Controlling a hot air element

[TimB]

Hi all

I'm not going to go into why just now.

I want to find the best way to control the heat generated by a hot air de-soldering element. What is the best method. What I will say is I need to get very fine control eg fractions of a degree. I can add mechanical buffering so heat output can be "averaged".

I was thinking to start with a Triac to turn it on every cycle. But that means a duty cycle of around 50-60 hz.
Otherwise its a Triac and chopping the mains up. I have done that before and it hurt my brain!!!

Other option is high current, High freq DC. Just not sure if an off the shelf 220v coil is going to require too much current if we reduce the V to say 50v.

Thoughts?

Thanks Tim
 

[top204]

An interesting project Tim.

Chopping the element is a good method, otherwise, as you stated, current will increase significantly if the element is controlled linearly. It doesn't need to be a Triac these days, it can be a high current MOSFET switch. I think they call them silicon relays or something. :-)

With a MOSFET, the chopping can be very fast, so PWM can control the "virtual" voltage to the element, and a standard PI or PID control to maintain the temperature as much as possible.

Is it streamed heat. i.e. A hot air flow?

[trastikata]

What I will say is I need to get very fine control eg fractions of a degree. I can add mechanical buffering so heat output can be "averaged".

Is it for crystallography? Do you really need the forced air circulation?

[Giuseppe MPO]

If you find it useful, I developed this code using the CIP of a PIC16F1619 (4MHz clock) without using the CPU.
I used two timers, a CLC and obviously a ZCD (zero crossing).
To control the partialization of the two half-waves it is sufficient to load the value (0-154) in the T2PR register.

Code Select Expand

'-----------------------------------------------
ZCD1CON  = %10100000
OPTION_REG.7 = 0 ' Enable all chip Pull-Up
WPUA.2 = 0 ' Disable Pull-Up PORTA.2
'------------------------------- ' CLC1 CONFIGURABLE LOGIC CELL
CLC1CON  = 0x80
CLC1GLS0 = 0x02
CLC1GLS1 = 0x00
CLC1GLS2 = 0x00
CLC1GLS3 = 0x00
CLC1POL  = 0x8A
CLC1SEL0 = 0x1A
CLC1SEL1 = 0x1A
CLC1SEL2 = 0x1A
CLC1SEL3 = 0x1A
'------------------------------- ' TIMER 2
T2CLKCON = 0x00
T2CON  = 0xE0
T2HLT  = 0x33
T2PR  = 0x5D
T2RST  = 0x08
T2TMR  = 0x00
'------------------------------- ' TIMER 4
T4CLKCON = 0x00
T4CON  = 0x80
T4HLT  = 0x11
T4PR  = 0x0C
T4RST  = 0x05
T4TMR  = 0x00
'-------------------------------
RC0PPS  = 0x04
'-------------------------------

[okmn]

you already know
I want to add with the aim of getting "source information" on the site.

one emi filter circuit ,one ssr thyristor/triac out (use it 100 hz Full Wave Rectifier circuit)  and
emi filter eval. board ;

https://www.schurter.com/en/datasheet/DKIH-EVB 
https://www.schurter.com/en/datasheet/typ_DKIH-EVB.pdf

one ssr thyristor/triac out (use it 100 hz full bridge)


ssr  ac-zero crossed out (for triac)
      ac-dc or only dc out for thyristor

CPC1976Y  RELAY SOLID STATE 2A 20-240V SPST-NO(1FormA)
CPC40055ST  RELAY SOLID STATE 5A 24-280V SPST-NO(1FormA)
 


CPC1968J  RELAY SOLID STATE 2A 0-500V SPST-NO(1FormA)
CPC1726Y  RELAY SOLID STATE 1A 0-250V SPST-NO(1FormA)


offcourse PI PID control (for fast speed use op-amp)  K Type Thermocouple 0-500 Degree Celsius
    Op Amp PID Controller  http://www.ecircuitcenter.com/Circuits/op_pid/op_pid.htm

    ME389_OPA01_TFSynthesis_Guideline.pdf
    _08_ELC4345_Fall2013_PI_Controller.doc
    lesson17et438a.7z

[Giuseppe]

If a resistance is controlled using the triac phase control technique, the emi interference is reduced

[TimB]

I'm going to reply in in sections

Les
"Is it streamed heat. i.e. A hot air flow?" Yes it is.  I want to build what is classed as a HOT BLOCK using hot air not heating an entire ali block.

trastikata
The use is in calibration. It used to be part of my job. The equipment was a heated block of ali with holes in it. They also blow air across it to speed up the cooling so the heater is in a constant dance with the cooling. The more cooling the quicker it cools so your PID heating loop is faster. This enables you to get finer control.
The issue with Ali blocks and holes is your probe placement needs to be just right and you spend ages trying to make sure your probes are in the right place.
Using hot air I can evenly heat the probe (with good air flow design)

okmn
Thanks! When I designed a speed controller for a client it used a mains Triac. The usual control systems, Speed was controlled by timing the switch on from the zero cross point. It worked and they are still in use in industry today, however it was never good at PID control eg put it under load and remove the load it took too long to react. Not my finest work.
I'm going to look your post in more detail. Really appreciate it.

Giuseppe and others

I did the maths 250w (the element rating) is ~21 amps at 12vDc. I can buy 250w 12/24v power supplies for <£30. What about using one of those with a beefy Mosfet to control it?

BTW when I say fine control I mean ~0.1oC (0.05oC even better) So it will work in conjunction with a PT100 probe.

[See_Mos]

Tim,

your reply to trastikata reminded me of my time in Aberdeen where I also repaired calibration and test equipment for one of the local hire companies.  I repaired several dry block and oil filled calibration units.  One brand I remember is the Ametek Jofra

[TimB]

See_Mos

Yes that was the block manufacture I used to use all the time. The same ones I had the issue with.

I need a new system as calibration of gear is REALLY expensive. Like £180 a pop. Kit costs £100 calibration £180
After building my meter I calibrate with precision reference resistors so they are spot on. But as they are paired with PT100 probes (class A) that can be 0.3oc off it means the meter can look 0.3oC out. Unless you calibrate as a pair. Only way to do that is with a hot block/ oil bath and a reference probe.
My distributor gets a lot of calibration done and is charged £100, yay! So I was going to go for that until I found out the do not calibrate the gear. They just test it. Even if it was 10oC out they would give you a cert.

I want to calibrate is first with my own hot block then get the lower cost cert.

[Tenaja]

If this is for a "one-off" and not a product, then use an industrial power controller that takes a 0-10v input.