5 ms·

Just a guess...maybe the 5V is supplied from a linear regulator with a maximum dropout of 1.2V, so the board is specified to require at least 6.2V from the wall

by hw-guy 3y ago

Just a guess...maybe the 5V is supplied from a linear regulator with a maximum dropout of 1.2V, so the board is specified to require at least 6.2V from the wall wart.

kurthr 3y ago
Yep, modern LDOs can maintain 0.2-0.3V with good regulation, but older cheaper ones are a diode drop (~0.6V) or two.
- makomk 3y ago
  Older cheaper voltage regulators are typically more like 2V drop and the optimum input voltage is somewhere around 7-9V for a 5V output. In practice a lot of devices used 9V power supplies because that was the lowest widely-available voltage that met the requirement. More modern LDO regulators would have plenty of headroom on plain 6V. It'd be kind of unusual for any electronic device to need the extra .2V in this particular case.
  - kurthr 3y ago
    I think you just described the 7805? https://www.tutorialspoint.com/what-is-voltage-regulator-7805-internal-block-diagram-and-applications https://www.tutorialspoint.com/what-is-voltage-regulator-780... It's a great old school design, but there are somewhat newer capless designs that are about 2 drops like the LM2940. There are a lot of other designs between a 0.2V current feedback pFET design, but not all of them have been so popular that their costs are close to packaging.
    - kurthr 3y ago
      That extra 0.2V is to go capless, which saves a few cents and some area on the PCB.
RobotToaster 3y ago
That was my immediate thought too.
crote 3y ago
That doesn't really make sense, though. If you need 5V, why use a 6.2V wall wart and a LDO instead of just using a 5V wall wart in the first place?
JohnFen 3y ago
That sounds entirely plausible. Many of the hobby electronics projects I make require at least a 6V supply because of exactly this. I usually use a 9V wall wart, though, because those are very commonly available (so are easy to scrounge up).
- arcticbull 3y ago
  For what it's worth in hobby projects it's time to throw linear regulators into the dustbin. There's 7805 pin-compatible switching modules now! Stuff like this. [1] [1] https://www.digikey.com/en/products/detail/cui-inc/VXO7805-500/7350288 https://www.digikey.com/en/products/detail/cui-inc/VXO7805-5...
  - stephen_g 3y ago
    Depends on what you're doing. For a lot of sensors and things you might want the power-supply ripple rejection (PSRR) you get through an LDO. I'm not doing hobbyist things, but in our designs we have a lot of places where we will have a switcher that generates an intermediate voltage to feed a bunch of ultra-low noise, high-PSRR LDOs for different components (isolates them from each other too). This is RF though, where that is especially important.
    - arcticbull 3y ago
      I just meant as a replacement for your main input power regulator - for the 7805 way too many people still use. You are absolutely right that there is a role for LDOs in modern devices. There's no reason for most hobbyists to be using super hot, super inefficient and super constrained high-dropout linear regulators from 1972 when there's a pin-compatible all-in-one switching module for roughly the same price. To pull more than a couple hundred mA at the lowest voltage differential you need a heatsink!
      - sidewndr46 3y ago
        Trying to use a switching regulator for everything becomes a complete nightmare when you have to debug a circuit. You scope something that should be a square wave or similar and it has the switching frequency of the regulator superimposed upon it everywhere.
jschveibinz 3y ago
I think you nailed it.
Gibbon1 3y ago
That's exactly it. Any less than the regulator won't regulate properly. More than that it'll dissipate too much heat.
- regularfry 3y ago
  Combine this with the sibling thread about lead acid battery voltages (2.1V per cell, 3 cells gives 6.3V), can we then say that this is the reason 5V circuitry is so common? 6.3V from battery ancestry plus a linear regulator?
  - Gibbon1 3y ago
    Yeah I think even earlier 6.3 volts was also the voltage used for the heating element on vacuum tubes. I think because very early radio's used 6V batteries for the coils. And a separate high voltage battery for the plates. Would not surprise me if there are roots going back to telegraph.

by hw-guy 3y ago

Just a guess...maybe the 5V is supplied from a linear regulator with a maximum dropout of 1.2V, so the board is specified to require at least 6.2V from the wall wart.

kurthr 3y ago

Yep, modern LDOs can maintain 0.2-0.3V with good regulation, but older cheaper ones are a diode drop (~0.6V) or two.

makomk 3y ago
Older cheaper voltage regulators are typically more like 2V drop and the optimum input voltage is somewhere around 7-9V for a 5V output. In practice a lot of devices used 9V power supplies because that was the lowest widely-available voltage that met the requirement. More modern LDO regulators would have plenty of headroom on plain 6V. It'd be kind of unusual for any electronic device to need the extra .2V in this particular case.
- kurthr 3y ago
  I think you just described the 7805? https://www.tutorialspoint.com/what-is-voltage-regulator-7805-internal-block-diagram-and-applications https://www.tutorialspoint.com/what-is-voltage-regulator-780... It's a great old school design, but there are somewhat newer capless designs that are about 2 drops like the LM2940. There are a lot of other designs between a 0.2V current feedback pFET design, but not all of them have been so popular that their costs are close to packaging.
  - kurthr 3y ago
    That extra 0.2V is to go capless, which saves a few cents and some area on the PCB.

RobotToaster 3y ago

That was my immediate thought too.

crote 3y ago

That doesn't really make sense, though. If you need 5V, why use a 6.2V wall wart and a LDO instead of just using a 5V wall wart in the first place?

JohnFen 3y ago

That sounds entirely plausible. Many of the hobby electronics projects I make require at least a 6V supply because of exactly this. I usually use a 9V wall wart, though, because those are very commonly available (so are easy to scrounge up).

arcticbull 3y ago
For what it's worth in hobby projects it's time to throw linear regulators into the dustbin. There's 7805 pin-compatible switching modules now! Stuff like this. [1] [1] https://www.digikey.com/en/products/detail/cui-inc/VXO7805-500/7350288 https://www.digikey.com/en/products/detail/cui-inc/VXO7805-5...
- stephen_g 3y ago
  Depends on what you're doing. For a lot of sensors and things you might want the power-supply ripple rejection (PSRR) you get through an LDO. I'm not doing hobbyist things, but in our designs we have a lot of places where we will have a switcher that generates an intermediate voltage to feed a bunch of ultra-low noise, high-PSRR LDOs for different components (isolates them from each other too). This is RF though, where that is especially important.
  - arcticbull 3y ago
    I just meant as a replacement for your main input power regulator - for the 7805 way too many people still use. You are absolutely right that there is a role for LDOs in modern devices. There's no reason for most hobbyists to be using super hot, super inefficient and super constrained high-dropout linear regulators from 1972 when there's a pin-compatible all-in-one switching module for roughly the same price. To pull more than a couple hundred mA at the lowest voltage differential you need a heatsink!
    - sidewndr46 3y ago
      Trying to use a switching regulator for everything becomes a complete nightmare when you have to debug a circuit. You scope something that should be a square wave or similar and it has the switching frequency of the regulator superimposed upon it everywhere.

jschveibinz 3y ago

I think you nailed it.

Gibbon1 3y ago

That's exactly it. Any less than the regulator won't regulate properly. More than that it'll dissipate too much heat.

regularfry 3y ago
Combine this with the sibling thread about lead acid battery voltages (2.1V per cell, 3 cells gives 6.3V), can we then say that this is the reason 5V circuitry is so common? 6.3V from battery ancestry plus a linear regulator?
- Gibbon1 3y ago
  Yeah I think even earlier 6.3 volts was also the voltage used for the heating element on vacuum tubes. I think because very early radio's used 6V batteries for the coils. And a separate high voltage battery for the plates. Would not surprise me if there are roots going back to telegraph.