Recent Posts

1

RF, Microwave, Ham Radio / Re: fake MPF102s from AliExpress

« Last post by G0HZU on Today at 11:13:12 pm »

Farnell still have stock of the BF256B JFET in the TO-92 plastic package. The price here in the UK is £0.40 each but Farnell sell them for less than $0.20 each in the US.
This is a process 50 JFET like the MPF102 but made to a tighter tolerance for Idss.

2

Power/Renewable Energy/EV's / Re: DIY Battery Bank from Recycled 18650

« Last post by FinOminal on Today at 11:12:12 pm »

more pics from their add on facebook

3

Projects, Designs, and Technical Stuff / Re: Passing 2 wires into a hot water cylinder

« Last post by ejeffrey on Today at 11:07:29 pm »

Thermowells have a number of big advantages that make them the overwhelming choice.  They precent the hot liquid from attacking the sensor, wires or insulation.  They prevent the sensor from contaminating the liquid (important if it's food!) and avoid nooks and crannies that can trap liquid and make it difficult to clean.  It's much easier  to remove and replace broken sensors or just take them out to calibrate. There are enough advantages that it's the most common way to do this and thus the easiest.

If you really need a feed through, swagelock fittings can be used for this.  Just use a solid rod the right diameter for the ferrule and connect to it on both sides.

4

Projects, Designs, and Technical Stuff / Re: Linear acceleration of stepper motor

« Last post by moffy on Today at 11:03:10 pm »

Just an FYI for Andre, there is a project that might suit called GRBL: https://github.com/grbl/grbl
which runs on an Arduino, it takes GCode commands and controls the stepper motors, it is used quite extensively and would probably suit your application.

5

Power/Renewable Energy/EV's / DIY Battery Bank from Recycled 18650

« Last post by FinOminal on Today at 11:00:24 pm »

Someone on facebook is selling these:

I attempted to let the seller know this is a dangerous build, but they don't seem to care.

I thought it would be worth while getting opinions (technical and rational) from the community so they can see that it's not just me being annoying.

What are your thoughts on using random batteries that are soldered together in this way?

6

Beginners / Re: LC filtering for combined Vref/VDD of ADC

« Last post by temperance on Today at 10:54:59 pm »

An LM4040 shunt reference maintains a dynamic resistance below 0.25 mOhm with a bias current of 1 mA up to about 350 Hz. With a 270 R bias resistor this equates into a 60 dB PSSR. Below 200 Hz the dynamic impedance is even lower and close to 100 mOhm or about 68 dB PSSR.

Does it outperform an LDO? When bypassed with a 10 µF ceramic capacitor is does for freq. above 1...10 K. But some LDO's are better than others. Some modern ones are very good while some are just bad. The REF2030 I mentioned earlier is a series reference. (My mistake) When it comes to high frequency noise, the LM4040 when bypassed with 10 µF outperforms the REF2030 for freq. above 1 K.

Besides, at low currents, the PSSR plot might still be valid because the LDO is not operating close to or in saturation. But the way they specify drop out can be tricky. I usually calculate the MOSFET on resistance. That seems a more valid approach to me. But that only works for LDO's with output voltages > 2.5 V.

A shunt ref with a very low dynamic resistance is the AZ431L. 50 mOhm from DC up to almost 50 KHz. This would translate onto 75 dB PSSR from DC up to 50 K without a bypass capacitor. Not bad.

But your question is valid because this turns out to be an edge case.

7

Projects, Designs, and Technical Stuff / Re: Passing 2 wires into a hot water cylinder

« Last post by tom66 on Today at 10:53:03 pm »

Why not just put a temperature sensor on the hot water output of the cylinder. It will be very close to the actual temperature of the cylinder water, since there will be some conduction even without active hot water flow.  If you insulate that area well, it will be even more accurate. 

8

Projects, Designs, and Technical Stuff / Re: (Yet Another) DIY Multislope ADC

« Last post by PCB.Wiz on Today at 10:48:29 pm »

NNNI may test that easily (including the new rundown) while running off the stock 12MHz crystal against the PLL version. A stable canned XO/TCXO would be even better.

An external Osc is certainly easy to try.
TCXO's need care, as some use digital corrections that give small jumps in frequency as the temperature varies.
Some vendors now call their better TCXOs analog compensated - eg
https://ecsxtal.com/news-resources/what-are-the-benefits-of-using-analog-compensated-tcxos/

I'm thinking the ideal is an oscillator that is (very) stable over the integration time.
That means long term drift is less important, & it is best to focus to avoid effects like micro-jumps or other disturbances.
The close-in phase noise is looking like the best indicator of that.

A bit of thermal mass around the oscillator can help reduce temperature slope effects, and a local low noise regulator can isolate supply variations.

The 48MHz is needed, however, as you want to upload the binary to the 2040 easily, and you will do it many times, sure.

You just need to be able to generate 48MHz for the USB, so more clock choices exist.
I've not found a very low phase noise oscillator at 48MHz yet, tho NDK say they can cover 20-50MHz in their NZ2520SDA family. Just needs some MOQ 

9

RF, Microwave, Ham Radio / Re: fake MPF102s from AliExpress

« Last post by Gerhard_dk4xp on Today at 10:43:48 pm »

Just curious. Is there anything special about MPF102? Could J304 be a suitable replacement?

MPF102 is traditionally the bin for the FETs that fail every test
but being a fundamentally working transistor.

Gerhard

10

Beginners / Re: LC filtering for combined Vref/VDD of ADC

« Last post by HwAoRrDk on Today at 10:43:45 pm »

How much bandwidth do you need?

I'm only measuring voltage rails on a device-under-test to determine whether their levels are in-spec, not any kind of varying signal, so hardly any bandwidth at all.

If not much, you might average it all out on the digital side and kinda not care.  It's not a great solution, as the noise will alias and there is an asymmetry to it (it's ratiometric, in particular in the denominator, not additive), which will cause some wandering baseline that doesn't average out, but it might be small enough not to mind (i.e. consider the effect of 3.35 vs. 3.25V ref).

I might do some averaging on the digital side, not sure yet. Although, I want to ensure the voltage rails under test aren't fluctuating so averaging over too much of a period will probably mask what I'm trying to measure. What I'm probably going to be looking at is not only the mean value of samples but also their std. deviation too.

What spectrum is the noise?  If it's switching noise, an LC filter is fine.  If it's all kinds of crunchy, from various causes, erratic time constants, likely you need an LDO.

I think it's switching noise from a DC-DC converter mainly. No idea what frequency the switcher is running at. I should probably try and analyse it in more detail than I've done so far: that is, poke it briefly with the 'scope and go "ew, that's dirtier than I was expecting".

Note the LDO can be very thin: say 3.0V, so logic levels aren't violated.  You don't want e.g. 3.6V+ beginning to forward-bias the ESD clamp diodes into a device at 3.0V supply, or 3.3 into 2.7, but 3.3 into 3.0-3.1 would be perfectly fine.  Downside: LDO PSRR is typically trash to begin with, and it's even worse operating so close to dropout.  Again, it's only a thing if you need low frequency filtering.

Bonus: LDO at least gives [the chance for a] more accurate supply/ref, potentially reducing calibration error.

I suppose an additional benefit of running the ADC and op-amps from a 3V supply is that I'll also get slightly higher resolution, as the input will be larger relative to the reference. But that's minor, and I don't particularly need super accuracy - even this 12-bit ADC is a bit overkill for my application.

I wouldn't know how to pick an LDO regulator with good PSRR, though. It seems difficult to make comparisons because the manufacturers all give specs according to different conditions. Some give it at 1kHz, some at 100kHz, etc. I'm not even sure what magnitude of PSRR is good. Is 50 dB good or poor? 70 dB? 90 dB?