LM35 EMITTER FOLLOWER OR ANALOG - Page 1
Forum section: electronics / beginners

author: neko efecktz
author_rank: Regular Contributor
timestamp: October 23, 2016, 03:47:54 am
content: I need a few LM35 Thermal sense chips for my power supply.They will be monitoring heat sinks and a couple of components that get a little warm.I have found that RS Components have a few versions.Some are from -40° to 110° and others from 0° to 100°I don't want to boil water on my heat sinks so the 0° to 100° would do nicely.My question is...There are two types available from RS.Emitter Follower and Analog.Is there much difference in the way they are used when driving an analog input of a PIC16F877A.?Thank you.BILL

author: JS
author_rank: Frequent Contributor
timestamp: October 23, 2016, 04:04:22 am
content: Please take the time and add a link to the data-sheet of the device you are asking about, if I'm taking the time to help you I expect not to be hunting for a given data-sheet.Without the data-sheet I can tell any LM35 would have an analog output, but it could be different output topologies. TI LM35 has an output stage with an emitter follower, also a much wider range. There are several ways to use the sensor, with offset for negative ranges, 2 wire connection for practicality, directly for low part count, low Z for noisy environments, etc. You''ll need to find what's best for you.Note that is an expensive part compared to other things that would do the job just right if the only thing you need to do is to control a fan to cool down your supply. The advantage it has is that is quite easy to connect it to an ADC and read the voltages and convert to temp with a linear relationship. If you need a comparator any silicon diode would do. We are talking over 50 times the cost, for DIY one off is ok, for production if you just need to make a comparison between the temp of your heatsink and a reference to turn on a fan, doesn't worth it.JS

author: neko efecktz
author_rank: Regular Contributor
timestamp: October 23, 2016, 06:54:28 am
content: gidday JSI havn't been very lucky in finding specific data sheets for the different versions of device.There are so many versions with the same prefix name of LM35.I just looked up LM35 in the RS Components on line shop.The link I attached is for TEXAS INSTRUMENTS LM35.It covers a general range of types.The main Purpose for the LM35 in my PSU is to monitor heat sink and a couple of components that get warn.If the temperature is over a preset value the temperature will be displayed on a 1602 lcd display.Texas Instruments LM35DZ/NOPB Temperature Sensor, 0 ? +100 °C, 3-Pin TO-92 is an ANALOG output deviceTexas Instruments LM35DZ/LFT1 Temperature Sensor, -55 ? +150 °C, 3-Pin TO-92 is an EMITTER FOLLOWER output devicehttps://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahUKEwiz7-vnqPDPAhVW12MKHbk2Ax8QFghIMAE&url=http%3A%2F%2Fwww.ti.com%2Flit%2Fds%2Fsymlink%2Flm35.pdf&usg=AFQjCNEYRFMfIX0Uy2rBPHutCN0J7Z9Zvg&sig2=nghUByVZk2RBUL0yAs8o-gthankyouBILL.

author: Zero999
author_rank: Super Contributor
timestamp: October 23, 2016, 07:59:44 am
content: Quote from: neko efecktz on October 23, 2016, 06:54:28 amI think you're confused about the terminology. All the LM35s have analogue emitter follower output stages. The difference is the packaging and temperature range.Look at the internal block diagram is on page 13 of the datasheet linked below.http://www.ti.com/lit/ds/symlink/lm35.pdfHaving an emitter follower output means it can only source current, not sink., i.e. it can only supply a positive, not negative current to the load. This means a load resistor can be connected between the output and 0V but not +V. The load should be as small as possible, to minimise internal heating of the IC, which would disrupt the measurement.

author: neko efecktz
author_rank: Regular Contributor
timestamp: October 23, 2016, 08:45:57 am
content: Thank you Hero999I was a little confused by the terminology used in the RS Component On line Store.Most of the devices listed were shown as Emitter Follower Output but one was shown as Analog Output.I will have a read of the data sheet and place an order for some chips in the next day or so.Thank you for your help.BILL

author: JS
author_rank: Frequent Contributor
timestamp: October 23, 2016, 03:02:50 pm
content: Quote from: neko efecktz on October 23, 2016, 08:45:57 amIt's quite usual for online stores to give different descriptions for the same item, don't trust that, go to the datasheet (and don't trust those so much either)The internal circuit is the same for all the variants, they behave the same. The only constrain is the range of temperatures they can handle, not because the circuit is different but because they are mechanically different or just calibration difference which makes it to comply the specs within certain range and not outside of it.JS