Ultra-Lite Digital SMU

Part 3 - Testing Zeners, LEDs, Thermistors

 

We've built an ultra-lite SMU to learn digital control and component testing. Why? A small system provides an easy on-ramp to grasp bigger ideas without getting lost in a myriad of circuit details. Later, you can develop more complex SMU's on a solid foundation.

 Part 1 - describes / simulates the essential blocks of a digital SMU.
 Part 2 - builds a prototype with an Arduino UNO and a few components.
 Part 3 - tests real-world components.

Check out EXCEL test plan/calculator link below.

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ZENER - REVERSE VOLTAGE TEST

BZX79-B2V7, 2.7V, ±5%

      

• Datasheet Specification: 
    Vz=+2.7V @ Iz=5mA
• Test Strategy:
    Force Current (FI), Measure Voltage (MV)
• Edit test params in Arduino setup() code:
   forceMode = FIMode;
   FI = 0.005;
   Rsense = 100;
   Kint = 20000;  
• Install
    Rs = 100
    D1 = Cathode to Vo, Anode to GND
• Test Limits
   Vo = 2.6V to 2.8V
• Expected Current Sense Voltage
   Vs = Io*Rs = 0.005*100 = 0.5V
• Check Voltage Headroom (must be greater than 0V)
    Vhr = Vsupply - Vs - Vo
          = 5V - 0.5V - 2.7V
          = 1.8V (Pass!)
• Expected Rise Time (Rdut = Rz~100 ohms)
   Tau = tr63% = 1/(Kint * Rs/(Rs+Rdut)*1/Rs)
          = 1/ (20000 * 1/2 * 1/100)
          = 10ms
• Run the program
  • Does the SMU servo the output current Io (MI) to ~5mA?
  • What is the measured voltage?  MV = 2.68V
  • Check the test limits:
    If 2.6V < Vo < 2.8V, then PASS , else FAIL.

 

ZENER - REVERSE LEAKAGE TEST

BZX79-B2V7, 20uA Max

• Datasheet Specification:
    Iz<20uA @ Vz=1V
• Test Strategy:
    Force Voltage (FV), Measure Current (MI)
• Edit test params in Arduino setup() code:
   forceMode = FVMode;
   FV = 1V;
   Rsense = 100k;
   Kint = 50;  
• Install
    Rs = 100k
    D1 = Cathode to Vo, Anode to GND
• Test Limits
   Iz < 20uA
• Expected Current Sense Voltage
   Vs = Io*Rs = 20uA*100k = 2V
• Check Voltage Headroom (must be greater than 0V)
    Vhr = Vsupply - Vs - Vo
          = 5V - 2V - 1V
          = 2V (Pass!)
• Expected Time Constant (Rdut = Rz~open)
   Tau = tr63% = 1/(Kint * Rs/(Rs+Rdut))
          = 1/ (20 * 1)
          = 50ms
• Run the program
  • Does the SMU servo the output voltage Vo (MV) to ~1V?
  • What is the measured current?  MI = 3.7uA
  • Check the test limits:
    If Io < 20uA, then PASS , else FAIL.

 

THERMISTOR - RESISTANCE TEST

NTCLE100E3 103JBE (10k ±5% @ T=25C)

      

• Datasheet Specification: 
    Rth = 10kohms ±5% TCR=-4.38%/C @ T=25C
• Test Strategy:
    Force Voltage (FV), Measure Current (MI)
    Calc Resistance (FV / MI)
• Edit test params in Arduino setup() code:
   forceMode = FVMode;
   FV = 1V;
   Rsense = 10k;
   Kint = 50;  
• Install
    Rs = 10k
    Rth = ~10k
• Expected Resistance at 21C (70 deg F)
    Rth = R25 * ( 1 + (T - 25C)*TCR/100 )
          = 10k * ( 1 + (21C - 25C)*-4.38%/100 )
          = 10k * (1.1752)
          = 11.75k
• Test Limits
    Rth = Rth +/-5% = 11.2k to 12.3k
• Expected Current
   Io = 1V/10k = 0.1mA
• Expected Current Sense Voltage
   Vs = Io*Rs = 0.1mA*10k = 1V
• Check Voltage Headroom (must be greater than 0V)
    Vhr = Vsupply - Vs - Vo
          = 5V - 1V - 2V
          = 2V (Pass!)
• Expected Time Constant (Rdut = Rth~10k)
   Tau = tr63% = 1/(Kint * Rth/(Rs+Rdut))
          = 1/ (50 * 0.5)
          = 40ms
• Run the program
  • Does the SMU servo the output voltage Vo to 1V? MV = 1.01V
  • What is the measured current? MI=0.086mA
  • Calculate the Resistance: R = MV/MI = 11.7k
  • Check the test limits:
    If 11.2k < R < 12.3k, then PASS , else FAIL.

 

LED - FORWARD VOLTAGE TEST

LTL-4224 Red LED, Vfwd=1.8V

      

• Datasheet Specification: 
    Vfwd=+1.8V ±0.3V @ Ifwd=5mA
• Test Strategy:
    Force Current (FI), Measure Voltage (MV)
• Edit test params in Arduino setup() code:
   forceMode = FIMode;
   FI = 0.005;
   Rsense = 100;
   Kint = 2000;  
• Install
    Rs = 100
    D1 = Anode to Vo, Cathode to GND
• Test Limits
   Vo = 1.5V to 2.1V
• Expected Current Sense Voltage
   Vs = Io*Rs = 0.005*100 = 0.5V
• Check Voltage Headroom (must be greater than 0V)
    Vhr = Vsupply - Vs - Vo
          = 5V - 0.5V - 1.8V
          = 2.7V (Pass!)
• Expected Rise Time (Rdut = Rfwd~1 ohms)
   Tau = tr63% = 1/(Kint * Rs/(Rs+Rdut)*1/Rs)
          = 1/ (2000 * 1 * 1/100)
          = 50ms
• Run the program
  • Does the SMU servo the output current Io (MI) to ~5mA?
  • What is the measured voltage? MV = 1.92V
  • Check the test limits:
    If 1.5V < Vo < 2.1V, then PASS , else FAIL.

 

EXCEL FILE

Explore the hands-on spreadsheet with the circuit calculations!

• Excel file:  SMU-FVFI-digital-lite-test-calc-1.xlsx
    Right Click on filename, select "Save link as...".
• Play in the sandbox, modify values, predict the impact on circuit!
• Copy to a new file - experiment!

 

 

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