PTC Thermistor: Time Interval to Trip Study by by David C. C. Wilson Owner Owner // Principal Principal Consultant Consultant Wilson Consulting Services, LLC April 5, 5, 5 Page 1-19
Table of Contents Description Page 1.: Introduction------------------------------------------------------------3-19 2.: Summary----------------------------------------------------------------5-19 3.: Statistical Analysis-----------------------------------------------------8-19 4.: Graph of Real Time Data (Lab)------------------------------------14-19 Page 2-19
Introduction 1.: Introduction Objective To determine how long it takes a PTC thermistor to trip after reaching the experimental threshold current. Information Sources Tyco Electronics Data Sheet RXE11-2 Literature on PTC Thermistors by Bowthrope Thermometrics, Thermometrics & Keystone Thermometrics Laboratory Equipment Volt-ohm meters Meter S/W Temperature Chamber Statistical Tools Minitab Excel Product under study PTC (xx327) Sample size = 6 Page 3-19
+ Laboratory Test Circuit Set-up V PTC Thermistor 1.1A Voltage meter + Introduction cont d Variable resistor Vs Input voltage _ I A Experiment Data Load Current Time to Trip Amps Seconds 1.77 21 1.76 73 1.76 61 1.76 61 1.76 25 1.74 41 _ NAC Circuit Current meter Page 4-19
2.: Summary Summary There were six PTC thermistors tested at the experimental threshold of 1.75 Amperes. The circuit application rating is 1. ampere and the MS-512 technical manual explicitly specifies 1. Amp load current. Laboratory Test Results Threshold current and time to trip confidence intervals (T-distribution model) Variables N Mean StDev SE Mean 95% CI Current (Amps) 6 1.75833.983.41 (1.742, 1.76865) Time (Seconds) 6 47. 21.3 8.7 (24.7, 69.3) Monte Carlo Simulation Test Results Threshold current and time to trip confidence intervals (T-distribution model) Variables N Mean StDev SE Mean 95% CI Current (Amps) 1 1.7585.125.32 (1.75787,1.75914) Time (seconds) 1 47.7 21.7.686 (46.3,49.1) The Monte Carlo simulation results indicates a more accurate assessment of load current versus trip time. For example, the confidence intervals, particularly the time it takes to trip are much smaller. The load current passed normality test when the Kolmogorov-Smirnov model was applied to the data. The the time to trip passed normality test when the Anderson-Darling model was applied to the data. Once normality was confirmed, parametric statistics models were applicable. Page 5-19
summary cont d Statistical Graphs Various plots of the data are on the following pages. There is no correlation between threshold current and time to trip. However, an attempt was made to use the exponential model anyway and it showed what was expected, a linear line for threshold load current versus time to trip. This device is a switching PTC thermistor and is intended to protect the board from an external short that might occur across the output NAC circuit. See figure 1 for the exponential graph. Additionally, see figures 3 and 4 for the correlation study. Additional statistical graphs are shown in figures 6 thru 14. The Monte Carlo simulation test results infer that if 1, PTC thermistors were tested at threshold current, 95% of the time, the time to trip will be between 46 and 49 seconds. Real Time Technical Graphs These graphs indicate the interval time period from trip current threshold to trip. The trip is where the avalanche takes place. There is a graph for each sample component. See figures 13 thru 17. Conclusion of Study There is a reasonable degree of practical and statistical certainty that the 1.1 Amperes device will not trip unless the output is driven 75% over the MS-512 load specification of 1. Ampere. See additional studies for this device in RP-66 dated 3/31/5. Summary cont d Page 6-19
Summary cont d summary cont d PTC Thermistors Plot of Load Current (amperes) vs. Trip Time (seconds) Time Expon. (Time) 1.78 1.77 1.77 1.76 Load Current (Am peres) 1.76 1.75 1.75 1.74 1.74 1.73 1.73 21 25 41 61 61 73 Figure 1 Trip Time (Seconds) Page 7-19
Statistical Analysis 3.: Statistical Analysis Probability Chart for Trip Time Probability Plot of Trip Time for PTC Thermistor Percent 99 95 9 7 6 5 4 3 Mean 47 StDev 21.28 N 6 AD.324 P-Value.395 1 5 1 4 6 Trip Time (seconds) 1 Figure 2 This graph also test for normality Page 8-19
Statistical Analysis cont d Empirical Data Correlation Plot Corrleation Plot of Amps vs. Trip Time Amps = 1.761 -.62(Time) Threshold Current (Amps) 1.77 1.765 1.76 1.755 1.75 1.745 S.18935 R-Sq 1.8% R-Sq(adj).% 1.74 3 4 5 6 Trip Time (seconds) 7 Figure 3 Comments: Although the linear line shows a slightly downward slope, the slope is statistically insignificant; therefore, there is no correlation between amps at threshold current and time to trip. Hence: R-squared is only 1.8%. Page 9-19
Statistical Analysis cont d Monte Carlo Simulation Data Correlation Plot Correlation Plot of Amps vs. trip Time Amps_1 = 1.758 +.(Time) Threshold Current (Amps) 1.79 1.78 1.77 1.76 1.75 1.74 S.12454 R-Sq.2% R-Sq(adj).1% 1.73 25 5 75 Trip Time (seconds) 1 125 Figure 4 Comments: Although the linear line shows a slightly upward slope, the slope is statistically insignificant; therefore, there is no correlation between amps at threshold current and time to trip. Hence: R-squared is only.296%. Page 1-19
Statistical Analysis - cont d Probability Chart for Trip Current Probability Plot of PTC Thermiser Trip Current Percent 99 95 9 7 6 5 4 3 Mean 1.758 StDev.9832 N 6 KS.151 P-Value >.15 1 5 1 1.73 1.74 1.75 1.76 Load Current (Amperes) 1.77 1.78 Figure 5 This graph also test normality Page 11-19
Empirical Data Statistical Analysis - cont d Density Curve for PTC Thermistors Trip Time Histogram of PTC Thermistors 1.2 1. Mean 47 StDev 21.28 N 6 2. 2 Mean 47 StDev 21.28 N 6.8 1.5 Frequency.6 Frequency 1. 1 1 1 1.4.2.5. 4 6 Trip Time (seconds). 4 6 Trip Time (seconds) Figure 6 Figure 7 Density Curve for PTC Thermistors (Triip Current) Histogram of PTC Thermistors Trip Current 2.5 Mean 1.758 StDev.9832 N 6 4 Mean 1.758 StDev.9832 N 6 2. 3 Frequency 1.5 1. Frequency 2.5 1. 1.74 1.75 1.76 Trip Current (A mps) 1.77 1.78 Figure 8 1.74 1.75 1.76 Trip Current (seconds) 1.77 1.78 Figure 9 Page 12-19
Monte Carlo simulation data Statistical Analysis - cont d Density Ccurve for PTC Tehermistors Trip Time (Monte Carlo Simulation) Histogram of Trip Time for PTC Thermistor (Monte Carlo Simulation) 9 7 Mean 47.67 StDev 21.69 N 1 1 Mean 47.67 StDev 21.69 N 1 Frequency 6 5 4 3 Frequency 6 4 1 4 6 Trip Time (seconds) 1 4 6 Trip Time (seconds) Figure 1 Figure 11 1 7 Density Curve of PTC Trip Current (Monte Carlo Simulation) Mean 1.759 StDev.125 N 1 Histogram of PTC Thermistor Trip Current (Monte Carlo S imulation) 9 Mean 1.759 StDev.125 N 1 Frequency 6 5 4 3 1 Frequency 7 6 5 4 3 1 1.736 1.744 1.752 1.76 1.768 Trip Current (Amps) 1.776 1.784 1.736 1.744 1.752 1.76 1.768 1.776 1.784 Figure 12 Figure 13 Trip Current (A mps) Page 13-19
4.: Graph of Real Time Data (Lab) Graph of Real Time Data Threshold current point PTC #1 Current (I) trip = 1.74 A Trip interval = 41 seconds Figure 14 Page 14-19
Graph of Real Time Data cont d PTC #2 Current (I) trip = 1.76 A Trip interval = 73 seconds Figure 15 Page 15-19
Graph of Real Time Data cont d PTC # 3 Current (I) trip =1.76 Trip interval = 61 seconds Figure 16 Page 16-19
Graph of Real Time Data cont d PTC# 4 Current (I) trip = 1.76A Trip interval = 25 seconds Figure 17 Page 17-19
Graph of Real Time Data cont d PTC # 5 Current (I) trip = 1.77A Trip interval = 21 seconds Figure 18 Page 18-19
Graph of Real Time Data cont d PTC # 6 Current (I) trip = 1.76 A Trip interval = 61 seconds Figure 19 Page 19-19