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Use Triac as SCR?

B

Bill Bowden

Jan 1, 1970
0
I'm thinking about reworking a 120VAC lamp dimmer so I don't have to
turn the brightness to max to turn it off. I would rather turn the
control to minimum and have the light go completely out and eliminate
the switch.

The dimmer installed turns on at full brightness and doesn't quite go
all the off at the minimum setting. There is an adjustment to set the
low level, but it's not very stable.

This circuit from an old 1960s book uses a full wave bridge and SCR and
can be adjusted to minimum so the light goes completely out. But I
would like to use the existing triac and just add the bridge and other
parts.

http://ourworld.compuserve.com/homepages/Bill_Bowden/page4.htm#acdimmer.gif

Is there any problem substituting the triac for the SCR?

-Bill
 
A

Andrew Holme

Jan 1, 1970
0
Bill said:
I'm thinking about reworking a 120VAC lamp dimmer so I don't have to
turn the brightness to max to turn it off. I would rather turn the
control to minimum and have the light go completely out and eliminate
the switch.

The dimmer installed turns on at full brightness and doesn't quite go
all the off at the minimum setting. There is an adjustment to set the
low level, but it's not very stable.

This circuit from an old 1960s book uses a full wave bridge and SCR
and can be adjusted to minimum so the light goes completely out. But I
would like to use the existing triac and just add the bridge and other
parts.

http://ourworld.compuserve.com/homepages/Bill_Bowden/page4.htm#acdimmer.gif

Is there any problem substituting the triac for the SCR?

-Bill

You probably could substitute a triac for the SCR, but that circuit is not
very green - it constantly wastes 7W in the 2K resistor, and "needs some
ventilation."

Can't you just reverse the outside connections on your on/off potentiometer
so it starts from minimum brightness?
 
C

Chris

Jan 1, 1970
0
Bill said:
I'm thinking about reworking a 120VAC lamp dimmer so I don't have to
turn the brightness to max to turn it off. I would rather turn the
control to minimum and have the light go completely out and eliminate
the switch.

The dimmer installed turns on at full brightness and doesn't quite go
all the off at the minimum setting. There is an adjustment to set the
low level, but it's not very stable.

This circuit from an old 1960s book uses a full wave bridge and SCR and
can be adjusted to minimum so the light goes completely out. But I
would like to use the existing triac and just add the bridge and other
parts.

http://ourworld.compuserve.com/homepages/Bill_Bowden/page4.htm#acdimmer.gif

Is there any problem substituting the triac for the SCR?

-Bill

Good morning, Bill. I'm not sure what you've got going with the dimmer
(I'm assuming it's a store-bought item, and most of them work the other
way, "off" being on the dim side of the dimmer pot).

I'm also not too sure you'll be happy with the circuit shown in the
link. There are a couple of problems here, which might suggest a
better circuit.

First, according to the text accompanying the schematic, the SCR will
trigger when the cap reaches about 8V. This will mean the SCR gate is
triggered by about 4 or 5mA of current (through the 1.5K resistor).
This is enough for some sensitive gate SCRs, and there are sensitive
gate triacs which will do this job (I would think something like the
Teccor/Littelfuse L6004L3 would work, 4A(RMS)@600V, I(GT) = 3mA max in
Quadrant 1), but if you want something beefier than that, I'm not sure.
I'm sure you would also like a circuit that uses the triac you already
have, which certainly isn't a sensitive gate triac.

Second, the triggering voltage itself is not going to be very
consistent. You can visualize it working well on the first half-cycle,
but the question is, what happens after that. There's no mechanism to
discharge the cap every cycle or every half-cycle to ensure consistent
triggering.

Third, the circuit itself is somewhat uneconomical, especially at
higher currents. You have to pay for power diodes that will conduct
the current of the load, just in order to get the benefit of control at
low voltages without "snap-on hysteresis" (that's the effect you're
seeing that's so frustrating when you turn the lamp dimmer down).
Power diodes aren't a big problem with a 60 watt lamp (you can just use
1N4003+), but at 20 amps, it kind of whispers in your ear that there
must be a better way.

There is a better way. I first saw this trick in the GE SCR Manual,
4th edition (another "old 60's book"), and it's just as good today.
Not only that, but you can probably scrounge the triac, diac and the
cap in your lamp dimmer, reducing your cost. Look at page 4 of Teccor
AN-1003, "Phase Control Using Thyristors":

http://www.littelfuse.com/data/en/Application_Notes/AN1003.pdf

and you'll see two little trick circuits on the lower right corner of
the page that'll do the job quite nicely. The key is that they
discharge the cap, leading to more precise control of load voltage at
the low end. Circuit a) in figure 12 is just the ticket for lamp
loads, because the two diodes only discharge the cap on the positive
half-cycle. It only costs two diodes and a 1/2 watt resistor. This
puts a major DC component into the lamp load, but there's no problem
with that. If you're thinking about running something that will not
like a DC component, try circuit b). It only costs two more diodes and
another 1/2 watt resistor. And all these diodes are only responsible
for discharging the cap -- they don't have to carry the load current.

Just for grins and giggles, I'd modify the app note circuit by adding a
Quencharc snubber across the lamp load or triac, and also put a high
current torroidal choke (you can scrounge these from PC power supplies)
in series with the works, to minimize the possibility of RFI problems.
You might also have the choke in the lamp dimmer -- look there, first.

You may want to consider having your switch in the circuit, just for
safety purposes, if nothing else. When you turn the pot up to maximum
resistance, the load is "off", but it's still "hot". This could make
someone very unhappy if they're, say, replacing the bulb. Be sure to
follow local code, and be careful about installing home brew line
voltage stuff inside the walls without consulting someone who knows
what they're doing. And of course, use appropriate safety precautions
during construction and troubleshooting.

I hope this has been of use to you.

Good luck
Chris
 
B

Bill Bowden

Jan 1, 1970
0
You probably could substitute a triac for the SCR, but that circuit is not
very green - it constantly wastes 7W in the 2K resistor, and "needs some
ventilation."

You may be looking at the wrong diagram. I don't see any 2K resistor.
It's the drawing titled "120VAC Lamp Dimmer"
Can't you just reverse the outside connections on your on/off potentiometer
so it starts from minimum brightness?

Yes, thought about doing that, but I want to eliminate the switch and
have the light go completely out at the extreme dim setting. And it
won't do that, because the phase needs to be exactly 180 degrees, and
it isn't stable enough to maintain 180.

-Bill
 
B

Bill Bowden

Jan 1, 1970
0
I'm also not too sure you'll be happy with the circuit shown in the
link. There are a couple of problems here, which might suggest a
better circuit.
First, according to the text accompanying the schematic, the SCR will
trigger when the cap reaches about 8V. This will mean the SCR gate is
triggered by about 4 or 5mA of current (through the 1.5K resistor).
This is enough for some sensitive gate SCRs,

Actually, the 1.5k resistor just establishes the trigger level. The two
transistors will conduct at the trigger level and then discharge the
cap through the gate at more than 20mA. The cap will discharge to well
below the trigger level before the transistors turn off.
Second, the triggering voltage itself is not going to be very
consistent. You can visualize it working well on the first half-cycle,
but the question is, what happens after that. There's no mechanism to
discharge the cap every cycle or every half-cycle to ensure consistent
triggering.

The triggering voltage should be consistent since the mechanism to
discharge the cap on every cycle is the dual transistor arrangement
that acts like a diac and becomes a very low resistance when triggered.
I think it's called a SCS (silicon controlled switch).

It works well, I tried it with a 25 amp SCR with good results.

-Bill
 
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