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Series Resistor or Voltage Divider ?

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  • Series Resistor or Voltage Divider ?

    As I have no formal training in electronics, I still have some gaps in understanding the basics, and I greatly appreciate any comments as they would help to solve my riddles !

    Questions :
    1. When would you use a resistor to lower or raise voltage, or when would you use a Voltage divider from two resistors instead, and what are the advantages / disadvantages of using a series resistor vs. a voltage divider ?
    2. Are there any other cautions to be considered using two resistors to divide voltage, provided they are sufficient in wattage / temperature ratings ?
    3. How does and to what extent does a resistor reduce AC ripple in an HT line, and what parameters of the resistor should be considered ?
    4. Are Voltage ratings in fuses important ?
    5. Do the coupling capacitors and the resistors dumping to ground just before power tubes create a low pass filter only or are the resistors doing other things as well, as in an Overall attenuation of the signal ?




    As always, Thanks for your help and your time, you gents are superb !
    " Things change, not always for the better. " - Leo_Gnardo

  • #2
    Harold....

    To me, Series Resistors and Voltage Dividers are synonymous. Take a look at this entry level video from the Khan Academy. I believe it is a good starting point for conversations.

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    • #3
      It’s all Ohms Law. And as Harold said it’s the same principle. It depends on the application.. especially the resistance of the load.

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      • #4
        In the case of a signal path, like between two tube stages, currents are tiny, so the voltage divider approach pertains. But when currents are involved, like say the resistors in the B+ line of a typical old Fender, I can change a 5k to a 10k say to drop the voltage to the preamp.
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          Originally posted by HaroldBrooks View Post
          As I have no formal training in electronics, I still have some gaps in understanding the basics, and I greatly appreciate any comments as they would help to solve my riddles !

          Questions :
          1. When would you use a resistor to lower or raise voltage, or when would you use a Voltage divider from two resistors instead, and what are the advantages / disadvantages of using a series resistor vs. a voltage divider ?
          2. Are there any other cautions to be considered using two resistors to divide voltage, provided they are sufficient in wattage / temperature ratings ?
          3. How does and to what extent does a resistor reduce AC ripple in an HT line, and what parameters of the resistor should be considered ?
          4. Are Voltage ratings in fuses important ?
          5. Do the coupling capacitors and the resistors dumping to ground just before power tubes create a low pass filter only or are the resistors doing other things as well, as in an Overall attenuation of the signal ?




          As always, Thanks for your help and your time, you gents are superb !
          You questions are very general. It would be much easier to give useful answers if you would specify circuit/application examples.

          Generally series resistors (meaning a resistor between a voltage source and a load) can only drop/lower not raise voltage and always cause power loss. The voltage drop depends on the current through the resistor and the value of the resistor. So a series resistor can be used to lower voltage if there is a well defined "load" current. If the load current is not steady or very low, an additional resistor to ground (and parallel to the load) is used which takes care of sufficient and better defined current through the series resistor (and thus voltage drop) - that would be a voltage divider. The specific application defines the appropriate resistance values and divider ratio.
          A series resistor alone can be a voltage dropper but certainly is no voltage divider.
          Last edited by Helmholtz; 07-24-2019, 08:50 PM.
          - Own Opinions Only -

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          • #6
            A single, SERIES resistor voltage "drop" is solely dependent upon the output/load current passing through it; more current yields more "drop" and less current yields less "drop" producing poor regulation.

            A two-resistor, DIVIDER voltage "drop" is dependent upon the RATIO of both the divider (R1/R2) and the output/load current, so is typically (but not always) a better *voltage regulator" circuit, ie: increased load current yields more "drop" while less load current just returns the voltage drop back toward the initial "no-load" (R1/R2) voltage value...acting like a maximum voltage source.
            Last edited by Old Tele man; 07-24-2019, 09:25 PM.
            ...and the Devil said: "...yes, but it's a DRY heat!"

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            • #7
              Originally posted by HaroldBrooks View Post
              [*]Do the coupling capacitors and the resistors dumping to ground just before power tubes create a low pass filter only or are the resistors doing other things as well, as in an Overall attenuation of the signal ?
              The resistors also provide a grid load for the power tubes. The tube data sheets have a maximum grid circuit resistance value for both fixed and cathode bias.
              There is some overall signal attenuation as the resistors form a potential divider with the output impedance of the driving stage.

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              • #8
                Originally posted by HaroldBrooks View Post
                As I have no formal training in electronics, I still have some gaps in understanding the basics, and I greatly appreciate any comments as they would help to solve my riddles !
                Good for you. Your riddles are Mother Nature's way of telling you that you would really, really enjoy some more electronics learning. Go for it! A few answers to questions in a forum won't do the job, but if you get some more formal training, the questions and answers in forums will help it all make sense.
                You've already received good advice, and my comments are just to reinforce what you've already heard.

                Questions :
                When would you use a resistor to lower or raise voltage, or when would you use a Voltage divider from two resistors instead, and what are the advantages / disadvantages of using a series resistor vs. a voltage divider ?
                A resistor cannot raise voltage - resistors always eat up energy all by themselves. Resistors may help other devices to increase voltages, but not by themselves.

                Another subtle point is that a resistor by itself cannot lower voltage. Resistors are the physical embodiment of Ohm's Law, which says that the voltage across a resistor is equal to the resistance times the current through it. If the current in the resistor is zero, so is the voltage across it. So a series resistor can only drop voltage if there is something after the series resistor eating the current. That is, >>there is always a load after the series resistor you're trying to insert.<< This is a point that a lot of people miss.

                If you insert a series resistor to drop voltage, you are making a voltage divider, using the series resistor as one of the divider resistors, and the loading of the circuit as the other resistor.

                Whether you use a single inserted series resistor or a two-resistor voltage divider really depends on what you're trying to do, and more subtly, how much power you can afford to waste to get your voltage down.

                Back at the idea that you might like some formal training, one of the things I still use a lot is the idea of a Thevenin equivalent circuit. A Thevenin equivalent circuit is a voltage source in series with a resistor that acts just like whatever it replaces. You can replace any voltage divider with its Thevenin equivalent circuit. Say you have 100V and you want to drop it to 50V. You can easily do this for no load conditions with any two equal resistors making a voltage divider. If you pick, say, 10K resistors, then the Thevenin equivalent circuit for 100V with a two-10K resistor divider is a 50V source in series with a 5K resistor. To the outside world, a 100V plus a 10K/10K voltage divider and a 50 plus 5K series are THE SAME.

                I'm sure I've just added a new spin to your questions, but consider it a bump in the direction of getting more formal training. You'll like it once you get past the homework and get to use it.

                [*]Are there any other cautions to be considered using two resistors to divide voltage, provided they are sufficient in wattage / temperature ratings ?
                Yes. You hardly ever need to do a voltage dropper with a constant load. The loads change on you. If you have a changing load, then the current eaten from the voltage source you're dropping varies, and so does the amount of voltage drop.

                A voltage DIVIDER lets you set the output VOLTAGE with a little bit of independence from what the load is doing. From that Thevenin model thing - the voltage in the Thevenin equivalent is the voltage set by the resistor divider, while the series resistance in the Thevenin model is the parallel combination of the two resistors. You can make the Thevenin equivalent series resistance arbitrarily small by using arbitrarily small resistors in the divider. This means that the current changes in the load don't matter as much because the voltage divider is setting the voltage, not the voltagae dropped by the series resistance. Of course, you pay for that. The way the voltage divider "stabilizes" the output voltage is by wasting a lot of power in the divider.

                Again - it only gets fun once you have some more training under your belt.
                [*]How does and to what extent does a resistor reduce AC ripple in an HT line, and what parameters of the resistor should be considered ?
                You're clearly after calculating dropping resistors running into decoupling caps. There are a few ways to look at this. The exact, formal way would be to write the equations and solve for the smoothing as a function of frequency. That's a PITA. You can do almost as accurate a job by approximating the dropping resistances and decoupling caps as "AC voltage dividers".

                A resistor in an HT line can't reduce AC ripple at all. It has the same impedance at all frequencies. If you combine it with a capacitor to ground in a "voltage divider", you form a simple filter. The capacitor has an impedance (i.e. AC "resistance") that drops linearly as frequency rises. By using a series resist and a cap to ground, the capacitor sucks ever-more AC to ground as frequency increases. The impedance of a cap is X = 1/(2*pi*F*C).
                So at DC, it's an open circuit. For a series R, shunt C filter, you can set the attenuation of ripple by setting the values of R and C.

                Let's estimate the ripple reduction of a 1K resistor and a 1uF cap at 120 hz (first ripple frequency of a FWB). The cap has an impedance at 120hz of X = 1/(2*pi*120*1uF) = 1326 ohms if I punched the right calculator buttons. So the attenuation of a 1K + 1uf cap at 120 hz is 1326/(1000+1326) = 0.57.
                So a 1K/1uF cuts 120hz ripple to 0.57 of its value before the filter. Since the equations are linear, we can figure other attenuations easily. A 10k resistor and a 1uF cap give an attenuation of 0.057, or a 94.3% reduction. A 10K and a 10uF give 0.0057, or a 99.4% reduction.

                It's the combination of resistance and capacitance that helps.

                [*]Are Voltage ratings in fuses important ?
                Yes. The voltage rating on a fuse is the highest voltage it can be relied on to interrupt. If you use a 32V fuse in a 120V or higher circuit, the fuse may melt, but still have a long lasting arc inside it that can start a fire. The fuse voltage rating needs to be big enough. Too big is OK
                [*]Do the coupling capacitors and the resistors dumping to ground just before power tubes create a low pass filter only or are the resistors doing other things as well, as in an Overall attenuation of the signal ?
                It's complicated.

                A series resistor and capacitor to ground shunts higher frequencies to ground better, so it's a low-pass filter. A series capacitor and shunt resistor makes it easier for high frequencies to get through, so it's a high pass filter. All passive filters also attenuate. This is another of those places where some formal instruction or self instruction with electronics theory would help you a lot.
                Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

                Oh, wait! That sounds familiar, somehow.

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