Did my subject line catch your attention? Yes, that's right -- if you source current from Ch3+ and sink it into Ch2-, the current flows through the voltage sense wires and induces a drop (and therefore error in voltage readout/regulation) of up to 57 mV, which is well outside spec (0.05% + 10mV = 10.5mV). Sense wires appear to have a resistance of 19 milliohm. Here's a specific example:
Configure both outputs to 1V, 3A limits. Attach voltmeter to Ch2.
A. Measure voltage. You get 1.000V. Win!
B. Short out Ch3+ to Ch3-. No worries, you still get 1.000V. Win!
C. Now, instead, short out Ch3+ to Ch2-. Now you get 0.943V, even though the display on the power supply still reports 1.00V. Boo!
Now you might be saying I'm being a bit silly, returning the current to the "wrong common terminal". Now I'm of the opinion that common terminals should be just that, common, completely identical in every way. But more to the point, if you configure a system with +/-12V rails for your power audio output (for example), and +5V for some other logic, then you've got a common ground all around and you should be able to just have one ground wire running from power supply to circuit-under-test. Having to carefully account for and make sure that every electron has an easy path back to its "home" negative terminal seems a bit nuts.
I mean, the whole point of sense wires is that they're supposed to have (practically) zero current, and therefore zero voltage drop, right?
What do you guys think? Am I being pedantic and making a fuss, is this standard practice for non-isolated channels on power supplies? Is it generally known in the profession to be bad practice to rely on common terminals being, well, common? Or is this a real problem particular to this power supply?
Update: Threw together a video to demonstrate this:
What do you guys think? Am I being pedantic
na, i dont think your being pedantic.
They should never have common'ed ch2 and ch3.
3 Isolated outputs is more useful and eliminates these sort of unexpected issues.
The longer this goes on, the more and more comfortable I become with my clunky, ancient, non-programmable, all-analog power supplies.*
*Before the torches and pitchforks come out.... yes, I know whether it's analog or digital has nothing to do with how buggy it is. I think it's more about time-tested designs, really - the crap doesn't last, so when you buy old stuff you know it's the one that was good enough to survive. I have yet to see an old analog power supply with control loop bugs like this.
The longer this goes on, the more and more comfortable I become with my clunky, ancient, non-programmable, all-analog power supplies.
+1
About the non-programmable part, if the ps has remote terminals for external adjustment for the voltage/curent loop, coupled with a digital controlled voltage reference, I wouldn't call it clunky & ancient anymore.
What do you guys think? Am I being pedantic
na, i dont think your being pedantic.
They should never have common'ed ch2 and ch3.
3 Isolated outputs is more useful and eliminates these sort of unexpected issues.
True. The trouble is, though, that to do that you need board space for isolation and well-separated heatsinks and so on, there's always going to be a considerable compromise in power/weight/size/cost when making truly independent supplies. What I don't get, though, is why they didn't use proper, thick (less than 38 milliohm!) wire to common the terminals directly, rather than via two sense wires and a crimp. Even 24AWG wire, at a length of 5cm, is 1.3 milliohms or so, low enough to make this problem reduce to being within spec. Incidentally, for those that don't mind voiding your warranty, that's exactly how you can fix this problem on your own DP832.
Yeah, i agree. cost savings will be the reason.
But a power supply is so critical for electronic design that it's better to have less features that are all done properly than more features where some have weird issues that might catch you out.
It's not like an advanced piece of test equipment where you might tolerate some differences in operation/usage between brands.
Having to carefully account for and make sure that every electron has an easy path back to its "home" negative terminal seems a bit nuts.
Strongly disagree.
Considering the return path of a current is part of good design. And it's even more important at RF. And these days most every circuit is an RF circuit. (If you think not, see if it works with a GSM cellphone next to it. Fluke 87 V anyone?)
Even at DC, you have to consider the I*R voltages in your "ground" connections and
decide if you can ignore them.
Orin.
If you built the circuit, you presumably know if you can ignore them. Basic testing of some random audio circuit (like the given example) isn't going to care about grounding. (Sure, it might need more attention to return paths to make it work well, but 99% of the time on the bench you just want to see that it works.) And I can make the decision to ignore that much faster than I can figure out exactly how I should caress the power supply to convince it to give me what I want.
Lets face it, RIGOL stuffed up in a big way with the LM317 . The spot light is now on and everyone is going microanalyse this bit of gear.
This is the price of not getting it right first time.
Frankly as the owner of one this is good, so before it gets sent back for whatever rework it is going to have I can decide if I just want to cut my losses and have a refund.
And IT WILL get sent back our consumer laws here in OZ put the responsibility on the seller/distributor so there is no hiding. I'm sure the US has even better "lemon" laws is some states.
http://www.accc.gov.au/consumers/consumer-rights-guarantees/repair-replace-refund
If you built the circuit, you presumably know if you can ignore them. Basic testing of some random audio circuit (like the given example) isn't going to care about grounding. (Sure, it might need more attention to return paths to make it work well, but 99% of the time on the bench you just want to see that it works.) And I can make the decision to ignore that much faster than I can figure out exactly how I should caress the power supply to convince it to give me what I want.
"If you built the circuit..."? Seriously, given the crap that is shipped, building a circuit means nothing. Remember, we are preaching to those that might not be circuit design gods.
Then you can't just design a circuit then slap quality (the last 1%) on as an afterthought. You have to design it in. And that includes how you draw power from your power supply.
Perhaps you have the intuition to get it right. I don't. I guarantee that any reasonably well designed circuit* will fail if I build it**. And the reason it fails is always one of these kind of gotchas. Call me on it... please... I'd like my projects to actually work right for a change just to prove me wrong. I have a stack of expensive test equipment just to work out why my projects don't work.
Orin Murphy.
*analog. The digital ones usually work.
**in some cases, they don't work right for anyone else, but they don't have the test equipment to realize it.
Sometimes I just want to see if it'll release the magic smoke...
I have a stack of expensive test equipment just to work out why my projects don't work.
Dangerous. You have a "stack of expensive test equipment" to make sure you'll never be able to use an "assortment of affordable equipment".
What is the isolation supposed to be between the 3 channels?
Having to carefully account for and make sure that every electron has an easy path back to its "home" negative terminal seems a bit nuts.
Strongly disagree.
Considering the return path of a current is part of good design. And it's even more important at RF. And these days most every circuit is an RF circuit. (If you think not, see if it works with a GSM cellphone next to it. Fluke 87 V anyone?)
Even at DC, you have to consider the I*R voltages in your "ground" connections and decide if you can ignore them.
Orin.
So of course, when you're designing a PCB, thinking about the ground return paths within the PCB is super-important. And when you're dealing with RF, the supply-decoupling cap-ground loop is super important, yes yes yes. But we're talking about an external power supply, if your circuit is subjecting your power supply to RF-specific conditions, then you've done your decoupling on your PCB wrong.
And at DC, I can calculate my I*R drop in supply cables, and I *should* be able to subtract those drops from the figures displayed on the DP832 readout. Like Dave said in the video, the great thing about these precision supplies is that they essentially have a multimeter built in -- but this one specific condition breaks this multimeter. And all of this is completely avoidable, if only they had properly shorted out the negative terminals with a strong, thick copper wire, this problem would just vanish. Even just 24AWG would massively help.
So yes, the way I worded that sentence is misleading, I'm not trying to claim that assuming ground is ground is the same voltage everywhere should be allowed, that's obviously a terrible idea in the context of designing and laying out PCBs -- but having two terminals marked as "common" which are so ridiculously far (38 milliohms) from being common, and so unnecessarily, is crazy in my books.
What is the isolation supposed to be between the 3 channels?
All channels are isolated from mains earth. CH1 is isolated from CH2 & and CH3, but CH2 and CH3 are *not* isolated from each other -- they have a common negative terminal. For completeness, all terminals are marked with "max +/- 240V to earth".
Lets face it, RIGOL stuffed up in a big way with the LM317 . The spot light is now on and everyone is going microanalyse this bit of gear.
This is the price of not getting it right first time.
The funny thing is, the reason I found this out is because of the first, non-critical teardown eevblog episode. I saw that the the current sense resistors were on the negative rails, and was wondering how that could possibly work. Turns out it's totally legit, except I noticed this unrelated problem!
if this is true ( current flowing through the sense wires) then this is a catstrophic design failure !
the normal sense configuration in a system is like this:
output
(+S)--o-----[50k]--------------------------
| |
[5k] |
| _________ ____|___
(+)---o--------------| | / |
|driver |_____/sense |
| | \control |
(-)---o--------------|_________| \________|
| |
[5k] |
| |
(-S)--o-----[50k]--------------------------
if you leave the sense wires floating the supply sense through the 5k and 50k resistors ( the values i give are average). most supplies i know have something in the range of 1k to 10k between sense and power terminals. from the sense lines towards to sense inputs themselves this is more in the order of 20k to 1meg and beyond ( you are looking ino an opamp so essentially it should be almost infinite impedance)
if you connect sense wires you create a short across the 5k resistor. the cable resistance is so low compared to the parallel sense resistance it has no noticable impact.
so, if you can indeed pull current from the sense wires ... please doublecheck it ... then this is a catastrophical failure.
now, it is possible there is a shunt diode !
output
(+S)--o----o---o------[50k]-----------------
| _|_ | |
[5k] /_\ _V_ |
| | | _________ ____|___
(+)---o----o---o-----| | / |
|driver |_____/sense |
| | \control |
(-)---o----o---o-----|_________| \________|
| | _|_ |
[5k] _V_ /_\ |
| | | |
(-S)--o----o---0-----[50k]------------------
in which case this behavior could be normal. there should be no more than 0.5 volts drop between sense and power terminals. the diodes will clamp that.
in that case you can pull current from the sense wires but there should be hal f avolt drop (0.3 if they used shottkys)
this is done to speed recovery of the sense amplifier and protect the control oop. ideally thse diodes should connect after the 50k resistor but i have seen them before at the sense pin itself ( mainly when they were inserted as an afterthought)
so, doublechekc that stuff before drawing conclusions. the diode may be there.
if it is really possible to suck the full current withouth the diodes ... baaaaad design !
Sometimes I just want to see if it'll release the magic smoke...
I have a stack of expensive test equipment just to work out why my projects don't work.
Dangerous. You have a "stack of expensive test equipment" to make sure you'll never be able to use an "assortment of affordable equipment".
OK, I'm guilty of seeing if a project lets out the magic smoke too
I should have left out the adjective 'expensive'. In reality, almost all* of the stack of test gear is used and I fix it myself. It would be expensive if I bought it new. It is in fact affordable or I wouldn't have it. I have no objection to affordable equipment if it works.
Orin.
*so I got an Agilent 34461A. First new equipment in about 15 years. Dave's review had no influence on that. No, none at all.
And all of this is completely avoidable, if only they had properly shorted out the negative terminals with a strong, thick copper wire, this problem would just vanish. Even just 24AWG would massively help.
But wouldn't that invalidate the readings from the low side (as you note in a later post) current shunts? They'd have to use a high side sense in that case with all the problems that entails.
Orin.
And all of this is completely avoidable, if only they had properly shorted out the negative terminals with a strong, thick copper wire, this problem would just vanish. Even just 24AWG would massively help.
But wouldn't that invalidate the readings from the low side (as you note in a later post) current shunts? They'd have to use a high side sense in that case with all the problems that entails.
Orin.
ehhh.. sensing is supposed to happen on BOTH sides ! or it's not sensing at all !
if this is true ( current flowing through the sense wires) then this is a catstrophic design failure !
So, to clarify, this supply does not have externally accessible voltage sense terminals, so the circuitry you're suggesting doesn't apply. What's happening with the DP832 is that the internal voltage sense wires, which are soldered directly to the output terminals (thereby measure voltage at the terminal rather than at the regulation PCB), are actually carrying 3A if you connect CH3+ to CH2-. Specifically, the CH2- and CH3- sense wires are tied together at the PCB (this is "fine" because the - terminals are common), but the point is that one connection point is the
only connection between CH2- and CH3-, hence it has to carry any current returning on the "wrong" terminal -- not that I think there should be any such thing as the "wrong" terminal. I'll upload a video soon that might give a touch more context.
But wouldn't that invalidate the readings from the low side (as you note in a later post) current shunts? They'd have to use a high side sense in that case with all the problems that entails.
Orin.
Forgive me, I'm not sure which later post you're referring to. So to answer this question;
a) The negative terminals are already shorted together by the sense wires, it's just a very high-resistance short (at 38 millohm).
b) In the instances where the two channels are used independently to power independent circuits, the voltage-sense wires and my proposed thick wire do/would both carry zero current, so there'd be no change; this scenario would continue to behave perfectly.
c) In the instance where the current is returned to the "wrong" terminal, the voltage drop between terminals is due to the high resistance of the sense wire. Reduce this resistance, and the problem reduces. There's no point in forcing the current to travel all the way to the PCB, to the point where the voltage sense wires are crimped together, and then all the way back again.
I'll post a reverse-engineered schematic in a sec.
Just to clarify things, I did some other testing and threw together and attached a rough reverse-engineered schematic.
Also, here's a video demonstrating the issue:
Shouldn't that be an obvious error for the designers ?
It seems like they didn't even bother to test the device before full on production...
Just to clarify things, I did some other testing and threw together and attached a rough reverse-engineered schematic.
I think i wouldn't make such a mistake. And i'm just a hobbyist. If i would do then i surely found it while testing.
PS: The boiling water demonstration is great!