Useless nameplate ratings? - Page 1
Forum section: electronics / projects

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 13, 2019, 08:12:06 pm
content: I have a Blackmagic Designs (BMD) Hyperdeck Studio Mini video recorder.It has a bog-standard IEC power input connector with the integrated fuse holder.The nameplate says: "100-240V 50/60Hz 0.5A"There is no power rating either on the nameplate nor on any of the published documentation.There is no indication of what fuse rating is recommended for 120V vs 240V mains voltage.I have several pieces of BMD gear working just fine here at home on 120V.But I need to take several pieces to 240V-land (Jordan) next month.The fuse is so small I don't even know if I can read the rating stamped on the end cap (if there is one)?I have tried to submit this post 6 times. But every time I try to upload the photos, the post just disappears into the bit-bucket. Even when I try to edit this post to attach photos, they disappear into the ether. Is something wrong with the system here?

author: Ian.M
author_rank: Super Contributor
timestamp: January 13, 2019, 08:26:19 pm
content: So what's wrong with that? It gives the permissible input voltage range, and that it needs AC and wont draw more than 0.5A, even right down at 100V. A fuse is too slow to save the semiconductors in a SMPSU if anything goes bad, so a 0.5A 250V T characteristic fuse should be suitable to prevent it burning out the cord or any internal wiring 'Weller style'. A faster lower current fuse would not be likely to reduce the damage if a fault occurs, but would increase the risk of nuisance blowing due to the power-on inrush current surge.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 13, 2019, 08:33:44 pm
content: OK, so if 0.5A is rated for 100V, then at 240V it is 2x over-rated and offers little or no protection.Not very reassuring for thousands of $$$ worth of gear when you are thousands of miles away from home (and no technical support).

author: soldar
author_rank: Super Contributor
timestamp: January 13, 2019, 08:37:37 pm
content: The fuse is not there to protect your equipment. It is there to protect the electric wiring and to prevent starting a fire. The fure is not there to prevent blowing a semiconductor. On the contrary, the semiconductor will blow first, thus protecting the fuse.There is no point in changing the fuse or even thinking about this.

author: DaJMasta
author_rank: Super Contributor
timestamp: January 13, 2019, 08:39:34 pm
content: Maybe so, but at that current level... are there that many faults that will trip the 500mA fuse at 120V input and could damage the hardware that wouldn't trip the 500mA fuse at 240V?If you're relying on the fuse for more than simple short protection, maybe, but in that case you probably shouldn't be relying on just a fuse, right? It's a bit lazy, but it probably makes no difference in the vast majority of the failure modes where a fuse could protect against damage.

author: Circlotron
author_rank: Super Contributor
timestamp: January 13, 2019, 08:51:52 pm
content: The buck stops with the fuse.After something else has already blown up, the fuse will then pop.In most situations it is unusual for a fuse to blow before anything else has gone wrong. If you are running 240v then a fuse rated at half the current might be okay, but it it may also be prone to nuisance blowing by inrush current at switch on.

author: dmills
author_rank: Super Contributor
timestamp: January 13, 2019, 09:09:49 pm
content: Inrush is the elephant in the room here and why you probably should NOT downsize that fuse.As others have said they exist to protect the cables from fire, not to protect electronics, and unless otherwise specified the stock fuse will do that across the full operating range.I would be more worried about BMD on a money job then the details of the fuse!Regards, Dan.

author: james_s
author_rank: Super Contributor
timestamp: January 14, 2019, 05:03:20 pm
content: At least 99.99% of the time, a component failure causes the fuse to blow, a switchmode power supply will be protected against downstream faults so if the fuse blows it's almost always a shorted chopper or rectifier. No sense in messing with the fuse, as has been said already it's there to prevent collateral damage.