Users` Manual - FSU High Energy Physics
Contents
1. If the Products do not conform to this Limited Warranty during the warranty period as herein above specified Buyer shall notify SVS Labs Inc in writing of the claimed defects and demonstrate to SVS Labs Inc satisfaction that said defects are covered by this Limited Warranty If the defects are properly reported to SVS Labs Inc within the warranty period and the defects are of such type and nature as to be covered by this warranty SVS Labs Inc shall at its own expense furnish replacement Products or at SVS Labs Inc s option replacement parts for the defective Products Shipping and installation of the replacement Products or replacement parts shall be at Buyer s expense Other Limits 1 FOREGOING IS IN LIEU OF ALL OTHER WARRANTIES EXPRESS OR IMPLIED z CLUDING BUT NOT LIMITED TO THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE SVS Labs Inc does not warrant eg ainst damages or defects arising out of improper or abnormal use of handling of tt Products against defects or damages arising from improper installation where wistallation is by persons other than SVS Labs Inc against defects in products lt ponents not manufactured by SVS Labs Inc or against damages resulting from Such non SVS Labs Inc made products or components SVS Labs Inc passes on to LE the warranty it received if any from the maker thereof of such non SVS Labs ac made products or components This war2. SET TEMP Allow about 5 minutes time on each setting for the temperature to stabilize and take the readings of temperature and current Draw the graph Inl vs 1 T PRECAUTIONS 1 v characteristics of LED should be drawn at very low current upto 1000 uA only so that the disturbance to Vois minimum 2 In T mode make sure that the oven switch is OFF and SET TEMP knob is at minimum position before connecting the oven 3 On each setting of temperature please allow sufficient time for the temperature to stabilize normally 5 6 minute is required 4 Though temperature of oven may go upto 70 C it is recommended that reading may be taken upto 60 C only to avoid excessive heating of LED 5 In case the LED is replaced please note that height of the portion inside the oven should not be more than 26mm otherwise it may strike the RTD REFERENCES 1 R Morehouse Am J Phys 66 1998 12 2 F Herrmann and D Sch tzle Am J Phys 64 1996 1448 3 LL Nieves The Physics Teacher 35 1996 108 ame TYPICAL RESULTS EXPERIMENT I Determination of Material Constant Sample Yellow LED Room Temperature 305K S No Junction Voltage V Forward Current I in Volts in pA 40 1 604 1 629 1 644 1 676 1 711 1 743 1 765 l V Characteristics Sample LED Yellow Ambient Temp T 305K 6 5 i 55 1 Alni 2 5 5 AV 0 124 3 1 4 1 6 1
3. 8 2 Junction Voltage V Graph no 1 From graph 1 Junction Voltage V vs InI we get AV _ 0 124V AlnI 2 5 Slope of the curve 19 Therefore qo 5 AV 1 60210 x0 124 kT AlnI 1 381x107 x 305 2 5 n 1 89 LXPERIMENT Determination of Temperature Coefficient of Current Sample Yellow LED 1 805 V Constant for whole set of readings Temperature Temperature Current LT X 10 C K mA K Lin ud T I Characteristics Sample LED Yellow V 1 805V Aln 0 52 In mA Graph No 2 From Graph No 2 1 vs Inl AlnI 0 52V Slope of the curve 7 a m AT x10 0 26 x 10 AlnI k AT x10 e Ve eons 100 1381107 ug 0 26x10 1 602 x10 Vo 1 805 2 0 x 10 x 0 862 x 107 x 1 89 Vo 1 805 0 326 Vo 2 13 eV 5800A as measured by diffraction grating how ex Vy xA_ 1 602 x 107 2 13 x 5800 x 107 h 3x10 h 6 60 x 1077 Joules sec LIMITED WARRANTY SVS Labs Inc a California Corporation having its principal place of business at 12262 C eta Avenue Suite 121 Saratoga California 95070 SVS Labs Inc warrants its PLANCK S CONSTANT APPARATUS Model PCA 01 89 products the Products as 4 1005 1 Limited Warranty SVS Labs Inc warrants that the Products sold hereunder will be free from defects in material and workmanship for a period of NINETY 90 DAYS from the date of purchase
4. As where the fraction x varies between 0 and 1 These materials are direct semiconductors The crystals are doped with small amounts of different impurities in adjacent regions to form a PN junction or diode These doped crystals emit light when a voltage is applied across the junction The colour of the emitted light depends on the exact value of x THEORY The basic idea in this measurement is that the photon energy which from Einstein s relation is E hv is equal to the energy gap Eg between the valence and the conduction bands of the diode The gap energy E is in turn equal to the height of the energy barrier eV that the electrons have to overcome to go from the n doped side of the diode junction to the p doped side when no external voltage V is applied to the diode In the p doped side they recombine with the holes releasing the energy as photons with E eVo Thus a measurement of V indirectly yields E and the Planck s constant if v is known or measured However there are practical and conceptual problems in the actual measurement Let us consider the LED diode I V equation I ac exp Vo V exp V V 1 RI ERIT where V nkT e T and e are Boltzmann constant absolute temperature and electronic charge respectively Vm is the voltmeter reading in the external diode circuit and R is the contact resistance The constant is the material constant which depends on the type of diode location of the
5. ab anion User s Manual PLANCK S CONSTANT APPARATUS Manufactured by Scientific Equipment amp Services 358 1 New Adarsh Nagar Roorkee 247 667 UA INDIA Ph 91 1332 272852 277118 Fax 91 1332 274831 Email ses sestechno com Website www sestechno com Model PCA 01 MOODY INTERNATIONAL ISO 9001 2000 Certified Company Authorised reseller and service providers in North America SVS Labs Inc 12262 Goleta Avenue Suite 210 Saratoga CA 95070 USA Phone 1 408 230 9381 Fax 1 408 517 0557 e mail info svslabs com website www svslabs com DETERMINATION OF PLANCK S CONSTANT FROM THE LED INTRODUCTION The Planck s constant is one of the universal constants which a student comes across quite early It is one of the basic ingredients of quantum physics Its measurement naturally has to be part of any college university physics laboratory program Traditional method of measurement has been a determination of current cut off voltage of a vacuum photocell irradiated by a monochromatic source of light Vacuum photocells are not easily available now and a reasonably strong source of monochromatic light is also difficult to maintain in an undergraduate laboratory An alternative method is however available It employs light emitting diodes LEDs which are widely used in various consumer products and are easily available Most LEDs are based on GaAs and GaP crystals with general composition Ga
6. he diode Fig 2 at room temperature from the relation n e KT AV A Inl NEU The Planck s constant 15 then obtained by the relation h eV A c 4 The contact resistance of the LED is usually around 1 ohm while overall internal resistance of the LED at applied voltage 1 8 V is few hundred ohms The factor RI in expression V V RI may therefore be neglected The value of Planck s constant obtained from this method is within 596 of accepted value 6 62 x 1079 Joules sec EXPERIMENTAL SET UP The set up consists of the following units 1 To draw I V characteristics of LED i Variable voltage source e Range 0 1 95 V Variable e Resolution e Accuracy 0 2 e Display 3 digit LED DPM ii Current Meter e Range 0 2000 pA e Resolution e Accuracy 0 2 e Display 3 digit LED DPM 2 Dependence of current 1 on temp T at constant applied voltage i Current Meter e Range 0 20 mA e Resolution 10 pA e Display 3 digit LED DPM ii Temperature Controlled Oven e Range Ambient to 65 C e Resolution 0 1 C e Stability 0 2 C Display 3 digit LED DPM iii Variable Voltage source of 1st section is used to apply a constant voltage DESCRIPTION OF APPARATUS 1 MAINS ON OFF SWITCH To switch On Off the instrument 2 VOLTAGE ADJ KNOB To adjust voltage 3 VOLTAGE TEMPERATURE DPM Read voltage in V I mode and temperature of oven in T I
7. mode 4 LED SOCKET To connect LED samples 5 CURRENT DPM Display current in pA in V I mode and in mA in T I mode 6 V I T I SWITCH A two way switch to switch the system between V I mode and T I mode 7 TEMPERATURE CONTROLLER a ON OFF Switch for oven b OVEN Socket on the panel is to connect the external oven c SET TEMP Knob to set the temperature d OVEN It is a small oven with built in RTD sensor PROCEDURE 1 To draw I V characteristic of LED i 1 ii Connect the LED in the socket and switch ON the power Switch the 2 way switch to V I position In this position the 1 DPM would read voltage across LED and 2 DPM would read current passing through the LED Increase the voltage gradually and tabulate the V I reading Please note there would be no current till about 1 5 V Draw the graph InI I in pA Vs V 2 Dependence of current I on temperature T at constant applied voltage 0 ii iii iv Keep the mode switch to V I side and adjust the voltage across LED slightly below the band gap of LED say 1 8 V for Yellow Red LED and 1 95 for Green LED Switch the MODE switch to T I side Insert the LED in the oven and connect the oven to the socket Please make sure before connecting the oven that oven switch is in OFF position and SET TEMP knob is at minimum position Now the DPM would read ambient temperature Set the different temperatures with the help of
8. oltage across the LED s at a single current all the LED s of the set are connected in series The intercepts of the tangents are except for an additive constant identical to diode voltages The additive constant may be eliminated by considering data from different LED s However the bulk of data collected from the original I V graph becomes irrelevant A basic drawback of these methods is the assumption that the barrier height V is constant equal to the gap energy E divided by the electronic charge E e which is true only when the external potential V is small or atleast less E e They further assume that the material constant is unity which is not correct It may have any value from about one to about two varying from LED to LED The present method is free from these infirmities The height of the potential barrier is obtained by directly measuring the dependence of the diode current on the temperature keeping the applied voltage and thus the height of the barrier fixed The external voltage 18 kept fixed at a value lower than the barrier The idea is that the disturbance to the potential barrier is as little as possible In our experimental set up the variation of the current I with temperature is measured over about a range of about 30 C at a fixed applied voltage V 1 8 volts kept slightly below V The slope of nI vs 1 T curve gives e V nk Fig 1 The constant n may be determined separately from I V characteristic of t
9. ranty also does not apply to Products seen which repairs have been effected or attempted by persons other than ritten authorization by SVS Labs Inc Exclusive Obligation 5 WARRANTY IS EXCLUSIVE The sole and exclusive obligation of SVS Labs Inc _ 1 be to repair or replace the defective Products in the manner and for the period provided above SVS Labs Inc shall not have any other obligation with respect to 2 e oducts or any part thereof whether based on contract tort strict liability or otherwise Under no circumstances whether based on this Limited Warranty or OL c 5e Shall SVS Labs Inc be liable for incidental special or consequential Ex NUR C er Statements 5 5 Labs Inc s employees or representatives ORAL OR OTHER WRITTEN ST T MENTS DO NOT CONSTITUTE WARRANTIES shall not be relied upon Wee Buyer are not a part of the contract for sale or this limited Entire Obligation T gt i Warranty states the entire obligation of SVS Labs Inc with respect to the roducts If any part of this Limited Warranty is determined to be vara or rd w aii der shall remain in full force and effect SVE rox
10. recombination region etc The energy barrier eV is equal to the gap energy Eg when no external voltage V is applied as pointed out earlier The quantities which are constant in an LED are the atom density the charge diffusion properties and the effective diode area The in the rectifier equation is negligible if I 2 2 nA and the equation becomes exp V V V exp e V V nkT ses 2 A direct method could be to apply a small voltage on the LED and increasing it till the LED is turned on This turning on could be detected by visually observing the light emission Plotting threshold voltage vs frequency of peak light output obtained from LED c4lasheets or from separate spectroscopic measurement provides the value of h e The visual observation of the emission on set is quite vague Use of a photo multiplier is sometimes suggested for this purpose but working with it raises maintenance problems and is quite costly Alternately a measurement of threshold current lt 107 A through the LED may be attempted but it is difficult and not entirely accurate due to inefficiencies of actual LED s Another procedure some times used is to draw a tangent to the I V characteristics of the diode and obtain its intercept This procedure may give reasonable good results if the tangents to the I V characteristics of all the diodes are drawn at the same current The method then really becomes equivalent to measuring v
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