Help needed with 'back to back' scr's

[Rekcirednos]

I am making a 1kw microprocessor controlled lamp dimmer that uses two
back to back scr's. I am having trouble finding a way to trigger them,
especially the one that switches the negative half of the ac sinusoid.
I have to use dual scr's in this design.

could anyone point me to a schematic or verbally tell me how to do
this?

any help would be greatly appreciated.

 

[Jim Thompson]

I am making a 1kw microprocessor controlled lamp dimmer that uses two
back to back scr's. I am having trouble finding a way to trigger them,
especially the one that switches the negative half of the ac sinusoid.
I have to use dual scr's in this design.

could anyone point me to a schematic or verbally tell me how to do
this?

any help would be greatly appreciated.

WHY do you "have to use dual scr's in this design"?

Use a TRIAC instead... it behaves as back-to-back SCRs, but is much
easier to trigger.

...Jim Thompson

 

[CFoley1064]

Subject: Re: Help needed with 'back to back' scr's

From: Jim Thompson [email protected]
Date: 9/13/2004 12:44 PM Central Daylight Time
Message-id: <[email protected]>



WHY do you "have to use dual scr's in this design"?

Use a TRIAC instead... it behaves as back-to-back SCRs, but is much
easier to trigger.

...Jim Thompson

Another homework assignment (sigh).

For a lamp load, a TRIAC is the obvious choice. Back-to-back SCRs are usually
used in switching inductive loads, where the current and voltage are out of
phase, meaning that the TRIAC won't turn off at voltage zero crossing. Since
B2B SCRs have only one thyristor conducting at a time, you can step past this
problem.

But, since "teach" specified B2B SCRs, you might have wanted to try a Google
search on "Back to back SCR" schematic. Page one of search results would have
showed:

http://www.web-ee.com/Electronic-Projects/data/moc3041.pdf

Look at page 4 of this appnote to get an idea of what to do. Note that the
opto specified, the MOC3041, is a "zero-crossing" opto-TRIAC, which means it
won't do for a lamp dimmer. You need phase control, meaning you have to turn
on the SCR in the middle of the AC cycle. We'll leave the voyage of discovery
to determine the right opto-TRIAC for the job to the OP -- he needs some
"ownership" of this design, I suppose. As a hint, I suppose I could say that
almost all of the alphanumeric characters of the IC you want are correct.
(sigh).

As part of your education, also try:

http://www.google.com/help/basics.html

Chris

 

[Spehro Pefhany]

I am making a 1kw microprocessor controlled lamp dimmer that uses two
back to back scr's. I am having trouble finding a way to trigger them,
especially the one that switches the negative half of the ac sinusoid.
I have to use dual scr's in this design.

could anyone point me to a schematic or verbally tell me how to do
this?

any help would be greatly appreciated.

You can connect an opto-triac between the gates of the SCRs. Or use a
dual-secondary pulse transformer with zero voltage or picket-fence
firing. Or use an alternistor. Or use a triac, perhaps with snubbing.


Best regards,
Spehro Pefhany

 

[Ken Smith]

I am making a 1kw microprocessor controlled lamp dimmer that uses two
back to back scr's. I am having trouble finding a way to trigger them,
especially the one that switches the negative half of the ac sinusoid.
I have to use dual scr's in this design.

could anyone point me to a schematic or verbally tell me how to do
this?

Get an anode gated SCR.

 

[Dana Raymond]

I am making a 1kw microprocessor controlled lamp dimmer that uses two
back to back scr's. I am having trouble finding a way to trigger them,
especially the one that switches the negative half of the ac sinusoid.
I have to use dual scr's in this design.

could anyone point me to a schematic or verbally tell me how to do
this?

any help would be greatly appreciated.

One way would be to use a fet output optocoupler.
HTH

 

[Bill Bowden]

I am making a 1kw microprocessor controlled lamp dimmer that uses two
back to back scr's. I am having trouble finding a way to trigger them,
especially the one that switches the negative half of the ac sinusoid.
I have to use dual scr's in this design.

could anyone point me to a schematic or verbally tell me how to do
this?

any help would be greatly appreciated.

I made a 1KW SCR controlled space heater. Schematic is
here:

http://ourworld.compuserve.com/homepages/Bill_Bowden/page2.htm#therm1.gif

It uses a small trigger transformer (3 windings) to gate 2 back to
back SCRs.

-Bill

 

[john jardine]

I am making a 1kw microprocessor controlled lamp dimmer that uses two
back to back scr's. I am having trouble finding a way to trigger them,
especially the one that switches the negative half of the ac sinusoid.
I have to use dual scr's in this design.

could anyone point me to a schematic or verbally tell me how to do
this?

any help would be greatly appreciated.

Years ago wasted a whole hour figuring it out from scratch :-(
It's simply a case of diode steering.

Supply
|
|
.-o.
Diode | |
___ .--------o-.|
Drive -|___|-.-->|-|Gate - ||
| | ^ || Thyristor #1
| '--------o-'|
| | |
| | |
| .--------|--o-.
'--|<-|Gate | V | Thyristor #2
| | - |
Diode '--------|--o-'
| |
| |
'-o-
|
|
Load

created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

 

[martin griffith]

I made a 1KW SCR controlled space heater. Schematic is
here:

http://ourworld.compuserve.com/homepages/Bill_Bowden/page2.htm#therm1.gif

It uses a small trigger transformer (3 windings) to gate 2 back to
back SCRs.

-Bill

Hi Bill,
Nice little site!
What software do you use for doing the circuit diagrams?





martin

Serious error.
All shortcuts have disappeared.
Screen. Mind. Both are blank.

 

[Bill Bowden]

Hi Bill,
Nice little site!
What software do you use for doing the circuit diagrams?

Hi Martin,

The drawings are done by hand using a pixel editor similar
to Windows Paint. It was time consuming at first, but now
I have a good supply of templates for new drawings. I can
usually find something close to what's needed and just move
things around, and reconnect the lines.

-Bill

 

[John Woodgate]

I read in sci.electronics.design that martin griffith
[email protected]>) about 'Help needed with 'back to back' scr's', on Tue, 14
Sep 2004:

Nice little site!

Indeed. I have caused consternation and incredulity by describing as a
'passive amplifier' a version of the 'long loopstick' using a frame
antenna instead of the loopstick.