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Berni:

--- Quote from: exe on March 05, 2021, 10:05:03 am ---
--- Quote from: bd139 on March 05, 2021, 07:55:32 am ---There are two cases where the crowbar comes in handy as well which are almost never mentioned as well. Firstly, and this does happen with RF stuff, when there is RF noise on the sense lines to the supply. Secondly, when the load is too inductive and the supply starts oscillating. Even the best of power supplies can be coaxed into oscillating (try running something like an SMPS on the end of a few metres of untwisted wire.

--- End quote ---
In both example, when crowbar fires the output drops to zero, is that correct? I'm not sure how useful is that, the load remains with no power. Probably, very special cases. If PSU is oscillating, the only way to "fix" it is to switch-off and do something about it. Like, adding more output capacitance, or reduce loop bandwidth, or use shorter wires, add filtering on sense wires, or something.

As of oscillating, I was thinking if power supply can detect that. I considered using window comparator, or ideal diode bridge and AC-coupling to rectify and sense peak-to-peak voltage, but it was too much added complexity to my liking. Probably, digitally sampling output voltage and processing in software is easier, but might be not as fast as a hardware solution.

I was also considering not a crowbar, but a down-programmer (essentially a load resistor triggered by comparator when output voltage, say 5%, above set voltage). Also quite tricky to make everything nice, fast and stable.

PS I'm currently making a power supply, and I made CV feedback with adjustable bandwidth: fast and slow. Slow is to drive troublesome loads. Also helps with stability at low output currents (below 1mA) when output stage has too much gain. I think a few commercial power supplies too offer switchable speed.

--- End quote ---

Yep this is why some of the HP supplies i mentioned have current sinking capability. As soon as you try to raise the voltage it starts to sink it to ground, staying in regulation without any jerky step response.

Adjustable bandwidth is a lot more rare but some do have it. For example the HP/Agilent 66332A has a 'Fast' and 'Slow' switch on the back. This is likely because this PSU is specialized for a fast response since it is capable of replaying voltage waveform from a buffer at up to about the rate of 100K samples per second. So it is likely also much more prone to oscillation compared to a regular PSU. To get this speed it  drives the output using a AB class amplifier output stage. So due to the constant biasing the heatsink gets fairly warm even when it is outputting zero amps.

bd139:

--- Quote from: exe on March 05, 2021, 10:05:03 am ---
--- Quote from: bd139 on March 05, 2021, 07:55:32 am ---
Usually, taking radio power supplies as an example, you set the voltage to 13.8V and the crowbar to 15V. The specification for the load sets the voltage tolerance. Typically this is max input voltage of LDOs, capacitors, ICs etc directly connected to the supply considering power dissipation as well. Some things designed for say 9v input will quite happily run at 12v depending on how the internal power is arranged.

--- End quote ---

In you example with blown zener and output voltage skyrocketing. Do you propose to have a second reference that would only be used for crowbar?


--- Quote from: bd139 on March 05, 2021, 07:55:32 am ---There are two cases where the crowbar comes in handy as well which are almost never mentioned as well. Firstly, and this does happen with RF stuff, when there is RF noise on the sense lines to the supply. Secondly, when the load is too inductive and the supply starts oscillating. Even the best of power supplies can be coaxed into oscillating (try running something like an SMPS on the end of a few metres of untwisted wire.

--- End quote ---

In both example, when crowbar fires the output drops to zero, is that correct? I'm not sure how useful is that, the load remains with no power. Probably, very special cases. If PSU is oscillating, the only way to "fix" it is to switch-off and do something about it. Like, adding more output capacitance, or reduce loop bandwidth, or use shorter wires, add filtering on sense wires, or something.

As of oscillating, I was thinking if power supply can detect that. I considered using window comparator, or ideal diode bridge and AC-coupling to rectify and sense peak-to-peak voltage, but it was too much added complexity to my liking. Probably, digitally sampling output voltage and processing in software is easier, but might be not as fast as a hardware solution.

I was also considering not a crowbar, but a down-programmer (essentially a load resistor triggered by comparator when output voltage, say 5%, above set voltage). Also quite tricky to make everything nice, fast and stable.

PS I'm currently making a power supply, and I made CV feedback with adjustable bandwidth: fast and slow. Slow is to drive troublesome loads. Also helps with stability at low output currents (below 1mA) when output stage has too much gain. I think a few commercial power supplies too offer switchable speed.

--- End quote ---

The point is to protect the load so return it to the safe condition of "unpowered". The point of the crowbar is to blow something upstream that is more permanent i.e. a fuse.

Regarding the conditions mentioned, usually you twist the power leads together as that gives some common mode isolation or move the regulation closer to the load.

balage:
Here is something about the original thread.
Do you see the screen of the instruments? At the bottom. Is that a windows bar? What da...
But it is interesting the picture is smooth everywhere except the screen that is sharp. They must have photoshopped the Windows bar down there.  :-//
They are freakin' playing with us.
The pic is from a facebook post.

Edit: The bottom right is the scope with so an unlikely screen. Also the Run/Stop button is not active with the Single button in the same time.

Someone:
Nice catch, probably screenshots from a UI simulator running on a PC. Then its down to the graphic designer/artist who doesn't fully understand the products and whoever approved the image.

Keysight DanielBogdanoff:

--- Quote from: balage on March 08, 2021, 09:29:49 pm ---Here is something about the original thread.
Do you see the screen of the instruments? At the bottom. Is that a windows bar? What da...
But it is interesting the picture is smooth everywhere except the screen that is sharp. They must have photoshopped the Windows bar down there.  :-//
They are freakin' playing with us.
The pic is from a facebook post.

Edit: The bottom right is the scope with so an unlikely screen. Also the Run/Stop button is not active with the Single button in the same time.

--- End quote ---

 :palm: You found it! I was wondering if that would get mentioned. Each of these will have a dedicated Windows app/.exe for remote control / logging etc. I was involved in this shoot, and we did it all with the screens off/blank (there's nothing connected to any of the inputs). We did have a shot of them booting up, but decided to not try to get anything pretty on screen. That's where my involvement ended. Well, it was decided to add screenshots/screens in post and this is what they were given/used.

For a promo/teaser image I think it's fine, but THE GEAR DOES NOT RUN WINDOWS ON THE BOX, it's all embedded OS stuff. Any of these silhouette images with LEDs/front panel lights one are almost certainly grabbed from the boot LED sequence and not actual run-time modes.

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