Help Please..LM311 comparator chip for temperature control of my mirror cooling

[davidpretorius]

Hi Guys,

before I go down the whole Bird way and use picaxe temp loggers etc, I am keen to build a little circuit that



Measures ambient
Measures mirror surface temp
if the mirror is greater than 1.5 degrees below ambient, shut down the peltier coolersI have a bounce back in mirror temps (the middle of the mirror is still warmer) of about 1 degree, so if i can keep the mirror surface 1.5 below, but no more, then i know i can turn off the cooling, wait around 10 minutes and i should have a nicely ambient mirror.

Now, trying to remember back 20 years........

I have a LM311 comparator (http://www.national.com/ds/LM/LM311.pdf (http://www.national.com/ds/LM/LM311.pdf))

I have a LM335Z temp sensor that turns temperature into voltage (http://www.jaycar.com.au/productView.asp?ID=ZL3336&CATID=&keywords=LM335Z&SPECIAL=&form=KEYWORD&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID= (http://www.jaycar.com.au/productView.asp?ID=ZL3336&CATID=&keywords=LM335Z&SPECIAL=&form=KEYWORD&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=))

I have also bought a relay so i can switch on and off the current to the peltiers which draw around 5-8 amps.

I have managed to get the LM335Z temp sensor to give out about 1.3V for 25 degrees and then if i apply a flame to them, then the voltage drops.

When i feed both the two sensors into the two inputs to the LM311, then I do indeed get a swing from say 8 volts down to 0 volts if i apply the flame to the relevant sensor.....................NOW

As soon as I attached the relay or a little computer fan or a big computer fan........the voltage out drops right down and does not react the same way with the load.

I am sure I am doing something really silly

[citivolus]

My gut feeling that that the LM311 doesn't drive enough current for your load and that you will need to run it into a switching transistor. See the schematic in the upper right corner of page 2 of the data sheet for an example driver circuit.

[davidpretorius]

thanks for the tip, I will have a look

[Starkler]

Sounds like the most likely cause. The data sheet shows that the lm311 is never going to source/sink more than 100ma. Also make sure you have a reverse polarity diode across the relay coil to dampen any switching transients.

[citivolus]

Actually I wasn't paying enough attention to the schematic I mentioned. The upper right of page 14 is really what you are after. Oops!

[citivolus]

A quick note: the LM335 takes a bit over 3 minutes to equalise its temperature to ambient in still air, so if you are doing a differential sensor with one sensor exposed to air and one on the mirror, you'll need to let the air sensor equalise in temperature prior to using it if the temperature differential from storage to use is significant. If you don't let it equalise, the Peltier cooler will bounce off and on as the ambient sensor cools.

Also, the LM311 may need a decoupling capacitor to isolate it from the power sag induced by the relay kicking in, especially if the Peltier coolers happen to be running off the same power supply.

[davidpretorius]

thanks guys, great advice.

shopping list

1. power / switching transistor
2. reverse polarity diode

I do have a computer power supply doing the powering of the 12 volts......so I can grab another yellow wire to power the relay