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A bit more information on this power supply, its design and the how and why, of agilent this was designed when they were still hp doing the things they did in this design. First off : those caps do not have their vents blocked.
It is silicone isolation to prevent possible shorting of the bare top metal of these caps to the heatsink of the 6 volts section underneath when the lid is closed. Not all supplies have this.
Originally there were capacitors that were shorter, only later models use longer caps so they solved it that way. Nevermind , was thinking of another supply in the same series. Here it is purely mechanical stability. They use a conversion technique called multislope III which is a charge balancing convertor. It is similar to a dual slope convertor. They charge a capacitor in an integrator circuit during a fixed amount of time from a reference voltage.
They then use the, to be measured voltage, to discharge back to zero and measure time again. The ratio gives you the weighted ratio of Vin versus Vref. The problem with this is that conversion time changes with input voltage. So, what does multislope do?
So they can speed up the discharge of the cap. They use a simple to do the switching. A high precision opamp for the integrator and a really good comparator for zero detection. The asic controls the current switches and holds the counters to measure the times. Time that is derived from a charge current derived from a voltage reference. The beauty of this converter is that all errors are self canceling. Only the precision of the reference is important. If you want more digits out of the convertor..
This done by increasing clockspeed and having counters with more bits. The absolute clockspeed doesnt even matter as it is a ratio between two numbers. If you get more leading numbers before division you get more effective numers post division. Now, the machines were designed at the same time als the multimeter and switch So for them , this was a freebie And if that display tells you 5. So poking around with a scope probe or voltmeter tells you nothing.
The multislope system mechanism keeps all voltage in the convertor at 0 by injecting or extracting currents. It only measures the channel that is beeing displayed on the screen, or temporarily switches over if a request is made through gpib.
The power supply control loops are fully analog. Core processor is an with external memory and sram. Proven design , and for them , very short design cycle as it is paste and repeat With some software changes. Now, the dac is another interesting beastie They use a 16 bit dac to do the setting of all voltages and currents. The dac has a multiplexer on the output followed by analog memories built around tl and capacitors. They continously replenisch the caps by scanning and reprogramming the dac.
Because when these supplies were designed one dac costed more than making 6 analog memories with caps and opamps as unity gain followers More interesting tidbits. The 6 volt ground is fully isolated from chassis ground, as is the 20 volt section. Only the gpib and serial port sits at chassis ground. The supplies float in respect to chassis ground, and each other. So , how do we get the info in and out? We are not going to replicate that and send it across a digital optical link That would be too expensive But wait!
We are HP! And we make optocouplers Even analog ones! Lets do that! The analog coupler uses an led and two matched photodiodes. Two simple opamps make a system that drives the led , measures emission using one of the photodiodes and makes a servo system.
The other photodiode is a mirror Simply amplify it and done! Cheap, elegant and very very clever! Now ,the communication bit.. We find the classical TMS ieee controller driven by an as well as standard rs port.
These talk through regular digital optocouplers with the The front panel has its own as well. That thing scans keyboard, rotary encoder and the VFD through high voltage buffers and talks serially with the via the asic that ontains a second uart This may all seem like overkill but it isnt There is again something clever going on.
The VFD requires an ac voltage to drive the filament and a relatively high dc voltage to drive the segments. Problem is that this requires some voltage shifting. The scanning, and high voltage switching, also creates electrical noise. If you blink a digits intensity you see the noise spectrum change. The display is scanned in a matrix.. We are HP.. Wait for it We can do some trickery with the power supplies for the VFD!
The whole frontpanel runs at volts in respect to the ground of the processor. We simply use 4 opamps to level shift the control signals between domains. This negative voltage allows us trickery with the display filament so we only need one additional winding on the mains transformer, which alread has a lot of windings..
For them it was very cheap and they actually cut costs. The asic they had, the display system too, the analog optos too. They worked really hard to make this as cost efficient as possible. Now the supplies themselves. These are fully analog control loops , because even HP would not dare attempt a digital control loop as they know that is always too slow. The control is very simple. Two opamps do the work. One works as overcurrent detector and can pull the output of the voltage servo downwards.
The make a constant current source with an lm and inject this in the base of the power transistor. All the regulation opamps do is deviate this current. And you do NOT want an additional gainstage behind the opamp as this makes the loop unstable. There is a couple of other tricks in that system. I know they had a software bug in firmare 1.
These things have been in production so long that at one point the high voltage display controller went obsolete. They did a revision but the scanning is slightly different and the needs to know that.
Agilent publishes repair notes that you can download for free with all this stuff in it. They also publisch service manuals with a full description and explanation of all the techniques used. Wanna really learn electronics? You will be amazed at what kind of a goldmine you are sitting on!
Any comments, or points of view expressed, are my own and not endorsed , induced or compensated by my employer s.
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