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Old 12-10-2015, 02:58 AM #49
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Default Re: Suggestions on variable DC power supplies.

Yeah, you're right about bandwidth mostly being about the 3db signal amplitude point, but it ties in with timebase as well. I hadn't heard of or seen a scope that can trigger on a significantly higher-than-bandwidth signal, but none of the CROs I've used have had a horizontal magnify feature either. At 2div horiz (0.2uS) the signal would be 2.5MHz, which is right around half of what I consider the maximum useable limit for this scope, which coincides well with f/10 rule of thumb. Definitely spot on about digital scopes samples rates though. I have heard some mega expensive ones have parallel sampling capabilities though so some/all channels aren't shared bandwidth.

Anyway, my picture was more aimed at old school triggering capability vs modern digital trigger, but I didn't explicitly say it before.


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Old 12-12-2015, 01:38 PM #50
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Default Re: Suggestions on variable DC power supplies.

How would you guys feel about me editing the title to include oscilloscopes or start a new thread on the topic?
I would like to discuss some questions and there Is excellent info in this thread both about power supplies and o-scopes.

Question- when scoping a driver say the SXB or similar drivers where do you put the probe? Across the LD- & LD+ of the driver or in series with a test load? Any parameters on the rigol it should be set to? Hopefully "auto" and DC coupling I'm sure.

Last edited by Down with Umbrella; 12-12-2015 at 01:42 PM.
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Old 12-12-2015, 03:29 PM #51
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Default Re: Suggestions on variable DC power supplies.

Don't use the AUTO feature. It's a lazy thing to do, and it will often scale improperly, especially on DC signals. There are plenty of tutorials out there on how to use scopes. It's not too hard once you realize horizontal and vertical settings are basically just "zoom" levels. A scope is essentially an obscenely fast volt-meter that displays the voltage in a graph as a function of time.

Where you put your probe depends on what you want to scope. Across the laser diode will measure the diode's voltage. That's not particularly useful since this value won't really change. For visualizing the current, you need the probe across a shunt (current sensing resistor)... either the driver's internal shunt or an external shunt you wire into the circuit. For example if you've still got that video I sent you on file, you'll see I had the scope across the 1-ohm shunt of the test load.
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Old 12-13-2015, 12:25 AM #52
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Default Re: Suggestions on variable DC power supplies.

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Originally Posted by Sigurthr View Post
Yeah, you're right about bandwidth mostly being about the 3db signal amplitude point, but it ties in with timebase as well. I hadn't heard of or seen a scope that can trigger on a significantly higher-than-bandwidth signal, but none of the CROs I've used have had a horizontal magnify feature either.
There might be a link between the features. It basically allows you to look at signals faster than rated for the scope, albeit with inaccurate amplitude.

Triggering such signals can be finicky - on automatic mode it will problably not lock on, but selecting the correct channel and manually adjusting it until it 'just catches' works.

Downside is that you will see little information: if you feed a 100 MHz square wave into a 20 MHz scope it will display something closer to a sine pattern since the input amps just can't handle the harmonics. It's mostly useful to determine if a signal 'is there' or not - which can be very handy if you don't really need to know its waveform or amplitude.


As far as sample rate per channel on digital scopes goes: More expensive units will have dedicated sampling per channel, but on sub-$1000 models you can expect the sample rate to be shared between channels.

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Old 12-13-2015, 01:35 AM #53
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Default Re: Suggestions on variable DC power supplies.

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Originally Posted by Cyparagon View Post

Where you put your probe depends on what you want to scope. Across the laser diode will measure the diode's voltage. That's not particularly useful since this value won't really change. For visualizing the current, you need the probe across a shunt (current sensing resistor)... either the driver's internal shunt or an external shunt you wire into the circuit.

Probably didn't mean to say shunt resistor as this is a parallel resistor and won't give you a picture of the current. I think you meant to say a series resistor. I unfortunately have mistakenly said one thing when I meant another. Just wanted to clarify this.
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Old 12-13-2015, 03:42 PM #54
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Default Re: Suggestions on variable DC power supplies.

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Originally Posted by paul1598419 View Post
Probably didn't mean to say shunt resistor...
I did mean to say shunt. General use of the word "shunt" is to bypass, but in electronics it means something else.

Current Shunt Resistors
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Old 12-13-2015, 05:40 PM #55
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Default Re: Suggestions on variable DC power supplies/oscilloscope discussion.

I read your link and it refers to the use of a resistor in an ammeter for measuring a part of current that would normally go through the entire meter. In other words, the resistor is shunting most of the current away from the meter coil so the coil doesn't carry the entire current through it. This is used to only allow a part of the current to be measured by the ammeter, thus limiting the current that passes through the meter. The current isn't measured by the resistor, but the ammeter coil. I don't see how this could be applied to a laser diode, however. If you shunt a resistor across anything in series with the diode, or across the diode itself, you can't get a picture of the current in the diode. If it is across another resistor in series with the diode you have changed the effective resistance of the resistor and are still measuring the current through the new effective series resistance. I checked your claim that shunt in electronics means something different than it has in the past, but couldn't find any authority to substantiate this claim. Could you explain to me how you would use a "current shunt resistor" in trying to measure the current in an LD? Outside of this use inside this company's ammeter, I don't see how this could work. Every authority I could find states that a shunt resistor is a parallel resistor, and by definition, cannot be used to measure current changing as a function of time. I'm not saying you are wrong. Just that I don't understand what you are doing.
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Old 12-13-2015, 05:58 PM #56
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Default Re: Suggestions on variable DC power supplies/oscilloscope discussion.

You place a resistor with a low value in series with the laser diode , measure the voltage across the resistor and with ohms law work out the current ,

if using a Scope you can see what the current is doing on start-up ect by watching the voltage across the resistor

some Laser diode drivers and LED drivers use a series shunt resistor to generate a voltage feedback for current regulation




the shunt resistor is in series with the current being measured and the meter is in parallel with the shunt resistor .
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Old 12-13-2015, 06:08 PM #57
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Default Re: Suggestions on variable DC power supplies/oscilloscope discussion.

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Originally Posted by paul1598419 View Post
I read your link and it refers to the use of a resistor in an ammeter for measuring a part of current that would normally go through the entire meter.
No, not normally. That's not how ammeters work.

If you want to nitpick about the wording, it's called a shunt because it shunts out the meter that reads it, sure. In the case I mentioned above, the scope would be shunted. Maybe if I draw it out...



Does that help? It's basic terminology. I shouldn't have to argue for basic terminology. In electronics, when we say "shunt", it refers to a current sense resistor, because that's how it is used. Search for shunt on ebay/digikey/mouser, and that's what you get.
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Suggestions on variable DC power supplies/oscilloscope discussion.-shunt.png  
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Old 12-13-2015, 06:50 PM #58
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Default Re: Suggestions on variable DC power supplies/oscilloscope discussion.

I wasn't arguing the existence of a shunt resistor, as everyone knows that you can shunt, or parallel current paths to have different current running through each path. The voltage drop across the entire shunt is always the same across each element. The resistor is in series with the load and therefore has the same current as the load. It is the measuring device that you are calling the shunt. It's semantics. I was confused by the term "measuring current across a shunt resistor". I understand now what you are trying to say, but it seems to me to be imprecise.
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Old 12-13-2015, 08:11 PM #59
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Default Re: Suggestions on variable DC power supplies/oscilloscope discussion.

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I was confused by the term "measuring current across a shunt resistor"
I didn't use those words. Or if I did, I don't see where. You'd be measuring the voltage across the shunt which is translatable to the current through the shunt... Just as long as we're on the same page now
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Old 12-13-2015, 08:35 PM #60
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Default Re: Suggestions on variable DC power supplies/oscilloscope discussion.

My mistake. I did mean to say measure the voltage across the "shunt" resistor. I guess we are on the same page, in as much as I understand what you are trying to say. The imprecision I was talking about is calling a resistor introduced into a circuit which is in series with the load a shunt. Once you have done that, the resistor, no matter how low the resistance is, becomes a current limiting resistor. Now, if you parallel a voltage measuring device with a larger input impedance such that R<<< input impedance of meter, scope, what have you done to the circuit? Effectively nothing. It still seems to me that the resistor is not a shunt resistor in as much as it is a dynamic part of the circuit and shunts nothing ( of consequence ).
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Old 12-13-2015, 08:43 PM #61
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Default Re: Suggestions on variable DC power supplies/oscilloscope discussion.

A picture can be worth a thousand words.

I find myself reluctant to give advise without being very specific, because if someone breaks an expensive toy I don't want it to be because I expected they would know what I meant without over explaining it.

The pics/schematics are a good idea I think.
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Old 12-14-2015, 12:36 AM #62
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Default Re: Suggestions on variable DC power supplies/oscilloscope discussion.

Oh well, semantics. Everyone in electronics knows excactly what a shunt is and how it is used. Surely you normally measure the voltage across one to determine the current through one.

The only important thing to notice when using a scope to measure both current and voltage is that they usually have a common ground connection between all channels, and the shunt needs to be on the 'low side' to measure things.

When used in drivers i prefer to call them 'sense resistors'. They act like shunts, but if someone were to connect an external shunt in series with the diode at the output of the completed driver this may lead to some confusion.

Also, on a typical transistor-output low-side driver the current through the sense resistor is the output current PLUS de base drive current. With a HFE of 100 or so this is only off by 1 percent, but it could matter to some. Same obviously applies to the sink current on a high-side driver if you built one.
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Old 12-14-2015, 04:20 AM #63
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Default Re: Suggestions on variable DC power supplies/oscilloscope discussion.

Benm, I can't disagree with anything you've said here. Thank you for this clarification.
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Old 12-15-2015, 09:06 PM #64
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Default Re: Suggestions on variable DC power supplies.

Older makers of scopes use a 1.1 dB down point for their ratings.

The Chinese use a 1.3 dB down point for their ratings. So a 200 Mhz Tek is easily usable near 300 Mhz. Yet both of my Rigols, fall off at right where they are rated + 10 Mhz.

I have DS1102 at work and love it. Yes, I work in the land of 11,000$ oscilloscopes at the university. My day to day cheapie for the new lab is an 1102, and I'd buy it again. For what it is, it has maximal bang for the buck if you don't care about perfect impulse response on fast pulses.

The 1054Z has a much more cluttered display, and quite honestly, dispite having the four channels, I wish I'd bought a 1102 for home. I have nice 300 Mhz analog and digital scopes at home, but I liked the idea of a cheap digital scope for road work. Part of the reason for buying a cheapie is to maximize the lifetime of my CRT scopes.

Gripes on the 1054Z:

Cluttered display menus that do not go away.
Extra resolution is there on the screen compared to 1102, but they load it up with often useless readouts.
Your not really getting a better view of the waveform.

After the bandwidth mod/hack , FFT resolution goes down, and quite honestly, I don't consider the bandwidth hack as such a great deal. It really eats memory depth.

Four channels is awesome if you program microcontrollers, but the 1102 is really a better deal in the long run, if general use, short pulses or laser diagnostic work is a concern.

DS1054's interface is a PITA for small signal work. I've been using scopes for 25 years now, and I'm going to the paper copy of the manual to figure out how to fix issues on the 1054. The control layout is spread across a lot of function switches, and not all functions are neatly grouped. I'm a veteran user, and I find myself hitting "AUTO SETUP" too much.. I use the AS button as a system reset. Some controls are deeply hidden or anything but intuitive.

When hacked, it is really a 70-80 Mhz oscilloscope, its not truely a solid 100 Mhz..

Whereas the 1102 is a Breeze to use.

Keep in mind, when using all four channels, the 1054Z is about a 23 Mhz oscilloscope. Plenty of bandwidth for beginners, but the display is really cluttered when using more then one channel or timebase.

On the low and medium end Chinese scopes, the advertised bandwidth is the single channel bandwidth. Every time you turn on another channel, your actual bandwidth is a bit less then BW over N, where N is the active number of channels. So a 100 Mhz four channel with all four channels active is a considerable bit less then 25 Mhz per channel.


You can find plenty of scope reviews on EEVBlog forums.. In fact it will overload a beginner to go there.

1102 for a beginner, yes! 1054Z, only if you REALLY need four channels or intend to hook it to a laptop for delayed readout (Slow!)

If buying a cheap scope, get the most memory depth (amount of fast RAM) you can afford.

Steve
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