Build Your Own Current Limiter for Protection when Repairing and Testing Electronic Equipment

greetings several people have suggested that I create a video showing how you can build your own current-limiting device to give you an extra margin of safety and peace of mind when you're working on amplifiers this is the unit that I've been using for many years on every pretty much every electronic device that I work on consists of about a 250 watt bulb and the larger the wattage the better in this situation a porcelain lamp base and an electrical receptacle and a 3 wire grounded plug I think a lot of people fail to understand that outlets are polarized for a reason the narrower prong connects to a black wire this is the hot of connection this is where the hundred and ten volts resides the wide plug hole is for a white wire which is what we will call a common wire and this one I guess most people realize is for the green wire or the ground okay so wide is white now let's take a look at a little schematic I've drawn up now this is a picture of them receptacle common is white it's the wider of the two prong inlets the smaller one is the black hot wire and green is at the bottom now here's my three wire plug with the cable coming out of it the narrower of the two prongs is the black wire and that I run through my bulb this will be through the bulb base that you buy in my case it's a porcelain one so you go in one lead of the bulb base and out the other the black wire will come over here and then go to the narrower of the two receptacle holes the common is the green wire it will come around here to the green or common ground hole on our sceptic 'el and the wide prong is the white wire which goes to the common wide receptacle hole so two of the three wires continue uninterrupted it's almost like an extension cord here from this plug to this receptacle with one difference and that is the black hot wire is forced to pass through the ball before it can get over here to the receptacle now how does this work well since the current has to pass through the ball before it can get to the device that you plugged into the receptacle this ball prevents a huge flow of current it's sort of like a restrictor on the flow of electrons that go to the device if you develop a dead short over here in the device as I will demonstrate soon the bulb will flash bright as bright as it can be and will warn you that there is a dead short and it will also prevent a huge flow of electricity here that can cause damage to the device or to you or scare the heck out of you okay so this bulb then takes all the heat literally and all the electricity and it actually works as a warning ball at the same time now if the three wire plug that you're using is not polarized it as you notice in this case both of the blades are the same width that doesn't matter because look if I set it up here with the diagram that I've drawn this should be the white wire and this over here should be the black wire let's use an ohm meter and see if that's the case okay I have the ground look at the bottom like in the drawing and the right hand blade should be black which is hot I've gone over here to the black wire and as you can see there zero ohms there is continuity if I go to the other prong as you'll see there is no connection so even though the blades are not different size are polarized it doesn't matter you can trust the color of the wires to be correct because this lug down here serves to polarize it it can only go in one way with this one being black and hot this one being white and common also I strongly recommend as high a wattage bulb as possible in this case it's a 250 watt photo a floodlight okay so this is used for F in photography studios if you don't have access to something like that you can always go to Home Depot and get a 250 watt one of those outdoor floodlights and put it in the reason you want a real high wattage bulb is the higher the wattage the lower the resistance of the bulb and you really don't want a lot of resistance because then you're going to cut down on your voltage to your device and you're going to cause it to act up so the lowest possible resistance is what you're looking for which would be the highest wattage bulb okay at 250 s about it now we simply plug the current limiter into either an extension cord or into the wall outlet and then we plug our amplifier into the receptacle okay here we have a snazzy ole premier amp plugged into the current limiting device let me switch the amp on and we'll wait a few seconds for it to warm up you'll notice that the light bulb doesn't glow at all simply because the amp is really not drawing enough current to illuminate the ball okay and the amplifier functions just as it should there's no change in it because it's plugged into the current limiter okay let's set up a worst case scenario test here I've got my current limiter plugged into the wall 110 volts probably about 30 amps can flow before the circuit breaker will flip which can cause all sorts of damage to an amplifier or any other electronic device plugged into my current limiter is this extension cord with these two alligator clips on the end now what I'm going to do is short it with this screwdriver I think we all know we've probably done this at some time or other when you short across 110 volts with a screwdriver it's no fun at all our very loud noise pop flash you park well the screwdriver to the alligator clip all sorts of bad things can happen okay and including to you if you're grounded so I'm going to do this right now in front of you and you'll see what happens as you can see virtually nothing happened there is no no electrical arcing no damage it's all very quiet very passive but the bulb comes on to absorb all the current and to tell us that a dead short situation has arisen if it were a partial short the bulb would come on not quite as bright but it would come on okay so that would warn us we have a partial short unplug this thing check it out so as you can see this is a pretty darn near foolproof method of saving yourself and your equipment and your the device you're working on and I just don't see any downside to it it's easy to make and I use these this particular device religiously what about does it for this video now everything I've said refers to electrical supplies here in the United States the 110 volt 60 cycle and I'll add people in Europe you may still be able to make a device like this and use it I don't know if you're a polarizing scheme is the same as ours you need to check into it but for American wall current of this device is just the greatest simplest thing in the world to protect you and your equipment okay I hope you enjoyed this found it interesting please consider subscribing to my channel and regardless I hope to see you again in the near future bye and thanks for your time and interest

Channel: Uncle Doug

Views: 298,812

Rating: 4.9421721 out of 5

Keywords: Current Limiter, Power transformer, Short Circuit, Electrical Safety, Shock Hazard, Test Equipment

Id: wRFRwOnLsZI

Length: 9min 13sec (553 seconds)

Published: Wed Jul 31 2013

Reddit Comments

[u/6C6F6C636174]

If you build one of these (and it looks like a good idea), don't connect the hot wires the way his diagram recommends. The hot wire coming into a socket should always be connected to the center pin, never the shell. It's a lot harder to hit the live center pin accidentally than it is to hit any part of the shell around it.

[u/3DBeerGoggles]

I'd very much recommend building one of these if you work on mains electronics, even if you have a variac and isolation transformer.

If you're looking to build one that's a bit more compact, I found a junction box with a socket that fits in a conduit knock-out (like those used for flood lamps) made for a nice compact package. When not used as a tester, I just screw in a 15A fuse instead:

http://imgur.com/ghYsMZX

Edit: Oh, and for those interested - this was common tool in the early days of test gear as well. Light bulbs were cheaper and easier than metering setups. Here's a restoration of one of these units: http://www.antiqueradio.org/ChristyTester.htm

[u/Tastygroove]

This is the rig you use to power on old tube radios the first time. Match the current draw to the wattage of the light bulb. Turns a zzzzzzaaaap POP into a mere fizzle.

[u/OneLungHo]

Don't stick one of these in front of a switch mode power supply. I watched someone do this once and it lost its shit almost instantly. Whatever the control loop in it was up to I don't know but it kept shutting down. I suspect this was due to the completely non-linear characteristics of the thing.

[u/tensai_76]

From a safety standpoint, bulb base is wired incorrectly. Live wires go to the centre pin. Neutral wires go to the Edison screw itself.

[u/CrackCrackPop]

Why not just use a fuse? This feels like oldtimer of car repair, but guess it works, yeah he basically removed the safety feature of the bulb

[u/jrblast]

This wouldn't protect you much from electrocution though, right? It's essentially just a couple ohms resistor in series, but your body is in the 100k range, right?

I'm not an expert though, so if someone who is could chime in that would great!