give up the microwave, it is a really good one, and I mean noticably better than most.
Some TVs had triacs but most had relays. Speaking of which a relay won't wo rk for you ? So you are modulating it by phase ? If so, that makes the tran sformers a bit of a problem.
What else might have a triac in it these days ? Certain things that still u se big iron for transformers maybe, but even most of them are relays.
You might just have to bite the bullet and give Digikey three bucks.
Sorry, I missed that part. Hell for that you can just use a bridge and a bi polar. You're only talking 60 mA. A typical bridge with 1N4007s and a 500 v olt 1 amp transistor would be fine.
But you have another problem. Now matter what you have to find a way to iso late it. Phone wires, well it might be named different there but phone wire s are class 2 I think ad mains are class 1. (?) Code, which is probably sim ilar does not even allow you to mix the two in a conduit. You are having bo th in a circuit there has to be reliable isolation between the hot side and the cold side otherwise someone down the street could answer their phone i n the basement ad get electricuted.
If you got the right transformer that won't overload the phone line (capaci tively coupled of course, if the phones there work like they do here) then you hook to the high impedance side and then use a rectifier ad filter on t he secondary to drive the base of the transistor, or gate of some sort of t hrysistor.
And if you use an SCR with a bridge you'll have to make sure it turns off. I don't know which exact type of SCRs you have but it looks like some of th em get down to 70 uA holding current. You'll need a resistor, but whatever value that is it is going to have to withstand 320 volts without dissipatin g ridiculous power. Plus some devices in the family are only rated for 200 volts, which is no good.
No, you can't trust the isolation unless they're made for it. If your doodad puts mains voltage to the phoe line, screws up teclo equipment and they can prove it they'll be all over your ass.
Although, thinking about it, it might not turn off, depending on holding current... since there will be no reverse voltage. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I'm looking for work... see my website.
Not necessarily... but I presume this is at 50/60Hz (since the OP is trying to emulate a TRIAC) so it should be OK. (It's a swept charge removal timing thing.) ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I'm looking for work... see my website.
I thought about that for a moment and realized the voltage drop across the diodes gives a brief deadband where there is virtually *zero* current guaranteed. I don't know how this would *not* turn off the SCR... unless the load were inductive.
Sounds like an inductive load so it's a valid concern. A triac may not turn off without a snubber with such a load, but at least triacs tend to be ornery with large trigger/holding currents. The thing the OP has is the son of the late GE C106B1 SCR, which has notoriously low trigger and holding current (like 20uA typically) so it doesn't take much to keep it on.
The original horrible C106 package looked like this: The leads are 0.05" wide nominally.
The diodes he's likely to use will take away another 5-10usec of the zero crossing with their sloth-like turn-off characterists so it could get dicey, especially at high temperatures.
Alternistors are better- more like back-to-back SCRs in their commutation behavior.
It might be possible to use an MOC3021 type opto directly, it's only
60mA, but the turn-off problem exists and I don't think they're available with higher than 400V PIV rating which is pretty marginal.
Another trick is to use a small mains transformer primary in series with the load, say 24VAC, and connect a bridge rectifier and MOSFET or whatever on the secondary. Turn the MOSFET on and the series reflected impedance drops greatly. THe MOSFET and diodes have to handle proportionally more current, of course.
--sp
--
Best regards,
Spehro Pefhany
Amazon link for AoE 3rd Edition: http://tinyurl.com/ntrpwu8
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.