Author Topic: Why use a voltage regulator rather than voltage divider?  (Read 1141 times)

Philtre

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Why use a voltage regulator rather than voltage divider?
« on: February 18, 2018, 04:55:16 PM »
Excuse the real noob question here, but I gotsta know.  :-\

Some circuits use a 78(L)05 voltage regulator to supply +5v to a chip such as the 2399. But why not simply divide the 9v voltage with a couple of resistors and get the 5v that way?

somnif

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Re: Why use a voltage regulator rather than voltage divider?
« Reply #1 on: February 18, 2018, 05:10:07 PM »
I suspect its so the chip can draw the current it wants, rather than relying on Ohm's law to play nicely? Also to get 5v out of 9 you need a .8 ratio of R1 to R2, which would probably require some rather funky values on one side or the other (not to mention resistor tolerances could be a bastard at that point). (Also to get the ~30mA a 2399 likes you'd need pretty small resistor values, and at that scale you'd start tipping close to the quarter watt dissipation we play with. Toasty warm boards smell lovely)

The regulator just gives you a steadier voltage, less board real estate, plenty of current to draw on, and simpler to design around.

Then again these are guesses/assumptions. There could be other, better reasons I'm not thinking of.

Philtre

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Re: Why use a voltage regulator rather than voltage divider?
« Reply #2 on: February 18, 2018, 05:18:05 PM »
I suspect its so the chip can draw the current it wants, rather than relying on Ohm's law to play nicely? Also to get 5v out of 9 you need a .8 ratio of R1 to R2, which would probably require some rather funky values on one side or the other (not to mention resistor tolerances could be a bastard at that point). (Also to get the ~30mA a 2399 likes you'd need pretty small resistor values, and at that scale you'd start tipping close to the quarter watt dissipation we play with. Toasty warm boards smell lovely)

The regulator just gives you a steadier voltage, less board real estate, plenty of current to draw on, and simpler to design around.

Then again these are guesses/assumptions. There could be other, better reasons I'm not thinking of.

That sounds about right, thanks!  8)

bsoncini

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Re: Why use a voltage regulator rather than voltage divider?
« Reply #3 on: February 18, 2018, 05:49:45 PM »
A voltage regulator will give you 5 volts whether your power supply is pumping out 9.5 or 8 volts. A voltage divider will fluctuate depending on the supply voltage.

Philtre

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Re: Why use a voltage regulator rather than voltage divider?
« Reply #4 on: February 18, 2018, 05:52:16 PM »
A voltage regulator will give you 5 volts weather you power supply is pumping out 9.5 or 8 volts. A voltage divider will fluctuate depending on the supply voltage.

Yep, that makes sense.

But what I don't understand is why a voltage divider is used extensively in circuit design, but the PT2399 seems to want the steady 5v?

bsoncini

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Re: Why use a voltage regulator rather than voltage divider?
« Reply #5 on: February 18, 2018, 06:06:32 PM »
The data sheep of the pt2399 says recommended minimum supply voltage is 4.5 volts. So a voltage divider won't cut it. You never know what supply people will plug in.

Opamp circuits will use virtually ground. Half the supply voltage around 4.5 volts for 9 volts to a have an even swing for the both sides of the fluctuating ac sgnal. But it will still be half the supply voltages with 7 volts also
« Last Edit: February 19, 2018, 01:45:45 AM by bsoncini »

madbean

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Re: Why use a voltage regulator rather than voltage divider?
« Reply #6 on: February 18, 2018, 08:21:48 PM »

But what I don't understand is why a voltage divider is used extensively in circuit design, but the PT2399 seems to want the steady 5v?

The simple answer is it's the safest choice to use given that the PT2399 has a very narrow range of voltage tolerance. As already mentioned, it requires a minimum of 4.5v and it will continue to operate up to about 6v. Over that, it might simply stop working or worse, damage it. A voltage divider is completely dependent on the voltage provided to the circuit. A regulator only requires about 3v minimum over the voltage it regulates to operate. You would not be doing yourself any favors by saving $0.15 on a regulator since the possibility for damage outweighs it.

I own madbeanpedals (duh). I am part owner of Function F(X).

gordo

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Re: Why use a voltage regulator rather than voltage divider?
« Reply #7 on: February 18, 2018, 09:01:06 PM »
Programmers count data sheep to fall asleep... ::)

Philtre

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Re: Why use a voltage regulator rather than voltage divider?
« Reply #8 on: February 19, 2018, 01:44:16 AM »
Thanks for the elucidation!  8)