why not use 24v to power your lights

[algerdes]

To clarify - the problem we experienced (several times) was the DC-DC Buck Converters failed, putting the full 24vdc to the inputs of the remotes. Placing this "crowbar" circuit between the Buck Converter and the inputs of the remotes (and anything else downstream) seems to be an added layer of safety.

At the time, we were also looking at using the power supplied from the POE switches (48vdc, if I remember correctly) to a buck converter (48 vdc to 12 vdc) to run some of the smaller props. This, of course, was scraped due to the failure of the 24 vdc to 12 vdc units.

FYI, though I don't regularly put a fuse between the power supply and the controller, I do fuse all outputs to lights and other props.
This new stuff may allow for resurrection of this old idea.

 

[TerryK]

It is a minor detail, an argument of sematics as it were but a 'crowbar' circuit does not exist 'between' a supply and the load. While visually it may appear to be so, a 'crowbar' circuit exists in parallel with the supply (typically output) which of course is also in parallel with the supply's load.

@Skymaster's design which I would call a hybrid of sorts incorporates a fuse which from his posts seems to be on the input side of the converter 'between' the converter and the 24 Volt supply. The 'crowbar's' 'crowbar' components appear to be after the fuse and in parallel to the converter's input while the 'crowbar' trigger portion looks to be on the converter's 5 Volt output, in parallel with the output.

Without a schematic or wiring diagram of @Skymaster's 'crowbar' some of this is an educated guess on my part but also brings to light a(nother) limitation. The 24 Volt supply capability along with wire resistance between the supply and converter and 'crowbar' fuse size all needs examined to insure if the 'crowbar' triggers that the 'crowbar' PCB fuse opens. Otherwise, the 24 Volt supply will will 'shove' power until its input goes away. Or something burns up.

 

[Skymaster]

It is a minor detail, an argument of sematics as it were but a 'crowbar' circuit does not exist 'between' a supply and the load. While visually it may appear to be so, a 'crowbar' circuit exists in parallel with the supply (typically output) which of course is also in parallel with the supply's load.

@Skymaster's design which I would call a hybrid of sorts incorporates a fuse which from his posts seems to be on the input side of the converter 'between' the converter and the 24 Volt supply. The 'crowbar's' 'crowbar' components appear to be after the fuse and in parallel to the converter's input while the 'crowbar' trigger portion looks to be on the converter's 5 Volt output, in parallel with the output.

Without a schematic or wiring diagram of @Skymaster's 'crowbar' some of this is an educated guess on my part but also brings to light a(nother) limitation. The 24 Volt supply capability along with wire resistance between the supply and converter and 'crowbar' fuse size all needs examined to insure if the 'crowbar' triggers that the 'crowbar' PCB fuse opens. Otherwise, the 24 Volt supply will will 'shove' power until its input goes away.

@TerryK from your understanding, I think you have the exact schematic in your head.
You're 100% right with the operation.

The fuse I have chosen on the 24V side for my deployment is 5A. This allows around 120W. My DC-DC converters are rated to 300W max, or 20A max on the output, which at 5V is only 100W. So, the theory being that normal operation in the order of a max of 80, maybe even 100W will not blow the fuse, however, when the crowbar is triggered, the 24V line will be brought the ground. The supply line will need to be able to sink the full 5A, otherwise (1) the fuse will never blow, and (2) I'd never be able to hit the desired output of the converter.

Now, as for the first point, if the fuse doesn't blow, as the resistance of the feeder is too high - this isn't a major issue, as the short across the input terminals of the converter would prevent it from outputting anything more. The SCR is rated at 30A, so it would be able to handle a decent sized feed. There is no heatsinking on the SCR however - which is something I need to test with for extended periods - carrying the full load would mean it would be dissipating a fair amount of heat and would cause eventual destruction of the device.
However because the fuse is ever so slightly larger than the expected load - if it got to this, it would mean that the feeder cable is too small to supply even the required load and would cause other issues.

 

[TerryK]

@Skymaster: Thanks. Good to know I get a thing or two right every so often

If you haven't, you might look at a Germanium SCR; they're a bit hard to come by though. I'm sure you know but for most other individuals reading the thread; a Germanium device will have a lower Vf (Voltage Forward). Here, for an SCR, its On State.

@Kent asked about leakage and mentioned the 5 Volt side. Bit curious myself, mostly just the LED perhaps? 5 to 10 mil?

 

[fuzbat]

@Skymaster I'd be keen on testing out your crowbar circuit if I could - happy to either buy them or get my own PCBs printed up and populated - or any combination of the two that suits you. Have you given any thought into supporting different crowbar voltages?

 

[tooms]

I looked into something similar a while ago as I've got a few 80amp 48v DC power supplies in the shed, it worked out just cheaper for me to buy AC power leads and power boards then what it is for all the guff to reticulate high voltage DC around my display and step down devices from the higher DC voltage back down to 5/12.

 

[fuzbat]

I guess one advantage would be where I need multiple voltages in one location, each of which is usually pretty low current - I *could* buy a powersupply with all the voltages I need and get it 240V, but a lower feed voltage with cheapish converters might be smaller, safer and more flexible. Then again I should just get rid of all the silly voltage lights and replace them with something sane..

 

[David_AVD]

I run 24V out to the prop boxes because I don't want 240V out in the yard, footpath tree or light pole !

 

[Croydon Lights]

I run 24V out to the prop boxes because I don't want 240V out in the yard, footpath tree or light pole !

Could you tell how you have your set up from 240AC to the 5 or 12 DC at you controllers including you set up in the prop boxes and if you have any pictures

 

[David_AVD]

I have power supplies and some controllers in wall (house) mounted switchboard boxes. From there I feed 24V out to some of the remote prop areas.

In those prop controller boxes I have 24V > 12V or 24V > 5V DC converters. Some are Mean Well and some are a no-name type.

Some of my setup is show in my videos: https://www.youtube.com/user/DavidAVD1