The Bang Preventer - aka DIY Current Limiter with isolation transformer
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- Опубліковано 30 чер 2024
- Some time ago I made this home made box which packs an isolation transformer and a current limiter.
Mr. Carlson's lab Isolation tranformer video
• Tech Tips Tuesday, Iso...
00:00 Introduction
00:13 Overview
01:28 Current limiter explained
05:06 Simulated Short Circuit
06:13 Test drive
09:00 The Isolation Transformer
10:36 Conclusion - Наука та технологія
I have found you only really need one current limiting device when troubleshooting with an isolated variable AC power supply. I use a non-inductive power resistor on a heat sink that can be switched in or out of the circuit by a shorting switch across it. If the device under test (DUT) has a blown input power fuse then the first power up would be with the current limiter because you suspect a short in the power supply. If the unit powers up without a short you can then switch out the current limiter and use the CB on the output of the variac (you have to protect the variac anyway), here I have three different trip level front panel switchable magnetic/hydraulic CBs in parallel to choose from (1 amp, 5 amp and 10 amp, my variac is rated to 15 amps). So, I can power the DUT at full voltage and if it shorts out the CB will protect the variac.
For your design you should have a shorting switch across your current limiting bulbs. This way you can essentially take all of the out of circuit, which may be needed to meet a surge startup current for the DUT. Also, to test a repaired DUT you would want AC power as close to perfect mains as possible (no waveform distortion, low impedance, etc.)
Hi - thanks for your comment! The box has a "bypass" switch to bypass all the lightbulbs indeed. About everything else, of course there are better ways for a current limiter, this is just an inexpensive DIY project! Thanks for watching!
That great, never thought of adding variations, The total resistance
𝑅 𝑡𝑜𝑡𝑎𝑙 R total is approximately 156.92 ohms.
The total current 𝐼 I through the circuit is approximately 1.47 amperes.
Great unit you've built here! I've been wanting to build something similar, but there's a few changes I'll make to my plans after seeing your build. Thank you! Cheers!
Thank you, I use it all the time and I really like it! I'm glad my design gave you some ideas!
Absolutely Good Gadget for the repairs ..
Thank you!
Great name for an essential bit of kit!
I'm getting creative LOL! Thanks for watching!
Thanks for sharing that Tony. I should probably make one for my workshop.
A pleasure!
Great safety device & explanation; many thanks. Now subscribed
Thank you so much and welcome!
I thought about making one of these with a isolated transformer, few bulbs, switches and current meter. See someone beat me to it? A very nice dim bulb tester. Now going to have to built one like this. Going to call my The Magic Smoke Preventer? Yes could built ones with PTC and resistors but the bulbs found to be better.
Sounds good! :) Thanks for watching!
When the device is fully shorted and you have 100w bulb in series all the power goes to the bulb. Almost no power goes to the device as the voltage drop on the device is close to zero.
To be honest I thought about that. Maybe I should have said that when shorted you have 416mA flowing in the circuit so assuming there is a component "catching fire" in your device under test, it will only get 416mA. So say yours is not exactly a short but a component very close to a short, the light bulb will limit the current in the circuit to maximum 416mA.
Is that more accurate? I am genuinely asking, I understand my shortcomings when it comes to electronic :)
@@tony359 Assuming your load is purely resistive (which may be a bit wild, but let's stick to it), you have maximum power transfer when the load resistance is equal to the series resistance, in which case half of the power goes to the lamp and half goes to the load. If you do the math, it turns out that the maximum power the load can receive in this configuration is equal to the rated lamp power divided by 4: for instance, a 100W lamp would constraint the maximum power on the load to 25W (and in this condition, the voltage at the load would be exactly half of the mains voltage).
However, there is another point that can become interesting: when cold, the lamp filament shows a much lower resistance compared to the nominal one. In other words, if you turn on a 100W lamp it will instantaneously behave as a lamp with powerage much higher than 100W (even 10 times higher) for the first few milliseconds, the time it takes to heat up and increase its resistance. This means that at power up your device may receive more power than you would expect (which is not necessarily a bad thing, because it also means that your device will have some extra juice to overcome inrush currents).
@@rudilussana9194 I understand and it makes sense. I'm not sure I understand the 1/4 of power though. I totally get the load would get 1/2 maximum, but why 1/4 then?
And yes, I also understand that when the filament is cold you have a "dead short" for a split second - which is good to be honest as you say.
I knew I should have thought the explanation a bit more! :) I hope nobody gets too mad at me! :)
@@tony359 At maximum power transfer condition, half the power from the mains goes to the limiting lamp and half goes to the load. But the total power from the mains is not the nominal 100W of the lamp, it is halved because you have two equal resistors in series. Therefore, the load receives half of the halved nominal power.
Another way to see it: at maximum power transfer, the lamp has 120 V across its terminals (because it is a voltage divider with two equal resistors), which means it will dissipate 1/4 of its nominal power (V²/R, where V is halved and R stays constant).The load draws the same power as the limiter, so in this case a nominal 100W lamp limits the total power to 50W, of which 25 go to the load (which is the fraction of interest) and 25 to the lamp itself.
Anyway, these are just technicalities; moreover, you will actually be able to use more of the 1/4 nominal power because of the cold filament, so I believe your explanation is perfect and the device flawlessly does its job. Bringing these tematics into the video would make it boring without adding really useful information, and the point of the device should just be to prevent bangs, not to be a perfect power source. No worries, nobody will get mad at you :)
@@rudilussana9194 These are very interesting technicalities! I should brush up on my Ohm's law! Thank you, it makes sense - even though it's not intuitive, hence very interesting. Indeed it would not make sense to explain that in a video but next time I'll try to give the right numbers at least! :)
Thanks again!
I did not realize that you can still buy incandescent light bulbs in the UK.
I will have to get my family to send me some,(to Australia!).
They are for traffic lights 🙂 indeed they’re unavailable in the average shop! Thanks for watching!
Hi Tony, very nice build even the home made box itself, but i have a concern about the heat development by those light bulbs
inside a closed box, and with the bulbs close to the plexiglass ?
i know that with all bulbs enabled, and with a dead short you won't leave it in that condition for hours offcourse, but imagine that you have just repaired a amplifier or something and do a stress test for a few hours. All bulbs are enabled, and the amplifier works fine at first sight, the bulbs are very dim and you leave it so for a few hours, and suddenly a short appears and all bulbs are bright while you are doing something else. Maybe adding a thermal fuse ?
You are right and that actually happened to me the other day.
Let me say that I would normally use the box for short testing. An amplifier would likely draw more power than the bulbs (or the isolation transformer!) would handle so I would probably use it with an amp for a quick test.
However, the other day I was testing an iMac I got from the trash (and which you'll see on a video at some point) and since I had no idea in what condition the system was, I hooked it to the box. All was good, I eventually bypassed the bulbs.
I then started installing Windows on that computer. It's an old machine so it was slow. After many hours I realised the transformer was getting very warm and I had to unplug the iMac - during the installation, sigh - as I was not comfortable in leaving it there for so long.
So yes, you are right. The box used to be fully open but it was also a safety hazard. The least I could do would be to drill some holes - I should have mentioned that on the video.
Bulbs are not a concern, if they light up... you have a short and you switch it off! :) But your point is solid.
One thing I might want to do is to add a "main bypass" which bypasses everything. At that point the transformer would not get hot.
Anyways, room for improvement indeed! Thanks for your comment and for watching!
@@tony359 , my answer seems to be disappeared. I'l building a similar device which takes already more then 2 years due health issues, and adding more and more featues to the project. For the dim bulb current limiter i gonna use 10x G9 halogen bulbs, because those are very compact, and hopefully the heat development will be lower then with big bulbs. i'm also adding a 500VA isolation transformer, a 500VA Variac, a self designed electronic circuit breaker (not the famous Glasslinger design) , a earth leakage detecting of the connected device, a feature to lower the Ghost voltage on the output of the isolation transformer, and a feature that hopefully prevents the total risk of electrocution. if you like, i can mail you the details. Grtz
@@BjornV78 well that's sounds like a proper 'bang preventer', mine is a toy in comparison! What is the ghost voltage out of the transformer? By all means I'd be curious to see the design of you allow me. And I hope your health gets better and better!
@@tony359 , the output voltage of a isolation transformer is "floating" and not referenced to earth/ground, so that's why it is safe if only touch 1 wire at the time.
But if do a voltage measuring between one of the 2 output wires and the earth/ground pin of your wall socket, you will measure a voltage that can be more then 50% of the mains voltage, this Ghost voltage has very low current (in the µA range),barely enough to light up a small LED, but can be enough to kill sensitive electronic components like Mosfets etc.... This Ghost voltage is voltage that leaks from the primary side to the metal core of the isolation transformator and from the core to the secundary side. Each transformer has this, but some more then other due difference in manufacturing. i will send you a mail with more details, otherwise this response gets to long.
@@BjornV78 amazing this seems very interesting! Thank you for that! My email is on the channel under 'about' I think
I build small version with 3 bulbs long time ago. As bulb selection switch I reused 3 position switch from construction fan. Bypass switch is separate. Always, before connecting unknown devices, I recheck switch positions as great practice to prevent accidents.. 😂 My device is in old CD ROM case with house wall socket as output. Bulbs are with E14 sockets. Only I purchased wrong panel meter for it - voltage part is OK, but current part is not, because it can't register currents below 1A and I figured it out when tested after purchase.. Documentation didn't say that..
Amazing! My issue with my volt/current meter is that it has to be powered from the circuit you want to monitor. So it's switched off every time I switch off the circuit. Ideally, I would have liked one with two inputs: one for mains and one for the measured circuit - so it's always switched on and ready to measure what I need.
But I am happy with it anyways and it's very flexible. I think I might need one further "bypass" switch to bypass the isolation transformer as well when needed.
I'm sure your version is as cool as mine! Thanks for watching!
@Arturs Mihelsons, hi, does your panel meter have a separate current transformer (donut) ?
If so, you can use a stiff copper isolated wire that can handle the maximum current you need, and do 10 windings instead of 1 winding through the current transformer.
That way you measure from 0,1A up to 1/10 of the maximum current rating of your panel meter. You can use less then 10 windings also, but then you are always calculating the real value. Like with 2 windings show 2A want there is only a current of 1A.
@@BjornV78 Yes, it have separate current transformer. I tested out 3 windings, just for fun, but then it showed wrong readings, because it's logical. Tested it out that way, to just figure out is it working and that's how I figured out 1A minimum. My idea was to fully rewind secondary part for current transformer on smaller core but it wasn't primary in jobs list. I to had idea to wind primary wire 10 times, instead of 1, but installed wire was to short for that.. 😂
@@tony359 Coolest part in my project is that it is small because I have storage issues.. Now I'm building separate isolation transformer from old 270W lamp TV transformer with option to select output voltage at one channel and option to power oscilloscope from separate channel independently. Now I'm still waiting analog panel meters from China. I decided to go old style way for that.
@@artursmihelsons415 old style is good! "Flux Capacitor V2" :)
Does the isolation transformer improve power factor, for dc power supplies?
I cannot answer on that I'm afraid.
A like and a comment for the algorithm.😁
Thank you, I do appreciate that!
@@tony359 I know, need to keep "it" happy to grow the channel😁
260V only if you used that isolation transformer, not directly on mains.
Indeed. The transformer has some adjustments - +/- 15V which I could use but considering the bulbs will normally introduce a voltage drop, maybe not a bad thing.
260 was probably read when mains was a bit higher than normal, say 245ish.