Recent Posts

1

General Technical Chat / Re: Cable Management

« Last post by thermistor-guy on Today at 10:40:51 pm »

Each individual cable loosely coiled inside a ziplock bag, with a descriptive label on a card insert. All the bags stacked filing cabinet style (on edge) so the labels are visible, in a drawer. Apart from mains leads, which breed when you don’t look at them.

I do something similar:

 - coil the cables with generous radius in large ziplock bags;
 - place the bags in stackable transparent storage boxes; label the boxes (3 labels: side, front, top of box);
 - have separate labelled boxes of cables meant for repair or scavenging;
 - have separate labelled boxes for connectors and unterminated cables;
 - have a catch-all box labelled "misc" for odd items.

The idea is to have stacked array of boxes, where it's easy to tell what's inside each one from a glance.

2

Beginners / Re: How Current Limitation is happening in the circuit???

« Last post by Xena E on Today at 10:40:24 pm »

Hello All,

I started constructing prototype of the below circuit, But 68ohm,2Watts resistance went to breakdown.
again i changed to 470ohm, 2Watts , again it started smoking.


I also changed the MJE2955, but 470 ohm started smoking, What is the problem in the circuit?

How to find my transistor has went to breakdown?

Thank you all.

Incredible!

Taking 470 ohms as the value of the resistor, you have to have over 30 Volts across it to even max out its design dissipation of 2 Watts.

Now, assuming that the base emitter junctions of the pass transistors are intact that means that the output would have to be shorted to ground and in that case almost all of that hypothetical voltage has to appear across the 0.1 Ohm emitter degeneration resistors, that will cause them to try to dissipate over 8.5kW each.

So as this isn't possible, (the transformer isn't that big is it?), it's either ignorance or Bullshit.

The circuit diagram as drawn won't work, in fact if the practical circuit is wired as drawn it would be the transformer that has smoke coming out of it, (you show the output of the rectifier on the 5.1Volt supply shorted to ground for starters).

If you really want help you need to furnish facts and details.

Regards,
Xena.

3

Projects, Designs, and Technical Stuff / Re: Control of 2 CAN Open DS402 motors on a single shaft

« Last post by moffy on Today at 10:40:09 pm »

The crux will be getting the torque motor to react quickly enough to fool the speed controlled motor into thinking that it has twice the torque or rather that the load is half of what it is as it will not be aware of the second motor. So the speed controlled motor will need to have a slower torque ramp rate than the torque control motor so that whatever torque it uses is quickly matched by the Torque controlled motor so that it does not see the full load. And hopefully this all happens without the two falling ass about face or oscillating.

I was thinking the same thing, two torques rather than one, but I assume since it is a semi intelligent controller there must be something like PID parameters that it uses and if so then you can adjust these to reflect the new reality and hopefully prevent overshoot oscillations. Can you get a regular report back from the controllers on motor current? That would provide a good insight into what is happening torque wise.

4

Metrology / Re: ADR1399 reference

« Last post by EC8010 on Today at 10:39:54 pm »

Thanks for your thoughts. That's what I'm thinking; it's too close to the limits of my present measurement ability. I keep wondering about an 8 digit DMM. Or better, an array of at least four temperature controlled voltage references that can be compared using existing kit.

The 1399 is in the cardboard box with foam the evaluation board came in, but not properly sealed; I have done much better thermal control on other experiments. This is early days. I'm also wondering about that plastic hat the device is in. For instance, is the plastic triboelectric where it wobbles and scrapes on the leads? Is the plastic hat there simply because it's cheap and easy to add by the manufacturer, but not genuinely optimised? Would foam be better? The wobblyness and rattling concerns me, too. At this level, microphony is often an issue.

The LM334 is pretty much as per data sheet. I have tried tweaking the ratio of the resistors but it didn't help, suggesting that its remaining tempco was actually down to the (metal film) resistors themselves. I could perhaps do better using low-tempco wirewound resistors and trimming, but I think that's a bit of a dead end. I only put the compensated LM334 in because I had one ready-made and set for 3mA. I think the bootstrapped op-amp solution for the Zener should be better.

5

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

« Last post by Alex Nikitin on Today at 10:39:44 pm »

MMBF4416 perhaps?

Cheers

Alex

6

Computers / Re: Format a 256GB pendrive with FAT32 using a windows 10 pc

« Last post by Halcyon on Today at 10:33:29 pm »

Windows is a system of limitations.

And it's only getting worse.

But as DavidAlfa pointed out, the HP tool is a good one for Windows users. Used it many times when Windows format just fails.

7

Projects, Designs, and Technical Stuff / Re: more modern version of tl494?

« Last post by 2strokeforever on Today at 10:33:28 pm »

AFAIK, 598 is just 494 with totem pole outputs -- better for gate driving.  But you might want a proper driver on it, still, anyway; it's old bipolar stuff.

Tim

From the data sheets the rise and fall times were much better than the 494, some of my preferred gate drivers require a sharp edge to trigger on and this will help.

I never drive anything without a proper modern gate driver

8

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

« Last post by graybeard on Today at 10:31:26 pm »

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

Cheers

Alex

I could easily design using a J304.  However if the MPF102 parts were good, 10 for $1.79 was a good deal.   J304s are 10 for $23.50.  AliExpress gave me an immediate refund.

I may get some newer JFETs anyway because if I publish my designs I would like other people to be able to get the parts I design with if they want to build it.

9

Test Equipment / Re: Troubles with HP-8903B

« Last post by srb1954 on Today at 10:31:17 pm »

This week, I purchased an HP-8903B from a local gentleman.  I went to his home to view it and he mentioned that he serviced the unit by replacing the few capacitors in it and it was working "as it should" and within specifications.

So far, I have two problems with the HP-8903B.

PROBLEM 1 - Frequency Inaccuracy

Is it normal for the unit to not "hold" on a frequency and to have, what I consider, an abnormally high THD when simply using a loopback cable?

For example, if I turn off all filters and then just turn on a 1 kHz sine wave, the display will show a frequency from ~998 to ~1001.5.  No matter what frequency I put in to try to compensate for the +/- 3-ish Hz, I can rarely get it to stick exactly on 1 kHz.  Also, the longer I leave the HP powered on, the higher the frequency drifts.

The oscillator in the HP8903B is not synthesised so you can't expect perfect frequency accuracy. However, it shouldn't drift appreciably either as the the frequency is adjusted by the microcontroller using a "count and tune" method. It might be a good idea to follow through frequency monitoring and switching path from the oscillator to the microcontroller to ensure that the signal hasn't been lost along the way.

An unstable oscillator frequency could be contributing to the high distortion readings as the notch filter in the distortion measurement circuit will not be able follow the frequency variations and will be unable to notch out all the fundamental frequency. Check the oscillator O/P with a scope to see if there is any jitter in either the frequency or amplitude of the oscillator O/P.

PROBLEM 2 - THD %

When I use a loopback cable (regardless of the cable I try), I'm getting a "high" THD... AND the THD seems to also be dramatically affected by the amplitude.

Example #1:  All filters off. Frequency set to 1 kHz.  Amplitude set to 150 mA.  Distortion reads ~ 0.1240.  If I change the amplitude to 250 mA, the distortion drops to 0.77.  If I change the amplitude to 1V, the distortion drops all the way down to 0.022.

Example #2:  If I turn on the 30 kHz filter (1000 kHz tone @ 150 mA), the distortion goes from ~0.1240 down about half to 0.065. Choosing the 80 kHz filter results in a 0.09 distortion reading.

Am I expecting too much out of this machine or is something wrong with it?  The main reason I purchased this unit was to measure distortion so if that is going to be unreliable, it is not going to be the machine for me.

It should definitely do much better than what you are seeing. For a direct loop back connection you should be seeing distortion figures of 0.005% or better.

A useful diagnostic is to connect a scope to the monitor O/P on the rear panel to view the residual signal after the fundamental notch filter. The presence of any discernible fundamental frequency components indicates the notch circuit is not tuning correctly resulting in an inaccurate distortion reading.

The fact that introducing the 30kHz and 80kHz LP filters significantly reduces the distortion readings indicates that there is significant HF noise in the system. This could be due to a noisy front end on the HP8903B or interference from external devices. Turn off all computers, cell phones, WiFi, routers and LED lights in the vicinity to see if that makes a difference. Again, look at the HP8903B monitor O/P to determine the nature of the interference.

You should also try the 400Hz HP filter to see if there is any hum contaminating the measurements. This will also be visible on the monitor O/P. 

10

Metrology / Re: ESI RV622A Resistor Repair

« Last post by gmilliorn on Today at 10:30:28 pm »

I could not find out if the wires inside are Manganin/Constantin or just copper.  Since the point of a KVD is to eliminate
the effects of low-ohm switches, as I understand it, then I assume the wires are the same?  All the internal wires are
soldered to the switches.

Little info I could find on soldering manganin, other than a weird pair of youtube videos doing nothing.

In any event, I cleaned the area w/1200 grit sandpaper and cleaned again with IPA, and was able to solder it
relatively quick with no issues.  Either I was luckly or there's no issue soldering it.
3x cleans after soldering with IPA and it seems to be fine.

I can dial in any voltage within limits of my uncalibrated Fluke 8842, below reads as 1.0000V so I guess it is
off about 70mV.