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General Technical Chat / Re: Model Trains
« Last post by andy3055 on Today at 04:48:16 am »The 3 rail system!
Recent Posts
2
Repair / Re: TDS744A: 1M input impedance measures: Ch3 = 160Kohm, Ch4 = 706KOhm
« Last post by John_ITIC on Today at 04:44:22 am »Thank you both for your suggestions. I have not seen anything unusual in my visual inspections under microscope.
All relay solder joints are perfect and clean. No visible damage to the hybrid circuit board.
The hybrids used in my TDS744A are H2462G's: https://w140.com/tekwiki/wiki/H2462
The repair procedure followed was described here (thank you Terra):
In the above tekwiki link, there is a block diagram of the hybrid. I see no control line for 1M input impedance configuration. So, if not 50 Ohm, terminated in 1MOhm on the hybrid? The same tekwiki link has a detailed description of the M753 amplifier used on the hybrid board. The initial picture has a block diagram of the x1, x10 and x10 resistive dividers used. Those resistive dividers must be the 1M input terminators.
https://w140.com/tekwiki/wiki/M753#/media/File:1990-08-23_New_Attenuator_scheme_m753.jpg
I have confirmed that the below are the DC offsets on Ch4 for the different gain settings. The x1, x10 and x100 are the different resistive divider inputs on the M753 amplifier.
x1 (10V/div, 5V/div, 2V/div) : 40mV offset, 700 KOhm input impedance
x10 (1V/div, 500mV/div, 200mV/div) = -240mV offset, 225 KOhm input impedance
x10 (100mV/div, 50mV/div, 20mV/div, 10mV/div, 5mV/div, 2mV/div and lower) = -13mV offset, 680 KOhm input impedance.
I will take the unit apart again to see if I can identify any resistive dividers on the hybrid board. Perhaps I can confirm the above resistances.
Thanks,
/John.
All relay solder joints are perfect and clean. No visible damage to the hybrid circuit board.
The hybrids used in my TDS744A are H2462G's: https://w140.com/tekwiki/wiki/H2462
The repair procedure followed was described here (thank you Terra):
In the above tekwiki link, there is a block diagram of the hybrid. I see no control line for 1M input impedance configuration. So, if not 50 Ohm, terminated in 1MOhm on the hybrid? The same tekwiki link has a detailed description of the M753 amplifier used on the hybrid board. The initial picture has a block diagram of the x1, x10 and x10 resistive dividers used. Those resistive dividers must be the 1M input terminators.
https://w140.com/tekwiki/wiki/M753#/media/File:1990-08-23_New_Attenuator_scheme_m753.jpg
I have confirmed that the below are the DC offsets on Ch4 for the different gain settings. The x1, x10 and x100 are the different resistive divider inputs on the M753 amplifier.
x1 (10V/div, 5V/div, 2V/div) : 40mV offset, 700 KOhm input impedance
x10 (1V/div, 500mV/div, 200mV/div) = -240mV offset, 225 KOhm input impedance
x10 (100mV/div, 50mV/div, 20mV/div, 10mV/div, 5mV/div, 2mV/div and lower) = -13mV offset, 680 KOhm input impedance.
I will take the unit apart again to see if I can identify any resistive dividers on the hybrid board. Perhaps I can confirm the above resistances.
Thanks,
/John.
3
Mechanical & Automation Engineering / Re: PID control and different sampling rates
« Last post by Siwastaja on Today at 04:42:41 am »Ah, the problem of having to control slow motor speeds with insufficient resolution for speed sensing. There is no good or simple solution this, or solution at all. By far the best idea is to use a higher resolution encoder.
You can, of course, interpolate the encoder position e.g. by assuming that the time difference between two latest events (i.e. speed) remains constant and thus keep ramping the position variable blindly.
Or just try to make the control loop as good as humanly possible, meaning extreme effort at tuning the PID variables but also adding other strategies like feedforwards. Anytime your feedback signal sucks, feedforward is the obvious solution, but then you need some kind of model of the system. Simplest types of feedforwards are, (1) just add a term with a ff_constant * setpoint, (2) add ff_acceleration_constant * (setpoint - prev_setpoint).
I had similar situation with a mobile robot with two motorized wheel with only the 6-steps per electrical revolution hall sensors. At high speeds it maneuvered fine but slow speeds were impossible to tune. FF(setpoint) + FF(derivative of setpoint) + P + I + D all carefully tuned made it relatively OK-ish but the final thing that fully stabilized it was another feedback, from gyroscope yaw angle angular rate directly to motor currents. That one reacts very fast, even to small changes. Like, if one motor was getting just a half of a hall step ahead of the other, that was already showing as significant rotation of the robot, drunk mode oscillations ensuing. Gyro feedback fixed that simply with just a few lines of new code. Much easier than trying to solve the problem by tuning the PID loop until cows come home.
I could have added feedforward component from the orientation of the robot; i.e., going uphill more current is needed. The advantage of feedforwards is, they react quick and can't cause oscillations.
So maybe see if you can find secondary sources (other than the measured rotor speed) for feedbacks/feedforwards. If nothing physical to measure, then feedforwards from the setpoint and setpoint derivative still help.
You can, of course, interpolate the encoder position e.g. by assuming that the time difference between two latest events (i.e. speed) remains constant and thus keep ramping the position variable blindly.
Or just try to make the control loop as good as humanly possible, meaning extreme effort at tuning the PID variables but also adding other strategies like feedforwards. Anytime your feedback signal sucks, feedforward is the obvious solution, but then you need some kind of model of the system. Simplest types of feedforwards are, (1) just add a term with a ff_constant * setpoint, (2) add ff_acceleration_constant * (setpoint - prev_setpoint).
I had similar situation with a mobile robot with two motorized wheel with only the 6-steps per electrical revolution hall sensors. At high speeds it maneuvered fine but slow speeds were impossible to tune. FF(setpoint) + FF(derivative of setpoint) + P + I + D all carefully tuned made it relatively OK-ish but the final thing that fully stabilized it was another feedback, from gyroscope yaw angle angular rate directly to motor currents. That one reacts very fast, even to small changes. Like, if one motor was getting just a half of a hall step ahead of the other, that was already showing as significant rotation of the robot, drunk mode oscillations ensuing. Gyro feedback fixed that simply with just a few lines of new code. Much easier than trying to solve the problem by tuning the PID loop until cows come home.
I could have added feedforward component from the orientation of the robot; i.e., going uphill more current is needed. The advantage of feedforwards is, they react quick and can't cause oscillations.
So maybe see if you can find secondary sources (other than the measured rotor speed) for feedbacks/feedforwards. If nothing physical to measure, then feedforwards from the setpoint and setpoint derivative still help.
4
General Technical Chat / Re: Need 13Amp variac device to 'modulate' a water boiler.
« Last post by radar_macgyver on Today at 04:37:32 am »So temperature regulation is out. If I needed temp regulation I could use the built in bi-metalic thermostat.Hmm, something like the SSRMAN board with a pot to control the duty cycle might work. You would pair this with an SSR that does *not* have a zero-cross switching circuit. Holy crap, those boards got expensive!
A PWM controller that supports a direct duty cycle setting would be more appropriate.
I assume by "process controller" we are referring to the little modules with red led displays and outputs for driving relays etc.?Yep. The fancy ones include features you likely don't need (ramp/soak control, etc). If you were using a controller, you'd pick one with a voltage output that's suitable for driving the industry-standard 3-32V input for an SSR. A cheap controller may cost less than the SSRMAN board linked above.
There are the expensive real ones (is it STC1000?) and then there are the cheap copies. I assum the later still work fine as they are very popular?
One of those which has a high enough output voltage to control, say, a 20A SSR ... just the question about "non-temperature manually variably PWM" availability.
5
Projects, Designs, and Technical Stuff / dc clamp meter design with 10mA resolution
« Last post by justarandomelectron on Today at 04:25:19 am »hello gang, 
I wanted to design a dc clamp for measuring current with a resolution of 10mA. I am just measuring stuff upto 5A with 10mA resolution. I have some 49e laying around and a iron core. Now I have 2 choices, either make a open loop design or a closed loop with compensation winding. I think that open loop design will be worse since the resolution will be worse
, and the core will be magnetize after measurement even though slightly. Though I searched online but I couldn't find a detailed design guide on closed loop design. Can anyone help me? Also will the 49e be sufficient for measuring flux near zero point in closed loop design?
Also I found out another method called self oscillating fluxgate sensor with a single coil using a simple op amp . Anyone has any ideas about it too?
I wanted to design a dc clamp for measuring current with a resolution of 10mA. I am just measuring stuff upto 5A with 10mA resolution. I have some 49e laying around and a iron core. Now I have 2 choices, either make a open loop design or a closed loop with compensation winding. I think that open loop design will be worse since the resolution will be worse
, and the core will be magnetize after measurement even though slightly. Though I searched online but I couldn't find a detailed design guide on closed loop design. Can anyone help me? Also will the 49e be sufficient for measuring flux near zero point in closed loop design? Also I found out another method called self oscillating fluxgate sensor with a single coil using a simple op amp . Anyone has any ideas about it too?
6
General Technical Chat / Re: Model Trains
« Last post by joeqsmith on Today at 04:24:43 am »My Lionel 2-6-2. Like my old Marx, these are not worth a lot. If you want to repair one, you may find a whole new engine is cheaper than that one part you need. 
I found two Lionel tinder cars in my stash but both were plastic in rough condition. More so than my other bits.
I found two Lionel tinder cars in my stash but both were plastic in rough condition. More so than my other bits.
7
Buy/Sell/Wanted / Re: SALE (AU/NZ): Multicomp Pro MP710086 (Owon P4305 rebadge) at element14
« Last post by 5U4GB on Today at 04:11:37 am »I wonder if they had a damaged shipment that was partly covered by shipping insurance and are now selling the units at a discount to cover the residual cost? Seems unusual that they'd have identical damage to two separate units.
8
Buy/Sell/Wanted / Re: SALE (AU/NZ): Multicomp Pro MP710086 (Owon P4305 rebadge) at element14
« Last post by plurn on Today at 03:58:38 am »Maybe they are just reusing boxes to safely package delicate equipment? Maybe the multicomp box was opened so they could pop an Australian power cable into it?
I thought that might be the case until I saw the CEE 7 and UK cables in there alongside the AU one, much better than the usual wrong-country cable and a dodgy adapter plug. It's possible the Farnell original had the EU and UK cables and the AU one was added in Australia, which would explain the open box.
There was a significant dent in the Farnell (second) box that carried through to the Multicomp (third) box but there's foam packing in there that prevented it going any further. However there was nothing on the first (element14) box so it was definitely repackaged at some point after being originally shipped in the Farnell box.
Finally received mine along with a bunch of other stuff I ordered. Pretty much the same packaging for mine. Intact element14 box containing the power supply boxes as well as the other ordered items - there was a big gouge/hole in the farnell box that got through and also gouged the multicomp box. Open box multicomp box. No damage to the power supply. 3 power cables.
I did get the 15% discount for this order. Tried using it again for a later order for other items and it did not allow me to apply it - perhaps single use per customer or it has expired. Tried the other 10% discount and it let me apply it but was later contacted and told I could not use the 10% voucher as all the items I selected were already discounted. Even without that 10% off the items were well discounted so no complaints. So I expect it depends on who reviews your order and how diligent they are as to whether you will receive a discount illegitimately.
9
Projects, Designs, and Technical Stuff / Re: One Capacitor Type to Rule Them All? Help?
« Last post by NT0Z on Today at 03:26:32 am »In a perfect world that's true. But this is far from the craziest idea I've had! 
--NT0Z
--NT0Z
10
Projects, Designs, and Technical Stuff / Re: One Capacitor Type to Rule Them All? Help?
« Last post by NT0Z on Today at 03:24:56 am »Yes, I don't need bypass caps to be NP0, etc, and I have thousands of those in my junkbox already. I tend to use through-hole parts and SMD parts together, all on one side of an SMT board or Manhattan-style on a piece of copper-clad board.
I am looking into some kind of affordable USB assembly scope/stand with an LCD display. My eyesight isn't great for binocular solutions, as my left eye is awfully ornery. I use a magnification app as necessary on my Android phone, which is fantastic, but the camera lens aims down and the screen aims up, which is about 90 degrees away from useful when trying to solder. For inspection, no prob.
Although embracing it after 45 years, I do not have much experience with SMD. My trial by file was to replace SOIC8 and SOIC20 chips on the PCB of an obscure GPSDO unit. It worked (golf clap inserted here!) but it was really pushing my neurology. I have the aforementioned ornery left eye, a recalcitrant right eye, and ulnar nerve damage in my right (master) hand. What I really need is an SMD assembly robotic fixture from 20 years into the future!
I guess it comes down to wanting a good supply of "widely usable" parts on hand so I don't have to wait for delivery every time I want to build something (and pay shipping, and shipping, and shipping). I have lots of Rs, plenty or toroids to make Ls, and I have lots of through-hole Cs. I am hoping that I can sort of standardize on NP0/C0G 1206s for HF/RF stuff.
I recently ordered a couple hundred 600-V caps in the usual values for restoring tube amps and radios. Now, when I need something along those lines I have a pile of 'em on hand. Setting aside bypass caps, electrolytics, and audio stuff such as tantalums, I'm hoping that laying in a supply of NP0s will get me squared away for oscillators, filters, etc.
Plus, it's still shocking how little space SMD parts-caches take up!
Thanks for all of the advice. I appreciate it,
--NT0Z
I tend to use through-hole parts and SMD parts together, all on one side of an SMT board or Manhattan-style on a piece of copper-clad board.I am looking into some kind of affordable USB assembly scope/stand with an LCD display. My eyesight isn't great for binocular solutions, as my left eye is awfully ornery. I use a magnification app as necessary on my Android phone, which is fantastic, but the camera lens aims down and the screen aims up, which is about 90 degrees away from useful when trying to solder. For inspection, no prob.
Although embracing it after 45 years, I do not have much experience with SMD. My trial by file was to replace SOIC8 and SOIC20 chips on the PCB of an obscure GPSDO unit. It worked (golf clap inserted here!) but it was really pushing my neurology. I have the aforementioned ornery left eye, a recalcitrant right eye, and ulnar nerve damage in my right (master) hand. What I really need is an SMD assembly robotic fixture from 20 years into the future!
I guess it comes down to wanting a good supply of "widely usable" parts on hand so I don't have to wait for delivery every time I want to build something (and pay shipping, and shipping, and shipping). I have lots of Rs, plenty or toroids to make Ls, and I have lots of through-hole Cs. I am hoping that I can sort of standardize on NP0/C0G 1206s for HF/RF stuff.
I recently ordered a couple hundred 600-V caps in the usual values for restoring tube amps and radios. Now, when I need something along those lines I have a pile of 'em on hand. Setting aside bypass caps, electrolytics, and audio stuff such as tantalums, I'm hoping that laying in a supply of NP0s will get me squared away for oscillators, filters, etc.
Plus, it's still shocking how little space SMD parts-caches take up!
Thanks for all of the advice. I appreciate it,
--NT0Z