Author Topic: Voltage Divider or LM7800 series?  (Read 1444 times)

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Offline Nexo

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Voltage Divider or LM7800 series?
« on: August 31, 2016, 03:33:49 pm »
Hello everyone!

I've been wondering if is there any difference on using one of the mentioned components, I mean, both do the same right?
I know it is way more easy and handy to use a LM7800 series's component, but could the same be achieve with a voltage divider when I'm in a hurry and don't have a Voltage regulator in stock?
Apart from the cost and time of using one of them, is a voltage divider reliable? I mean, how accurate can it be in comparison to a Voltage regulator which was manufactured specifically to do that?

Sorry if this question is silly but I love to ask  ^-^

Thanks in advance!
 

Online Ian.M

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Re: Voltage Divider or LM7800 series?
« Reply #1 on: August 31, 2016, 04:23:51 pm »
A potential divider typicaly requires a standing current ten times greater than the load current.  This is practical when you just need 1 or 2 milliamps, and dont care much about regulation, but if you need a stable voltage at a greater current up to about half an amp, 78xx series regulators or a LM317 or other linear regulator are more practical.  At greater load currents and/or higher voltage drops you almost invariably want a switching regulator as the heat dissipation from a linear regulator rapidly becomes difficult and expensive to manage.
 

Offline Zero999

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Re: Voltage Divider or LM7800 series?
« Reply #2 on: August 31, 2016, 05:55:09 pm »
Ian is right. With a potential divider, the output resistance is equal to that of the resistors in parallel, so reducing 12V with two 1k resistors will create 6V in series with 500R so the voltage will drop by 0.5V per 1mA of current drawn. It also wastes power by drawing 6mA when no load is connected.

Go for the LM317, if you don't want to stock a vast range or regulators and the LM317L is also good if you need to save space because it comes in the TO-92 package but it's only rated for 100mA.
 

Offline NANDBlog

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Re: Voltage Divider or LM7800 series?
« Reply #3 on: September 01, 2016, 08:19:33 pm »
A voltage divider can source or sink up to a few miliamps, but your voltage changes. A buffered voltage divider can source and sink a few miliamps, and your voltage does not change. An LM78xx can source a few hundred milliamps, but it cannot sink.
 

Offline Zero999

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Re: Voltage Divider or LM7800 series?
« Reply #4 on: September 01, 2016, 09:33:45 pm »
If all you need is a voltage reference which can sink or source no more than 20mA, then the LM10 is for you:
http://www.ti.com/lit/ds/symlink/lm10.pdf
« Last Edit: September 01, 2016, 10:17:01 pm by Hero999 »
 

Offline danadak

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Re: Voltage Divider or LM7800 series?
« Reply #5 on: September 02, 2016, 12:38:14 am »
The 7800 series has a high value of ref leg current, 8 mA, and
it experiences a significant change with Vin, 1.3 mA. So the R
network has to have lower values in order to keep the ref pin
looking like its attached to a V source. To maintain regulation.
That means you burn more power needlessly.

Whereas the LM317 has low ref leg current, 100 uA, and its change
is 5 uA. So net of this should allow one to use higher values for R
network, thereby lower power.

You should do an error budget on this to make sure your design
goals are met.

Regards, Dana
Love Cypress PSOC, ATTiny, Bit Slice, OpAmps, Oscilloscopes, and Analog Gurus like Pease, Miller, Widlar, Dobkin, obsessed with being an engineer
 


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