Suggestions on variable DC power supplies/oscilloscope discussion.

[Sigurthr]

Re: Suggestions on variable DC power supplies.

Old scopes do take up a tremendous amount of room, be forewarned. My scope is roughly 14" x 6" x 26", with the long axis parallel to the surface you put it on. It's basically half of my electronics shelf.

Edit: Oh and it weighs over 35lbs.

 

[Down with Umbrella]

Re: Suggestions on variable DC power supplies.

My first experience with an O-scope was at an antique store that specialized in vintage electronics. ( I don't think they tested the equipment, just slapped it on the shelf with the other junk) anyways, this scope was old. Very old. The unit was the size of a small suitcase and had a round screen IIRC with few knobs or dials.
If I had to guess I would say it predated the 70s if not early 70s. Judging by the wiring in the back.

I will admit, I am still new to these tools. If I bought something used off eBay I might not have the technical know how to trouble shoot an old CRT. Or run the risk of it not working at all and I loose money on a boat anchor.

I still haven't chosen between an digital or analog. Still researching for my applications.

 

[Illuminum3415]

Re: Suggestions on variable DC power supplies.

Old scopes do take up a tremendous amount of room, be forewarned. My scope is roughly 14" x 6" x 26", with the long axis parallel to the surface you put it on. It's basically half of my electronics shelf.

Edit: Oh and it weighs over 35lbs.

Gotta love Tube scopes though, immune to surges, keeps the workshop warm

 

[Sigurthr]

Re: Suggestions on variable DC power supplies.

Hehe, that they do. I love properly working tube equipment really, I mean I REALLY do, but they are hard to come by and expensive to repair.

As much as I love my old scope I'd pick up a rigol in a heart beat if given the chance. I mostly do various oscillator work (TCs, radio, etc) and the features new scopes have are immeasurably useful. Some times I just can't get this old scope to trigger where I need it to and I'd fudging info out of fading and free running traces.

 

[ARG]

Re: Suggestions on variable DC power supplies.

I mostly do various oscillator work (TCs, radio, etc) and the features new scopes have are immeasurably useful.

Digital circuit troubleshooting has been made much easier with the newer scopes. Most of them these days can decode the data being sent over common protocols, so I don't have to count the bits and do the math to determine is the data being sent is as expected.

 

[paul1598419]

Re: Suggestions on variable DC power supplies.

Yeah, though I love my scope, I've been thinking of getting a new digital one. Only problem, the ones I want are in the thousands of $s.

 

[Down with Umbrella]

Re: Suggestions on variable DC power supplies.

Paul, is that a Hewlett Packard in your profile picture?
We haven't discussed HPs yet in this thread.
I see a few of these on eBay for reasonable starting bids
HP Agilent 54602A 150MHz 4 CH Oscilloscope Analog Digital Storage Good Working | eBay


I would like to bring an O-scope to work and see which welding Machines run best on a tig/stick setting. The machines just don't run right. I'd love to see their current characteristics on a screen.
Tesla coils, drivers, welding machines, my roommates audio amplifier woes...all things I could use this tool for.

 

[Cyparagon]

Re: Suggestions on variable DC power supplies.

I would like to bring an O-scope to work and see which welding Machines run best on a tig/stick setting.

Many modern welders have a high voltage starting pulse to strike the arc. This will blow the ass out of the scope's front-end. Proceed with caution.

 

[Down with Umbrella]

Re: Suggestions on variable DC power supplies.

Yes, some machines have a high freq. start and/or an arc control setting. The increased voltage helps initiate the arc across the air gap. Have to avoid that.
There must be some way to test welding machine's out put Assuming the arc control is zeroed out.
What type amperage/voltage tolerances do common oscilloscopes have?

 

[paul1598419]

Re: Suggestions on variable DC power supplies.

Down with Umbrella, Yeah, it's a 100 MHz delay sweep HP. I went through it when I first got it and now it works like brand new. A 4 channel 150 MHz HP should be a good scope if it is still functional. I've used these for over 40 years.

I checked out the scope you are looking at and it is a DSO, but a beginner's scope. It didn't cost the $1200.00 mine did new. Depends on what you want to do. For working on an arc welding machine, I'd have to say this is not the scope for you. Also, I'd be very careful trying to scope any arc welder as the HV could be disastrous. Once you have an arc, the current isn't an issue. It's only the HV you need to worry about. I'm not too familiar with these machines and don't know if once you have an arc, it would be safe from any additional HV should you lose the arc while testing.

 

[Benm]

Re: Suggestions on variable DC power supplies.

Oh well, they can be bulky. I have this dual trace 20 MHz hitachi scope here that is not quite THAT big, but compared to the digital ones it is very heave and deep. The front of it is just a bit bigger compared to the typical budget rigol option, but its over a foot deep.

I do lik the 'no BS' operation of these things: each channel has its own buttons for scaling and positioning, timebase is very simple to use, trigger is a breeze.

If you have them set on your bench they're just fine, unless you need memory, single shot and such. On the road i'd obviously prefer a digital, especially as some of them come with battery packs so no need for a mains outlet.

 

[Sigurthr]

Re: Suggestions on variable DC power supplies.

To scope a welding machine there's no off-the-shelf scope for the job, what you need is a current probe and then any capable scope set up properly. You pass the conductor through the current probe and then the voltage across the probe is proportional to the current through the conductor. Only really works for AC current as you might have guessed, but any change in DC current would be seen as AC current superimposed over an (invisible) DC bias. This way the scope is completely isolated from the HV and the welding current. EasternVoltageResearch sells some decent current probes on the cheap, I have one of his larger ones and it works fine up to 100A or so. You can change the operational range by changing the onboard resistor, the hard part is just finding a resistor bulky enough as there's quite a bit of power (wattage) through a burden resistor when you're talking about hundreds of amps.

The less-correct but cheaper way is scoping across a resistive element in series with the load, but you have to take into consideration that this will produce a relatively consistent loading that the welder would normally never encounter, so results may not reflect actual situations. This way there is no isolation, but you can directly watch DC current profile. You'd have to set the tig machines to scratch-start (no HV/HF) and use a very low resistance current sense resistor of major wattage, probably just a length of steel cut to a certain size so the resistance is known. If for example you expect to be running at 300A; you'd want half a volt or so for your scope to see at most, so R = V/I; R = 0.5/300 = 1.66mOhm (milliohm). This of course requires you have an ohmmeter capable of such resolution/accuracy.

Generally though, if it's an inverter welder the issues are from thermal buildup on the switching elements causing increased voltage drops and current fluctuations, which leads to momentary arc quenches or pool solidification and varying penetration depth. Older transformer welders generally don't have problems other than bad HV starters unless the interturn insulation is cooked to nothingness from poor ventilation. If the TIG torches are watercooled, check the conductivity of the circulating water, as electrolyte and corrosion buildup can cause issues. Make sure the collets are all in good condition and that the O-rings seal properly too.

 

[Down with Umbrella]

Re: Suggestions on variable DC power supplies.

current probe and then the voltage across the probe is proportional to the current through the conductorThe less-correct but cheaper way is scoping across a resistive element in series with the load, but you have to take into consideration that this will produce a relatively consistent loading that the welder would normally never encounter, so results may not reflect actual situations. This way there is no isolation, but you can directly watch DC current profile. You'd have to set the tig machines to scratch-start (no HV/HF) and use a very low resistance current sense resistor of major wattage, probably just a length of steel cut to a certain size so the resistance is known. If for example you expect to be running at 300A; you'd want half a volt or so for your scope to see at most, so R = V/I; R = 0.5/300 = 1.66mOhm (milliohm). This of course requires you have an ohmmeter capable of such resolution/accuracy.
Older transformer welders generally don't have problems other than bad HV starters unless the interturn insulation is cooked to nothingness from poor ventilation.

Interesting. This could be worth pursuing. Realistically I'd be running at 80 amps but I understand the equation. With that said I don't have an ohmmeter that sensitive.
Often those old machine run so much more reliable. Cheers to that.
Maybe after some work on the other projects I'll revisit scoping the welding machine and ways this could be achieved. It's not a pressing issue; only a curiosity.

The power supply I ordered from Cyparagon arrived today. Seems to power up and work fine. After I make some leads I should be up and running.

More than likely I will order the rigol ds1054Z or the E model.
Benm nailed it on the head when he said it earlier interface of the rigol is very "noisy" to look at, this is slightly annoying. I really will probably only need two channels. Hmm do you think the features on the 1054Z are worth the extra $50? ARG stated a firmware update & The larger screen would be nice
The DS 1054E is 2 channels 50MHz for $345 (+50 for 100MHz)

 

[Sigurthr]

Re: Suggestions on variable DC power supplies.

If it were my money I'd spend the $50 on the newer model. Btw, you never think you need 4 channels until you do. Haha. Oh and also you can scope live connections by doing the "poor man's differential probe" trick, which ties up two channels PER POINT you want to scope. So, say you wanted to watch the output of a Bridge (inverter) AND the gate drive going to the bridge; that takes 4 channels (or 2 channels + an expensive differential probe).

 

[Sigurthr]

Re: Suggestions on variable DC power supplies.

Sorry for double post, it's not letting me edit.

"Annoying visual noise" was brought up regarding the entry level DSOs, I'd like to show what the CRO version is;

I was testing a possible SRVTTC hybrid design and this is what I had to contend with:

Clean trace is Grid drive signal, "shaded" trace is Primary/Anode current. It's not supposed to be shaded, but the triggering circuit couldn't get a good lock on it except right at the edge of the trigger range, where the traces are too faint to really see even with intensity jacked up. Phase was reversed because I just phased the isolation transformer backwards, but I wasn't looking at phase, just amplitude and cleanliness.

Also note this is a 1MHz signal displayed on a 40MHz scope set to it's fastest timebase. You don't get any where near the listed bandwidth as far as useable range, there's often a 10:1 downscaling factor at best.

 

[Benm]

Re: Suggestions on variable DC power supplies.

That actually varies. Back in the analog day, the bandwidth of a scope was more or less defined to how accurate it displayed voltages at a given frequency. The cut-off frequency was often the point where it went all the way down to half the proper amplitude.

This says very little on how fast the timebase can be though, it depends a bit on the make and model. Many also have a feature where you pull out the timebase potentionmeter to magnify the horizonal axis 10 times.

Practially this usually means that you can see a 10 MHz signal on a 10 MHz scope such that the full sine wave spans 2 divisions or so when pulling out magnification. I've had a 20 MHz one trigger on and display a 100 MHz signal though - with a full sine wave spanning about a division with magnification pulled active.

One thing to really consider in digital scopes is that they have a maximum sample rate, which is divided over the channels. So a 1000 MS/s scope can be usable up to a theoretical maximum frequency of 500 MHz using one channel. Using 2 that drops to 250 MHz and using 4 it drops to 125 MHz. Those frequencies would be right on the nyquist rate, so expect lower (about half) values in practice. This makes it perfectly possible for your 1000 MS/s scope being unable to capture a 100 MHz signal when running 4 channels - switching to single channel operation would however solve that.