So my vaporizer broke and I thought
I might try to fix it. At first I
thought the heating element was the cause because there was some brown
discoloration on the wire side of it, but then upon inspection of the external
power supply and components I discovered that almost everything was destroyed,
probably from a transient. This quickly
turned into a complete rebuild. All
temperature mentioned in this article is in Fahrenheit.
So all I have to start with is a
heating element. I am surprised to see that
heating elements like these are hard to find, expensive, and only come from
china. If anyone knows where I can buy
small cheap ceramic elements please tell me.
My instant thought about heat
control was pulse width modulation, common to most micro-controllers. A digital to analog module would probably
work better. This pwm signal would
control a beefy transistor to control the amount of current to the element. I am aware that the two small conductors
protruding from the element are some sort of sensor. They provided garbage when I measured them
and I didn’t know if they provided a voltage or a resistance so I did not use
them. This means that there is no
feedback in this project! Gasp! If
anyone can explain how those are used for temperature measurement please tell
me.
I tried to do this project as cheap
as possible. I would have liked to use
an old computer PSU, but I need at least 15V at 3A. Even after bridging the +12V and -12V lines,
the PSU would give out because the -12V line can only handle 800mA. I tried the reference adjustment trick but it
didn’t help that much. So I had to buy a
Mean Well rs-100-24 power supply. If any
of you are lucky enough to find old high output power supplies keep them, they
are valuable and expensive, and could go in your next project. I used the case of a computer PSU because it
had the fan, AC receptacles and a switch I needed.
With the power supply solved I
moved to power transistors. I broke open
a few computer PSU to find that many have power transistors inside, but some do
not. After experimenting with a few I
decided to use a pair of 13009 NPN transistors.
Amazingly they were already screwed to the same heat sink. The benefit of using these over MOSFETS is
that the output is linear and it makes controlling the heat much easier. I took other heat sinks and screwed them all
above and underneath the pair until I had a heat sink that weighed about a
pound. Even with a fan on it, it reaches
140 degree F. The main challenge in this
project it seems is heat management. The
factory circuit of the vape used a pair of F7416 P-channel mosfets which have a
very low effective resistance. As you
can see from the only factory circuit
board: there are no heat sinks. It
appears that buying very efficient transistors is worth the money.
Note that I use a weird
Darlington-like circuit for my microprocessor interface. These 13009 transistors needed about 30mA
each through their base to fully turn on.
I was worried that PWM would harm the power supply so the 100uF capacitor
smoothes the waveform to be more like DC.
I did not have an oscilloscope but that would definitely help.
I chose the 9s08 because it is
cheap, the programmer is cheap, and it has an internal oscillator. But with only 14 I/O I could not make an extravagant
display, even with external ICs. Since
the selectable heat range is 250-375 I decided to make a binary display with 1
representing 250 degrees F and so on.
This way, everything can be displayed accurately.
Math time! There is no relation between power
dissipation and thermal energy. I
believe this is because power is linear but heat is volumetric. So I had to find a relation experimentally. I made the voltage across the heating element
10V. From now on I will call the heating
element the coil. The coil has a DC
resistance of 6.6 ohms. So the coil
current is 1.515A. The coil was
contained in its original housing with the top closed off so no air could
escape. After allowing the coil to heat
for a generous 30 minutes, I took the measurement with an Extech 42505 infrared
thermometer (the kind you point and shoot).
The temperature read 280 degree F.
Take 1.515A/280 and we get 5.41mA per degree F. With this relation I can now accurately
predict the temperature.
Because of my slow bus clock, and
the need for high precision PWM, I struck a balance. So each time a button is pressed the
temperature will change by +/- 1.4 degrees, and the display will verify this.
With all these new electronics, I
needed a new case to hold it all and be sturdy.
It’s not technical so I’m not going to talk about construction, just get
‘r done. I made it in three pieces so it
could be taken apart without tools for cleaning or repair (and you better believe it will
need cleaning). A bit bulky but solid. I got the small amount of scrap plexiglass I needed for free from my local Lowe's! I used that piece of pottery on top of the bowl to contain heat so I could measure it. Enjoy!
Code:
INCLUDE 'derivative.inc'
XDEF _Startup, main
XREF __SEG_END_SSTACK
MY_ZEROPAGE: SECTION SHORT
MyCode: SECTION
_Startup:
main: LDHX #__SEG_END_SSTACK
TXS
CLI
LDA #%01110011
STA SOPT1
MOV #%01000100,ICSC1
MOV #000000,ICSC2
MOV #$FF,PTBDD
MOV #$F9,PTADD
MOV #$06,PTAD
MOV #001000,MTIMCLK ; prescaler= 64
MOV #16,MTIMMOD ; 15625/256=61 61/MTIMMOD=Hz
MOV #000000,MTIMSC ; TOIE=0, TRST=0
LDHX #312 ;10Hz
STHX TPMMODH
LDHX #220 ;300d=10.7V=70.6% #220
;STHX $0064 ;SAVE DUTY
STHX TPMC0VH ;PIN 16
MOV #$28,TPMC0SC
MOV #$08,TPMSC
LDHX #0
STHX $0062
LDA #49
STA $0060
STA PTBD
LOOP BRCLR 1,PTAD,III
BRCLR 2,PTAD,DDD
BRA SK
III JSR III2
BRA LOOP
DDD JSR DDD2
BRA LOOP
SK JSR TIMER
LDHX $0062
AIX #1
STHX $0062
CPHX #$5460
BHI P_OFF
BRA LOOP
P_OFF LDHX #0 ;TURN OFF THE ELEMENT
STHX TPMC0VH
MOV #$00,PTBD
HERE BRA HERE
SWI
;*****************************************************
; ROUTINES
;*****************************************************
III2 LDHX TPMC0VH
AIX #1 ;INCREASE TEMP AND DISPLAY
CPHX #266 ;HIGH LIMIT=85%
BHI H_LIMIT
STHX TPMC0VH
;STHX $0064
LDA $0060
ADD #1
STA $0060
STA PTBD
JSR TIMER ;DEBOUNCE
H_LIMIT RTS
DDD2 LDHX TPMC0VH
AIX #-1 ;DECREASE TEMP AND DISPLAY
CPHX #172 ;LOW LIMIT=55%
BLO L_LIMIT
STHX TPMC0VH
;STHX $0064
LDA $0060
ADD #-1
STA $0060
STA PTBD
JSR TIMER ;DEBOUNCE
L_LIMIT RTS
TIMER BRCLR 7,MTIMSC,TIMER ;WAIT FOR TOF
BCLR 7,MTIMSC ;CLEAR TOF
RTS
