# 24vdc multi tap supply

#### ChiefD

Joined Nov 2, 2021
4
I am looking to specify (e.g.: outsource design of) a product one feature of which includes overcurrent protection for class 2 wiring supporting 12vdc, 24vdc, and 48vdc circuits.

One issue I'm concerned with, however, is what happens if a user (installer) short circuits as I am trying to avoid manual intervention to reset the circuit.

My question is whether there is a way to design a OCPD that does not need to be manually replaced (fuse) or reset (breaker)? I have been told there are automatic resetting breakers which is one option, but I'm not sure what their downsides are? I assume they come with a higher price which my application may support, but what about reliability?

#### LowQCab

Joined Nov 6, 2012
1,918
Simply use a Current-Limiter, as well as a Voltage-Regulator.
Some Voltage-Regulators have Current-Limiting "built-in" to their design.
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#### ChiefD

Joined Nov 2, 2021
4
well that seems simple enough ... looking at an online article it appears it will work if I select the correct device for my circuit, which makes me question where this is widely used. Why, for example, have circuit breakers in a homes load center when you could just have a series of 120v current limited voltage regulators you connect to and never have to reset a tripped breaker?

#### LowQCab

Joined Nov 6, 2012
1,918
Thermal / Magnetic Circuit Breakers cost about ~$2.oo to manufacture, and require manual resetting if they are tripped. Electronic-Regulators that can handle that much Power may cost ~$10.oo, or much more, to manufacture.

Sometimes an "Automatic-Reset" of a Circuit can create a dangerous situation.

Some devices can not tolerate the reduced Voltage that a
Current-Limiting-Circuit might create during an over-load condition.

Automatic-Thermal-Only type Circuit-Breakers tend to get weaker the more times they are tripped,
and they usually produce Heat under normal operation, which is wasteful of Power.

are two very different conditions that must be handled in specific ways.
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#### dl324

Joined Mar 30, 2015
14,310
Welcome to AAC!

Have you considered PTC (Positive Temperature Coefficient) fuses, ceramic or polymer, or eFuses?

It would be helpful if you provided more details (currents involved, what you're trying to protect, etc).

#### ChiefD

Joined Nov 2, 2021
4
class 2 circuits, particularly for LED lighting but other devices can be loads on the circuit.

One NEC class 2 acceptable method is to power limit the circuit to 100w, so +/- 4a @ 24vdc - some inspectors allow fuse protection, some don't because it is not "power supply limited" as the user can put a bigger fuse in, so right there is a reason I'm looking for a different solution (although a user can always put a 30A breaker on a 15A wire in a load center, I don't see the difference but inspectors do, probably because they figure homeowners do not mess with load centers much).

On a recent project I'd say we went through 40 fuses, incredibly easy to do as the guys get careless wiring low voltage and they tend to work with live circuits.

So I want to design a module that, without going into all the other details, you can homerun class 2 wiring to, it will ensure 4a max @ 24vdc, and it won't require replacing a fuse or resetting a breaker every time somebody shorts a circuit.

Really looking for a EE consultant who can design PCBs to work with, IDK if it is frowned upon in this forum to ask?

#### dl324

Joined Mar 30, 2015
14,310

#### BobTPH

Joined Jun 5, 2013
4,754
One issue I'm concerned with, however, is what happens if a user (installer) short circuits as I am trying to avoid manual intervention to reset the circuit.
So instead if getting an immediate indication that he has done something wrong, the device is fed a lower voltage that still might be enough ti start a fire? Not a good isea in my opinion.

Bob

#### ChiefD

Joined Nov 2, 2021
4
Interesting point I had not realized fire was an issue at the device - however it isn't usually that the device is connected wrong, it is that while stripping wiring they let the wires dangle and one of them touches. That said, fuse/breaker would do, I was trying to avoid them only because it is inconvenient to leave the room and go to the panel to reset.

Dennis, as for consultants I've worked with 1000+ but my background is CS, not EE. An experienced engineer with the right skillset could flesh out the requirements and design the intended product in no time but I'd look for is a NDA, review meeting, then proposal. I love working with retired guys because they usually like smaller fun projects with no corporate BS and if they are good they have the experience to go right to workable solutions and understand issues like cost/benefit and ability to manufacturer.

If you have the right background and are interested does this forum have a mechanism for private communication, I do not see one?

#### dl324

Joined Mar 30, 2015
14,310
I love working with retired guys because they usually like smaller fun projects with no corporate BS and if they are good they have the experience to go right to workable solutions and understand issues like cost/benefit and ability to manufacturer.
Doing piece work isn't economically feasible. Having an attorney look at a contract will cost thousands of dollars and that's not counting the time any prospective taker would spend understanding requirements, doing a time and cost estimate, iterating on changing requirements, product support, and having to carry liability insurance.