This maybe isn't the right place to ask but I'll give it a shot anyway since there is a lot of knowledge here in a lot of different fields.

I was helping a friend a couple of months ago trying to calibrate cheap ebay 4-20mA transmitters for Pt100 temperature sensors. A while back I found ultra precision resistors (0.01% 2ppm) of 100 and 120 Ohm. If we convert these resistor values to Pt100 temperatues it is 0,00 degrees and 51,57 degrees Celsius +-0,03 degrees or thereabout.

We connected each resistor to a 4-20mA transmitter and applied voltage. The transmitters were connected to shunt resistors on an Arduino Uno. After letting the system stabilize for 24 hours we wrote down the A/D values and switched resistors between the transmitters to get two calibration points and repeated the procedure.

After this was finished we assumed we had a fairly good compensation for any miscalibration in the cheap transmitters.

Then we connected two cheap Pt100 sensors also bought from China on ebay. They showed two different temperatures that were nowhere near the actual room temperature so we decided to make an ice bath calibration to get the offset for each sensor. We assumed this offset would be fairly linear and that once we subtracted or added the offset the sensor would show roughly the correct temperature in the 0-50 degree range but the temperature reading was of by ~3 degrees at regular room temperature.

Since calibration equipment is expensive I have no way of verifying that the sensors are close to the specs they SHOULD have. Should we have calibrated in a different manner? The resistors are brand new but has few years on them so there might be some ageing but not this much I think. Also, two Pt100 devices bought from the same seller at the same time should be at least within on degree from each other I think.

I sometimes have a need to weld stuff and I always have to turn to people for help since I don't have the knowledge/skill or equipment for it. The latter can be remedied by getting a welder and what better way than to build one, right 

What I have that may or may not be useful

* A large DC power supply capable of 150A continous @ ~45VDC
* Some film capacitors taken from different welders
* A couple of BIG stud diodes (I think they are rated at 300A)
* Two HUGE chokes, one at 25mH @ 50A and one at 35mH at 50A. They are built on what I believe to be silicon steel sheets so maybe not suitable for high frequency.
* A lot of IXYS IXFN520N075T2 mosfet trench gate modules rated at 480A each

From what I understand a stick welder works best with a constant current power supply so I was thinking that I could build some kind of low frequency PWM controlled constant current curcuit and attach it to my big power supply. Can this be done in a crude manner with the parts I have together with some kind of simple microcontroller to monitor the current and adjust the PWM duty cycle?

There are many circuits out there using arduino and some extra components to drive nixie tubes but I am trying to figure out a cheap way that don't take up too much space so I would like to use ULN2003 if possible.

When looking at the IN12 Nixie tube drive voltage seem to be around 145V DC and recommended current is 2,5-3,5mA. Do these thing behave like a resistor? Can I assume each digit is like a ~48k resistor? If so I should be able to use a bussed resistor array with 7 pcs of 22k resistor to make sure the ULN2003 is never exposed to more than ~45V (50v is ABSOLUTE MAX according to the data sheet).

Assuming that the internal DC resistance of each digit is never LOWER than ~48k I should be safe right?

The other potential problem is the current when the digit is NOT lit. There will still be close to 2mA running through each digit. Will they be dark or will the probably be dimly lit?

Just found these. I am extremely interested in buying one myself but I just have too many other projects at the moment

https://www.ebay.com/itm/Induction-Heating-Capacitor/153605789972

The other day I received an arduino shield for my motorcycle tachometer project. It will be used to convert the ~12V system to 5V and also provide a little backup power.

Since there is a fair amount of noise on the electrical system I figured it would be a good idea to add some kind of filter on top of the filtration the shield will provide for me so I bought a DC choke of 320mH rated for 600mA with a specified DC resistance of 7.8 Ohm. My goal is to remove some of the worst spikes and dips but do I NEED a capacitor as well and should I place it before or after the choke? I know a capacitor will make the filter circuit perform BETTER but what I would like to know is if I MUST place a capacitor just before or after or if it's just good practice. Can a choke WITHOUT a capacitor even make things worse and possibly break certain components?

My initial plan is to use my oscilloscope and measure before and after the choke to see what kind of difference it makes but if just a single choke can actually make things worse and damage my shield I do not want to do that before I have checked with people who has a better understanding about this than I do. All information I can find about chokes refers to using the after a rectifier bridge @ 50/60Hz

I would like to user fixed resistors as calibration for a couple of Pt100 and Pt1000-to-4-20mA current loop converters and I was wondering if a fixed resistor (metal film or wire wound) with extremely low temperature coefficient changes value or if it can be considered stable?

For example, if I order a 0.1% resistor specified as 100 Ohm, the actual resistance may vary between 99,9 and 100,1 which isn't much but when translated to degrees Celsius it's roughly -0,2 to +0,2.

My question is: Apart from temperature drift, will a fixed resistor shift in resistance value or can I use a low ppm fixed resistor of any value and type and attach a small note to each stating the corresponding temperature once I have been able to accurately determine that?

I found a small, embedded machine that I wanted to turn in to a small firewall. It's an x86-based industrial computer with 3xNIC and a 9 pin D-sub connector for console access. The OS  and applications are stored on a CF card and with the software I intend to use I needed to get the serial access going so I could make the initial configuration but no matter what baud rate I tried I could not get it to work. 1200 baud was the "best" but still only gibberish so I had to figure out a way to examine the signaling on the serial port and determine the actual baud rate from that. I found some web sites that explained how an oscilloscope could be used so I tried it. It worked OK but I wanted to get more data and also the signal so I connected the transmit pin to an optocoupler (through a constant current limiting device) and fed the signal to my logic analyzer. At first I tried the "auto baud rate function" and a lot of different settings but then I looked at the signals and looked for the "shortest time" or "highest frequency" which was 680Hz. Half of that time equals 1360 which is the baud rate that was used. After I figured that out I could decipher the information sent on the serial port and start configuring my device.

I thought this information could be useful to others.

I have a fairly good grasp of many components but inductors baffle me.

I try to read up on common mode and differential chokes, DC filter inductors and all kinds of different configurations but I don't understand the fundemental basics of them.

I re-used the HUGE inductor/choke (which is it?) from the welder I modified to a power supply. I connected it between the rectifier and my capacitor bank assuming that it would help smoothing out the ripple in a LC filter configuration. Another thing I believe it helps with is some kind of "soft start" since the DC resistance of the winding should charge the capacitors a little more gentle or am I completely wrong?

I may have a need for a three phase filter of some kind for "analog" SSR that controls a resistive heater load. I have looked at three phase filter chokes which are basically a toroid core with 3 separate windings, one after another, and also three phase inductors that seem to be much large and probably insanely expensive.

What kind of interference can I expect from the proportional triggered SSR and what type of filter components should I try to find or make to reduce the noise on the grid? Am I right in assuming that the frequency of the noise is never higher than the driving frequency? A lot of components seem to be manufactured to filter out noise from SMPS but I need to filter mains frequency (50Hz).

A while ago I started a build of a simple (or so I thought) magnetic stir plate. I ordered a cheap motor on ebay and a friend helped me to 3d-print a holder for the neodymium magnets. I use and arduino with a mosfet to get speed control by PWM.

The problem was that the magnets locked on to the screws I used to mount the motor in the plastic case so it took quite a bit of force to get it to start so I thought I could use a closed loop to control the speed of the motor. I bought a new motor with a built in magnetic encoder that sends pulses when the motor spins. I wrote a small program for my arduino and connected the pulse output from the motor to one of the hardware interrupt pins. When I tried it I got very erratic behavior that I believe may be caused by the inertia of the rotating axel . Would you agree and do you have any good ideas what I can do to remedy the problem?

To understand the pictures better I will try to explain the code I used. Every time I got a change (either rising or falling edge) from the encoder, the mosfet channel is turned off and the arduino resumes the regular loop that has a pause time (to regulate speed) and the turns the mosfet on again. In the pictures channel 0 is the encoder on the motor and channel 1 is the mosfet driver. The last screenshot is from a test I did where i let the motor spin for a few seconds and the turned the mosfet off to see how long after the power was cut to the motor it still turned.

I am setting up a very small off-the-grid installation with a 5W solar panel and a PWM charger

http://www.epsolarpv.com/en/index.php/Product/pro_content/id/773/am_id/136

Since my panel is VERY small I thought I would install a small film capacitor between the panel and the charger, but how should I think when choosing the correct capacitor? I know I want a MKP film cap, but do I need to measure the frequency of the charger before I can properly decide on capacitance value? I guess I want something like 2-5uF.