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PICList Thread
'16A heater control? Triac? SCR?'
1999\05\14@082025 by Jason Wolfson

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Today I'm trying to control a 1800W electric heater with a PIC.
About 16A @ 110VAC, I was thinking of using a 25A Triac and
optocoupler/driver.
My question is how hot will the Triac get? is there a better way? cheaper?
thanks

Jason
P.S.
I've archived all the PID discussions for the next step of this project.....

1999\05\14@102725 by Harold M Hallikainen

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       I think a triac is about the most efficient way of handling the problem
(unless you want to go to an electromechanical relay).  The voltage drop
across the triac would be about 1.5 volts, so multiply that by the
current to get dissipation in watts.  Multiply the dissipation by your
heat sink's thermal resistance to ambient (degrees C/watt) to get
temperature rise above ambient.
       For typical drive circuits for the triac, see the datasheet on the
MOC3020 or similar opto couplers.  Also, for simplicity, you can just buy
an opto coupled solid state relay.  The manufacturer of the SSR has
solved all the drive and snubber problems for you.  You can drive the SSR
directly from a PIC.

Harold



On Fri, 14 May 1999 08:16:16 -0400 Jason Wolfson <spam_OUTjasonTakeThisOuTspamLIPIDEX.COM>
writes:
{Quote hidden}

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1999\05\14@110529 by Richard A. Smith

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On Fri, 14 May 1999 08:16:16 -0400, Jason Wolfson wrote:

>Today I'm trying to control a 1800W electric heater with a PIC.
>About 16A @ 110VAC, I was thinking of using a 25A Triac and
>optocoupler/driver.
>My question is how hot will the Triac get? is there a better way? cheaper?
>thanks

1) Don't for get about the inrush currents.  Within the 1st
half cycle you will have a current of approx 2.5 times the
rated load or 40 amps in your case.  Make sure your device
is rated for this surge current or you will cause
pre-mature failure.

2) How much of an inductive load is the heater going to
present?  Triac's have a habit of not shutting off under an
inductive load unless the snubber cap is designed properly.
According to the Motorolla Thyristory databook snubber
desing is kind of an art form.  The equations get you close
but the rest is trial and error.

Perhaps you should consider an inverse parallel SCR design.
No inductive problems and as a bonus you share the load
across 2 devices which should ease your heat concerns.

I am interested in seeing whatever PID code you come up
with.  I just finished a project where I control a 192kW
cheston heater.  It will heat a steel bar from ambient to
bright glowing red hot in 6 seconds! The original spec did
not call for any kind of PID control but of course it
really did require it.  So I had to hack in a kind of
bastard PID to keep the inrush from blowing fuses.



--
Richard A. Smith                         Bitworks, Inc.
.....rsmithKILLspamspam@spam@bitworks.com               501.521.3908
Sr. Design Engineer        http://www.bitworks.com

1999\05\14@125155 by Lawrence Lile

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I'll follow this thread closely.  I've got an old idea to replace a
mechanical relay controlling a 1200 watt load with a triac.  Real similar
appliaction.  I kept running into problems with heat sinking (it has to
squeeze into a thimble, of course, with no air circulation and no metal
parts to bolt a hot triac onto...)

At 1200 watts, 120V I've got a 10 amp load, with a 1.5 volt drop Triac that
would be only 15 watts.  Hmmm.

I've also got some ideas about PID, when we get to that part.    Keep
talking, Jason!





{Original Message removed}

1999\05\14@153643 by Reginald Neale

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>Today I'm trying to control a 1800W electric heater with a PIC.
>About 16A @ 110VAC, I was thinking of using a 25A Triac and
>optocoupler/driver.
>My question is how hot will the Triac get? is there a better way? cheaper?
>thanks
>
>Jason
>P.S.
>I've archived all the PID discussions for the next step of this project.....


 Jason:

 The SHARP S116S02 is an SSR I have used, although this would
 run it at max. Has built-in opto isolation, heat sinkable.
 If you could split your heater load in two and use two SSRs
 this would be more reliable. They're less than three bucks.

 Reg Neale

1999\05\14@155351 by Thomas McGahee

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I have had to build some systems that drew about 20 amps continuous
and 40 amps peak (turn-on). Not wanting to have to put up with
either arcing relay contacts or excessive heat at the Triac,
I chose to design a circuit that has the best features of both.

I have a control circuit that upon receipt of a closure command
waits for zero crossing of the AC mains, then activates BOTH
a heavy duty Triac (with NO heat sink!) and a relay capable
of handling the 20 amps continuous current. Naturally the Triac
will respond first. Some tens of milliseconds later you can be sure
the relay contacts have closed (with NO arcing, due to the Triac
already being on!)

Triac control was via optoisolators. Relay control was direct,
but the relays had a separate power supply and ground buss. The
PIC and relay ground busses then shared a common connection
so the relay driver transistors could operate properly. This
saved me from having to use a bazillion additional optoisolators.

When the "open" command arrives I keep the Triac on, but turn off
the relay. I wait several tens of milliseconds to ensure that
the contacts have opened (with NO arcing!), and then I turn
off the Triac at a time such that inductive effects are minimized.
Keeps the lines "clean".

The Triac does not need any heat sink at all, because it actually
conducts for only a few tens of milliseconds during turn on and
turn off. The rest of the "on" time it is the relay contacts
that carry all the current.

Simple, effective, and right up a PIC's alley.
Of course, I have also designed PIC-less versions of this
circuit using monostable timers and an XOR gate, but the
PIC version is nice because it can control MANY such Triac/Relay
sets at once. Using shift registers it is amazing how many
such sets can be controlled with a resolution of a millisecond
or so. An RS232 type serial port set allowed remote control
of all the Triac/Relay sets and even allowed a deferred and
synchronized mode where the data for all sets was loaded and
then implemented upon receipt of a syncronizing command.

This made it easy to control a wide variety of machinery.

Fr. Tom McGahee

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{Quote hidden}

1999\05\14@170939 by Richard A. Smith

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On Fri, 14 May 1999 15:37:14 -0500, Reginald Neale wrote:

>  The SHARP S116S02 is an SSR I have used, although this would
>  run it at max. Has built-in opto isolation, heat sinkable.

Crydom also makes a line of SSR that would fit the bill.
There available from Digi-key, although expensive.


--
Richard A. Smith                         Bitworks, Inc.
EraseMErsmithspam_OUTspamTakeThisOuTbitworks.com               501.521.3908
Sr. Design Engineer        http://www.bitworks.com

1999\05\17@060053 by Peter Marinsek

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Thomas McGahee wrote:

{Quote hidden}

Could You send me the schematics of  this project? I'd like to make a power swit
ch
controlled by PIC, but with zerocrossing detect.
Thx, Peter

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