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| Acmlm's Board - I2 Archive - General Chat - Physics/Electronics |
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goodall Double hammer Level: 11  Posts: 36/38 EXP: 5405 For next: 580 Since: 05-19-04 From: the UK Since last post: 189 days Last activity: 194 days |
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| im not sure if this is the right place to post this, but here goes. im trying to do a piece of coursework for physics, and ive been off for a while so im not to sure with this module. anyway. i have been given a set of volatges for a thermistor and the temperatures that match up with them, the thermsitor was also in a potential divider circuit with a 4.65Kohm resistor. anyway i need to find an equation that allows me to find the resistance of the thermistor at a certian temperature. can anyone help? |
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Kefka Indefinitely Unbanned Level: 81 Posts: 2740/3392 EXP: 4826208 For next: 166641 Since: 03-15-04 From: Pomona, CALIFORNIA BABY! Since last post: 4 hours Last activity: 4 hours |
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| Steinhart & Hart: T = 1 / ( a + b.ln(R) + c.ln(R)3 ) I actually somewhat remember that equation because my science teacher leaves all his physics equations up on the board for some reason (there are SO MANY), but the only reason I was able to recall that at all was because of a quick Google search.  a, b, c = constants In( ) = natural logarithm R= resistance in ohms T= Temperature in K (so that would be Celsius + 273 if I recall right) I should quit acting like I know what I'm doing now  Anyway, that should do it  |
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goodall Double hammer Level: 11 Posts: 37/38 EXP: 5405 For next: 580 Since: 05-19-04 From: the UK Since last post: 189 days Last activity: 194 days |
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| i did actually use google. thats not what i was looking for, i believe i actaully saw the site you quoted. anyway, im looking for an equation giving me the resistance of the thermistor, based on a fixed resistor, and 2 voltage values. thanks for your help though | |||
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Sliver X Level: 8  Posts: 5/19 EXP: 1942 For next: 245 Since: 05-01-04 Since last post: 1 day Last activity: 3 days |
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| Considering thermistors are built to different specs, you need to know some information about it (Usually this is accomplished by looking at the "White Sheet" for the component; I hope your instructor gave you this information). The formula is as follows: R = Ro exp( Beta/T - Beta/To) R Thermistor resistance at T (K) T Thermistor temperature (K) Ro Nominal resistance at To (K) To Temperature where Ro is measured Beta Thermistor material constant Once you have the resistance for the thermistor, you can deduct the following: For the thermistor: I through thermistor will be V(total) / (R(thermistor) + R(fixed resistor)) V drop across the thermistor will be I * R(thermistor) Since the thermistor is in series with the fixed resistor: V drop across the resistor will be V(total) - V drop (thermistor) I through the resistor will be the same, since it's a series circuit. (edited by Sliver X on 04-26-05 12:50 AM) (edited by Sliver X on 04-26-05 12:53 AM) |
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goodall Double hammer Level: 11 Posts: 38/38 EXP: 5405 For next: 580 Since: 05-19-04 From: the UK Since last post: 189 days Last activity: 194 days |
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| once again thanks, but not whta im looking for. i need a formula where temperature is not involved. consider the thermistor a fixed resistor. maybe that helps | |||
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Sliver X Level: 8 Posts: 6/19 EXP: 1942 For next: 245 Since: 05-01-04 Since last post: 1 day Last activity: 3 days |
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| Ok, assuming the two voltage values you're given deal with the voltage that would be measured across either of the resistances (in series), all you need is simple Ohm's Law and Kirchoff's Voltage Law. Ohm's Law works like so: (Voltage = V) (Current = I) (Resistance = R) V = I * R I = V / R R = V / I Kirchoff's Voltage Law states that any resistances in series' voltage drops will equal the sum of the total voltage being applied to them. So, if you have a 12V source going to the thermistor and the resistor, and the resistor is dropping 4V across it, then the thermistor *must* drop the remainder of V source: 8V. Therefore, to determine the resistance of the thermistor, use Ohm's Law: If the voltage drop across the fixed resistor is known, divide that by its resistance to get total current (Any linear resistances in series will have the same amount of current flowing through them). Now use Kirchoff's Voltage Law to get the V drop across the thermistor. Divide this by total current, and viola, you have the thermistor's resistance. If this isn't what you're talking about, please post what exactly the problem you've been given is: it'd make this *so* much easier for me to explain that way. (edited by Sliver X on 04-26-05 10:30 AM) |
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