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There is a kinda woven semirigid and a solid semi rigid (like a copper pipe). The solid one does not put up with bends for very long.
Beginners / Re: If you had to pick just a handful of capacitors..
« Last post by SkrillBill on Today at 08:09:20 AM »
Wow, i'm learning a lot more than i thought i was going to. This is awesome.

For a simple capacitor made of two plates,

The capacitance is proportional to the area of the plates and inverse proportional to the distance between the plates

C =  k   A/d

When  you put two caps in series,  you double d, so  you divide the capacitance by two

When you put two caps in parallel, you double A, so you  double the capacitance

See for more.

Another thing to note is, in a set of capacitors connected in series, the smallest capacitor always sees the highest voltage. In fact, the same formula used for a resistive potential divider, can be used for two capacitors in series, if reciprocal of each capacitor is taken first.

C1 = 47nF
C2 = 22nF
Since both capacitors are in nF and it's only the ratio between them that matters here, we can ignore the nano part, which stops us having to deal with big or small numbers.

Take the reciprocal of each capacitor:
Y1 = 1/C1 = 1/47 = 0.02128
Y2 = 1/C2 = 1/22 = 0.04545

Now use the usual formula for a resistive potential divider:
V(Output) = Y2*V1/(Y1+Y2)

V(Output) = 0.04545*12/(0.02128+0.04545) = 8.173V

The only trouble with the above calculation, is it assumes each capacitor is perfect and doesn't leak any current. In real life capacitors have a leakage current, which be unpredictable and will start to dominate, once the capacitors have charged up. In the previous example, if C2 has a higher leakage current (lower equivalent parallel resistance) than C1, the voltage across C2 will settle to a lower value, than C1 after awhile. If there's no DC present, for example the two capacitors are across a purely AC voltage source, then the above formula will work. If the voltage rating of both of the capacitors is higher, than the supply voltage, and it's not being used as a voltage divider, then the unequal leakage current can be ignored..

If the voltage ratings of the capacitors is below the total supply voltage, then the uneven leakage current could be an issue. The solution is to add voltage sharing resistors across the capacitors. If the ratio of the resistors is the same as the reciprocal of capacitor values, then the steady state and initial voltages will match. In the above example, if a 2.2M resistor is connected across C1 and a 4M7 resistor across C2, then the steady state voltage, will be similar to the initial voltage when the capacitors are rapidly charged.

One short cut which is useful is, if the values of both of the capacitors are the same, then the total capacitance halved and any voltage sharing resistors used can be the same value.

i'm making notes of all this information guys. It's the same as me reading it but having it explained this way seems to make it easier for me to understand because i do.
I would suggest you take out the motor from the system so that the noise can be isolated and traced.

Yes, this is on the way and should hopefully be in my mailbox tomorrow. Should make it easier to remove motor, no need to disassemble the whole drivetrain...hopefully
RF, Microwave, HAM Radio / Re: HDSDR, RTL-SDR, and power
« Last post by cdev on Today at 08:05:51 AM »
You should be using another means of measuring your signal generator.

Not a TV receiver. The way the front end of the little $5 TV dongles is designed, its just not linear.

Also you would need an attenuator. Although I would be surprised if 1.8 volts "fried" it. 1.8 volts is still very high for any kind of receiver.

I have an RTL-SDR dongle hooked up to HDSDR.  The spectrum display shows decibels of power on the signal.  I understand they aren't calibrated, but is there any information on what they are relative to?  It doesn't seem to be dBm.

I ask because I'm trying to look at a signal that, on the scope, shows as 1.8Vpp (but not centered on ground, rather 0-1.8V)  Which should be about 9dBm.  On the HDSDR it's coming in around -50dB.  (the signal is 50 Ohm source and it's fed to the dongle via a 50-75Ohm adapter, since the dongle is 75 Ohm).  Admittedly, it's an ugly square wave, so lots of the power could be in the various harmonics its throwing (and it's throwing a lot of them), but that much?

I don't have any other way to measure power that low at those frequencies - my power meter can only resolve down to about .1 Watts - 20 dBm.
Probe the motor input with the scope.

Will do.

When the pinion rotate, it sounded like the motor is rubbing onto something. Mechanical noise.
I would suggest you take out the motor from the system so that the noise can be isolated and traced.
Also the speed controller seems to be trying to rotate a stuck rotor.
You need to power the motor not from the speed controller, but external power supply, best is variable voltage power supply. Adjust voltage from 0v to 24 volts to just the motor, both unloaded and loaded externally situation.
Hope to see the video soon.  :D

Other Equipment & Products / Re: reMarkable: The Paper Tablet
« Last post by Tsippaduida on Today at 08:02:12 AM »
  • Bluetooth is not supported. haven't checked if it actually has required hardware, but at least software does not support it.
  • My Financial (or what ever it is in english) teacher once said "It's not expensive, it just feels like it if you don't have the money". He did not get friends from us students.
  • Tips wear down, same for the sony's device. Even the tip replacement package seems same including the replacement tool.
  • Never tried to phone, just used the webshop ("shut-up and take my money" -button)
  • There are only few darkness levels, at least for now. It seems to me that the technology limits this, could not confirm it via rapid googling.
  • Yes, it isn't quite A4, I would also liked that size more, but maybe the next version?
  • Not based on kickstarter, but a crowd funding project in any case. Contrary to many such projects, this one delivered nearly on time and a working product at that.
  • Resolution is enough, it is not the highest you you can get on e-ink, but quite good, imho. E-ink has excellent contrast which makes the display look crisp. My e-reader has slightly higher ppi. High resolution e-ink -displays have quite low ppi if you compare them to ordinary displays.
  • I also suspected that the internal storage would run out in an instant, but I was wrong. Naturally if you want to keep everything you create in the device, you will eventually run out of space. But transferring files between the device and a laptop (I use Linux) is relatively easy.

If you have a change to try out Sony's devices you could then use one of the reviews "Sony vs. reMarkable" to help decide which one to get.

I do not draw, well given ruler and compass I can do a thing or two, but freehand? Perhaps my stick figures are passable. I get by as reMarkable has plenty of templates you can use. I like e.g. Isometric template for some of the technical doodlings I occasionally need to do.
If there is a commutator problems, your motor will jerk, the torque would be jerky and usually sparking at the commutator.
In your case, your motor rotate smoothly, So don't waste your time on this idea.

It's not moving smoothly anymore. It's making noise when it's not moving at all, which means it's having trouble starting. This could definately be your commutator.

However, it could also be an electronic issue. If your motor is getting electrical noise, there may not be enough power in the spurrios impulses to make it move, but enough to make the coils vibrate like a speaker. Probe the motor input with the scope.
If you have any shading, everything turns to crap fast. MPPT ain't going to save you except in a really long string and parallel strings are just lost. A lot of opinions from people that have never used solar. Being off grid makes you look at every watt.
General Chat / Re: Water Heater for an Airline Trolly?
« Last post by james_s on Today at 08:00:00 AM »
For my religious friends, during the 24 hour Sabbath when they cant use any electricity/gas/wood for heating of water, they have a huge thermos/Dewer which holds something like 25L of water, which plugs in and boils it.  When unplugged, (since they don't use electricity during the Sabbath), it keeps the water hot for 24 hours if they only use half of the water.

My oven has a "Sabbath mode" which I accidentally triggered once, I had to read the manual to figure out what the heck it was and how to disable it.

Some of those contraptions are really pretty funny. It always struck me as a bit odd too, I mean if you actually believe that god doesn't want you to do any work on that day, don't you think he'd be like "Hey smartass, nice try!" when one tries to exploit various loopholes to get around literal interpretations of their religious text?
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