What is the purpose of a 1 ohm resistor in this circuit?

Thread Starter

FlamingWallaby

Joined Apr 23, 2019
1
Hello

For a lab project we were instructed to do a full wave rectifier with a capacitor with these parameters:

upload_2019-4-25_1-59-50.png
https://imgur.com/a/U90mfxV

Then we were instructed to add a 1 ohm resistor in series with the capacitor and notice a difference, we didn't notice anything different from the 2 circuits in terms of Vmax and Vripple aside from a more "stable" signal, what is the 1 ohm resistor doing specifically that allows for a more stable signal? and are there other differences that me and my lab partner are missing?

Thanks.
 
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Ylli

Joined Nov 13, 2015
865
Was the resistor place in series *only* with the cap, or was the resistor placed between the rectifiers cathodes and the rest of the circuit?
 

DickCappels

Joined Aug 21, 2008
6,073
I think you are looking at the 1 kOhm resistor (1,000 ohms). It is a bleeder and it can be used to keep the peak voltage across the capacitor from charging to higher voltages than would be expected when the load is applied and/or to bleed down the voltage across the output capacitor when power is removed from the circuit.
 

Ylli

Joined Nov 13, 2015
865
Assuming you have the resistor in series with just the capacitor... While the capacitor charges and presents the output with near the peak voltage, the 'ripple current' from the rectifiers is conducted through the capacitor. If a 1 ohm resistor is placed in series with the cap (or the caps internal resistance increases by 1 ohm) then the ripple current flowing through the cap produces a greater ripple voltage across the cap than if it wasn't there. So take a second look at the ripple voltage with and without the added resistor.
 

WBahn

Joined Mar 31, 2012
25,060
Hello

For a lab project we were instructed to do a full wave rectifier with a capacitor with these parameters:

View attachment 175793
https://imgur.com/a/U90mfxV

Then we were instructed to add a 1 ohm resistor in series with the capacitor and notice a difference, we didn't notice anything different from the 2 circuits in terms of Vmax and Vripple aside from a more "stable" signal, what is the 1 ohm resistor doing specifically that allows for a more stable signal? and are there other differences that me and my lab partner are missing?

Thanks.
What was more "stable" about it?

The capacitive reactance of the cap at 50 Hz is only about 3 Ω, so that would be a pretty heavy load on the supply if it were AC. An additional ohm of pure resistance is a significant change. But I don't have a good feel for this particular case to offer much beyond that without doing some sims and/or measurements.
 

MrAl

Joined Jun 17, 2014
6,930
Hello

For a lab project we were instructed to do a full wave rectifier with a capacitor with these parameters:

View attachment 175793
https://imgur.com/a/U90mfxV

Then we were instructed to add a 1 ohm resistor in series with the capacitor and notice a difference, we didn't notice anything different from the 2 circuits in terms of Vmax and Vripple aside from a more "stable" signal, what is the 1 ohm resistor doing specifically that allows for a more stable signal? and are there other differences that me and my lab partner are missing?

Thanks.

Hello,

The 1 ohm resistor is most likely used to simulate a rather large capacitor ESR value.
That resistor makes the capacitor less effective in filtering the full wave rectified sine wave.
For some numerical results, see the attachment.
Note that the top calculation was done with no 1 ohm resistor, and the bottom calculation had that 1 ohm resistor (C_ESR).
Also note the effect it has on the ripple and the percent ripple (lower right corners of each set). It goes from 0.91 to 3.33 percent just because of that one resistor addition.

This is probably an exercise in spotting a capacitor that had the ESR go up high.
 

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MisterBill2

Joined Jan 23, 2018
4,819
I understand exactly what it was, what should have changed, and most likely why it was missed. Posts #4 and #8 did get what it was, others ignored the reality. The 1 ohm was indeed to simulate a higher internal resistance of the capacitor, and it certainly did increase the ripple some. But at the light load of the two resistors, about 15mA, the increased ripple was difficult to spot, and if they had an analog meter then it would have been very difficult to spot.Also, the average voltage out would have dropped just a very small amount. So another lesson would have been that some meters will not be able to display what you need to know in every instance. My old analog multimeter would have been really challenged to display the change but my 4 1/2 digit Beckman meter would have told the story quite well. In some instances resolution is very important, while in other cases it is accuracy that matters the most.
 

MrAl

Joined Jun 17, 2014
6,930
Hi,

Just to note, my program may have overestimated the percent ripple for the 1 ohm value because it wasnt made to work with that high ESR.
The actual percent ripple probably changed more like from 1 percent to 2 percent with the addition of the 1 ohm resistor.
 
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