Little things that clear up mysteries...

Thread Starter


Joined Jan 27, 2019
Experience includes a bunch of little things—facts—that can make the mysterious or prevent wrong thinking when trying to work out how and why things act the way they do.

They are the sort of things that, when you learn them in the context of struggling with a problem, become bricks in the foundation of not-being-baffled.

Here's a couple of examples off the top of my head, what gems do you have buried in your mind?

  • The gate of a MOSFET is a capacitor.
  • The negative terminal of a battery is not ground.
  • Batteries are resistors
  • ...


Joined Oct 2, 2009
Here are some more:

  1. It is the load that determines the current, not the power supply.
  2. An LED needs a current source, not a voltage source.
  3. An ideal voltage source has zero internal resistance.
  4. An ideal current source has infinite internal resistance.
  5. The driver impedance should be lower than that of the load.
  6. The impedance of a voltage divider should be 10 times lower than that of the load that it is driving.
  7. The voltage at the inverting input is the same as that at the non-inverting input of a properly biased operational amplifier.
  8. The open-loop gain of an op-amp falls off to unity at the high end of the Gain-Bandwidth-Product.
  9. It is the circuit resistances that determine the gain of a BJT amplifier, not the beta of the transistor.
  10. Use beta = 10 to switch a BJT into saturation mode.
  11. Yes, you need 1μF capacitors (check the datasheet first) on the input and output pins of 3-terminal linear voltage regulators.
  12. Yes, you need 100nF capacitors across Vcc and GND pins of every IC.
  13. Yes, you need 10μF capacitor across Vcc and GND of 555-timer IC.
  14. Peak current in rectifier diodes increases when you increase the value of the reservoir (smoothing) capacitors.


Joined Aug 9, 2016
When working with high voltages, put one hand in your pocket.
I prefer to keep both hands in my pockets and bark commands at my buddy...

I’ll reiterate the one about the load determining the current. It is amusing to think way back and being afraid to connect something to a large battery because it had little wires even though the device is rated for the voltage. Not sure, but I think I expected it to melt down.


Joined Aug 9, 2016
A microcontroller runs at a speed that....well it is really fast. I can count somewhat under my breath to ten in about a second and it feels fast. 10Hz? That’s really, really slow. Coming to grips with what 8MHz is has really helped me.


Joined Dec 2, 2017
Always turn your bench supply back to 5 volts before turning it off.

loads should be attached to a bench supply thru a switch, and the switch should be used to turn on the load, not the supply power button.

Oh...and voltage dividers should NOT be used to power a load. ;)

This is how you divide a voltage when powering a load...
Last edited: