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bd139:

--- Quote from: exe on March 04, 2021, 03:38:19 pm ---
--- Quote from: bd139 on March 03, 2021, 07:20:26 pm ---Disagree entirely. Crowbar is important when the following equation is true:

cost_of_load > cost_of_power_supply

A year or so ago I had what i consider to be a fairly decent quality supply (HP) blow the reference zener. This caused the output to bang itself at the collector supply voltage. You should plan for such failures.

--- End quote ---

How does a crowbar work? I assume the expensive load is also sensitive to overvoltages. In this case the crowbar should engage very fast and at voltages not so much above the set voltage. If reference circuit doesn't work,  how does the crowbar know when to trip?

If crowbar set to trip at, say, 30V, then it's already late, imho.

--- End quote ---

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.

Crowbar usually has a voltage ref and comparator and SCR trigger and that’s about it. They are extremely fast.

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.


--- Quote from: tautech on March 05, 2021, 07:41:46 am ---Why is it that foldback current limiting isn't incorporated in a 'quality' PSU FFS, even a $30 SMPS has such a feature as it's hardly a new leading edge technology when it's been around for decades.

--- End quote ---

HP6236/6237 have fold back limiters on the third supply. This is only used above about 500mA where the load is likely to blow up in your face or turn incendiary. I think it’s more a “let’s not burn the place down until the next engineer does a walk past of this test rig” feature b

tautech:

--- Quote from: bd139 on March 05, 2021, 07:55:32 am ---
--- Quote from: tautech on March 05, 2021, 07:41:46 am ---Why is it that foldback current limiting isn't incorporated in a 'quality' PSU FFS, even a $30 SMPS has such a feature as it's hardly a new leading edge technology when it's been around for decades.

--- End quote ---

HP6236/6237 have fold back limiters on the third supply. This is only used above about 500mA where the load is likely to blow up in your face or turn incendiary.

--- End quote ---
Wow, on the 3rd supply.....well I guess that's something.  :clap:
It might seem the designers couldn't count when the only are 2 more outputs.   |O

bd139:
Well the other two are only 500mA so  :-//

Also it's difficult to rationalise foldbacks when you have a variable current limit. I mean you can't go over-current. What if it's a CC source you want (charging SLAs). Foldback would be a problem.

nctnico:

--- Quote from: tautech on March 05, 2021, 07:41:46 am ---Why is it that foldback current limiting isn't incorporated in a 'quality' PSU FFS, even a $30 SMPS has such a feature as it's hardly a new leading edge technology when it's been around for decades.

--- End quote ---
How would that be compatible with constant current mode? How is a outputing a random voltage at a random current going to help?  AFAIK foldback current limiting is used to limit the dissipation inside the PSU in case the heatsink is not big enough.

Besides that SMPS don't have fold-back current limiting; they just go into 'hickup mode' sending pulses into the load.

Berni:
Well the HP 6032A does have a dedicated foldback button.



It works like a normal bench PSU normally, but if you push the foldback button the LED next to it lights up and this locks the PSU into the current mode of operation. If you are currently in CV mode and the supply hits CC mode then it will instead shut down and wait for the output to be disabled and enabled again. If you are in CC mode (say powering a big LED with a set current), push the foldback button and then disconnect the load, causing the PSU to go back into CV mode it will shut down too.

Such a feature is probably even more desirable on such a massive PSU that can supply 1kW 60V 50A.

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