and yet another thermostat question...

[andefault]

hello people, i have slight dilemma. i need to build a thermostat to control a heating element in an incubator (for an engineering project), and i'm not sure of whether or not my design will work. i have a TC622EAT temperature sensor configured as following:
Vdd to +6v
Gnd to gnd....
Tset - 123.3kohm resistor - Vdd
Out - gate on a mosfet

i know a little about electronics (i'm a mechanical eng student though) but if i have the drain and source outputs connected to ground and +12v respectively; can the mosfet act as a switch for the heating element safely? It will be switching on and off a 55W (ish) load. ie around 3-4amps. Oh yes i have separate power supplies for both the TC622 (4AAA batteries - 6v) and the heating element, (12 DC car battery).
So... will my circuit work?
Cheers

 

[Boncuk]

Please do some more and post your schematic. This helps the members to help you.
Also don't forget to post the desired trip point temperature.

Boncuk

 

[andefault]

schematic

ah yes a schematic... sorry about that. n00b mistake - trust me i haven't any drawing programs installed at the moment so we'll have to look at my nicely hand drawn one. yeah the trip point temperature is 38deg C = 311.15deg K... which i substituted into the formula on the data sheet for the TC622 (also on my picture) to give a value of R equal to 123.3kΩ.
enjoy

 

[andefault]

as an aside: can anyone suggest a good program for drawing schematics with?

 

[ericgibbs]

ah yes a schematic... sorry about that. n00b mistake - trust me i haven't any drawing programs installed at the moment so we'll have to look at my nicely hand drawn one. yeah the trip point temperature is 38deg C = 311.15deg K... which i substituted into the formula on the data sheet for the TC622 (also on my picture) to give a value of R equal to 123.3kΩ.
enjoy

hi,
Whats the FET type and why is the fuse in the 0V line.?

 

[ericgibbs]

as an aside: can anyone suggest a good program for drawing schematics with?

Google for expresspcb its a free schematic and pcb layout package.

You can also add your symbols.

Also get pdfcreator [free] so that you can save the expresssch in pdf format.

Also get ScreenHunter 5.0 [free] grab a section/all the screen as 'gif'

OK.?

 

[andefault]

well to tell you the truth i wasn't sure where to put the fuse... it looked ok there at the time. i'm guessing it should be between the light and s on the fet? 50% chance of being right for the mosfet i was thinking an IRF520; which has a tolerance of up to 8A. oh yes that means i need to reduce the fuse to 7ishA too... cheers for the program suggestions too ericgibbs i'll need it for my report. 20000 words due in tomorrow! yew!

Also get ScreenHunter 5.0 [free] grab a section/all the screen as 'gif'

haha shift+command+4 on mac

 

[andefault]

i just got served... expresspcb isn't supported by mac.
lol

 

[Boncuk]

i just got served... expresspcb isn't supported by mac.
lol

but Eagle is, http:www/cadsoft.de

Isn't 38 degrees C a bit hot for an incubator? Makes fried eggs though.

Also the transistor should be connected to /out. Connected to out the circuit won't stop heating at the trippoint.

 

[andefault]

nah the temperature for incubation is a constant 37.5degrees Celsius... which is the same as our core body temperature. haha you should be worried if you can smell pork... but half a degree is ok when we consider the hysteresis of the TC622

anyways... any verdicts on my design?

 

[andefault]

Also the transistor should be connected to /out. Connected to out the circuit won't stop heating at the trippoint.

cheers Boncuk; your help is greatly appreciated

 

[ericgibbs]

cheers Boncuk; your help is greatly appreciated

hi,
The datasheet for the TC622 states that it will work from 4.5 to 18V, so why are you powering the TC622 from 6V and the lamp from 12V.?

 

[andefault]

hi,
The datasheet for the TC622 states that it will work from 4.5 to 18V, so why are you powering the TC622 from 6V and the lamp from 12V.?

well i thought that the risk of the TC622 being fried if it were powered by a car battery too high; as it can only tolerate up to 600µA. However... do you think it may be safe to get rid of the 6V (4* 1.5v AAA batteries)? i shall include a schematic of my original circuit... which instantly fried the TC622 when turned on... and you can suggest where i went wrong or maybe some mods i could make to make it go...
bear in mind that i only have 4 unfried TC622's left and i had to get them from america.
lol

 

[ericgibbs]

well i thought that the risk of the TC622 being fried if it were powered by a car battery too high; as it can only tolerate up to 600µA. However... do you think it may be safe to get rid of the 6V (4* 1.5v AAA batteries)? i shall include a schematic of my original circuit... which instantly fried the TC622 when turned on... and you can suggest where i went wrong or maybe some mods i could make to make it go...
bear in mind that i only have 4 unfried TC622's left and i had to get them from america.
lol

hi,
Did the TC 'fry' when it was actually driving the FET.?

If its a cause for concern you could use a 78L05 or 78L09 100mA voltage regulator to drive the TC.

BTW: a low current device like the TC drawing about 600uA could be happily powered by a 12V 120AHr battery. It will only draw the current from the battery that it needs to operate.
Think about your house lights, some are 40Watt some 100Watt, all connected to say 240Vac.

 

[andefault]

mm i'm not sure if it died whilest it was driving the FET. it all happened rather quickly. however; yes it was connected to said fet. to test if the temperature sensor will work safely with the big battery shall i connect one up w/out the fet (as i don't have one of those anymore ) and put my multimeter between ground (-ve battery terminal) and the /out... and see if the voltage goes to zero once the temperature trip point is reached?

and if it can do that without smoking... well theoretically it should easily drive a fet shouldn't it? (like in the second diagram that i attached earlier)... which could only mean that i somehow bollocksed the first prototype up somehow...?
thanks so much for all yous guys and gals help btw... without it i'd be down to my last sensor. come to aus and i'll buy you a beer

 

[ericgibbs]

mm i'm not sure if it died whilest it was driving the FET. it all happened rather quickly. however; yes it was connected to said fet. to test if the temperature sensor will work safely with the big battery shall i connect one up w/out the fet (as i don't have one of those anymore ) and put my multimeter between ground (-ve battery terminal) and the /out... and see if the voltage goes to zero once the temperature trip point is reached?

and if it can do that without smoking... well theoretically it should easily drive a fet shouldn't it? (like in the second diagram that i attached earlier)... which could only mean that i somehow bollocksed the first prototype up somehow...?
thanks so much for all yous guys and gals help btw... without it i'd be down to my last sensor. come to aus and i'll buy you a beer

hi,
I would test the TC as you say, on the bench, no FET attached, with a voltmeter to the Iout pin, check that its switching at temperature OK.

You could also check the FET driver on its own with the load connected.

Connect a 100K from Gate to 0V, then connect [touching] a 1K from the Gate to +12V. That should switch the FET ON/OFF and the light.

 

[andefault]

thanks eric... i'll give it all a go in a short while... gotta finish writing about it first in my report lol. i'll let everyone know of it's success and post a final schematic for anyone else who needs to make a temperature dependent switch once it works