I understand that voltage dividers make lousy power supplies, such as when directly attached to rectified stepped-down mains power. But in this case I've got a number of 12-volt DC supplies laying around so assuming that they're fairly well regulated and I don't exceed the maximum rated amperage then I should be able to wire these up to a simple voltage divider arrangement to give me the range of voltages I need for the various projects I'll be working on, right? So I could just plug one of the leads from the divider into the breadboard to select either 12v, 9v, 5v, 3.3v, etc, depending on whatever voltage that particular circuit requires. Should that work okay?

 

It's not dangerous but it is inadvisable because the load will essentially form part of the divider and your voltage will change as your load changes.

A linear regulator and a couple of caps will cost you a buck or two, well worth the cost.

 

It's not dangerous but it is unadvisable because the load will essentially form part of the divider and your voltage will change as your load changes.

A linear regulator and a couple of caps will cost you a buck or two, well worth the cost.

Oh, okay so if I understand you correctly these DC power supplies don't typically provide a constant-voltage source (but rather engineered to supply the correct voltage for the specific device they were designed to power in mind). Is that correct?

 

Any load you place across the divider will change the value of the resistor in the divider. This is why dividers won't work for your application.

Ron

 

Any load you place across the divider will change the value of the resistor in the divider. This is why dividers won't work for your application.

Ron

Alright, so I just need to build a voltage-regulated supply circuit then. I know of a simple setup which uses an op-amp with a zener as a voltage reference, maybe I could just feed the output of my 12v supply into that to get a constant-voltage source? That way I wouldn't have to bother with the whole business of stepping down the mains and rectification.

 

Oh, okay so if I understand you correctly these DC power supplies don't typically provide a constant-voltage source (but rather engineered to supply the correct voltage for the specific device they were designed to power in mind). Is that correct?

Ummm, no. Assuming it's not a constant current supply, they are spec'd to deliver a given voltage within a (hopefully) narrow specificed range up to their max specified load current.

 

Alright, so I just need to build a voltage-regulated supply circuit then. I know of a simple setup which uses an op-amp with a zener as a voltage reference, maybe I could just feed the output of my 12v supply into that to get a constant-voltage source? That way I wouldn't have to bother with the whole business of stepping down the mains and rectification.

Use an off the shelf voltage regulator. Start with the LM78xx series.

An LM7805 costs $0.29 from Jameco. Caps are cheap.

Look at the LM317 to build a simple adjustable supply.

 

Use an off the shelf voltage regulator. Start with the LM78xx series.

An LM7805 costs $0.29 from Jameco. Caps are cheap.

Look at the LM317 to build a simple adjustable supply.

Definitely economical, but how stable is the output? I know the op-amp regulator is extremely so (of course for my purposes the LM317 will probably work just fine, but just curious). Not to mention I received a boatload of LM324 op-amps today but not a single linear regulator handy so...unless the LM324 just won't do here (not sure, haven't read the datasheet yet) I may end up going that route for now.

 

Very stable but learn to read the datasheet which will give you chapter and verse on how it behaves under various conditions.

 

Definitely economical, but how stable is the output?

Download the manufacturer's data sheet for the 7805 and see for yourself.

Always read the data sheet. Always read the data sheet. Always read...

Short answer: very stable.

 

You can also use the LM317 to generate all the voltages so you would only have to buy one part type.
It just requires a couple of resistors to select the desired output voltage.
Here's a calculator to determine the resistor values for a desired output voltage.

Note that typically R1 should be 121Ω±1% to provide sufficient bias current so that the output stays regulated with no load.
R2 should also be the closest ±1% resistor for the calculated value so the output voltage is close to what you want.

You could also use a trimpot to adjust each voltages to what you want.

 

Download the manufacturer's data sheet for the 7805 and see for yourself.

Always read the data sheet. Always read the data sheet. Always read...

Short answer: very stable.

Or, as I have been heard to say (to myself AND others), RTFDS.

 

Or, as I have been heard to say (to myself AND others), RTFDS.

Yup. Just like in my .sig...

 

Yup. Just like in my .sig...

Oh yeah, so it does...

 

so...unless the LM324 just won't do here (not sure, haven't read the datasheet yet) I may end up going that route for now.

The LM324 can only output a few tens of milliamps of current maximum.
And it does not have a built-in voltage reference as the voltage regulators do.

 

You can also use the LM317 to generate all the voltages so you would only have to buy one part type.
It just requires a couple of resistors to select the desired output voltage.
Here's a calculator to determine the resistor values for a desired output voltage.

Note that typically R1 should be 121Ω±1% to provide sufficient bias current so that the output stays regulated with no load.
R2 should also be the closest ±1% resistor for the calculated value so the output voltage is close to what you want.

You could also use a trimpot to adjust each voltages to what you want.

You can also use the 7805 as an adjustable VR though the floor is 5V. You just need to put your intended voltage minus 5V on the middle pin.

 

The LM324 can only output a few tens of milliamps of current maximum.
And it does not have a built-in voltage reference as the voltage regulators do.

Okay, well scratch that then.

Is there any decent reference available that lists the operating characteristics of various op-amps?

 

The voltage regulators recommended are designed for the job so just go with them. They have internal reference and OpAmp as well as other goodies for protection.
Unless you really need to build your own, and if so, Google is you friend.
All OpAmp manufacturers publish data sheets so download some and have a read. Also look at the voltage regulator data sheets and app notes.

 

Is there any decent reference available that lists the operating characteristics of various op-amps?

I don't know of any comprehensive reference. However, most of the op amp manufacturers have "selection guides" on their web pages that serve that purpose. Here's one example.

 

A scavenged ATX power supply might be worth considering.