Looking to Cross ref a Diac & Triac set

What load and operating voltage?

Generally a 10 amp triac and any 'normal' diac will do. This isn't precision engineering.

Tim, The triac is not critical almost any will work but the diac is one that triggers at 28 volts + or - 4 volts. Ray

They're ancient, that'll be the problem !

Graham

Hmmm, this circuit was drawn in 2005, August 18th to be exact. It's strange that the guy used old parts in the design. It also appears that he has spec'd and drawn a triac, but the pins are marked cathode, anode and gate, like an SCR. I think it should have MT1, MT2 and gate for a triac.

The schematic is here;

- Tim -

Yes, I figured I'd use the parts I have kicking around the shop, and I thought mebbe someone would reconize the parts and tell me the ratings on 'em.

The schematic is here if you are interested in the project;

So is that what the -28 is for? Kinda makes sense. I am planning on using some TO-220 15A triacs I have here. I'll be sure to get a 28v diac tho.

The schematic is here if you are curious as to the application;

Thanks for the info,

- Tim -

Tim,

Is this a diode on the ac input of of this circuit? Also is the load a transformer? If either is true then the circuit will not work. You can not control the primary of a transformer with a triac or two scr's at least over the full range. you can go from zero to aproximately 75% depending on the inductance vs the load and wheather or not the load is resistive or rectified with a cap filter.

Ray

I had intended to use this as a variac to lower the operating voltage to an RCA TV, to allow me to reset it's eeprom. The set uses a switching power supply inside. It's a trick that some guru came up with to solve the eeprom programming issue without a proper chip programmer. Apparently these sets had a problem with this chip getting scrambled due to bad grounds, bad soldering, and power flucuations.

I don't have access to a real variac, nor the funds to buy one, even if one was available locally.

- Tim -

You can with the right design. Fan controllers do it.

Yes,

Fan motors are high slip and less inductive and more resistive. This similar to a transformer with a full resistive load ( pure resistive). The fan motor does not have as much mutual/leakage reactance that looks inductive to the primary as the transformer with a pure resistive load. Ray

On Tue, 17 Apr 2007 20:50:12 GMT, Tim put finger to keyboard and composed:

MPT-28 is a 28V diac:

I don't see a MAC2-4 in either my 1984 Motorola catalogue or my NTE

1990-91 catalogue, but the "-4" appears to refer to a 200V device. FWIW, the MAC20-4 is a 200V, 15A triac.

- Franc Zabkar

--
Please remove one 'i' from my address when replying by email.

There is a very good circuit for controlling the primary of any inductive load. It consists of a switch in series with the ac input to the transformer that pwm's the load. During the off time a second switch turns on in parallel with the load. Each switch consists of fets or IGBT's either in anti series or across the output ( + and - ) of a 4 diode bridge( fast diodes ) . The fets/igbts are transformer driven ( both switches are driven with the same transformer ) This will prevent the two switches from conducting at the same time ( shoot through ). The output can be filtered so the output looks like the output of a variac. The use of scr/triacs can not be filtered very well and reek havoc with the switchers with cap input filters. Ray

I have a 1979 mot catalog that does not list the MAC2-4 ----I have looked in a 1981 D.A.T.A. BOOK with no results. The lowest numbers for the MAC is MAC15-4. Ray

It mnay simply be what he had in his box of bits !

It should.

They are non-critical as previously mentioned by others. Choose a triac to suit your load. It's basically a triac based dimemr. Check any other similar designs online for comparison.

Did you spot the deliberate mistake btw ?

Graham

That may not work very well actually.

Graham

Deliberate mistake? In the drawing? I can't see anything obvious...

- Tim -

I don't think it's a deliberate mistake. I suspect it is just poor drawing of what is supposed to be the output connector.

Ed

I got the TV set re-programmed without needing the variac thing. I pulled the chip and re-programmed it in my eprom programmer using data from a fellow tech on the net.

I did not need to build the circuit after all, but I appreciate everyone's input on the project.

- Tim -