24v to 5v power issues, again.

Other Hardware General Electronics
1

I requested some help a couple of days ago regarding power regulation (Powering Arduino from 12v 1A linear regulator? - General Electronics - Arduino Forum); and was hence steered towards using a stepping regulator instead of a linear one.

Anyway, the buck regulator and other required parts have arrived from RS components and I've assembled a circuit on the bread board according to Schematic 2 on page 4 of this document: http://docs-europe.electrocomponents.com/webdocs/0aab/0900766b80aab9ee.pdf

For the sake of sanity checking, here is a photo:
https://drive.google.com/file/d/0B-iyJRLLlHbfcURRSHZUcnh2dGM/edit?usp=sharing

Anyway, this arrangement seems to be even worse than with the linear regulator in terms of heating up and cutting out and something just doesn't make sense.

Measuring the current, my Chinese knock-off Arduino + Ethernet shield is drawing 230mA. The arrangement runs for about 15 seconds before the regulator thermally cuts out (by which time it is well over 100degC; I can sense the heat from it on my face sat next to it). The 5V output otherwise seems to be very stable according to my multimeter.

I realise the documentation states that a 'heat sink will be required for most applications'; but Christ, I'm only drawing 7% of what the regulator's supposed to be able to put out (3A); how bloody big a heat sink does it need?

The fact that the capacitors and inductor are large enough for midgets to use as dumbbells only confounds my dissatisfaction at the design. I only want to convert 24VDC to 5VDC and I just feel that this should be much easier and better than I'm making it!

So anyway, back to the drawing board and to elaborate on what I'm doing...

I'm trying to design a general purpose shield that will provide primarily 8 optocoupled digital outputs and 4 optocoupled digital inputs and power to the Arduino; for interfacing with 24v circuits and a 24v DC power supply. I therefore need to design power circuitry to step down 24v to 5v with enough welly to drive the outputs (signal only) and perhaps 10 LEDs.
I hope to use the shield with industrial motor controllers and other components which will share a 24V power supply.

Linear regulators got to hot and fell over; and this design, well, gets too hot and falls over.

I would greatly appreciate any advice as how to better achieve this then by my current efforts!

[Edit] For the record, I do fully appreciate and am thankful for the help & advice I received the other day; it's unfortunate that despite this I haven't yet been able to get where I'd like to be with the design.

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I think you put a wrong link in there - I see no document.

Switching regulators and breadboards don't mix too well. The datasheet should give a recommended PCB layout to keep everything in specifications.

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Link updated, sorry.

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Ok, as can be seen in the photo, I rather embarrassingly did not connect the diode correctly. Correcting this error seems to have solved the problem; the regulator now runs at a steady 35degC.

I'd still appreciate any suggestions to improve the circuit; ideally to reduce the size/number of components for power regulation.

5

I was just going to ask about where the diode was connecting to...

I'm not familiar with that specific regulator. I am much more used to working with much smaller components.

You're not going to need anywhere near the amount of power that regulator can provide, that's for sure. You're not using it to power external things, just interface with them, yes?

I'm more used to using things like the MCP16312 which is a tiny SMD device, and takes a few equally small SMD devices. Here's a typical layout with one I am working on for a project (regulator is U2, input is top, output is bottom):

So, I'd say, to reduce the size of the components, pick a regulator that works with smaller components.

sreg.png