Electrical – Hysteresis threshold changed when connecting a transistor to the output of a comparator LM393,

transistors

schematic

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Hi all, I have built an inverting comparator with hystersis using LM393. First I calculated \$R1,R2,R3\,R4_{pull-up}\$ and got the exact upper and lower hysteresis threshold \$V_{TH}=1.8V\$ and \$V_{TL}=1.39V\$. However, when I connect a transistor \$Q1\$ (I want to use it as a switch, when its ON, let as much as possible current flow through the load \$R6=20\Omega\$), the upper hysteresis threshold has decreased to \$V_{TH}=1.47V\$ and \$V_{TL}\$ didn't change.

Here are my questions:

  1. I thought I need to take \$R5,R6\$ into hysteresis calculation. But how? The load resistance \$R6=20\Omega\$ cannot change.

  2. How to make \$Q1\$ saturate when it is ON? I have iterated \$R5\$ with many different values from \$100\Omega\$ to \$10k\Omega\$ but cannot make B-C junction forward bias. My goal is to get maximum current flow through the load.

Best Answer

schematic

simulate this circuit – Schematic created using CircuitLab

Note that since transistor inverts output, you use (Vin-) for positive (+) feedback.

LM393 open collector NPN has an equivalent ESR.

Saturation Voltage Vout 700 mV max at I SINK ≤4 mA,

  • thus ESR= 700mV/4mA = 175Ω max "ESR or Rce = Vce(sat)/If."
  • if using a BJT switch you correctly choose Rb/Rc=10 but since it is internal to IC equivalent Rce, R5 can be 0 Ω and Q1 must be PNP.

Right now with NPN your effective Rb/Rc = R4/R6 and is too high thus hysteresis problems.

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