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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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Thermal control question
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 |
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"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 |
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I would guess that the heater puts out a few watts (12V 1A or so).
"NSM" wrote in message news:dD2Gd.97965$dv1.33586@edtnps89... "Robert Wolcott" wrote in message ... | This is more of a design question, but there are some pretty sharp people on | this board so I'll post it here. I am designing a closed loop thermal | control unit that would like to be very accurate (+/- .2 degrees C). I plan | to use a PID controller with an RTD sensor and a small tape style heater. | | Most PID controllers only provide a signal out and do not directly power the | heating element. If I had a PID controller providing a low voltage signal | (mV), what sort of intermediate circuit would I need to power a 10-50 watt | heating element? My main goal is not necessarily accuracy, but stability. I'd be inclined to make a closed loop controller which is controlled by the PID output. What volts/amps for the heater? If you can find an old GE SCR manual you should find some good starting points. N |
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Robert Wolcott wrote:
I would guess that the heater puts out a few watts (12V 1A or so). .... so about 12 watts. "NSM" wrote in message news:dD2Gd.97965$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 . |
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"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 |
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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 |
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"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. .. |
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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 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. . |
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"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 | 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. "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: |
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"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 |
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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 |
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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 |
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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 |
#14
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"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 |
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"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 | 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. |
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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: 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. |
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"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 |
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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. |
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