Triac & DC Control

[gaut...@my-deja.com]

Hi Guys,
Does any one know how one can control a Traic using
DC current, without any of these expensive and hard
to get Optocouplers (MOC's). To be used just to turn on
and off the triac with DC control of the gate, should be
simple and easy to make something similar to the Light Dimmers
available everywhere.
Is this very elementary or am I missing something.
Thanks ahead for the help
Gautham Nalamada

Sent via Deja.com http://www.deja.com/
Before you buy.

[Rene Tschaggelar]

You may have forgotten that one of the pins is hot, meaning there is
line voltage. You can use an impulse transformer to trigger the triac.
Just don't have some electronic on the line voltage.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com

[Robert]

The most economical technique is to use the DC voltage to gate on
a high frequency astable oscillator and then capacitively couple
this to the gate. An astable frequency of 100KHz and coupling
capacitance of 1nF with a resistive pulldown from Gate to MT1 is
often used. The trigger circuit should also be referenced to MT1.
The resistor must be low enough in value so that the maximum gate
voltage due to leakage current remains below the minimum trigger
threshold voltage.
Another technique is the use of a reed relay. In this case, the
relay contacts are placed in series with a current limiting
resistor between MT2 and Gate.

[Patrick Timlin]

gaut...@my-deja.com wrote:
> Does any one know how one can control a Traic using
> DC current, without any of these expensive and hard
> to get Optocouplers (MOC's).

Expensive and hard to get? 30 cents to a buck or so and you can get them
anywhere practically.

> To be used just to turn on and off the triac with DC control of the
> gate, should be simple and easy to make something similar to the Light
> Dimmers available everywhere.
> Is this very elementary or am I missing something.

Yes you are. Triacs can not be turned off per se, but instead they turn
themselves off when the current going thru them goes to zero. So they are
usually used in AC applications. Either you just hold them ON, then
remove the signal and they turn themselves off when the AC current
through them goes to zero. With light dimmers, what you are doing is
chopping the AC wave. You keep turning it on at a certain point in the
wave form, and the triac turns itself off when it passes through zero,
then you turn it on again. Since this happens MANY times per second, the
light looks dimmer to us since power is only applied for part of the
time.

Now you didn't specify, but if you try and use a triac to turn on and off
a DC load, one of the first things you learn is that one you turn it on
is STAYS on until you cut the DC power. <grin> So unless your load is
either an AC load or a DC load that is turned off somehow after you
activate it, a TRIAC won't work for you.

--
Patrick Timlin --- pti...@yahoo.com
http://www.geocities.com/ptimlin/

[JW]

Ummmm, you mean a.c. control.

The optocouplers are really inexpensive - from Jameco - I believe
MOC3010 but you won't find the data sheet on Motorola's website anymore.

gaut...@my-deja.com wrote:

> Hi Guys,

> Does any one know how one can control a Traic using
> DC current, without any of these expensive and hard

> to get Optocouplers (MOC's). To be used just to turn on

> and off the triac with DC control of the gate, should be
> simple and easy to make something similar to the Light Dimmers
> available everywhere.
> Is this very elementary or am I missing something.

> Thanks ahead for the help
> Gautham Nalamada
>

[igla...@my-deja.com]

Hi,

here anotehr option with Scr instead of Triac (not the cheapest but
all depend what you need to do) :

use 2 scr which connected as followed :

Scr1 Anoda to R1 (your load)
Scr2 Anode to R2 (dummy resistor)
Scr1 Anode to C1 (non-electrolitic type)
Scr2 Anode to C1 (second "leg")
both Scr catode togther tie to the (-)
both R second "leg" connected to the (+)
the gate of Scr1 throu a push-button connect to the (-) and this will
be the "ON" button and the same with Scr2 which will be the "OFF" button

IF the dummy resistor is choosen such as it's resistance is "big"
enough than it not need to be "big" as the dissipation power on it will
be very small.

the above circuit work in general as followed:
you first press the OFF push button than the current charge the
capacitor and once it is charged than the current go below Ihold and
the Scr are shut off (or if the resistor R2 is not big enough the
current keep going throuh it, the resistor can be a led or any other
load, of course if this is inductive load don't forget to use diode in
parallel).
Than when you press the ON push button incase Scr2 is still on the Cap
potential "appear" on Scr2 thrugh Scr1 in reverse polarety causing it
to shut off and while the capacitore is descharge and re-charge in the
reverse polarity the current keep going through the load and you get
your "on state".
when you want to shut off press the off push button and similar effect
will cause Scr1 to be closed and so on.

Have a nice day

Illan

In article <8c3od8$96b$1...@nnrp1.deja.com>,
Patrick Timlin <pti...@yahoo.com> wrote:

> gaut...@my-deja.com wrote:
> > Does any one know how one can control a Traic using
> > DC current, without any of these expensive and hard
> > to get Optocouplers (MOC's).
>

> Expensive and hard to get? 30 cents to a buck or so and you can get
them
> anywhere practically.
>

> > To be used just to turn on and off the triac with DC control of the
> > gate, should be simple and easy to make something similar to the
Light
> > Dimmers available everywhere.
> > Is this very elementary or am I missing something.
>

> Yes you are. Triacs can not be turned off per se, but instead they
turn
> themselves off when the current going thru them goes to zero. So they
are
> usually used in AC applications. Either you just hold them ON, then
> remove the signal and they turn themselves off when the AC current
> through them goes to zero. With light dimmers, what you are doing is
> chopping the AC wave. You keep turning it on at a certain point in the
> wave form, and the triac turns itself off when it passes through zero,
> then you turn it on again. Since this happens MANY times per second,
the
> light looks dimmer to us since power is only applied for part of the
> time.
>

> Now you didn't specify, but if you try and use a triac to turn on and
off

> a DC load, one of the first things you learn is that one you turn it
on
> is STAYS on until you cut the DC power. <grin> So unless your load is
> either an AC load or a DC load that is turned off somehow after you
> activate it, a TRIAC won't work for you.
>
> --
> Patrick Timlin --- pti...@yahoo.com
> http://www.geocities.com/ptimlin/
>

[gaut...@my-deja.com]

Thanks guys for all the suggestions given, but let me clarify
my problem in more detail.
I am trying to develop a solid state on-off switch, which will
either turn on or off the AC Power under certain conditions. This
I am doing by logic control of the MOC, which in turn
controls the Triac. The problem area here being, the MOC is the most
expensive component of the circuit, more than the Triac and delivery
times of most suppliers is about 12 weeks.
So I was wondering if I can take a light dimmer circuit, eliminate
the Preset, change the gate current to DC and control it from my logic
circuit. This circuit doesnot seem to be working, any help.
In the overall circuit, MOC is very expensive, hence I was looking
for other alternatives.
Any help anybody.

Gautham Nalamada

[John Woodgate]

<8cc48l$2oj$1...@nnrp1.deja.com>, gaut...@my-deja.com inimitably wrote:
>Thanks guys for all the suggestions given, but let me clarify
>my problem in more detail.
>I am trying to develop a solid state on-off switch, which will
>either turn on or off the AC Power under certain conditions.

You cannot turn off a triac: it turns off only when the main-terminal
current falls to nearly zero.

This solves your problem: you need to change the triac to a GTO or IGBT,
which is more expensive. So the MOC won't then be the most expensive
component. (;-)
--
Regards, John Woodgate, OOO - Own Opinions Only. Phone +44 (0)1268 747839
Fax +44 (0)1268 777124. http://www.jmwa.demon.co.uk I wanted to make a fully-
automated nuclear-powered trawler,but it went into spontaneous fishing.
PLEASE do not mail copies of newsgroup posts to me.

[Ed Ngai]

gaut...@my-deja.com wrote:
> Thanks guys for all the suggestions given, but let me clarify
> my problem in more detail.
> I am trying to develop a solid state on-off switch, which will

> either turn on or off the AC Power under certain conditions. This
> I am doing by logic control of the MOC, which in turn
> controls the Triac. The problem area here being, the MOC is the most
> expensive component of the circuit, more than the Triac and delivery
> times of most suppliers is about 12 weeks.
> So I was wondering if I can take a light dimmer circuit, eliminate
> the Preset, change the gate current to DC and control it from my logic
> circuit. This circuit doesnot seem to be working, any help.
> In the overall circuit, MOC is very expensive, hence I was looking
> for other alternatives.
> Any help anybody.
>
> Gautham Nalamada

hummm ...
1. you have dc logic control to MOC ... whats a MOC ?
2. MOC controls triac
3. triac controls ac power, ... 120vac ?

simpler get a solid state relay. you control the ssr w/ 3 to 20 volts
dc, like 5 mA ? the ssr is connected to the 120VAC line.
the SSR turns on and the 120v gets switch on.

ues your logic ckt to switch on off the ssr.

[Robert]

Ed Ngai wrote:

>
>
> hummm ...
> 1. you have dc logic control to MOC ... whats a MOC ?
>

MotorolaOptoCoupler

[Ed Ngai]

> MotorolaOptoCoupler

inside a solid state realy is a triac. so why can't you use
a ssr to control the 102vac ? it'll be easier i think ?

all you have to do is pulse in to the dc control lines of the ssr
pulse in 5 volts , blink , blink, blink goes the led inside
the ssr, which turns on the triac part of the ssr, now you have
triac control ...?

[Patrick Timlin]

gaut...@my-deja.com wrote:
> The problem area here being, the MOC is the most expensive component
> of the circuit, more than the Triac and delivery times of most
> suppliers is about 12 weeks.

I am not sure why you are convinced that these parts are both expensive
and hard to get. Maybe you are looking at the wrong thing. Example,
Digikey carries the MOC3009 (by QT Optoelectronics) for less than 60
cents each and has almost 400 of them sitting on the shelf. Unless you
are getting the rest of your ciruit for pennies, 60 cents doesn't seem
very expensive.

Besides, even at as much as say two-dollars, the opto is going to be
cheaper than buying a dimmer switch and trying to modify that as you
asked about.

--
Patrick Timlin --- pti...@yahoo.com
http://www.geocities.com/ptimlin/

[Charles W. Shults III]

The fact is, most "solid state relays" are intended for AC power control and
work exactly like a triac or SCR- they will not turn off until the current
ceases or reverses.
That said, let's examine the triac/SCR thing. For DC control, what you must
do is to either remove or reverse the current. This can be done with a pulse
transformer, as applying a brief but negative going pulse of current to the
anode of the device can turn it off. This must be done while power is applied
to it, but it can be done with a gated high voltage circuit.
Now, the MOC is the name for a family of optically coupled control devices
that are intended to provide a simple interface between your circuits and a
triac or SCR. Inside, there is often an optically coupled DIAC or bilateral
trigger device.
Using a solid state relay give you all of the items in one package, thus
simplifying design and reducing package count, but the same rules apply for
turning on off. Build a circuit that develops a voltage that is counter to the
AC line and slightly higher, then gate it through (perhaps) a pulse transformer
or another smaller SCR...
Try to locate an applications handbook from Unitrode or Motorola.

Cheers!

Chip Shults

SPAM free Email - aic...@gdi.udu.net but remove the .baryon

PGP \\ 8B27 CFD5 AAD5 67EA BF00
Key // 7529 9CF6 C3D7 233C D4D9

[Robert]

Chip, You still don't get it. The TRIAC is controlling an ac driven load and he is
trying to find a way to convert his DC control voltage to a turn-on signal for the
gate without the use of an opto-coupler.

[Charles W. Shults III]

Use a transistor, capacitor, and a diac.

[Dan Fraser]

If you apply DC to the triac's gate through a resistor it will turn on
and stay on until the AC voltage crosses zero the first time the signal
is removed from the gate. The gate voltage can be positive all the time,
even when the AC voltage goes negative.
Gate has bias - Triac ON
Gate has no Bias - Triac off the next time the AC voltage crosses zero.

About 47 to 100 ohms is good with a 5V drive. Note that most logic will
not supply the 100mA or so needed and you will have to drive it with
something like a 2N4401 tramsistor.

However this means that the logic is floating at AC voltage. The least
hiccup in the circuit and the AC will get loose in the logic and blow it
up real bad. The MOC are an essential part unless the drive logic is
entirely enclosed and floating. If there is any connection to the
outside world, the MOC is required for safety. If you have any outside
connection, you left out the MOC and anything happens you will get your
butt sued off and may even get someone killed.

The MOC is, if nothing else, lawyer repellent. Eliminate it at your
peril. Elimation of the MOC just means having to pay the money later
anyway except for replacement logic too much and if you are not lucky,
lawyers.

I've been there. I built circuitry with triacs in the pre MOC days and
blew way more money on service calls that I would of saved. As soon as
they came out in 77, I started using them and would NEVER NEVER
interface triacs from logic without then.

BTW, a solid state reley is nothing but a triac and a zero crossing MOC
in one case and at a higher price.

do NOT eliminate the MOC. They are cheap compared to paying a lawyer.
You pay either way. However, it is best to avoid lawyers if you can.

--
Dan Fraser
From Beautiful British Columbia in western Canada

[gaut...@my-deja.com]

Thanks guys for all your replies, some to the point, some missing
the point and some condescending replies.

For now ignoring the reasons as to why I am not wanting to use an MOC,
could somebody answer my main question,

is it possible to control the Triac without an MOC,
I have tried using a Transistor, Diode, Capacitor and Diac, but since I
am using the load in series to get the gate drive, the load is getting
uneven voltage and is flickering, is there anyway I can get around the
problem.

Any technical help please, no commercial replies.

Gautham Nalamada

[Robert]

gaut...@my-deja.com wrote:

>
> I have tried using a Transistor, Diode, Capacitor and Diac, but since I
> am using the load in series to get the gate drive, the load is getting
> uneven voltage and is flickering, is there anyway I can get around the
> problem.

I cannot figure out what you are talking about with this description. Are
you using the DIAC in series with the gate so that C charges through the
transistor into MT1 up to the DIAC breakover? The load in series? Does this
mean the load is on MT2 to the high voltage AC input and MT1 is connected
to AC return? If this is the case then a flickering load means you are not
triggering long enough or with enough current. If you synchronize your
trigger with the AC line so that the gate current pulse- delivered by the
DIAC- occurs when the AC across the load results in enough TRIAC current to
sustain latch-on- this means Vinput/Rload>>Ihold specification for TRIAC-
the "holding current" specification ( do you know what that is?) , then the
turn on will be reliable. Now if your load is inductive then this will
create problems and you must increase the gate drive duration so that the
TRIAC is triggered on until the inductance allows the load current to build
up sufficiently to sustain latch-on. This means you increase C and add a
current limiting resistor in series with the DIAC. You should also have a
pulldown resistor from Gate to MT1 to prevent false triggering, and an RC
snubber in parallel with MT1-MT2 to limit the dV/dT to less than 20V/us.
Why don't you post a schematic together with details on the load.

[Charles W. Shults III]

You can use a small pulse transformer to do it, and this will provide the
isolation you must have. You will often have quite high differential voltages
present on the gate lead, which is usually fatal to a lot of small, low voltage
parts.

Sometimes, you can find -fast- SSRs and they can be used for this, but don't
hold your breath.

[John Fields]

gaut...@my-deja.com wrote:
>
> Thanks guys for all your replies, some to the point, some missing
> the point and some condescending replies.
>
> For now ignoring the reasons as to why I am not wanting to use an MOC,
> could somebody answer my main question,
>
> is it possible to control the Triac without an MOC,

> I have tried using a Transistor, Diode, Capacitor and Diac, but since I
> am using the load in series to get the gate drive, the load is getting
> uneven voltage and is flickering, is there anyway I can get around the
> problem.

---

Well, now, let's see if we can find out why you're having trouble.

"Using the load in series to get the gate drive" makes very little sense
to me.
Using the load in series with what?

Here are the only two sensible ways your load can be connected:

MAINS>----[LOAD]-----+
|
MT2
MT1 A
|\G
|
|
MAINS>---------------+

MAINS>---------------+
|
MT2
MT1 B
|\G
|
|
MAINS>----[LOAD]-----+

Are you using A or B?

---

John Fields, Austin Instruments, Inc.
El Presidente Austin, Republic of Texas
"I speak for the company" http://www.austininstruments.com