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Power Supply: Build or Buy?

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mawyatt:
A story related to the power supply output capacitance that some folks might find interesting. Back in 70s and 80s much effort went into nuclear survivability for electronics, and research found the levels of minimum energy in a supply line or power supply that would damage/destroy a chip. Basically a nuclear detonation will turn on all PN junctions of exposed semiconductors, this includes diodes, transistors and junction isolated chips and why for high radiation environments (space for example) insulated substrate chips are preferred.

For survivability the supply line and power supply had a maximum available capacitance and discharge profile which often dictated utilizing a "Crowbar" type Power Supply, where the Crowbar (we used an SCR) would act upon an event leading edge. The available capacitance was a complex assessment of the capacitance that could quickly discharge during an event leading edge from the power supply and PCB distributed decoupling capacitances, and recall this was something on the order of ~90uF for a 10mJ energy level for a 15V supply (vaguely recall this as the maximum available discharge energy to cause chip damage).

Anyway, this is kind of related to the output capacitance of a Power Supply and why one prefers a lower capacitance as an "event" could be a malfunction of the test circuit, device or even operator (scope ground lead dangling and shorting something on the PCB, been there done that) :o

We must remember that the power supply output capacitance, or a portion of such, is generally outside current limit circuitry and the current limit can't help with the instantaneous discharge levels for the output capacitance, so lower is better :-+

Best,

 

David Hess:

--- Quote from: mawyatt on May 11, 2023, 01:33:21 pm ---It's interesting to see discussions of power supply active sinking capability, IMO this should be a prerequisite in any modern quality Bench Level Supply, but few have this built-in capability. Same goes for output capacitance, this should be as low as possible in a quality Bench Supply. It doesn't take much energy to destroy/damage a modern IC, and even with a low current limit set point the output capacitance energy storage is sufficient to quickly dump into a circuit and take out various chips, as as we know Murphy will direct that energy to the most expensive and elusive chips!!
--- End quote ---

In a majority of applications, minimum output capacitance and active sinking are not required.  And both have at least the increased potential of creating oscillation with certain loads.  Of course either or both may be indispensable in specific cases.

Active sinking is nice, but so are readouts of current and voltage, and I can get by without the later as long as I have an indicator that the power supply is current limiting.  None of these things are something that I consider indispensable.


--- Quote from: mawyatt on May 11, 2023, 02:23:05 pm ---For survivability the supply line and power supply had a maximum available capacitance and discharge profile which often dictated utilizing a "Crowbar" type Power Supply, where the Crowbar (we used an SCR) would act upon an event leading edge. The available capacitance was a complex assessment of the capacitance that could quickly discharge during an event leading edge from the power supply and PCB distributed decoupling capacitances, and recall this was something on the order of ~90uF for a 10mJ energy level for a 15V supply (vaguely recall this as the maximum available discharge energy to cause chip damage).
--- End quote ---

"Intrinsically safe" designs are subject to a similar analysis.  Systems which support hot-plugging, including USB, also have limits on total capacitance to prevent high inrush currents which could cause glitches.

rsjsouza:

--- Quote from: james_s on May 06, 2023, 05:52:08 pm ---
--- Quote from: br4n_d0n on May 06, 2023, 04:20:06 pm ---
--- Quote from: BeBuLamar on May 06, 2023, 02:31:19 pm ---I don't see much point in using 24VDC power supply to power a soldering iron. Get an AC powered soldering iron and save the 24VDC iron for when you want to use it on battery.

--- End quote ---

I've had the soldering iron for a few years now so purchasing another one would be a waste of money. Having a bench PSU that can do double duty when I feel like it would be more worth it IMO.

--- End quote ---
So lash up a salvaged transformer, rectifier and filter capacitor (assuming the iron contains electronic temperature regulation) and call it done. If the iron is not a temperature controlled soldering station then IMO it's not really worth having except as a backjup.

--- End quote ---

Perhaps this was mentioned before but, since you are in the US, there are several relatively cheap sources of 24V power transformers that would only need four diodes, a capacitor plus a few parts to make a voltage regulator to tame the input voltage (LM317 is quite versatile and can even perform current limiting and high output current applications). Look for the typical 40VA transformers for HVAC and house alarm systems. Forget about doorbell transformers, which are probably too weak for a soldering iron.

br4n_d0n:
 :popcorn: Well, I either made a good deal or a very expensive (for me) mistake. I'll see when it arrives.

james_s:
What did you get?

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