Re: I’m an idiot!
 

I recently killed my trusty Fluke 177 but I won't say how I did it other than it is scrap!

Re: I’m an idiot!
 

Done that!

Re: I’m an idiot!
 

I did a really silly thing yesterday.

I recently purchased through the Forum a working 12 volt 12.5 amp switched mode power adapter, which just needed a kettle type mains lead.

To test it I borrowed a cable from one of my cassette recorders but the power adapter appeared dead.

I was thinking that I would have to go inside the adapter and start fault finding when the penny suddenly dropped.

I fuse (the mains plugs) of most of my equipment with 1 Amp fuses, occasionally going up to 3 Amp. Checking the fuse it was indeed 1A and blown (it was working before I borrowed the cable). So even though the adapter had no load connected it was obviously drawing > 1A

Fitted a 13A fuse and then all good.

Edit Update - The label on the adapter shows the mains input to be 1.8A, so should be able to drop the fuse down to 3A


David

Re: I’m an idiot!
 

Some of the better meters, ie, modern Flukes, will beep at you if you select a non-Amps range while the leads are still plugged into the Amps sockets. I don't know how they know.

Re: I’m an idiot!
 

Regarding that PSU. Remember that the device is probably (in fact almost certainly) a switched mode power supply. So when first switched on, regardless of any output current, there will be a surge to charge the capacitor on the input. That will be pretty high albeit for a short time. You can't simply rely on the device ratings.

Re: I’m an idiot!
 

I always assumed there was a switch in the bottom of the socket. Didn't stop many muppets where I worked.

Re: I’m an idiot!
 

I will now confess to what I did with the aforementioned meter, it was being used to test current flow from a solar panel to charge controller, a battery test was then performed without changing the red lead to volts instead of the 10A socket, the internal fuse was dead so I used a piece of fuse wire instead, the PCB track was the fuse!
The only settings to work are Volts and that's it, the other settings are giving random readouts.

Re: I’m an idiot!
 

idiotic tricks!
i recently had a stroke and already have dementia. the doctors asked me to send video of arm (medication reaction). still cant find folder for camera stuff, i was high on tech all my life, so feel a right gonk

Re: I’m an idiot!
 

Not me but a colleague. he was measuring the filament current on the electron gun on a linac. This is normally about 8A. When the machine is generating x-rays the gun is pulsed to -40kV. guess what happens to a meter in series with the gun.

Re: I’m an idiot!
 

Good point.

I dropped the fuse in the plug from 13A down to 3A.

Out of interest I connected an AVO in series with the fuse to see what current it was drawing.

Surprised that it is as low as 25mA and no obvious sign of a surge at switch on, it is maybe too fast to register on the AVO.

David

Re: I’m an idiot!
 

A well-designed SMPS will draw a seriously-low current when the output is unloaded; it doesn't have the static 'magnetising current' of a copper-and-iron transformer to deal with; that's why they are so popular in these times of energy-saving.

A typical AVO is so damped that it won't really respond to initial switch-on surges [which can only be half an AC cycle...] to properly respond to such surges you really need an undamped 'Ballistic Galvanometer', or a fast storage-scope with time-domain integration/differentiation functions to see what is going on.

In times-past one of my clients had people in over the weekend to do PAT on all their gear, which included a bunch of what were then top-end 21-inch Sun Microsystems monitors. The PAT-people looked at the stickers on the back of these monitors and, seeing that they were rated for 600 Watts, replaced the 13A fuses in the kettle-leads wiuth 3A ones.

After a few days the client was unpleased - monitors were failing at switch-on. The PAT-people had failed to account for the combined effect of a SMPS PSU capaacitor-charging current-inrush *and* the degauss-circuit activating at switch-on. I sent a couple of guys out to the local DiY sheds to buy up every 13A fuse they could find, and paid them triple-time to fit these fuses in every plug.

The PAT-testing company did _not_ get paid for their work!

Re: I’m an idiot!
 

Yes, too fast for you to see with a meter. Bear in mind that at switch on if the main capacitor is empty it will need to charge to rectified mains voltage, and it's resistance (well impedance really) will be pretty low. In theory that will happen within a half cycle of the mains at 50Hz. So milliseconds.

It's a different situation, but had a problem with a Weller soldering station. Kept blowing input fuses even though they were technically the right current rating. I learned something about transformers. You would think, that transformers are basically inductors so shouldn't have a surge on switch on. But it's more complicated than that (and I never really understood the full detail). But bottom line is, if you switch on at the right point in the cycle (I think peak voltage, but zero current flowing because it's been switched off so far) then there's actually a much higher current peak than you might expect. So a time delay fuse is really important even for what you might consider something that wouldn't surge.

Hopefully, someone can explain a bit more anout my ramblings as TBF I'm a low voltage DC or logic man.

Re: I’m an idiot!
 

For some reason which I'm sure someone will explain toroidal transformers in amplifiers can blow the fuse severely (blackened or shattered) but work perfectly well when it's replaced.

Re: I’m an idiot!
 

But there was an obvious sign of a surge - you blew your 1A fuse, remember?

1A fuses have thin wire, so they won't survive much of a surge, either in amps or in milliseconds.

Not necessarily half a cycle - there's almost always some deliberately-added series resistance to reduce the magnitude of the surge, which is bypassed shortly after switch-on. Sometimes it is something as simple as a self-heating thermistor, which starts high resistance and then reduces as it warms up. A supply like this can exhibit a much higher inrush surge than normal if it's switched off and switched back on a few seconds later, because the thermistor will still be hot and low-resistance.

The worst time to switch on a transformer is actually the zero-crossing, not the peak - because then the voltage applied is the same polarity for the longest possible time (10msec with our 50Hz mains). That makes the magnetic flux in the core continually change in the same direction, saturation is then most likely, and that is when current shoots up.

Toroidal transformers are particularly prone to this, because from the last switch-OFF, they can retain core magnetism (remember core memories?), so if you switch off at the wrong time of the cycle, and switch back on, at the wrong time - even weeks later - expect a thump!

Re: I’m an idiot!
 

Yes realised that, I should have said no sign of a measurable surge using the AVO.

David

Re: I’m an idiot!
 

About 20 years ago I decided to make a 160m am tx, all solid state with mosfet pa for around 10 to 15W dc input. Mains psu supplying various required voltages. A bits of this and that circuit design cobbled together.
I struggled to get more than a watt or two indicated rf output, despite dc pa input being a great deal more. Even occassional rf burns and sparks as well.
After several hours, the penny dropped.
I was using my Bird termaline power meter. Freq range 30 to 500MHz! (I did mostly vhf 2 way radio repairs).
Changing to my Racal Dana 1 to 1000Mhz power meter showed more like the expected rf level of over 10W.

My previous top band tx was built around 1972, using a BD123 I think as the pa It was for 12V mobile only and worked well. May have been either a PW or SWM design?.
Rob

Re: I’m an idiot!
 

I think we have all made embarrassing mistakes in our past. It’s a case of learning where we went wrong and hopefully not doing it again.

Re: I’m an idiot!
 

My apprentice and I had been evaluating a new transistorised NDB transmitter - changing the taps and tuning capacitors on the inbuilt tuner and getting a few amps into the antenna on our test frequency. We knocked off for lunch and afterwards I went back to carry on and the boy came along later. For the life of me I couldn't get the power out that we had before lunch - until the boy came along and said "you've left the earthing wand on the antenna feeder" so I just grabbed the handle and took it off. The spark I drew was about 6 inches long before the power supply breakers popped. Fortunately everything survived except my nerves. The burn marks on the bare copper feeder and the wand are probably still there to testify my stupidity

Re: I’m an idiot!
 

I used to work on X-ray machines, these went up to 150kV. The cables between the transformer and tube were about 2 inches thick and the plugs and sockets were oil filled.
The oil would start to break down after a while, the best check for this was to sniff the end of the plug. Problem was that the cable had significant capacitance and would hold a charge for a very long time. Standard procedure was to discharge the cable by holding the plug against ground.
Most engineers I worked with had a small scar on the end of their nose where they had forgotten and drew a large spark between the cable and nose.

Peter

Re: I’m an idiot!
 

Anyone can make a mistake.

The real questions are:
"Did you learn from it"?
" Will you make the same mistake again"?
This is how we learn. Sometimes with dire consequences, and sometimes not.