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Help from electronic guys (again)

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  • Help from electronic guys (again)

    A while back i asked for some aid from the electronic guys about stableizing the voltage reading on my home made bike trainer and got many great replies on how to do so --- even a diagram or two and you guys were right on the money, I built it with some resistors and a capasitor and walla -- it worked great, very stable -- but iv got a glitch, its actually very unstable through the course of the time its being used for testing,

    A brief discription is this, I now have two good size generators that I power, one directly and then this is linked to the other by an oldham coupling that I built, they are both perfectly matched and wired together in parralel, I use there power to operate a massive ford electric fan off of a 3.8 liter tourus, It rocks, I got tons of air to keep myself cool and the voltage and load is about perfect, its also surprisingly stable even after much dyno time, , I have a pedal cadence guage, a heart rate monitor and my voltage meter to keep track of many of the variables, But when I try to use the little electronic unit to stablize the sample voltage for use to keep the digital voltage meter from jumping all over the place im now at the mercy of two flaws, #1; I imediatly take a hit in voltage drop and dont get the true reading that the generators are putting out to the fan (that would actually be acceptible except for the #2 addage) ----- #2; the resistors in the little stableizer unit i built are causing an unbelivable long term fluctuation due to them being subject to much variation because of temperature varience,

    I know this because of my cadence reading, Even if I keep it dead on and keep my bike in the identical gearing it will read different after much use, but when I disconect the stableizer unit and just measure generator output its as close to the same readings from the start to finish (as much as I can tell because the meter is searching its last .00 digit and is not stableized but its very very close)

    So -- How can I get a stable meter without using resistors? would not just running a capasitor do it? I recall one of you guys stating that I should bench the digi- meter and go with a needle, I will consider doing so if I cant bring this around, thanks for the input...
    Last edited by A.K. Boomer; 12-05-2007, 12:30 PM.

  • #2
    Perhaps the simplest solution would be using different resisters. Could you use larger ones, or a different type (wire-wound instead of carbon film for instance?)


    • #3
      I read the past posts, but I don't know for sure how your final circuit turned out. Could you describe it, with the resistor values?

      There is something wrong, if your resistors are getting hot, or you can notice any change when connecting a volt meter, even with a filter circuit.


      • #4
        AKBoomer, Thermal effects on the resistor are not likely to be a cause of your problems since they were only part of an RC filter if I recall correctly. There are several things which will impact your readings, the first easy to fix, the 2nd not so easy but I'll suggest a simple possible compromise.

        1. If the R in your RC is very large and so is the capacitor value, the capacitor leakage current may shift your readings and it WILL change with temperature.

        2. The shape of the voltage waveform you are cranking out, if more of a pulse shape, will read differently on almost anything you measure it with. Accurate measurements would be difficult and would require more sophistication than even a good DMM.

        That said, you could try a rapid attack, slow decay circuit. Reduce your resistor value and parallel it with an ordinary silicon rectifier diode (from the Shack) with cathode pointing toward the positive of your meter. The diode will allow your cap to charge rapidly and cause your meter to indicate the voltage a bit better if it is in fact more pulse shaped. The resistor can be adjusted to give you a decay time that makes the reading appear smooth enough for your liking.

        You could also add a 2nd resistor in series with the diode. Now, this allows you to adjust the attack and decay times and perhaps give you the cheapest approach to matching the other reading.

        All of this assumes that I understood the problem correctly



        • #5
          I tend to agree with nheng..... at least if my mental picture is correct.

          Maybe you could draw the circuit with values and post a pic? or some good description?

          Resistor values do NOT vary like that with temp, within ratings, and for normal resistors. So clearly something else is going on.

          You SHOULD get a result that is equal to the average of the voltage, if the R and C are such as to have a several second time constant.

          There is also the chance that they are telling the truth, and some other effect is causing the rock-stable reading of the meter.

          Unless you are a very disciplined cyclist, which you may very well be, I am inclined to doubt the ultra-stable reading. I'd believe SOME more substantial variations, but you may be seeing larger ones than I imagine.

          Numbers and pictures always help.........

          Keep eye on ball.
          Hashim Khan


          • #6
            The large capacitor in conjunction with the resistor that you have now is great at filtering low frequencies and damping longer period variations. They're not worth a damn at filtering high frequencies and so those are probably showing up at the meter which is taking snapshots of them. A way to improve things is to add some .01 mfd capacitors in parallel with the existing capacitor. They have excellent characteristics with high frequencies and will help swallow spikes. Use several. You can also put a clamping zener diode across the capacitor(s) to swallow large spikes. This in concert with the new smaller capacitors stand a good chance and cleaning up the voltage your meter is recording. The zener voltage needs to be higher than the highest expected voltage you expect to see else it will go up in smoke.


            • #7
              Seeing the circuit will help a lot. There are a few things that could be at fault, and the most likely is the electrolytic cap, if there's one in the circuit. If so, it would be better to re-design this low pass filter to enable the use of a non-electrolytic cap, and not a tantalum either. To keep the time constant in the appropriate range, the resistor values would have to go up, and this is better anyway because they would heat less and have a better chance of remaining stable. Modern resistors are pretty stable as is, but there's no point in having them heat significantly- if indeed they are. The dvm has a high enough input resistance that it won't load the cap much, so a reasonable reading can still be had.

              Carbon resistors have a notorious habit of changing resistance, especially if they're old. It doesn't matter that they may not have been used at al- if you've used any in a time constant circuit, change them to metal film types.

              Something which bites me from time to time is a failing battery in a meter. Just the other day I was trying to get a temperature reading from a thermocouple, and the reading didn't seem right after a short time. This particular meter doesn't have a low battery warning, that I could find anyway, and it was too weak to operate the circuit properly. I think that's why I acquired this meter- the previous owner was getting faulty readings 'cause they didn't have a way of knowing when to change the battery. Hm.

              Anyway, there's no reason why an analog meter can't be used. I find there are many times when a moving pointer gives a far better indication than a dvm, and this would be one of those applications, in my opinion. Because an analog meter has a much lower resistance than a dvm, the time constant circuit would have to be designed around the meter on hand. No big deal.

              Back to using the dvm- you can measure the output voltage from your generators directly, so having the right range on the meter is not a problem. You could wire a rectifier only in series with the meter, and you'd see a slight reduction in the reading, by about .5 to .7 volt-then you add the capacitor from that voltage to ground. Using no resistors whatsoever, you can rely on the dvm to slowly drain the capacitor so when you quit pedalling the reading does drop. I just did a quick test with one of my dvms- using either the 20 v range or the 200 v range, a cap value of about 1 to 1.5 uf holds the voltage well enough to keep the reading reasonably readable. Really It's easy to make up this cap from two or three film capacitors.

              I can't see you having a problem with reading the full generated voltage instantly, but if that's a problem, a series resistor with the rectifier would change that.

              There, I've probably just said what everyone else has said, just used different words.
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


              • #8
                Any electrolytic worth a hoot should do just fine with the output of a generator. Especially with a large resistor in series, it will filter JUST FINE at any useful frequency in that situation......

                I do NOT think that effect is the problem, although it can happen in other situations. Electrolytics of good but not abnormal construction are commonly used for filtering output of SMPS operating at over 100 kHz, they do not fall far short in those usages.

                Keep eye on ball.
                Hashim Khan


                • #9
                  I will tinker around, maybe I got something wired wrong, from as close to what I can tell I built DP's unit -- the one he sugested way back in the "calling all electrical guru's" thread,,, ;


                  I just deleted a multi-paragraph dissertation on voltage regulators and perm-mag generators. Bottom line is, any regulator is going to waste power and you don't want that - you want to turn that fan!

                  Because such generators produce d.c. pulses (on a scope they have the same shape as the McDonald's Golden Arches , and because the regulators provide output only when the input voltage exceeds the output voltage what you get is pulsed d.c. out (imagine the top half of those arches being the conducting part of the generator output). If you don't also put some filtration there you get switching noise when those pulses dump into the load. The fact is you really don't need to filter anything except the volt meter.

                  Go to Radio Shack and buy a 250uf capacitor (voltage rated at twice your generator output) and a 47k ohm resistor. Wire the resistor and capacitor in series and attach the resistor to the generator output (assume positive pole), and the negative lead of the capacitor to ground, or negative. Put your voltmeter across the capacitor and it should clean up the measured voltage rather well. If it is still bouncing, try a 100k resistor. Or even higher. The digital voltmeters are very high impedance devices so even using a 1 meg resistor is viable.

                  What you have at this point is a simple single pole low-pass filter that should damp the variations caused by your irregular stroke.

                  Edit: Or do what Wes said Great minds and all that!


                  A quick re-cap,,, the generators and fan unit would be very stable but powering them are not, I got a nice little flywheel on one of the generators and spin the generators aprox. 5 times the speed of my bike cranks, Im still experiencing the humanoid biomechanical flaws on the meter and the variations in the pedal stroke as its hard to make refined power for 360 degree's of the pedaling cycle --------- DP's unit totally smoothed this out but im paying a long term price somewhere in the resistors used or the cap.

                  I will tinker some more when i get some time, thanks for all the input --- I can't understand why just using a cap between the meter and the gens wont smooth this out, I know i must of tried it before though --- It's mostly greek to me, You guys would be amazed at the power output --- its unreal, and very "free", the fan would make anyone lose thier hat,------ upon coast down the fan has momentum and because its a perm. mag unit it will actually turn into the generator and drive the cranks! Not only that --- hook up a tiny little AAA 1.2 volt battery and the entire rig comes to life with fan spinning and cranks turning from the gens... never thought that to be posible as these are massive units, the generators themselves have the resistance of 8 brushes to overcome


                  • #10
                    'just using a cap between generator and meter'- the generator will raise the voltage on the cap just fine, but the load, which is your fan, is always in circuit, so it's discharging the cap as quickly as the generator slows and the voltage falls. If you de-couple the cap from the fan, the problem goes away. This is the job of the series rectifier (diode). The voltage on the cap will rise to the level generated, minus .7v or so for the forward voltage drop of the diode, then that voltage relies on something else to drain it- whether the meter itself, or added resistors.

                    If you want to hold a peak reading, use the series diode but no resistor in parallel with the cap. The cap should then be an electrolytic of good size, say 100 uf or more, and selected for low leakage. You'll be able to keep a peak reading for tens of seconds, maybe minutes, depending on values, etc.

                    JTiers is right that an electrolytic would be just fine on the output of a generator, capable of filtering without problem. Some types are better, of course, where the frequencies are higher, and that's why switch mode p/supplies use low esr types, etc. rather than the more commonly found types. If the job for it was simply to slow down the changes in meter readings so you could at least see a stable reading for a second or two, then the cheap type would likely be fine. In any event, the generators output can be considered to be dc, and if brush noise is a problem, the simple cheap cap is more than capable of handling that.

                    But we're talking about a reading that seems to change after some time, and trying to resolve that (at least that's what I was talking about) . Electrolytics have some characteristics that make them change how they respond to various voltages on them- not what you need in a circuit where you want consistency. For example, you can charge an electrolytic, then discharge it- then watch it build some voltage back up, on its own- in this use it could tell you that you're pedalling slowly, even if you aren't at all. Thus my bias against electrolytics in this application.

                    That being said, this characteristic probably isn't what's making the circuit drift (if that's what's going on). More likely is the non-linear leakage characteristic- a lot depends on the resistor values you've used. And we haven't yet ruled out operator fatigue-
                    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


                    • #11
                      Thanks for explaining Darryl, I'll give the diode trick a try, it makes sense and sounds like it will be much less effected --- also sounds like I can utilize various cap sizes to dial in the delay i want??? its not operator fatigue as im holding the same cadence throughout testing, I will get it dialed now with the help you guys sugested, just got to find the time now, its a crazy unit, I gutted out my old house fan and used its pole and big circular base to mount it on, its also very heavy as i had to make mounts out of some pretty thick steel and plus the fan motor is 3 times as heavy as the old house unit, when i first started testing I was focused on my volt meter and wasnt paying attention to the fan (just the wind it was creating), as I took it past twelve volts in high gear it took much effort and the fan tipped over on its stand and went into the wall peeling off paint and some drywall paper, good thing its got a circular ring around all the blades or it would have been disintigration of the blades and some of my wall

                      I'll say this much, some of it was my best guestimation but most of it was luck --- its a really good match between generators and fan motor and fan blades (9 of them) --- its right at the operating voltage I was shooting for and the Gens just get a little warm on the shaft part by the brushes after 15 minutes of moderate pedaling, they will be seeing much more duration at much more effort but they should do just fine --- Im glad I got two though - i think i would have burnt one up just because they have to turn slower to come up with the goods.


                      • #12
                        Use the output to electrolytically plate copper from a copper anode to a copper cathode in a 20% solution of copper sulphate and water. The weight change of the two plates will be a very close approximation of the work done in relative terms.

                        This is an actual method used in some types of hour meters.
                        Free software for calculating bolt circles and similar: Click Here


                        • #13
                          Now that IS a reasonable idea.

                          If the voltage is pretty constant, the plate-out will track the total coulomb transfer.... i.e current integrated over time, and would correspond to net joule output.

                          Keep eye on ball.
                          Hashim Khan


                          • #14
                            Thanks guys but iv got enough things around the house that resemble some kind of "lab experiment" and im not even a science teacher, besides anything liquid can spill and anything that can spill will and that means the pig farmer is going to drink it and im not comfortable with that... at least diodes are kinda crunchy and that will tip me off that she's eating something she shouldnt be...


                            • #15
                              Got It --- (i think)

                              I tried everything, I tried diodes and tried to isolate the power into large and small caps and then hooked up "drains" (resitors) that bled off almost as much as what my digi-meter consumed or built up a little more, I actually kinda had it, but the voltage was so low compaired to what the gens were actually putting out that it discusted me, plus it was still at the mercy of a small heat up over time ------- UHHHHGGGG, my mind dont work this way --- as i look at my two gutted out old tape decks that Iv been canabalizing for parts over the years whenever i have to venture into this area of the unknown I get frustrated all over again, any of you guys who posted could probably hook something up in five minutes that would maybe get me by, I cant stand being stupid, I just cant stand it --- I also cant stand wasting allot of time ------- I wish it was mechanical, id have it done and ready in no time, why can't electronics be mechanical? I asked myself after my third glass of wine when my head hit the pillow --- Next morning I woke with the answer ---- It is!!!
                              Five minutes later the tape decks are torn down in an area theve never been ripped into before, you got it --- the mechanical area, another five minutes and I got both little permanent magnet motors strapped to a test board with holes in it and wire ties holding the motor/generator set down firmly with one of the belts linking them together, I hook the one motor to the Dynometers on the bike trainer and the other is hooked to the digital volt meter, Bingo --- Never have I had a unit this stable even without a flywheel!
                              Iv effectivly mechanically TOTALLY removed the electrical link between the trainer gens and the digital meter, It has to go mechanical first to achive any results, NOW iv got you you little bastard! Now your in my arena!!! I now have the ability to gear or pulley the gen/motor set to mimmick the aprox. values -- I now have the ability to add a small little (1 or two oz.) flywheel to the generator if my reading bounces around at all and what I believe to be the best part is both the motor and the generator will be "free running" --- their might be an initial change just in running it at first but there really will be no price to pay in heating up over time - therefore my values should remain pretty consistant, the motor will really have no load after it accelerates the generators flywheel up to speed (it should be an aprox. ratio of 1 to 3 (1 motor/3 gen. hence the flywheel being on the gens side, plus the added bennie of the drive belt elasticity ) because the only load on the generator will be the digital volt meter which is nothing, I should add that small as they are these a high torque little motors, they really dont run at very high RPM, the Gens drive motor wont even feel it!
                              My initial voltage reading is very low with a 1 to 1 ratio hence the "welcomed" reason for increasing the Gens RPM's -- welcomed because this will be a nice place to add the flywheel...

                              Ok, --- JT, Ev, Darryl,DP, what am I missing, give it a little time would you, let me be happy for a while...