The presence of C20 doesn't seem to make a huge difference in CV mode, but it definitely affects CC operation perhaps for the reasons xavier60 mentioned? That being said, I have a few more questions. I know that some output ripple is expected, but I want to be sure I understand things: First off, the 60 Hz sine wave in CV mode at TP 43 was a result of poor probing and close proximity to the input transformer.
Again, I don't have the best probes for high-sensitivity measurements, but the averaging function on my scope got me close enough to where I could trigger on a waveform and be reasonably assured of a signal on those test points. Out of curiosity, I compared the It's definitely noisier and seems to correspond to the output ripple waveform. Even though the supply is working within its specifications now, could I assume that the output ripple is coming from the I would guess that some component of the waveform feeding the regulator TP22 is also proportional to the sawtooth wave coming into the pass transistors, i.
Lastly, I have a question about probing VR4, the 4. When I touched my scope ground lead to either side of VR4, the output of the supply went to zero, probably due to some kind of internal short. This seems strange to me, since I didn't have any other probes connected and the supply was essentially floating. I had to measure the What would cause this? I didn't have any trouble probing directly across the 24 V, 53 V, and 62 V rails from the transformer.
Quote from: iroc86 on June 02, , pm. Quote from: xavier60 on June 03, , am. Iroc, you could cobble up a 1X probe good enough for low frequencies from a coax cable with a BNC connector. Use a 1K resistor on the probe end to help isolate the cable's capacitance from the circuit. Xavier pointed out some limitations in the pass transistors that allow AC ripple to sneak by. A consequence of the Early effect is basically that increasing the collector to emitter voltage will increase collector current.
The feedback circuitry tries to counteract this but it has limited gain and can't eliminate it all. The intrinsic series impedances of the capacitors Xc and ESR and zeners Rz limit how well they can reduce ripple. The electrolytic caps are getting old and could be replaced with new ones with larger values.
I wouldn't think that grounding the other side of the current sense resistor would shut the supply down - but who are you gonna believe, assumptions or your eyes? I wonder if there's a connection between the circuit somewhere and the chassis - maybe the pass transistor heat sink insulators.
I'd measure with an ohmmeter and expect something north of 1 Mohm. Anything less indicates a problem and the possibility of future problems. It might explain why the diode in the CC circuit died - maybe one of the back panel terminals is grazing the chassis.
If you see a 1. This is super informative. I'm learning way more about analog circuit troubleshooting than I ever expected by posting this issue. I really can't thank you guys enough for sharing your knowledge.
I'll try the resistor trick on the probe that you fellows mentioned. Those unobtanium semiconductors have proven to be especially resilient with my amateur probing, but I don't want to push it. I did pull out the pass transistors at one point to check them, and I was careful to put the mica gaskets and pin standoffs back in, but it might be time for replacement insulators.
New caps are next on my list. It'll be interesting to profile the supply before and after to see if there were any changes. I'm also going to pick up a variable power resistor and some current shunts to adjust the supply per the manual. I have two other HP supplies that seem to work okay but could use a thorough evaluation, so it'll be a good learning experience.
Regarding the new capacitors, how critical is the ripple current rating and ESR for these old supplies? Short of buying computer-grade caps, I was going to put a few smaller electrolytics in parallel to get a similar advantage.
Is there anything to watch for in doing this? Unfortunately, I couldn't find any specs on the original caps, so I'm kinda shooting from the hip with respect to ripple current and ESR. I suppose I could calculate the ESR and in-circuit ripple current using an LCR meter and the waveform from my scope, but it may not be accurate if the capacitors have degraded. It's a massive topic, electrolytic capacitors.
For many of the rules there are almost as many exceptions. Low ESR capacitors generally use a water based electrolyte which tends to be less chemically stable than conventional electrolytics. Best not to unnecessarily use low ESR capacitors. I would rather not indiscriminately replace electrolytics, mainly those that show physical signs of problems. For the rest I do in circuit ESR tests. While ESR can be an important parameter in circuits, it also can be an indicator of capacitor health.
Low ESR capacitors can cause regulator instability in some cases. Experience is the best source of perspective regarding this. I tend to discard old capacitors that have ESR much above 1 Ohm. But smaller units often exhibit higher values. Capacitance readings are important. If its value is much above rating I won't use that as criterion for trash; tolerances are often sloppy.
Leakage can be important; if too much it will cause internal heating and even thermal runaway. Especially when an increase in applied voltage causes more than proportional leakage current increase. Old capacitors often recover after a period of use. But they can fool you by suddenly deciding to fail. There is no measurement of which I am aware that can predict this. Certainly replacing all the electrolytic capacitors in a unit is a good idea sometimes but it sets you up for making errors such as bad solder joints, reverse polarity, and miswiring.
Plus it's time consuming and sometimes expensive. The dimensions of modern capacitors prevent an easy replacement due to the mounting differences. The modern cans are much smaller and have different mountings. And I have mroe than once found a brand new capacitor defective. I have one here rated for Volts that had excessive leakage above about 75 Volts.
It was fine up to that stress but any more and it ran away thermally. In summation, there is no simple answer and any course of action has its pitfalls. No product matches found - System Exception. Product Status:. Request Service. Replacement Product. View Product Details. Buy or Rent Options. Sort by : Date. Date Title.
Filter Results. No information found for this product. Tapping around on the board didn't seem to affect anything, either. I made sure to put a high-value resistor across the output terminals to avoid any current surges. I was able to verify operation up to 40 V with no issues. I also tested the current output with a 10 ohm power resistor and was able to output the full rated current! The output ripple was completely normal and did not exhibit any of the 60 Hz modulation as when in CC mode.
So, the next step is to start troubleshooting the Current Input Circuit. I read through the theory section in the manual again. HP describes a transient scenario to explain the way the feedback loop regulates the output. I highlighted the sentence that suggests it's a voltage-operated loop after all.
What do you think? To check this behavior, I set up the following test: CR4 removed per above , 10k resistor across the output terminals, voltage control set to 10 V, and a 10 ohm resistor ready to short the 10k.
I'm externally triggering my scope off the floating 10 ohm resistor lead and measuring the collector of Q1A, which is essentially the CV input to the OR gate.
I'm using a second channel to observe the output voltage of the supply. When I connect the 10 ohm resistor, the scope triggers and captures the regulation happening in the Voltage Input Circuit to maintain 10 V despite the X increase in output current 1 mA to 1 A.
This all happens so fast that I couldn't see it before, and I assumed something was messed up because the OR gate was always at a stable value. As shown in the picture, the output voltage drops slightly due to the instantaneous load change, but Q1 alters its conduction to stabilize the voltage again Iroc, you're right about this being a voltage operated circuit too.
My experience with diode OR gates and transistors in general is that it doesn't take much of a change in voltage to cause a large change in current and I understood from your observations that the voltage variations were quite small.
I would think that in CC mode, there should be almost no variation in any of these TPs. The latest 'scope shot shows an interesting double pulse where the second pulse is of lower amplifude. It'd be interesting to see where it's coming from.
I'd start with TP Also, the Voltage Clamp circuit p 4. Perhaps there's something interesting on TP 42 and I have a Kikusui power supply that probably uses a similar design to hp's. It would be clean at low and no output current but would put out a weird AC ripple at mid to high currents, but not at all voltages.
It turned out the capacitor on the output terminals was bad. Your supply has a uF cap across the output. May I have you try putting to uF across the output, vary the current limit and see what happens?
Quote from: xavier60 on May 26, , am. It'll probably be a few days until I can get on this again, but I wanted to post back with some comments. Maybe we'll learn something if I set up a similar test as with the CV circuit.
Also, regarding that second pulse, it may have just been a loose contact when shorting the output with the 10 ohm resistor leads, almost like switch bounce. I'll run through the triggering a few more times and see if it's repeatable. That's an interesting story about your Kikusui supply. I don't have an LCR meter and can't really test any of these caps, but I do plan on replacing them down the road for good measure.
In so doing, I saw the output ripple double to around 2 mVrms, so I guess the capacitor is still doing something useful. Can you elaborate on how you've come up with the numbers? I'm also not entirely sure how the summing points at A5 and A6 are supposed to work.
This is the Harrison or floating type of design that you might have noticed others also mention. Most linear lab and bench supplies are based on very similar principles. Some voltages given are approximate. The Base of Q2b is tied to 0V so assume that the Emitters are R22 goes to the The voltage on A5 doesn't change much, actually the bit it does change by doesn't matter at all. The voltage across the Current Programming resistor and so its current remains constant.
This current causes a negative voltage on A5 in proportion to the resistance of the Current Control. The voltage drop caused across R24 results in 1. It looks like 1. The increasing voltage drop across the Current Sampling resistor that an increasing load current would cause, makes A5 change in the more positive direction. As the voltage at Q2a's Base approaches 0V, it begins to conduct, leaving less and less current for Q2b causing its Collector voltage to rise, and eventually taking control of the output from the CV loop.
CR5 is a likely fault suspect. Extra: Just occurred to me that Q2a's Base will not quite get to 0. Anyways, a voltage reading of Q2a's Base will be useful. Iroc, I probably can't improve on xavier's description of the Current Input circuit. I'd probably overcomplexify and confusicate it just trying. These days, I seem to confuse desktop stuffing and stovetop publishing all the time. It's important to look at the waveforms on TP14 thru 16 while in CC mode.
When I think about it, any ripple on these supplies is coupled coupled directly into the veto signal and thus affect the output. A quick test is to connect your uF cap across C10 check polarity! Since you're seeing 60 Hz, i'd suspect a diode or connection. I have been ignoring the ripple because I am hoping that it is the result of the power supply operating open loop because of a fault in the Q2 area. The 60HZ component could be due to primary to secondary capacitive coupling although it does seem to be more than would be expected.
Thanks for explaining the circuit theory, guys. I think this makes a little more sense now. How does -S, the output "common," play into this? The operation is different than other control circuits I'm used to, where the reference points are fixed. What's the advantage of a floating design? As for the troubleshooting, we are making some progress!
It turned out that CR5 was indeed faulty shorted as xavier60 suggested. I presume that this short was causing Q2 to conduct constantly? After fixing CR5, the current limit now reaches the full rated amount 1. However, I am still experiencing the weird 60 Hz modulation in CC mode.
I took duak's advice and checked all of the voltage references again. I admit that I don't have the best probes for high sensitivity measurements, but I couldn't really detect any 60 Hz ripple on the rails. The 6. There wasn't any change in the output ripple, although the ripple on the Either way, it's in spec, but at least I know my extra cap isn't toast.
I also put the capacitor across the other filter caps, C12 and C14, and did not see any significant change on the output ripple. So, I don't think the ripple is a filtering issue. So, thus far, I had only been able to observe 60 Hz ripple on the output.
I couldn't find it anywhere else in the circuit.
0コメント