Recent Posts

1

Beginners / Re: Bulk input caps and impedance

« Last post by selcuk on Today at 11:02:35 am »

You may use inrush current limiters for this purpose instead of a regular resistor. That is either a NTC or ferrite. They have ratings for inrush currents in their datasheets.
If you want to use a resistor, there are "Pulse Withstanding" resistors on the market. These have inrush current ratings in their datasheets as well. An inductor will probably help but you need to check the DC resistance.

2

Beginners / Re: Strange noise/signal on scope

« Last post by gf on Today at 10:58:23 am »

Your sample rate is relatively low. What you see can be higher frequencies which are folded into the first Nyquist zone by the sampling process. Since the sample rate is different in both screenshots, the folding of aliases is expected to be different as well.

3

Beginners / Re: Bulk input caps and impedance

« Last post by paulca on Today at 10:55:29 am »

If I was to use an inductor... as I am basically trying to "smooth" the current...

Then am I correct in saying the ideal capacitor will "instantaneously" appear as 0Ohm, thus pull an ideally infinite current -ESR which will produce a "rising edge" with a VERY high frequency.

"Inductors effect voltage to resist a charge in current."

So on the DC barrel jack connecting the current will "attempt" to spike but the inductor will instead charge it's magnetic field reducing the slope of the current into the classic inductor curve until it hits saturation, but which time the capacitor should hopefully be charged a good bit.

Is that the "gist" of the idea?

I assume then to actually calculate it you need all the values including the parasitics?

4

Beginners / Bulk input caps and impedance

« Last post by paulca on Today at 10:49:46 am »

A split out question from my other project.  Trying to pick out some of the basics I might be missing.

If I put a cap directly across a DC barrel jack input, then when I connect the power to it, I will get a spark and the supply will basically see a 0 Ohm short for a fraction of a second and a load which seems to want to pull an ideally infinite amount of current.

So, it seems to make sense to put at least a "token" value resistor in series before it.  That or one of them magical to me, inductors.

As it is a bulk input cap, it has to be able to pass at least the average current of the load.

If I place an upper limit on the PSU of 200mA and choose a quarter watt resistor, then using ohms law....

I want a resistor that will drop 0.25W maximum across it at 200mA under normal operating conditions.

P / I = V
0.25W / 0.200A = 1.25V

R = V / I
R = 1.25V / 0.2A
R = 6.25Ohm

Now, for the "empty cap" short.  Initially the resistor will be dropping all 36V of the input.  So the "instantaneous" current through the resistor is limited to

I = V/R
I = 36V/6.25O
I = 5.76A

P = V*I
P = 36 * 5.76
P = WAY TOO MUCH

However.  It's transient.  How long does it take for that volt drop to reduce as the cap charges?

Well, we know that initially we would be putting 5.76A into the capacitor.  This I believe can be input into the capacitor charge equation and produce a charge time / curve.

I think this is the end of the line and as far as I can go, without dusting off the simultaneous equations I haven't explored in quite a while!

As the cap charges the voltage drop across the resistor reduces... so the current passed with reduce as well.

With 5.7A in and 0.2A out and, say, a 220uF cap.  How does one calculate the total transient power dropped across the resistor and how do you determine if it's "sensible"?

Can I assume that a quarter watt resistor will take a few micro seconds of 100W+?

5

Manufacturing & Assembly / Re: Ribbon Laser Bonding on PCB

« Last post by wraper on Today at 10:38:35 am »

Dunno about 5x5mm but smaller ones are easily available https://www.digikey.com/en/products/detail/harwin-inc/S70-138305045R/6211249

6

Beginners / Re: Strange noise / FFT signal on scope

« Last post by DanielDaniel on Today at 10:37:54 am »

I don't agree. Why should the displayed spectrum suddenly change completely if you change the frequency range only slightly?  
Further, I dug out an old (rather historic) spectrum analyzer and could not detect said strange signal component with it.
For me that shows that the FFT implementation is     
At any rate, it think that the FFT results of the RTB2004 should be treated with caution.

7

General Technical Chat / Re: ultra sonic transducers

« Last post by eutectique on Today at 10:35:24 am »

You might get some ideas from these videos:

8

Microcontrollers / Re: SD Card reliability in SPI mode

« Last post by Miyuki on Today at 10:35:02 am »

It seems like an implementation issue.
SD Card and Fat systems are fragile and must be written really carefully.

I have SD Card logging in one recent project, it buffers data internally and then flushes it in 5 to 10 second intervals to the card. It works fine for many weeks.
But as it does not have RTC or any date, but just a fixed value, it will totally crash if any modification is done to the files or even when they are simply deleted.
It uses FatFs library and when this happens it results in total garbage on the card. 

9

Projects, Designs, and Technical Stuff / Re: No interrupt from AS6501...

« Last post by mino-fm on Today at 10:32:06 am »

0xC0                  ; CFG2: common & blockwise read
This is for LVDS output you don't use.


Let me show you an init sequence for both channels without PIN_ENA_RSTIDX.

const uint8_t AS6501_init_tab[] = {
      0x15,           // stop-A, stop-B und ext. REFCLK einschalten
      0x85,           // stop-A und stop-B auswerten, 1x Auswertung
     
      0x00,           // LVDS nicht aktiv
      (uint8_t)(REFCLK_AUFLOESUNG),            // LSB 
      (uint8_t)(REFCLK_AUFLOESUNG >>  8 ), // MSB-1
      (uint8_t)(REFCLK_AUFLOESUNG >> 16), // MSB
      0x00,           // LVDS nicht aktiv
      0x00,           // dto.
      0xa1,           // interne Konstanten
      0x13,
      0x00,
      0x0a,
      0xcc,
      0xcc,
      0xf1,
      0x7d,           // bis hierhin
      0x04            // CMOS-Pegel Eingänge
    };

You always have to read out each FIFO to enable INTERRUPT again. They should be cleared after init.

10

Manufacturing & Assembly / Re: Ribbon Laser Bonding on PCB

« Last post by Kean on Today at 10:25:18 am »

I'm guessing these are too small... or as you say, not flat.

https://www.keyelco.com/category.cfm/Test-Points-Tips-Probes-Clips/SMT-PCB-Jumpers/p/518/id/1206
https://www.keyelco.com/userAssets/file/M70p60.pdf

The largest seem to be 2.0x2.54mm, 1.7x3.0mm, or 1.8x5.5mm on top surface.  Maybe they could make something custom one for you.