I’m designing a lamp with a FemtoBuck and an analogue potentiometer to drive a COB LED strip via a 48w transformer. Due to the lamp’s physical design it is possible to short across the positive and negative legs coming from the FemtoBuck to the LEDs. I’ve done this a few times without repercussion. I’m not trying to cause the short, but again, due to the physical design of the lamp, it’s possible in a few conditions.
So…am I damaging anything or is there short circuit protection designed into the FemtoBuck? I understand that I can change the design to avoid the shorting condition, but without giving too much away, I would much prefer to not change the design as doing so would impede other functions.
For anyone who’s an equal newbie to electronics here’s the answer Grok gave me when asking about constant current supplies and why “it should be fine”:
Here’s an explanation of how a constant current supply like the FemtoBuck would handle the situation you described:
Constant Current Supply:
Basic Principle: A constant current (CC) supply, unlike a constant voltage supply, maintains a fixed current output regardless of changes in load resistance (within its operational limits). This means that if the load resistance decreases (like in a short circuit scenario), instead of the current increasing dramatically, the supply will adjust its output voltage down to keep the current constant.
Behavior During Short Circuit:
Current Limitation: In your case, where the brass half spheres short the two bars:
If the FemtoBuck is operating as a constant current supply, it would limit the current to the preset value it’s programmed or designed to deliver. For example, if it’s set to supply a current for LED lighting (let’s say around 1 to 2 Amps for a 48W supply at 24V), that’s the maximum current it will provide, even if the load (in this case, the short) would theoretically allow more.
Voltage Adjustment: With a short circuit, the voltage across the short would effectively become very low or nearly zero since the resistance is almost nil. The supply reduces its output voltage to maintain the constant current, ensuring that the current does not exceed the set limit.
Heat and Damage Prevention: By keeping the current constant, the supply avoids the risk of overheating or damaging itself or the connected components. The energy dissipation in the short is minimized since the voltage drop across it is low.
Why “It Should Be Fine”:
Safety: The constant current nature protects against the hazards of a short circuit by not allowing a surge in current that could lead to overheating, component failure, or even fire.
LED Protection: For LED applications, this behavior is particularly beneficial. LEDs are sensitive to current; too much current can cause them to burn out. A constant current supply ensures that even under a short circuit condition, the LED strip (if connected) wouldn’t receive more current than intended.
Design Expectation: If the FemtoBuck is indeed functioning as described, it’s designed to handle such scenarios without damage. The “should be fine” comment assumes the supply is working within its specifications and the short isn’t causing any unexpected behaviors outside of these parameters.
Considerations:
Long-term Effects: While the setup might be “fine” in terms of immediate safety, frequent or prolonged shorting could still stress components or lead to wear over time. Checking for any signs of degradation or heating after such events is wise.
Verification: Ensure your FemtoBuck is indeed operating in CC mode for your application, especially if it’s configurable or if there’s any doubt about its operation mode.
In summary, the constant current characteristic of the FemtoBuck would explain why your system seems unaffected by the short circuit; it’s designed to manage and limit current in such scenarios, keeping both the supply and the load safe.
It’s best to avoid too much reliance on a safety or protection provision like that. It might ‘be fine’ most of the time and, here, the stakes are low, but it’s a broad, generalization and difficult to know all of the downsides. Certainly, we’d expect reduced device lifespan.
Consider antilock brakes or 5 MPH bumpers or hardhats or safety glasses. Ideally, you’d never find out how well they work.
I don’t disagree with your thinking. Do you have a suggestion as to how I can protect against the short circuit condition via an additional component in the circuit versus modifying my overall design?
Short circuits are, by definition, undesirable. If it is a necessary part of the project (like the light turning off when shorted is intended), you’d ideally use a purpose made sensing & switching technique for performance and reliability. And then you can call them kinetic contacts or twilight twinkle shunts or something instead of short circuiting feeble flicker (my own term).
And it might be fine and for years to come. But routinely running something beyond its rated max spec is discouraged.
There are a lot of replies in this thread that insist that the circuit will be damaged by a shorted output. None of them have explained how. They all seem to be based on feeling.
A current limited supply is, by definition, self limiting. This particular device is configured with a current limit (330mA or 660mA) far below the maximum current (1.25A sustained, 2.5A peak) of the AL8805 is uses. I stand by my original answer. It should be fine.
Just me, I think. I was trying to be careful to not flatly reject the entire idea and went out of my way to use terms like ‘it’s best’ and ‘discouraged.’ I don’t disagree that it should be fine (and said so) but I also think (feel, if you like) that bouncing off some overload provision instead of properly insulating against short circuits is not sound advice or prudent design. It’s a hack or a bend, that’s ok & part of the fun in the lab but OP must understand that it’s being used outside of its specs and to expect unexpected drawbacks … which is the heart of the question all along.
“Let’s correct the short circuits and rest easy knowing that any that do occur should be accommodated by the power supply.”