Safety components - Page 1
Forum section: electronics / beginners

author: pcblearner
author_rank: Contributor
timestamp: April 26, 2015, 10:59:58 am
content: HiI have made a circuit that has a large number of LEDs connected in series. The circuit can consume up to 50W, this is with a 5VDC AC-DC adaptor power supply (up to 100W). So I put a fuse in series with the power input rated at 15A. Is there anything else I need to do or should research doing in order to consider this circuit "safe". (By "safe", I guess I mean it is not going to injure someone or cause a fire.)Am I kind of already safe since the input is 5VDC?Thank you for your help in advance

author: lutkeveld
author_rank: Regular Contributor
timestamp: April 26, 2015, 01:34:03 pm
content: A large number of LED's in series on a 5V supply? Don't you mean parallel?It's better to use a PTC as fuse, it's more userfriendly because it's selfresetting.Maybe add polarity protection, or is it impossible to switch polarity on your design?Make sure heat can't cause problems, but that's probably not a problem with only LEDs.

author: pcblearner
author_rank: Contributor
timestamp: April 27, 2015, 05:15:51 am
content: Quote from: lutkeveld on April 26, 2015, 01:34:03 pmSorry, I did mean parallel. Thanks for the tip on the PTC, I will look in to them further.

author: Wilksey
author_rank: Super Contributor
timestamp: April 27, 2015, 10:45:29 am
content: No it is not "safe" just because it is 5V.If it is a reputable off-the-shelf AC-DC PSU then it should be fine.Remember, it's not the voltage that kills, it's the current.

author: lutkeveld
author_rank: Regular Contributor
timestamp: April 27, 2015, 11:06:56 am
content: But to produce any significant current through a human with skin (high resistance) you need high voltage. 5V can't do any harm.

author: Cliff Matthews
author_rank: Unknown
timestamp: April 27, 2015, 11:57:44 am
content: Can you be more specific? Eg: Layout, voltage and current of each LED, limit resistors, etc..There is point at which a series-parallel combinations can have more efficiency and simplicity.I'm thinking fire safety since 50w at 5v needs 10 amp wire... you get kink or a thin point and maybe burn your house down...

author: pcblearner
author_rank: Contributor
timestamp: April 27, 2015, 12:13:57 pm
content: Quote from: Cliff Matthews on April 27, 2015, 11:57:44 amEach LED is PWMd through a controller. Each LED is 5V, with an approx 80ohm resistor on it. I am concerned about your comment of burning a house down... what's the standard solution to this.. use 20A wire?

author: madires
author_rank: Super Contributor
timestamp: April 27, 2015, 12:36:06 pm
content: Quote from: pcblearner on April 27, 2015, 12:13:57 pmLet's see, a LED with a 82 Ohms series resistor at 5V would sink roughly 30mA. When the whole circuit needs 10A, you would have a lot of LEDs. Do you really drive each single LED with a dedicated controller or do you drive a bunch of LEDs with one controller? Sorry to be picky, but the details matter.

author: pcblearner
author_rank: Contributor
timestamp: April 27, 2015, 12:44:02 pm
content: Quote from: madires on April 27, 2015, 12:36:06 pmA bunch of LEDs with one controller.

author: Cliff Matthews
author_rank: Unknown
timestamp: April 27, 2015, 12:55:45 pm
content: I think that's >300 LED's.. and 5v is such a poor distribution voltage.Can you star-wire your lighting solution and get away from costly wiring? What is the length you wish to cover?

author: pcblearner
author_rank: Contributor
timestamp: April 27, 2015, 01:00:38 pm
content: Quote from: Cliff Matthews on April 27, 2015, 12:55:45 pmYes I can star wire. What do you mean by 5v is a poor distribution voltage? Should I use 24?

author: lutkeveld
author_rank: Regular Contributor
timestamp: April 27, 2015, 01:11:41 pm
content: Or 19V. To keep current down: yes that would be good.30mA per led is A LOT. Most modern LEDs can perfectly run on a couple of mA's.

author: Cliff Matthews
author_rank: Unknown
timestamp: April 27, 2015, 01:18:16 pm
content: Quote from: pcblearner on April 27, 2015, 01:00:38 pmThat depends on the PWM controller.Without specs or part numbers, this is still only a little less nebulous than the first post

author: madires
author_rank: Super Contributor
timestamp: April 27, 2015, 01:21:38 pm
content: Quote from: pcblearner on April 27, 2015, 01:00:38 pmYes, something like that. WIth the parallel wiring and a resistor for each LED you're burning a lot of power: 2.5V * 30mA * 300 = 22.5W. So half of the power of your PSU is turned into heat by the resistors.

author: zerorisers
author_rank: Regular Contributor
timestamp: April 27, 2015, 01:33:48 pm
content: Quote from: Wilksey on April 27, 2015, 10:45:29 amI believe it is around 30-40V at least to travel through the body. somebody can hold 50,000 Amp leads at around 2-4 volts and not get hurtresources of info(Photonicinduction's 50,000 Amp transformer videos, Experienced co-worker)

author: Wilksey
author_rank: Super Contributor
timestamp: April 27, 2015, 02:18:10 pm
content: Yes, the voltage has to be higher to "kill" you.A few mA's across your battery (heart) can kill you if the voltage is right.But 5V @ 50,000 amps can still cause a fire, which in turn, can kill you (hypothetically).That current kills,not voltage was more of a footnote, 5V can still be unsafe in other respects.