Is this safe for everyday use? - Page 1
Forum section: electronics / projects

author: cart
author_rank: Contributor
timestamp: November 18, 2021, 01:16:53 pm
content: Hi Everyone.Reference to attached picture OR this imgur link ->https://imgur.com/a/HSzZ75MI built this very simple stair lights circuit. I had some issues with getting the Fet to fully saturate at first but then Iconnected it to the 5V directly and when the 2n2222 is off the 5V will just drain via the 1Kohm resistor.My Question is. Is this a safe circuit for every day use?There are carpets etc close to it and the wires connecting everything will be RJ25 cable.The Led lights are rated at 4.8Watts and 5V which is kind of scary.I have a 2A fuse right behind my Power Supply which in this case would be a cellphone charger or any other 5V power supply.I just dont want to burn the house down on some obvious noob mistakes.ThanksCart

author: brabus
author_rank: Frequent Contributor
timestamp: November 18, 2021, 02:59:09 pm
content: Safety has very little to do with the bare schematics. It could run perfectly for decades or catch fire in one minute, just because of some minor details in the assembly, just as any other electronic device out there.If you don't feel 100% confident about your build, my suggestion is to avoid running any risk.Invest some time investigating and perfecting your technique; you will learn the best practices and produce working, robust, reliable circuits.

author: CaptDon
author_rank: Super Contributor
timestamp: November 18, 2021, 04:08:10 pm
content: What is the 1mhz clock signal used for??? Is it pulse width modulated for dimming?? That is a goofy circuit. Why would anyone try to drive the gate of a FET at 1mhz to operate a lamp??? No wonder you had trouble driving the FET!!!

author: ejeffrey
author_rank: Super Contributor
timestamp: November 18, 2021, 05:18:20 pm
content: I don't see any likely *dangers* here, there are no high voltages or high currents. Total power should be no more than 5W. Assuming the 5V supply is appropriate (something like a 7.5W USB charger, not a 20 amp computer power supply) it's very unlikely to be a fire hazard. To be clear, you absolutely can start a fire with 1 amp / 5 watts, but it is relatively unlikely.That said, this is a weird circuit. Why is there a 1 MHz clock? Simple PWM would normally be done at max of several kHz. What kind of LED is that? Is it a 5V lighting module with a built in current regulator or is it a bare LED? LEDs are normally fed with current sources not voltages, although maybe the built-in resistance of your supply is enough to limit the current to 1 amp. If the lamp has a built-in regulator it probably has built-in capacitance that will filter out switching at 1 MHz. Or is the "clock" just representing an external on/off control voltage and the 1 MHz is just a red herring? Why are you using a BJT to drive the FET? You could just use a logic level FET. Do you just need to invert the signal? That FET is *way* overkill for a 5W LED. It's rated for 120V, 72A, and 45W power dissipation and costs a few dollars each in small quantity. OK if that is what you have on hand I guess, but 5V is kind of marginal to drive it.

author: coppercone2
author_rank: Super Contributor
timestamp: November 18, 2021, 08:03:38 pm
content: you want to study fire electrical safety and fuses.The way you prevent electrical fires from spreading is with a fuse. You can add an appropriate fuse to look for a short circuit condition and break the circuit.I recommend if you do electronics at all you just buy yourself a fuse kit and a bunch of fuse holders and just fuse everything you make, its cheap. This will work so long there is not a hazardous impedance path created by something like a bad connection that overheats in an area in such a way that does not trip a fuse (i.e. a wiring/interconnect fire). You mitigate this risk by careful positioning of the wires and stuff so even if they decide to smoke up and melt the insulation and everything, they do so in an area that is not a fire risk, and by studying reliable interconnects and keeping stuff clean and new looking. The good stuff is built to be fire resistant or retardant and often does its job quite well when used properly. In most cases things 'run away' and the fuse works fine, but itsalwaysa consideration.I have seen things in portable electronics that run on low voltages that certainly seemed shady, usually in the form of a smoldering contact or something. Often they don't add a fuse because they don't know how to pick one out that works with a crappy high inrush load, and of course cost, but I can tell you that even with home battery powered stuff, some stuff that seems benign has fuses (good manufacturer) while other stuff does not (bad manufacturer, in most cases).

author: WattsThat
author_rank: Frequent Contributor
timestamp: November 18, 2021, 11:21:05 pm
content: One issue is the mosfet, it’s not appropriately rated as it requires 10 volts of gate drive to achieve its minimum on resistance. It may become very hot, don’t know if it could become hot enough to ignite something. It would probably fail before that happens, that’s where a fuse ends the drama, if one has been installed and is of an appropriate rating. It’s really hard to say because the only datasheet I can find online I would not trust.The correct mosfet for such a circuit is referred to as a logic level device because they turn on fully and achieve their lowest on resistance at 5 volts or less on the gate.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 18, 2021, 11:34:26 pm
content: Quote from: ejeffrey on November 18, 2021, 05:18:20 pmFrom the schematic, it isn'tanykind of LED, it's an incandescent----though it is called a LED in the accompanying text!As such, 1MHz drive makes even less sense.Maybe the OP just "bodged in" bits of existing schematics, & forgot to edit them properly.

author: cart
author_rank: Contributor
timestamp: November 18, 2021, 11:49:21 pm
content: Hi everyone thanks for the replies!Just want to clear up some things.The signal generator was purely for clarity reasons but we can assume it's a PWM on 100% DC and the MHz shown can be voides.Yea it's not the greatest circuit since I haven't touched electronics in 5 years or so.I do have a 2Amp fuse behind the PSU and if any current should rush it would be straight from the PSU. There are no big caps that needs on startup charging so it should be fine.The light is indeed an LED STRIP. I just gave that symbol cause I didn't really know of another one that will work better.Is there some fire retardent material I can line the casing with perhaps?

author: cart
author_rank: Contributor
timestamp: November 18, 2021, 11:51:23 pm
content: Do you perhaps have an example of a Fet that will work with the specs you have in mind?The reason for a fet is heavy duty and less sparky sparky.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 18, 2021, 11:54:34 pm
content: The only thing that is worrisome to me is "RJ25 cable".I would suggest the use of "figure 8" automotive style cable.

author: cart
author_rank: Contributor
timestamp: November 18, 2021, 11:56:44 pm
content: Haha yea it's overkill cause I have this idea of ober engineering for less smoke and fires. Also I just had them lying around.Thouht I can just put them to use.Yes the HZ in that PWM can be omitted.I tried several easy circuits but having a 5V supply it was hard fully saturating the Fet to het mac light out of the LEDS. The voltage over the Leds always ended up being 3.7 or so and was clearly visible even at 100Duty Cycle. So then I opted to drive the 5V directly into the Leds and just break the circuit with the npn.

author: Hiemal
author_rank: Regular Contributor
timestamp: November 19, 2021, 07:51:26 am
content: I'm sort of surrpised you're having issues with turning the fet on fully; no, it's not a logic level fet, but in the datasheet it shows you can support ~ 50 A with 5V on the gate. Far off from the actual rating of the fet but if it's what you have, it should be able to handle a measly 1 amp load without getting too hot/dropping too much voltage.How were you driving it before, like, with what circuit? Just a pushbutton, or an arduino, or something else?Oh, and as far as what to recommend, IRF44 are fairly easy to come by on most websites.You're really not dealing with much power at all, so honestly, pretty much any logic level fet that can handle over 5 amps (for safety margin) would probably do the trick fine.

author: ejeffrey
author_rank: Super Contributor
timestamp: November 20, 2021, 05:31:56 pm
content: I'm guessing the input was 3.3V logic level, and the BJT is converting that to a 5V swing for the FET but the OP didn't say what the input was. 5 V should be enough to drive that FET in this application but a lower power true logic level FET would be a much better choice.

author: coppercone2
author_rank: Super Contributor
timestamp: November 20, 2021, 10:12:13 pm
content: the circuit boarddoes not like to burn, the correct way to protect the chassis from heat is with standoffs. If you want to be ultra sure, metal standoffs connected to a metal base plate which is connected to screws. Then it will just smolder and overheat suspended in air over fiberglass until the circuit breaks. More standoffs is better because the board should sag less. Heavy sections need to be supported so they don't sag like lasagna and short out to the chassis.