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Topic: Peltier temp control PWM method - sensor analog reading value (Read 5088 times) previous topic - next topic

dc42

As well as checking that Rds(on) is quoted at logic level gate-source voltage (i.e. around 4.5v, not 10v), you need to check that it is quoted at at least the drain current that it will pass in your circuit (17A).

Unlike bipolar transistors, mosfets share current well when connected in parallel. So, if you have problems sourcing a mosfet with a very low Rds(on) at 17A current, you could use two or three mosfets with a higher Rds(on) at one half or one third the current.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

geoland

Thank you.

RFD12N06RLE - This fits the specifications. http://www.mouser.com/catalog/specsheets/RFD12N06RLE.pdf

dc42

#17
Nov 17, 2011, 12:44 pm Last Edit: Nov 17, 2011, 07:18 pm by dc42 Reason: 1

RFD12N06RLE - This fits the specifications. http://www.mouser.com/catalog/specsheets/RFD12N06RLE.pdf


Not really. At 4.5v Vgs they are only good up to 8A, and the Rds is rather high (75 milliohms max). You could use 3 in parallel, but they would still get quite hot.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

dc42

#18
Nov 17, 2011, 01:10 pm Last Edit: Nov 17, 2011, 07:19 pm by dc42 Reason: 1
PS - this one http://docs-europe.electrocomponents.com/webdocs/0f35/0900766b80f35037.pdf is inexpensive and quite good for your application, although it will still dissipate around 5W at 18A, so you might choose to use 2 in parallel.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

cjands40

I read the original requirement as 8A so one of those should be fine (~1W dissipation).  My quick search shows it to be a little bit cheaper than the IRLB8743 so back to the OP: where are you located and what are your cost constraints?  Would something like the STP60N3LH5 be easier/cheaper for you to source?

Back to your sensor question (since this is the Sensors forum  8)), you mentioned wanting at least +/- 0.5 deg C accuracy but the TMP36 appears to only be +/- 2 deg C.  Is that going to fit your needs? The DS18B20 robtillaart mentioned earlier does spec +/- 0.5 deg C accuracy.

geoland

Thanks again... The problem is freight costs to Australia. However, I have found JK Devices who do post at a very reasonable price. I balk at paying $$$$ for freight for small cheap components. I will follow up your suggestions. I think I have enough to work with.

I must check the specs on the TMP36 again. 0.5 is preferable for dark frame consistency (low light photography). +/- 2 means shooting many more dark frames to better the average.

dc42

You can get better accuracy than +/- 2 degC from the TMP36 if you are prepared to calibrate it (e.g. in an ice bath). However, you will be limited to around 0.4C resolution when you read it through the ADC, as previously outlined.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

robtillaart

Quote
The problem is freight costs to Australia.

Check - http://arduino-direct.com/sunshop/ - they might be cheaper ...
Rob Tillaart

Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -
(Please do not PM for private consultancy)

geoland

Thanks again to you both. I am talking to Avnetexpress about their shipping costs to see if regular mail is an option, as they have the STP60N3LH5 and IRLB8743.

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