DC Supplies Output Resistance

[Roam]

Hi guys!

I'm a bit confused. I have a question regarding the effective dc output resistance of dc power supplies. For a "good" power supply, should this internal resistance be large or small?

Well, I used to think that a good power supply should have a small internal resistance, so it would maintain a constant terminal voltage until exhausted before droping to 0. However, I did an experiment and found that regulated power supplies (which give a more steady outoput with less ripples), tend to have a higher output resistance than the unregulated power supplies. So does this mean the dc power supplies with larger resistance are more desirable?

I greatly appreciate it if anyone could confirm this, and also explain briefly why higher internal resistance reduces the output ripples

 

[srizbf]

For a "good" power supply, internal resistance is small.

have you done the test on the same supply ?
one unregulated and the other with regulation added?

or is it two diifferent supplies.?

 

[btbass]

The output impedance of a power supply should be low, otherwise you will get significant drop when you draw power.
I dont believe the output resistance of a regulated power supply is high.
They are normaly a series power device with a feedback circuit that maintains a constant output voltage over the rated load. The feedback amplifier helps to reduce the ripple.

 

[chuckey]

Most power supplies are "voltage" supplies they should provide the same output voltage right up to the point they blow their fuse. A half decent low voltage supply, say 0 - 100V will have an output impedance in the order of milli ohms, i.e. drawing one amp from them will only drop the terminal voltage by less then a hundredth of a volt. The unloaded voltage of a power supply is its EMF (Electro Motive Force), its a bit of a theoretical unit as anything connected to measure the terminal voltage will draw some microscopic current.
An unregulated power supply will have an output voltage that will vary greatly, say -10% from 0 amps to its rated output. Take a 12V power unit. With no load its is likely to be about 17V with 12V at one amp. So in coarse terms it has an output impedance of 5V/1 = 5 ohms. But it will also contain a very large capacitor, so taking a large AC current of say .5A, will not change its output voltage at all because the capacitor will maintain its terminals at the mean voltage.
Frank

 

[Syncopator]

Hi guys!For a "good" power supply, should this internal resistance be large or small?

An ideal voltage source has zero impedance.

When performing circuit analysis it is always considered so, and the positive and negative rails may be drawn connected together.

The corollary is that an ideal current source has infinite impedance.

 

[Roam]

For a "good" power supply, internal resistance is small.

have you done the test on the same supply ?
one unregulated and the other with regulation added?

or is it two diifferent supplies.?

Well, they are two different supplies. However, I also did an experiment on the same unregulated power supply, and I varied a resistive load which was connected across the supply output. With the heavy load (1k ohm) I got a more smooth output, and for the lighter load (220 ohm) I got ripples with larger amplitude (looking at the oscilloscope). So, my question now is that the heavier this resistance, the more linear/steady output will be?

 

[Syncopator]

With the heavy load (1k ohm) I got a more smooth output, and for the lighter load (220 ohm) I got ripples with larger amplitude

You have things back to front.

220 ohms is a heavier load than 1k. It draws 4.5 times more current.

Your results are exactly what should be expected. The higher the current drawn, the more the smoothing capacitor will be discharged between peaks, and that equals more ripple.

 

[srizbf]

for a supply with capacitor filter , the ripple increases when you approach(or load it) the max load current.

now the point is on 'ripples'.
and not on o/p resistance.

 

[FvM]

Referring to your intial statement

Well, I used to think that a good power supply should have a small internal resistance, so it would maintain a constant terminal voltage until exhausted before droping to 0. However, I did an experiment and found that regulated power supplies (which give a more steady outoput with less ripples), tend to have a higher output resistance than the unregulated power supplies.

The assumption about good power supplies is generally correct. Your experimental result contradicts the expectable behaviour of a regulated supply. Either you're operating the supply beyond it's specifications or it isn't actually regulated.

DC output resistance refers to the ratio of average voltage and current and hasn't to do with ripple. You'll possibly find unregulated supplies with additional ripple filtering (e.g. RC or LC filter), that can have a higher output resistance.

You also noticed, that the output voltage change of a unregulated supply isn't a linear function of current, but that's another topic.