Recent Posts

1

Repair / Re: Keyboard flexi PCB

« Last post by soldar on Today at 08:03:46 am »

I did not think of using masking tape which is so obvious... I hope I will remember for next time.

I could have used Post-it notes which will come off easily.

2

Repair / Restoring vintage asynch motor (DOs and DON'Ts?)

« Last post by thexeno on Today at 08:02:26 am »

Hi all,

Fist forgive me if I am in the wrong part of the forum, I just need expert advice on vintage electric motors, so feel free to move this thread.
We are trying to bring up safely an old vintage 3N industrial asynchronous motor - is quite heavy, has a few HP, sadly I don't have the label right now. I have an idea on the steps, but I'd like to get  an opinion. The motor mechanically seems perfect and spins with no friction. It seems to have coils connected in star configuration.

My idea:
1. cleaning the old dirt. open if necessary
2. measure insulation, coil-to-coil, and chassis (with multimeter or even high voltage tester if I can)- If needed open the star connection to measure all individually.
3. replace bad wiring, check the internal wirings that are going to the motor, including PE and chassis
4. get a variable frequency controller (rated for such motor), we have 1N and 3N with differntial protection available to the controller
5. playing around, starting slowly? <- this is the part I am not sure about, on top of wanting to double check the previous steps.

Thanks for any 2 cents given! Let me know if you want to know something more in details, I'll provide the info.

3

Repair / Re: Please help, how can I adjust this transistor?

« Last post by daisizhou on Today at 07:59:31 am »

Ether is something that does not exist. This is a medium in which radio waves, gravitational forces and other phenomena incomprehensible to modern science propagate.

So, we have -5 volts from nothing.
Check the resistor R241 -1MOhm.
Look for flux residue around the trace from R241 to JFET Q29.

I checked the connection lines, please see the picture.
Most of it is clean, except for the two yellow ceramic capacitors.I used PCB cleaning water to clean the bottom of the two ceramic capacitors.

4

Projects, Designs, and Technical Stuff / Re: Brushed DC motor closed loop speed control

« Last post by Zero999 on Today at 07:54:53 am »

You can try this.


EDIT:

Multimeter says 4.3 ohms.

Is the resistance of the cables already subtracted?

Using an N-channel source follower like that will require a 5V or more higher voltage power supply to the op-amp. It also appears to be a linear controller, which sill dissipate a lot of power.

5

Projects, Designs, and Technical Stuff / analog pulse shaping strategy question

« Last post by john23 on Today at 07:50:33 am »

Hello i need to convert a 0-3.3V 120ns 500ns period pulse into -2v to -5V pulse to turn on and off CGHV1A250F.
A 100pF load is the load which represents the gate of the CGHV1A250F.
I got the circuit shown below ,but its very odd.
I need to control the upper level lower level and keep the output pulse as square as possible.
Is there some new circuit configuration you could reccomend me to use instead the circuit i got?
Thanks.
https://pdf1.alldatasheetde.com/datasheet-pdf/download/1673867/WOLFSPEED/CGHV1A250F.html

6

Buy/Sell/Wanted / WTB (UK) HPAK 3458a Faulty / Damaged

« Last post by Oldtestgear on Today at 07:47:53 am »

I am looking for a faulty or damaged 3458a for a sensible price. It must be complete, power on, & have a usable display.
I am not bothered about faults or errors as I do not want to use it as a working DMM. 
Cosmetics are not important.  I am happy to consider anything including abandoned projects or the dreaded BER instruments.

I know that repairing one of these can be very time consuming & expensive (Keysight charge over £3k to repair these)  & am prepared!

Please message me off list if you can help.

Phil Parsons

7

Test Equipment / Re: Show us your square wave

« Last post by guenthert on Today at 07:43:12 am »

You can see how the anomaly builds as the signal goes through the system.

Can we?  To me the signals look different.  Almost as if the first blue trace is less steep than the yellow one.  Is the signal's rise slower or is it just rendered differently?  At that sample rate, there are just 40pts per division.  Squeezing sinc interpolation in there will cause artifacts.    I'm not familiar with this specific model, but if it is unable to show the actual samples, it might still export them to a file, which can than be rendered on a PC.

I don't expect pre-rise samples below the base level.  You're not measuring tachyons.

8

Dodgy Technology / Re: Ashton Forbes Over Unity Challenge

« Last post by racemaniac on Today at 07:41:55 am »

I love how he thinks these kind of devices are not regulated.

If it really is over unity, shouldn't power increase exponentially, and this is thus a bomb, which i'm pretty sure is regulated everywhere that you shouldn't build bombs XD.

9

Beginners / Re: schematic review_Switching MOSFETs.

« Last post by clocksticking on Today at 07:29:51 am »

Why would you even need to worry about that? STM32F GPIO is guaranteed to be 0.4V or less when sinking 6mA, implying about to 60 ohm impedance, and in practice it's much lower than that. You're unlikely to draw much more than a couple mA for an indicator LED.

Or do you intend to drive some serious current into the LEDs here?

I'm not so worried about the LEDs current. I'm worried that when the signal is LOW, the gate voltage will be over Vth.
Which will result in activating the relay even when the signal is LOW.

10

Test Equipment / Re: Show us your square wave

« Last post by egonotto on Today at 07:29:21 am »

Hello,

I wanted to highlight the capabilities of the Siglent SDG7102A and Siglent SDS7404A combination by showing a rectangle with the PicoScope 4262.
However, the PicoScope 4262 also has strengths. It is a true 16-bit device.

Best regards
egonotto