Best video on YT for a dim bulb tester hands down.

Hi Eric! I, too, have watched many of these dim-bulb/current limiting device how-to videos. Yours is definitely the most comprehensive. Most just show how to build one. Some explain why. Yours actually demonstrates how and why. Plus, your video is the only one I've seen that demonstrates the optimal order of isolation transformer, variable AC auto-transformer, dim-bulb/current limiting. As an aside, I was fortunate to pick up a pristine Heathkit IP-5220 Isolated Variable AC Power Supply. Internally, it uses the same order as you've shown. All I need to do is to plug in my dim-bulb into the IP-5220 and it does everything you've shown in your final configuration. Great video! Thanks!

Just discovered your channel. This ans. All my questions about how to set up a safe work bench.

And that's why you always get multiple sources of information folks. I've seen Mr Carlson's video about a build like that but he didn't go into detail about the order of the components. Thank you for your very detailed and easy to understand video I'm quite surprised that Mr Carlson didn't arrange it that way but there might be a reasoning behind it.

Nice video Eric. One thing I want to add about the isolation transformer you generally buy that I see most people skip over in their videos. I have 1000w hospital grade Tripp Lite (I believe the hospital grade ones already have the neutral isolation), but to have a true isolation transformer you need ground isolation as well. The input voltage line’s ground is connected to the metal case of the transformer. Even if you disconnect the ground wires on the output receptacles, the stock receptacles (as do most) have their ground lug connected directly to the mounting tabs. So when that receptacle is screwed securely to the metal case of the Tripp Lite it is indeed still grounded to the main input ground. So you would either need to get plastic isolation tabs and use plastic screw, or what I did was replace the stock receptacles with special ones where the ground lugs are not connected to the metal mounting tabs. These receptacles are generally orange in color. So I have complete isolations from hot, neutral, and ground.

EXCELLENT EXCELLENT video. Yes, you certainly have the absolutely BEST isolation transformer/dim bulb/variac video done. Great job Sir. Thank you.👍

Putting the iso transformer first is best. In this configuration the variac does not have to supply excitation current to the iso transformer which depending on the size of the iso transformer can be mA to Amps.

Hi Erik I qualified as an electrician now technician that was a great explanation of how to configure the tester 👍

Thank you. I got my "dim bulb fix" for the night... :) And that's the 2nd biggest transformer I've ever seen! Thank you for the (always) informative video(s).

Thank you, this was the most lucid presentation about how to arrange the major components, and the pros/cons of each arrangement.

The most comprehensive video l have found on the subject. Great job!

excellent work mr Erik, thanks

Great video Erik, really clear.

Absolutely excellent vid. I now know the optimal setup. Thank you.

Great job on your explanation.

This is by far the best and most comprehensive discussion I've seen on this subject. I'm going the rearrange the components of my dim bulb test system to match your recommended setup. Isolation transformer to autotransformer to bulb to device under test. thanks!

Very good explananation. I liked how you showed each configuration. I just wish I saw this 1week ago. As the incandesant bulb ban just went into effect.

Very well done. Thank you

Very interesting video on this subject , you have explained it extremely well. Being a electronicien myself this is something I had learned when at CEGEP (school), and I used it all the time. Thanks

Great video!!

Great video. Im buiding one now using a small variac and a 1 to 1 pyramid transformer. I havent decided how to wire it yet but im going to use a motor to turn the variac which has the same 3 amp ouput as the transformer. I also have an old hammond boat anchor 1kva which is earth connected on both sides but i havent found a use for it. I have alot of recording gear and i may use it when i record to avoid hum when tracking guitar.

very very good video thank you

great video.. I like your variac, mine shows voltage not amps. this is good.. glad to know my setup ( the 4th one) is good.. and I need to check inside the ISO and fix that !

Hi Eric, interesting and informative video; thanks. I see the logic in arranging the items as you do on your bench and indeed i used to have mine configured in the same order but, quite frankly, I couldn't put up with the noise. Since moving house, I now have the space to move the 'lab', such that it is, from a very drafty, cold garage to a nice warm ground floor room in the house. Whilst setting up, I decided I needed to do something about the 'growling' noise emanating from my toroidal iso-transformer: it really is quite offensive and not the sort of thing one would want next to their ear for any extended period!! I it put the first in the chain originally since it is a 2kVA unit and my mechanical engineer's logic suggested it would be better to put the 50VA variable behind it. Anyway, long story short, when I started the new setup I reversed the positions and had the variac first in chain. Lo & behold, the horrendous 'growling' ceased. It does return to a degree when I give it full mains voltage but below about 90-95% full power it is almost unnoticeable now, though I do not have the bulbs in circuit yet. They have always been last in the chain. Any ideas as to what the cause may be? I surfed the 'net' for hours trying to find clues as to what aggitates the toroid enough to make it angry, though without noticeable success. It does seem voltage dependent but I am at a loss with this one. Any advice or suggestions would be much appreciated as I do not like being 'growled' at by 240 of His Majesty's finest volts. Keep up the good work. 👨‍🎓

Hi Eric, i have seen many video's the last 3 years about this subject, because i'm building for almost 3 years (due health issues) a project with a isolation transformer , dim bulb limiter, variac and electronic circuit breaker (own design) in 1 chassis, but this video is one of the best and comprehensive explanation on the different configurations that i have seen so far. From the beginning i had already the last configuration in mind to use in my own project, but now i have seen what effect of the other configurations are. Regarding the isolation transformer itself, did you do some modifications to lower the "ghostvoltage" from the secundary side to ground ? Grtz from Belgium

I would like to add some protection. The lamp switch should be a 2A circuit breaker. Because, in case the switch is on and something bad happens, it is good to have an automatic circuit breaker. 2A is sufficient for many projects. I would also add an RCCB on the primary of the transformer (I choose your last configuration). If something fails in the transformer, I hope not! This RCCB would act before the home RCCB. Makes sense? I would also add a wattmeter, a PF meter, and a needle ammeter at the end, just to take note of the consumption of the radio or amplifier. For me, the needle is the best indicator for debugging these old junk. Nice video, risk is on me, but thanks for sharing.

Thanks!

Excellent explanation, thank you! I have been trying to find a definitive answer on this issue for a long time. I had arrived at the third set up myself, partly because I wanted to ground my variac, so it had to go before the isolation transformer, but I now believe your fourth set up is better. BTW, wouldn't the 'bulb always on' problem also apply to the first set up when using a low wattage bulb?

Great video. I've always had my Isolation Transformer, Variac, and Dim Bulb in the 2nd configuration you've shown and I guess since I'm using a 300w bulb, I haven't run into the issue it potentially could have. However, you've convinced me to try out the 4th config and will soon when I clear out the work I have going on. One configuration you didn't touch on is: Isolation Transformer > Dim Bulb > Variac. Any thoughts on that config? Thanks for the tips!

Great video. What VA rating would br ideal to work on antique radios using non-polarized cords. Pretty=much limited to 5-tube am radios..

Great Video Thanks! I’m in the process of changing all the old electrolytic’s in my HI-FI pre and power tube amps ( previously working fine) and am a bit anxious about switch on. What still concerns me is if I have a setup like this and say have a capacitor that decides to go faulty, is it still going to go bang all of a sudden even if I bring the voltage up slowly in stages-as there probably is still enough current flow? Also my amp has voltage regulation with 85V reference tubes so I assume the fun might not begin until I have the Variac at 85V and above? Many thanks for any knowlegeable answers 👍 Martin UK.

Thank you for this video! As a noob hobbyist, my primary concern is a) not killing myself and b) not blowing up my oscilloscope. So this is very helpful. One thing I did learn, and correct me if I am wrong, is that using a powered differential probe is safer than using this setup because you can then still use the ground for the Oscilloscope, while isolating the probe connections from the scope, which in theory should allow you to probe a dead short on the test device without electrocuting yourself or blowing up your scope, right? Or is that wrong?

Hi Erik. I've found a problem. When switching this large isolation transformer, I've noticed that sometimes the house circuit breaker trips. It seems that the inrush current plays an important role, especially when the power is turned on at a point other than when the mains is crossing 0V. I've been thinking about connecting an NTC in series and creating some kind of controller to power up the entire system. The controller should manage the NTC's temperature, include a bypass relay after a few seconds, control a contactor for switching on and off based on conditions, a crossing 0V detector, (arduino project)etc... Perhaps there is something already available in the market.

Excellent video ! I'm getting ready to build my setup, just like you did yours. I was thinking of adding a receptacle between the variac and isolation transformer with full line voltage so as not to have to use the variac all the time? I want to add an Amp and Voltmeter after the variac with the bypass switch as you did. My transformer is a beast ! Big Hammond 115v primary 782VA and 115VCT Single phase. I was wondering if I could just bond the ground and neutral together on the primary side, but not ground the chassis ? The primary side has three terminals. H1, H2 & H4. H1 & H4 = 125V from my 125V main. Which is how I'm going to wire it. H4 & H2 = 125V ? H1 & H4 = 242V on the primary side ? Secondary is 125V X1 & X3, which is how I am going to wire it, center tap equals 62.5V each side out of phase. It's an older model. A single phase. I could not find a schematic on it anywhere ! I tested it and everything works fine. I was confused at the 242V on the primary side center terminal? And what would be the effects of running it at 125V line as opposed to the 115V rating? Would ot do or possibly harm the trany ? Just a little more heat ? I don't plan on running anything that long that would saturate the filaments. I'm planning on putting a 6A fuse between the line and the transformer with a 5A fuse between the variac and transformer. Any advice or information you could tell me, much appreciated, thanks again for a great video !

Can you explain how to reference a ground on an oscillscope since the bnc ring is connected to house ground itself. Thanks very much! I will be trouble shooting a NAtional NC183D

would it make sense to have a isolation transfer before and after the variac?

Hi Eric, Any advantage to using this bulb tester on vintage transistor receivers/amplifiers and tape decks.

👍

I’d like to see what the current does on the DUT when you short out the bulb?

ISI DIM BULB TESTER VIDEO…I use the 4th and final version of the setup… I now own an oscilloscope and learning about ISO power. Question…does adding a GFCI or AFCI “after” the setup, add any safety? I can’t find any video demonstrating why the GFCI won’t work. Testers give false reading because most of us lift ground. The tester can’t send current to ground…so it won’t test correctly. What am I missing…or is the GFCI a redundant safety feature not needed? THANKS

Your setup do not have a Residual Current Device (RCD) after the isolation side to mitigate potentially serious electrical shock to an operator. This can happen when testing DUT, the operator connects one of the terminals after the isolation to Ground (for some reasons) and then accidentally touch the other terminal. The most probable situation is that you connect GND lead of an oscilloscope to the DUT; and even if you connect light bulb load in series with the DUT, you may still get a serious shock if you accidentally touch any point that is live in the DUT. Perhaps adding MCB after the isolation is a good idea to provide extra protection in case of short circuit or an overload especially when the light bulb load is accidentally removed or intentionally bypassed from the test circuit.

For the 3rd configuration (variac-transformer-bulb). If it is CT transformer on secondary ... than i should use 2 bulb, one for positif and one for negatif, am i right?

a question: how you source your oscilloscope, RF generator, AC voltmeter, etc? with or without ground?.

another question. Which is the best illumination? I always have filament bulb at 230VAC, but perhaps now are other options. The noise of the new LED bulbs is crazy...

Good luck finding a suitable incandescent bulb. I was told they aren't being made anymore.

Do microwave ovens have a suitable tranformer to use as an isolation transformer? Cheers PeaceFromOz

Logically it's obvious, but "for science" it would have been informative if, during the setup with the lamp in the middle, you had shown the voltage level on the Input of the isolating transformer also.

TLDR: Isolation Transformer > Variac > Light bulb

I have a nice iso transformer but no variac. I have been looking at a vac SC-10T from ebay that is a isolated variac but have not bought it because it seems everybody has seperate devices. I would like to know if the advatages and disadvantages of using a seperate variac and transformer vs a "all in one". I have a small bench so that would be one advantage for the SC-10T.

Someone please help me out here. I totally understand the dim bulb and the variac. They make total sense to me. Please help me understand the isolation transformer though. Now, I totally understand step-up, and step-down transformers, and so 1-to-1 also makes sense. I understand that the secondary side has to be COMPLETELY ISOLATED from the primary side; no connections between neutral, ground, etc. What I DO NOT understand, and what I can’t seem to wrap my head around is how this isolation makes me, my devices being serviced, or my tools (oscilloscopes for example) ANY SAFER… 120 volts is 120 volts, right? The secondary has 120 volts on it too, just like the primary, and if I grab one leg of that 120v with one hand, and the other leg of that 120v with the other hand, I AM going to shoot 120v through my heart, whether I am on the primary side, or the secondary side of the transformer. The secondary side isn’t magically turned into human-resistant voltage. It’s STILL DEADLY, so SOMEONE PLEASE explain to me how the isolation does any good. Ok, so I’m no longer connected to the breaker box neutral and earth ground (why can’t a 3-prong adapter/ground lift adapter plug do the same thing?), but I’m STILL dealing with that 120v on the secondary, so how am I any safer? This has boggled my mind for a while, and I know it shouldn’t…. but… it does… Thanks in advance for any insight…

so I assume your secondary stage of the iso transformer has no ground reference , therefore the variac will have no ground reference and neither will the dim bulb and device under test. so besides placing two ground probes on the dut which can cause voltage potential if not in the same place, are their any other vunerabiltys , can yo be shocked by touching the metal casing of the Variac as it is not grounded or something of that sort. thank you.

ERIKS, Why is the variac meter measuring "In rush current spikes", is the variac transformer or isolation transformer causing the In rush current spike? Transformers shouldn't cause in rush current spikes. The Test Set up Config#2 with the Current Limiting Light Bulb in the middle will light up at very low line voltage which means that the isolation transformers primary has "leakage current"?

Nice Chanel

Having the variac feeding the IT will cause problems . At low variac voltages the primary of the IT will struggle to generate secondary voltage due to back EMF. Could you do a vid on the difference between a tech IT and a medical IT.

Is there a simple analog soft start circuit?😅 plan to use for angle grinder