Path: utzoo!attcan!uunet!lll-winken!ames!xanth!nic.MR.NET!csd4.milw.wisc.edu!leah!bingvaxu!sunybcs!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: Looking for isolated DAC chip
Summary: Simpler solution for lamp dimmer application...
Message-ID: <2930@kitty.UUCP>
Date: 15 Jan 89 17:32:30 GMT
References: <1378@ucsd.EDU>
Organization: Recognition Research Corp., Clarence, NY
Lines: 56

In article <1378@ucsd.EDU>, brian@ucsd.EDU (Brian Kantor) writes:
> I've got 7 or 8 bits of TTL on a latch output that specify a lamp 
> brightness level, and I want to control a TRIAC or other simple 
> light-dimmer circuit with it.  The idea is to have a microprocessor
> control a lighting panel.
> 
> I've seen some somewhat complex circuits involving optoisolators and
> time ratio controls, but I'm looking for a simple circuit (i.e., a
> complex chip is just fine, but the amount of glue needed to use it has
> to be small).  I'd like to control about 1 to 15 amps at 120 VAC per
> instance.

	The "proper" design solution is probably a phase control firing
circuit with zero-crossing detection, and which takes an analog voltage
input to set the firing angle of output pulses.  Isolating the output
pulses using a pulse transformer is trivial.  No isolated DAC would be
required here.

	However, you asked for "simple", so here it is...  The heart
of this approach is the GE H11F1 optocoupler having a bilateral analog
FET output.  In simple terms, changing the LED current on this device
changes a two-terminal output resistance.

	Begin with a simple 8-bit DAC connected to your TTL latch.  You
probably want to run the DAC output in a current mode, so you will need
an op-amp (plus maybe a FET or additional op amp as a buffer) to drive
the optocoupler LED.  Realistically, you probably want to drive from
2 to 20 mA; anything less than 2 mA is off.

	Now, you've got your H11F1 being driven by the DAC in a controlled
fashion.  Take the output of the H11F1 and use it to replace the resistor
in a typical two-terminal TRIAC dimmer circuit using a diac as a trigger
device.  You may have to add a series resistor for "zero" compensation,
and you may want to add a parallel resistor for "keep-alive" current.

	The above approach works.  I have used it for simple proportional
control of electric heating elements, which is close enough to your
application.

	The "price" you pay for the above "simple" circuit is a truly
abysmal shape of the transfer function between DAC setting and effective
lamp brightness.  However, since you obviously have a microprocessor,
you can neatly use a table or algorithm to correlate the relationship
between DAC input value and output birghtness; that's what software is
for, ain't it? :-)

	As an amusing aside, for many years I used to design rather complex
linear circuits intended to render precise transfer functions between a
a control voltage input and process parameter output.  Now with the use of
microprocessors for almost everything, I say screw it, if the transfer
function don't look right, I'll fix it up in software. :-)

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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