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I build a temperature circuit in PSpice with a TLV3701, a MOSFET and 3 light bulbs (\$R_6\$, \$R_7\$, \$R_8\$). The \$R_9\$ resistor is 108.18 Ω (can be bought, I checked). As soon as the room temperature falls below 21 ℃, the comparator switches through, and when the temperature rises, it switches off. I now have two questions:

I first installed a series resistor \$R_1\$ because I read in the data sheet of the TLV3701 that it should not output more than 10 mA. I've found that this 10 mA doesn't flow at all even when \$R_1\$ = 1 Ω. May I remove this resistor?

As you can see from the red label, 18 mV are left at the drain input. How does that happen? I know it's a semiconductor, there are physical effects, but what exactly happens there, so that 18 mV are “left over”?

circuit

Edit: Question № 1 is OK. It's called \$R_{DS}(On)\$.

In real life I thought of this MOSFET: https://asset.conrad.com/media10/add/160267/c1/-/en/000156110DS01/datenblatt-156110-stmicroelectronics-stp16nf06l-mosfet-1-n-kanal-45-w-to-220ab.pdf But: PSpice is not yet set that way.

R9 is equal to 108.182 Ω and R3 is said to be similar to get 3.118 V.

Also, why do you decide to add a pull-up resistor? I found out that the TLV3701 needs one. I didn't work without…

winny
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Daniel
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    What you see is the voltage drops across Rds(on) = 18mV/2.5A = 7.2mΩ. What type of a MOSFET did you decide to use? As for the R1 = 6V/10mA = 600Ω = 680Ω – G36 Nov 14 '21 at 16:53
  • _Rds(on)_ Thank you, got to know another parameter again. Now I know what to look for. This is just a generic model. – Daniel Nov 14 '21 at 17:01
  • I mean real-life part number for a MOSFET. – G36 Nov 14 '21 at 17:07
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    Also in real life, you need to add some hysteresis to your comparator. https://electronics.stackexchange.com/questions/465430/waveform-at-the-negative-terminal-of-an-opamp/465585#465585 Also, why do you decide to add a pull-up resistor? And why R3, R9 have such low values? – G36 Nov 14 '21 at 17:15
  • @G36 I've edit my question. – Daniel Nov 14 '21 at 17:37
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    A 108.18 Ω resistor for that sort of circuit is overkill. A 100 Ω and 10 Ω resistor in series should be fine. However, dissipating nearlt 0.2 W just for a voltage divider like that seems extravagant: you could multiply the values of the resistors by 22 and add a potentiometer for fine adjustment.. – Andrew Morton Nov 14 '21 at 18:02
  • Ok, I'll work on that ^^ – Daniel Nov 14 '21 at 18:27

1 Answers1

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I first installed a series resistor R1 because I read in the data sheet of the TLV3701 that it should not output more than 10mA. I've found that this 10mA doesn't flow at all even when R1 = 1Ω. May I remove this resistor?

Yes. In high-speed, high-power switching circuits such as switching power supplies and motor drive amplifiers, a resistor in series with the gate dampens unwanted oscillations cause by the gate capacitance and the wiring inductances. This does not apply to a simple power switch application such as yours.

As you can see from the red label, 18mV are left at the drain input. How does that happen? I know it's a semiconductor, there are physical effects, but what exactly happens there, so that 18mV are “left over”?

When a power MOSFET is fully enhanced (saturated), the channel has a minimum possible resistance. This is labeled Rds(on) on the datasheet: The Resistance from the Drain to the Source when the transistor is fully "on". With Ohm's Law, you can calculate Vds, the voltage from the drain to the source, at any load current.

AnalogKid
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