This is more of a design question, but there are some pretty sharp people on
this board so I'll post it here. I am designing a closed loop thermal
control unit that would like to be very accurate (+/- .2 degrees C). I plan
to use a PID controller with an RTD sensor and a small tape style heater.

Most PID controllers only provide a signal out and do not directly power the
heating element. If I had a PID controller providing a low voltage signal
(mV), what sort of intermediate circuit would I need to power a 10-50 watt
heating element? My main goal is not necessarily accuracy, but stability.

Thanks,
Bob

"Robert Wolcott" wrote in message
...
| This is more of a design question, but there are some pretty sharp people
on
| this board so I'll post it here. I am designing a closed loop thermal
| control unit that would like to be very accurate (+/- .2 degrees C). I
plan
| to use a PID controller with an RTD sensor and a small tape style heater.
|
| Most PID controllers only provide a signal out and do not directly power
the
| heating element. If I had a PID controller providing a low voltage signal
| (mV), what sort of intermediate circuit would I need to power a 10-50 watt
| heating element? My main goal is not necessarily accuracy, but stability.

I'd be inclined to make a closed loop controller which is controlled by the
PID output. What volts/amps for the heater?

If you can find an old GE SCR manual you should find some good starting
points.

N

I would guess that the heater puts out a few watts (12V 1A or so).


"NSM" wrote in message
news:dD2Gd.97965$dv1.33586@edtnps89...

"Robert Wolcott" wrote in message
...
| This is more of a design question, but there are some pretty sharp
people
on
| this board so I'll post it here. I am designing a closed loop thermal
| control unit that would like to be very accurate (+/- .2 degrees C). I
plan
| to use a PID controller with an RTD sensor and a small tape style
heater.
|
| Most PID controllers only provide a signal out and do not directly power
the
| heating element. If I had a PID controller providing a low voltage
signal
| (mV), what sort of intermediate circuit would I need to power a 10-50
watt
| heating element? My main goal is not necessarily accuracy, but
stability.

I'd be inclined to make a closed loop controller which is controlled by
the
PID output. What volts/amps for the heater?

If you can find an old GE SCR manual you should find some good starting
points.

N

Robert Wolcott wrote:
I would guess that the heater puts out a few watts (12V 1A or so).

.... so about 12 watts.


"NSM" wrote in message
news:dD2Gd.97965$dv1.33586@edtnps89...

"Robert Wolcott" wrote in message
...
| This is more of a design question, but there are some pretty sharp
people
on
| this board so I'll post it here. I am designing a closed loop thermal
| control unit that would like to be very accurate (+/- .2 degrees C). I
plan
| to use a PID controller with an RTD sensor and a small tape style
heater.
|
| Most PID controllers only provide a signal out and do not directly power
the
| heating element. If I had a PID controller providing a low voltage
signal
| (mV), what sort of intermediate circuit would I need to power a 10-50
watt
| heating element? My main goal is not necessarily accuracy, but
stability.

I'd be inclined to make a closed loop controller which is controlled by
the
PID output. What volts/amps for the heater?

If you can find an old GE SCR manual you should find some good starting
points.

N







--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form .

I am not going to design this for you.

I am assuming that you want to run an AC operated heater. I am taking it
that you know how AC light dimmers and speed controllers work that are Triac
driven.

What you have to do is make a VCA (Voltage Control Amplifier), which is used
to drive the an optical isolator that is in a circuit to be used to do the
replacement of the pot (sort of speaking) to control the phase drive of a
Triac, that is driving the heater unit. It is important that you maintain
proper electrical isolation between the VCA output, and the power driver
that is on the AC side (Triac circuits).

If your heating element is a DC voltage type, then you can use the VCA to
drive a DC power amplifier circuit that would be able to drive your heating
element.

Normal heating systems that have a window of about 2 degrees F are able to
use a power relay on the output of the thermostat system, and thus
eliminating the complexity of a VCA, and an output stage of some type. The
power relay would be a lot easier to drive, and would require less
precision.

There are commercially available thermostats that can do what you want to
do. I am sure that if you research this out, you will find some. They are
very precise, and well engineered. Going by the fact that you have to ask
this question, you are going to have a bit of a complicated job to do, to
make this work accurately. There are a lot of design aspects to be
considered.


--

Jerry G.
=====

"Robert Wolcott" wrote in message
...
This is more of a design question, but there are some pretty sharp people on
this board so I'll post it here. I am designing a closed loop thermal
control unit that would like to be very accurate (+/- .2 degrees C). I plan
to use a PID controller with an RTD sensor and a small tape style heater.

Most PID controllers only provide a signal out and do not directly power the
heating element. If I had a PID controller providing a low voltage signal
(mV), what sort of intermediate circuit would I need to power a 10-50 watt
heating element? My main goal is not necessarily accuracy, but stability.

Thanks,
Bob

"Robert Wolcott" wrote in message
...
| I would guess that the heater puts out a few watts (12V 1A or so).

DC power or can you run it from AC? ISTR that the GE manual had a very
precise temperature control circuit that ran on half wave AC.

N

"Robert Wolcott" writes:

This is more of a design question, but there are some pretty sharp people on
this board so I'll post it here. I am designing a closed loop thermal
control unit that would like to be very accurate (+/- .2 degrees C). I plan
to use a PID controller with an RTD sensor and a small tape style heater.

Most PID controllers only provide a signal out and do not directly power the
heating element. If I had a PID controller providing a low voltage signal
(mV), what sort of intermediate circuit would I need to power a 10-50 watt
heating element? My main goal is not necessarily accuracy, but stability.

Do you have a specific PID controller in mind? Are you designing from
scratch or will be using a commercial controller (Omega, etc.?)?

As others have said, if your controller only provides a voltage or current
output (not pulse width modulation or phase control), you need to provide
a circuit to convert it. Some controllers provide a pulse width output
of sorts to drive a SS relay or logic input. If this is a one-off and
you don't mind using a commercial controller, they seem to be going for
about $30 on eBay.

Converting to pulse width modulation from a DC voltage is nothing more
than an amplifier and a single IC like those used in SMPS controllers,
or a 555 ramp generator and comparator.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
| Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

Note: These links are hopefully temporary until we can sort out the excessive
traffic on Repairfaq.org.

Important: Anything sent to the email address in the message header above is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.
..

Howdy Robert.....Based on my own experience designing a similar
situation I'd want to know what the thermal mass is that you're trying
to heat to get a handle on the stability issue. Based on my own
experience, if you can get away with something simple like a relay to
switch your heater, use that (could be a solid state relay etc).
Depending on the anticipated repairability of this circuit, you could
design your own switching based on everything from bipolar, Power FET to
the SCR's or TRIAC's as others have alluded to.

Because you mentioned stability, the amount of times you'll have to
switch on versus heat loss will be the basic issue assuming you have
sufficient heat input to overwhelm a quickly cooling load. If the heat
load is small and the heater has a lot of output in relation, then
you'll be playing with overshoot a lot more.


Gord

"Robert Wolcott" bravely wrote to "All" (14 Jan 05 21:47:29)
--- on the heady topic of "Thermal control question"

RW From: "Robert Wolcott"
RW Xref: aeinews sci.electronics.repair:7183

RW This is more of a design question, but there are some pretty sharp
RW people on this board so I'll post it here. I am designing a closed
RW loop thermal control unit that would like to be very accurate (+/- .2
RW degrees C). I plan to use a PID controller with an RTD sensor and a
RW small tape style heater.
RW Most PID controllers only provide a signal out and do not directly
RW power the heating element. If I had a PID controller providing a low
RW voltage signal (mV), what sort of intermediate circuit would I need to
RW power a 10-50 watt heating element? My main goal is not necessarily
RW accuracy, but stability.
RW Thanks,
RW Bob

If it is AC power then you need to convert the PID voltage level into
a phase angle in order to control a TRIAC. This can be done brute
force with on/off control, slightly better by varying a pulse rate, or
precisely with a countdown circuit driven by a pll sync'd to the AC
phase. It depends on application.

A*s*i*m*o*v

.... Power is obtained by current meeting resistance

I had planned on using an Omron (E5CK?) controller. They are usually
available on Ebay for $25-$40. Do you know of a source for schematics of
the circuits you mentioned?

Thanks,
Bob


"Sam Goldwasser" wrote in message
...
"Robert Wolcott" writes:

This is more of a design question, but there are some pretty sharp people
on
this board so I'll post it here. I am designing a closed loop thermal
control unit that would like to be very accurate (+/- .2 degrees C). I
plan
to use a PID controller with an RTD sensor and a small tape style heater.

Most PID controllers only provide a signal out and do not directly power
the
heating element. If I had a PID controller providing a low voltage
signal
(mV), what sort of intermediate circuit would I need to power a 10-50
watt
heating element? My main goal is not necessarily accuracy, but
stability.

Do you have a specific PID controller in mind? Are you designing from
scratch or will be using a commercial controller (Omega, etc.?)?

As others have said, if your controller only provides a voltage or current
output (not pulse width modulation or phase control), you need to provide
a circuit to convert it. Some controllers provide a pulse width output
of sorts to drive a SS relay or logic input. If this is a one-off and
you don't mind using a commercial controller, they seem to be going for
about $30 on eBay.

Converting to pulse width modulation from a DC voltage is nothing more
than an amplifier and a single IC like those used in SMPS controllers,
or a 555 ramp generator and comparator.

--- sam | Sci.Electronics.Repair FAQ Mirror:
http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ:
http://repairfaq.ece.drexel.edu/sam/lasersam.htm
| Mirror Sites:
http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

Note: These links are hopefully temporary until we can sort out the
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Important: Anything sent to the email address in the message header above
is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.
.

Gord,

I haven't really sat down and gone through the numbers for heat transfer
yet, but the physical mass would be around 30-50 grams of aluminum. It
would not be exposed to much air flow either. I plan to use a tape style
heating element (http://www.minco.com/products/heaters.aspx) that will be
sandwiched between the two pieces of aluminum.

Thanks,
Bob


"GS" wrote in message
...
Howdy Robert.....Based on my own experience designing a similar
situation I'd want to know what the thermal mass is that you're trying to
heat to get a handle on the stability issue. Based on my own experience,
if you can get away with something simple like a relay to switch your
heater, use that (could be a solid state relay etc). Depending on the
anticipated repairability of this circuit, you could design your own
switching based on everything from bipolar, Power FET to the SCR's or
TRIAC's as others have alluded to.

Because you mentioned stability, the amount of times you'll have to switch
on versus heat loss will be the basic issue assuming you have sufficient
heat input to overwhelm a quickly cooling load. If the heat load is
small and the heater has a lot of output in relation, then you'll be
playing with overshoot a lot more.


Gord

"Robert Wolcott" writes:

I had planned on using an Omron (E5CK?) controller. They are usually
available on Ebay for $25-$40. Do you know of a source for schematics of
the circuits you mentioned?

Search on: TL494 or SG2524. I'm sure there are better ones nowadays but
those part numbers come to mind. The datasheets will have enough info
to design a simple PWM controller. They can directly drive common MOSFETs.

In fact, with one of those chips, you can design the entire thing without
any fancy Omron controller.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
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"Sam Goldwasser" wrote in message
...
"Robert Wolcott" writes:

This is more of a design question, but there are some pretty sharp people
on
this board so I'll post it here. I am designing a closed loop thermal
control unit that would like to be very accurate (+/- .2 degrees C). I
plan
to use a PID controller with an RTD sensor and a small tape style heater.

Most PID controllers only provide a signal out and do not directly power
the
heating element. If I had a PID controller providing a low voltage
signal
(mV), what sort of intermediate circuit would I need to power a 10-50
watt
heating element? My main goal is not necessarily accuracy, but
stability.

Do you have a specific PID controller in mind? Are you designing from
scratch or will be using a commercial controller (Omega, etc.?)?

As others have said, if your controller only provides a voltage or current
output (not pulse width modulation or phase control), you need to provide
a circuit to convert it. Some controllers provide a pulse width output
of sorts to drive a SS relay or logic input. If this is a one-off and
you don't mind using a commercial controller, they seem to be going for
about $30 on eBay.

Converting to pulse width modulation from a DC voltage is nothing more
than an amplifier and a single IC like those used in SMPS controllers,
or a 555 ramp generator and comparator.

--- sam | Sci.Electronics.Repair FAQ Mirror:

"Robert Wolcott" wrote in message
...
| I had planned on using an Omron (E5CK?) controller. They are usually
| available on Ebay for $25-$40. Do you know of a source for schematics of
| the circuits you mentioned?

Have you looked at the app notes on http://www.microchip.com ??

N

I couldn't reply to your email, so I'll post it here.

Gord,

This is very early in the design stage so I'm really
only trying to line up how I will approach the
problem. The temperature I am looking for is 30-33
degrees C so I might be way off on the wattage
estimate. I haven't even looked into the possible
wattage values and I have little experience with
thermal systems. I know it will be a low power system
though. The control voltage and current have not yet
been determined.

Since this is a personal project (for my hobby) cost
is a concern. My approach is:

1. Find out if it can be done.
2. How much will the major components cost
(controller, TC, various circuits)
3. Spend time determining exact requirements
(wattage, etc...).
4. Physically design the system (CAD modeling etc...)
5. Order the parts.
6. Fabricate it.

I'd suppose I'm currently at stage 1 or 2. I have
heard of Peltier elements being used for systems like
this. Do you know anything about these?

Thanks,
Bob



"GS" wrote in message
...
Howdy Robert.....Based on my own experience designing a similar situation
I'd want to know what the thermal mass is that you're trying to heat to
get a handle on the stability issue. Based on my own experience, if you
can get away with something simple like a relay to switch your heater, use
that (could be a solid state relay etc). Depending on the anticipated
repairability of this circuit, you could design your own switching based
on everything from bipolar, Power FET to the SCR's or TRIAC's as others
have alluded to.

Because you mentioned stability, the amount of times you'll have to switch
on versus heat loss will be the basic issue assuming you have sufficient
heat input to overwhelm a quickly cooling load. If the heat load is
small and the heater has a lot of output in relation, then you'll be
playing with overshoot a lot more.


Gord

"Robert Wolcott" wrote in message
...

| I'd suppose I'm currently at stage 1 or 2. I have
| heard of Peltier elements being used for systems like
| this. Do you know anything about these?

http://www.allelectronics.com/cgi-bi...rch&item=PJT-1

Needs WELL filtered DC.

N

"NSM" writes:

"Robert Wolcott" wrote in message
...

| I'd suppose I'm currently at stage 1 or 2. I have
| heard of Peltier elements being used for systems like
| this. Do you know anything about these?

http://www.allelectronics.com/cgi-bi...rch&item=PJT-1

Needs WELL filtered DC.

If you know your device will be above ambient, you don't need a Peltier
device (thermo electric cooler, TEC). Yes, they can heat as well but
heating tape will be cheaper.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
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Important: Anything sent to the email address in the message header above is
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contact me via the Feedback Form in the FAQs.

This is for temperature control of an LBO crystal used in doubling a yag
laser (externally). I have heard that there is an optimal temperature for
maximizing efficiency in these and I'm sure there will be some heat
generated by the 1064nm beam within the crystal. My concern is that the
beam's heat will actually drive the crystal's temperature over the optimum
value and cooling might be needed, which would require a means of cooling as
well as heating.

How much does the temperature affect the conversion efficiency anyways? If
it is ~20%, designing a good thermal control system would be worth the
effort, but not if it is only 2-3%. For my purpose, an elaborate thermal
control system might be overkill but it is still interesting to learn about
nonetheless.

Thanks,
Bob



"Sam Goldwasser" wrote in message
...
"NSM" writes:

"Robert Wolcott" wrote in message
...

| I'd suppose I'm currently at stage 1 or 2. I have
| heard of Peltier elements being used for systems like
| this. Do you know anything about these?

http://www.allelectronics.com/cgi-bi...rch&item=PJT-1

Needs WELL filtered DC.

If you know your device will be above ambient, you don't need a Peltier
device (thermo electric cooler, TEC). Yes, they can heat as well but
heating tape will be cheaper.

--- sam | Sci.Electronics.Repair FAQ Mirror:
http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ:
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Important: Anything sent to the email address in the message header above
is
ignored unless my full name is included in the subject line. Or, you can
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"Robert Wolcott" writes:

This is for temperature control of an LBO crystal used in doubling a yag
laser (externally). I have heard that there is an optimal temperature for
maximizing efficiency in these and I'm sure there will be some heat
generated by the 1064nm beam within the crystal. My concern is that the
beam's heat will actually drive the crystal's temperature over the optimum
value and cooling might be needed, which would require a means of cooling as
well as heating.

In that case, you really should be using a TEC since you don't know
which direction it will need, heat or cool. In addition, that 30-33C may
turn out not to be the optimal temperature.

How much does the temperature affect the conversion efficiency anyways? If
it is ~20%, designing a good thermal control system would be worth the
effort, but not if it is only 2-3%. For my purpose, an elaborate thermal
control system might be overkill but it is still interesting to learn about
nonetheless.

It could be very significant. For KTP, it could be 50 percent or more.
Don't know specifically for LBO but the principles are similar.

You're going to all the effort, it certainly makes sense to do a proper
job of the temperature control. However, you have to decide if you really
want the learning experience given all the time and effort it will take,
or just get a TEC driver from Wavelength Electronics or elsewhere
for about $100 and be done. I think using an Omron or Omega PID unit
is gross overkill if it were even suitable, but given the new information,
I'd tend to recommend against that approach.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
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"Robert Wolcott" bravely wrote to "All" (15 Jan 05 14:47:44)
--- on the heady topic of " Thermal control question"

RW From: "Robert Wolcott"
RW Xref: aeinews sci.electronics.repair:7310

RW This is for temperature control of an LBO crystal used in doubling a
RW yag laser (externally). I have heard that there is an optimal
RW temperature for maximizing efficiency in these and I'm sure there will
RW be some heat generated by the 1064nm beam within the crystal. My
RW concern is that the beam's heat will actually drive the crystal's
RW temperature over the optimum value and cooling might be needed, which
RW would require a means of cooling as well as heating.

RW How much does the temperature affect the conversion efficiency
RW anyways? If it is ~20%, designing a good thermal control system would
RW be worth the effort, but not if it is only 2-3%. For my purpose, an
RW elaborate thermal control system might be overkill but it is still
RW interesting to learn about nonetheless.


In that case you could use a couple of matched diodes, one to sense
the temperature and the other as reference. The voltage difference
between the 2 diodes will be aproximately linear with temperature at
about 2.5mV/'C. Add some gain and an offset and you can control
any set temperature. A PID is overkill for such a simple task.

A*s*i*m*o*v

.... If all else fails, hurl it across the room a few times!

Here are some pictures of the proposed design. I bought the mirror mount
but will machine the LBO mounting fixture.

http://oregonstate.edu/~wolcottr/Las...bling%20setup/

Bob


"Sam Goldwasser" wrote in message
...
"Robert Wolcott" writes:

This is for temperature control of an LBO crystal used in doubling a yag
laser (externally). I have heard that there is an optimal temperature
for
maximizing efficiency in these and I'm sure there will be some heat
generated by the 1064nm beam within the crystal. My concern is that the
beam's heat will actually drive the crystal's temperature over the
optimum
value and cooling might be needed, which would require a means of cooling
as
well as heating.

In that case, you really should be using a TEC since you don't know
which direction it will need, heat or cool. In addition, that 30-33C may
turn out not to be the optimal temperature.

How much does the temperature affect the conversion efficiency anyways?
If
it is ~20%, designing a good thermal control system would be worth the
effort, but not if it is only 2-3%. For my purpose, an elaborate thermal
control system might be overkill but it is still interesting to learn
about
nonetheless.

It could be very significant. For KTP, it could be 50 percent or more.
Don't know specifically for LBO but the principles are similar.

You're going to all the effort, it certainly makes sense to do a proper
job of the temperature control. However, you have to decide if you really
want the learning experience given all the time and effort it will take,
or just get a TEC driver from Wavelength Electronics or elsewhere
for about $100 and be done. I think using an Omron or Omega PID unit
is gross overkill if it were even suitable, but given the new information,
I'd tend to recommend against that approach.

--- sam | Sci.Electronics.Repair FAQ Mirror:
http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ:
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"Robert Wolcott" writes:

Here are some pictures of the proposed design. I bought the mirror mount
but will machine the LBO mounting fixture.

http://oregonstate.edu/~wolcottr/Las...bling%20setup/

You should probably put just the inner LBO mount on a TEC.

P.S. What CAD program are you using?

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
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The CAD program is Solidworks and it is very easy. Designing the mount
probably took about four hours because there was design work involved.
Disassembling and modeling the mirror mount only took about 2 hours because
I was just measuring parts. I think it is extremely useful and worth the
time.

I have heard that LBO is hydroscopic, is this true? Do you know if the
mount will need to seal it from the atmosphere? That is a reason that the
mount is designed the way it is. If it does not need to be sealed I will
simply cut of the left half of the pink piece and merge the rest of it with
the yellow piece, allowing plenty of room for the TEC wires and heat sink.

The current setup allows for five DOF adjustment (3 axis on the LBO mount,
X-Y translation of the lens on its mount). I plan to use the lens
adjustment to aim the beam at the crystal. It should be very precise.


"Sam Goldwasser" wrote in message
...
"Robert Wolcott" writes:

Here are some pictures of the proposed design. I bought the mirror mount
but will machine the LBO mounting fixture.

http://oregonstate.edu/~wolcottr/Las...bling%20setup/

You should probably put just the inner LBO mount on a TEC.

P.S. What CAD program are you using?

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Thanks for the reply. I'm sure a PID is overkill for this but at $30 one
can't go wrong. For that price I get a digital display with all kinds of
bells and whistles and a unit that works like clockwork. I also like being
able to set the temperature where I like it and then just forget about it.
If it weren't for Ebay I wouldn't even consider a PID for this. Not at
retail price :-)

Bob



"Asimov" wrote in message
...
"Robert Wolcott" bravely wrote to "All" (15 Jan 05 14:47:44)
--- on the heady topic of " Thermal control question"

RW From: "Robert Wolcott"
RW Xref: aeinews sci.electronics.repair:7310

RW This is for temperature control of an LBO crystal used in doubling a
RW yag laser (externally). I have heard that there is an optimal
RW temperature for maximizing efficiency in these and I'm sure there
will
RW be some heat generated by the 1064nm beam within the crystal. My
RW concern is that the beam's heat will actually drive the crystal's
RW temperature over the optimum value and cooling might be needed, which
RW would require a means of cooling as well as heating.

RW How much does the temperature affect the conversion efficiency
RW anyways? If it is ~20%, designing a good thermal control system would
RW be worth the effort, but not if it is only 2-3%. For my purpose, an
RW elaborate thermal control system might be overkill but it is still
RW interesting to learn about nonetheless.


In that case you could use a couple of matched diodes, one to sense
the temperature and the other as reference. The voltage difference
between the 2 diodes will be aproximately linear with temperature at
about 2.5mV/'C. Add some gain and an offset and you can control
any set temperature. A PID is overkill for such a simple task.

A*s*i*m*o*v

... If all else fails, hurl it across the room a few times!

"Robert Wolcott" writes:

The CAD program is Solidworks and it is very easy. Designing the mount
probably took about four hours because there was design work involved.
Disassembling and modeling the mirror mount only took about 2 hours because
I was just measuring parts. I think it is extremely useful and worth the
time.

I have heard that LBO is hydroscopic, is this true? Do you know if the
mount will need to seal it from the atmosphere? That is a reason that the
mount is designed the way it is. If it does not need to be sealed I will
simply cut of the left half of the pink piece and merge the rest of it with
the yellow piece, allowing plenty of room for the TEC wires and heat sink.

Looking at various manufacturers of LBO, it seems to have "low hygroscopic"
sensitivity and should be handled in dry conditions. Definitely not as
robust as KTP. However, some say it (at least theirs) is "non-hygroscopic".
I'd err on the side of keeping it sealed.

Search on "LBO hygroscopic"

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Asimov wrote:

"Robert Wolcott" bravely wrote to "All" (14 Jan 05 21:47:29)
--- on the heady topic of "Thermal control question"

RW From: "Robert Wolcott"
RW Xref: aeinews sci.electronics.repair:7183

RW This is more of a design question, but there are some pretty sharp
RW people on this board so I'll post it here. I am designing a closed
RW loop thermal control unit that would like to be very accurate (+/- .2
RW degrees C). I plan to use a PID controller with an RTD sensor and a
RW small tape style heater.
RW Most PID controllers only provide a signal out and do not directly
RW power the heating element. If I had a PID controller providing a low
RW voltage signal (mV), what sort of intermediate circuit would I need to
RW power a 10-50 watt heating element? My main goal is not necessarily
RW accuracy, but stability.
RW Thanks,
RW Bob

If it is AC power then you need to convert the PID voltage level into
a phase angle in order to control a TRIAC. This can be done brute
force with on/off control, slightly better by varying a pulse rate, or
precisely with a countdown circuit driven by a pll sync'd to the AC
phase. It depends on application.

A*s*i*m*o*v

... Power is obtained by current meeting resistance

its common place to simply use a solid state switch which is a triac in
side.
most heat controllers will give you a relay & logic out, all that is
needed is to turn on the solid-state switch via the logic level.