Yes, we are all waiting for the Vedolyzer sequel like Christmas. Therefore I have been forced to watch reruns of all MrCLab videos. And you learn new things every time. Thanks and wish you lots of energy for the darkest weeks of the year.

I built one kind of like that in my lab, I do have all connections covered . I used a variac, isolation transformer and a three bulb vanity fixture, so I can unscrew bulbs to lower the current, now after watching this I may revamp mine a little. great video!

I have watched so many of your videos! always enjoyable and I always learn. I saw this unit in 1 of your recent videos and was very curios. Today I searched and found this video. Great stuff! I have been Interested in ham radio for some time. (that's how I found your channel) You make things that seemed scary , understandable which makes them a lot less scary or intimidating. This seems much better than any "isolating" transformers that are readily available. Thank you for sharing your seemingly endless knowledge.

Dear Mr. Carlson, As I continue focusing on repairing that tube radio, I remember seeing this video, and, realizing how serious it is to take safety precautions when testing such a radio, I decided to build a dim bulb tester of my own. So, I went to an electronics surplus store and managed to find in stock a 120V/100VA isolation transformer (with power and female cord attached) and a 5A variac which happens to not come with any housing, and so I decided to build one out of scrap wood and mount that and the whole system on a scrap board base. And I also made a three-inch wooden handle with eight finger slots and a chicken head pointer which I glued on top of the handle and mounted it on the arbor of the variac. I also bought a 3-prong to 3-hole extension cord, a light switch, two sockets, two 150W light bulbs, and some electrical boxes. And then I made it in three stages. The first stage is the isolation transformer, of course. The second stage is the variac with the 3-prong end of the extension cord wired to it and another length wired from that to the third stage. The third and final stage is the switch, light bulb, and female end of the extension cord. I set it up so that if the switch is in the off position, current limiting begins as the current is bypassed through the two light bulbs hooked in series so that they do not get too luminescent to balance the current load properly. With the switch turned on, the switch provides a _direct_ path to the female end of the cord. I plugged a small drill into the new device and used it to test the thing and it works!!! :) I also used a voltmeter to test that the voltage goes from 0 to 120V with or without the current limiter system, making sure that no voltage registers if one or the other bulb is unscrewed. The only thing is that light bulbs tend to stop electric drills from spinning. I hope electric drills just require more power than tube radios. If so, then, provided I manage to fix the radio, I can use the new device to test the radio. :) :) :)

Great job! The only suggestion I have would be to replace the light switches with old knife switches to complete the old look.

Thanks for the time and effort you put into these very excellent videos. You're not only a first rate tech, but also a very masterful craftsman. This project is as awesomely steam punk as it is useful. Kudos comrade ! Your work is a thing of beauty.

Mr. Carlson, I recently stumbled across your channel and I have been delighted by your work, wit and ingenuity. I attended a technical school here in the states and studied digital electronics. During the first phase of the courses we learned about LCR, RF, and basic electrical circuits before we dove into logic and processors. That was back 35 years ago and my career path diverged wildly from electronics. Since a young age I experimented with electronics and your channel has renewed my interest and I am currently digging out my old power supply and test meters. Thank you for your very informative and entertaining videos. Best Regards, Gary

Very cool. I have a separate section in my lab that is HV, Variacs, 10KV Hipot, 2KV DC Supply and etc. I like to keep all the HV isolated to it's own circuit and bench. Cool build!

I am thinking of rebuilding the current limiter I built yesterday adding in my variac plus a Power On lamp. Nice job, Sir.

More CRAZY STUFF!!!... Lovin it Mr C, thanks for your time :-)

It does look cool... You have developed the unconscious habits that keep you out of trouble. If I do a version of this, you bet I am using an isolated enclosure! Many thanks!

Nice tool :-) I suddenly get the urge to yell out "it's alive it's alive"

Thanks for sharing this - I really liked the look and the schematic was a huge bonus! Off to the bench 😃 to make mine!

Your is the best I have seen yet! Love your vids. They are very analytical and in depth! I love them👌

Thank you for detailing this for us. I really like the current limiting aspect.

Nicely done, I like the looks of it too.

Very nice use of the lamps for current limiting - thanks for sharing Cheers Jim

amazing tool, you inspired me to build one for my own :) .... I suggest replace the on-off switch with a relay, making a 'set-reset' to bypass the lamps instead of the key, so you will never need to remember to turn of the lamp bypass switch as it will be disconnected every time you turn off the AC, putting the lamps in series...

I really like your design. Thank you

Mr carlson. The more I watch your videos. The more I understand. Your level of intelligence. I am only 32 years old. But wow. It's people like you who keep the world working. I work as a machinist. But have a passion for electronics. It's hard for me to understand stuff at times. When I can't see it. But you have a great way of explaining what you are doing. It's really sad how trades people who know what they are doing can't teach in a public school system. Because they don't have a masters degree here in the us. The us is just making every one stupid. And they think every one is going to push a pen around.

Nice video - thanks for the work!

Mr. Carlton’s understanding even shines through with this simple but practical test set up. It reminds me of things as seen in 1950s era popular science magazines or QST’s varnish 2X4 antenna projects. Perhaps there is an influence derived from the first Westinghouse generating station at Niagara Falls. Large knife switches meters and pilot lights, and if you’re stupid enough to put your hand on open switch or terminal you deserve a shock. (No disparity meant to lawyers) but why are instruction manuals written with 1 page of technical information and 10 pages of warnings like don't use your hairdryer in the bathtub. Thank you Mr. Carlson keep up the good work

Great job on this thanks for producing it.

Mr Carlson, thanks for this explanation and schematic. 😎

thank you paul''''i really like how you cover & recover to make sure we get it'''''that is very nice of you'''''its great and your a great'''''''class act''''

Steam punk cool! One thing I would change is to turn the dual gang box 90 degrees so the outlets are up and down. Maybe omit the 220 Volt trans. unless your in Europe! Keep it safe. Cover the business below the top bulbs with a clear acrylic plastic. Awesome Paul!

Great tool. Simon Spiers channel just posted a 6-minute example of how Mr Carlson's dim bulb tester can save heart-ache (and expensive output transistors) when playing around with output bias circuits.

Fantastic as always!

Paul, thank you for this video, very informative ..

This is very cool and useful! I'm gonna have to build something like this myself! Across what wires or terminals do you connect the capacitors, as you didn't show that in the schematic?? I couldn't quite tell exactly when you showed the demo of the caps dimming the light bulbs out, due to the lighting in that room you were in not being quite bright enough to clearly see what terminals you placed the caps across... Does it even matter at all, or do the caps have to go across only specific terminals of that transformer in order for them to actually help?

Just thought I would make a quick comment on my own experience with shorting out mains wires. I think I was going for a Darwin award at the time. One day I decided to short out two mains wires (240v, touching active to earth) to see what would happen and to test the safety systems. It was more violent than I was expecting, a loud bang, big flash and then the house plunged into darkness. The safety systems worked fine, but I wasn't expecting 5 circuit breakers to trip! Lesson learnt. :)

Very nice. I built a much simpler version of this that serves my purposes. I just have one bulb i can change of necessary. One suggestion i would make is to have a double pole switch for complete isolation on your primary so you don't have to unplug the DUT every time you go to solder or move things. 😊

Great tool! I have to build one! THX

Hi Paul. Is there any particular reason to have the variac first and the Olsen after it? Here in Scandinavia the plugs are not polarized and a variac becomes even more unsafe. Therefore I have used my variac after the isolation transformer (which is of course just 1:1, no extra windings).

Hi Paul, Two comments that others eluded to but didn’t get clear answers to: Why is the auto transformer before the isolation transformer? Only to allow variable control over all the isolated “secondary” windings at one time. Otherwise, isolation is isolation. If your isolation TX has only 2 windings like mine, put the Variac on either side. Trickier issue, is the fusing of the Variac, eluded to by others. Paul has fused the primary at 8 amps. Most integrated variable auto transformers fuse the output (wiper), as does my Staco 3PN1010. Auto transformer operation takes some thought. In normal 2 winding isolated TX all of the secondary power (VA) is carried by transformer action. Not so in an auto transformer. Some is carried by transformer action and some is carried directly through the unisolated shared upper winding. The ratio depends on the wiper setting. When at the top, obviously the secondary is directly connected to the primary main terminal. Let’s look at the wiper at half way. Primary turns Np = 2 * Ns (sec turns) so ratio Np/Ns = a = 2. By auto transformer action, primary to secondary current transformation is Ip/Is = 1/a, or Is = Ip * 2. So if Paul has his Variac primary fused at 8 amps, and set to 50% and the light limiters are switched out, the wiper at 50 % will carry 16 amps before the fuse blows. It better be rated for that. and it will be higher at lower settings. I don’t know his Variac model or capacity or if the wiper is internally fused. But where ever you put a Variac, always fuse the wiper at around the rated VA on the name plate. Paul is a smart, articulate guy, so I know he knows this. But do you?

I finally found it, thank you.

Another way to look at he "matching" effect is that a transformer with no load is still an inductor and so will draw current. This current lags the voltage by 90 degrees of phase and so consumes no power of itself. However, any resistance in the primary circuit will consume some power, including the light bulbs.The added capacitors also draw a reactive current which *leads* the voltage by 90 degrees, so the inductive and capacitive currents are 180 degrees out of phase and cancel each other out. This nets out to no current on in the input (variac and bulbs) so the bulbs do not light.There is still a resonant current flowing between the capacitors and transformer primary so the transformer itself will still dissipate the same amount of power as heat in its primary winding due to the winding's resistance. The ideal capacitor value will indeed be the one that, with the inductance of the transformer primary, forms a circuit that resonates at the supply frequency of 60 Hz.

i have seen a whoollle bunch of mr carleson videos where ya mention this power supply and i finally got to see it. i was always curious about what ya were referring to.

Neat project. Although it's not something I do frequently but when I power up an old radio of unknown status I usually connect it with a variac. I haven't done any current limiting method other than a fuse. Some series lamps is a good idea. I have a 10A variac but it has a convenient fuse holder and I can put in whatever I like. If I know that unit shouldn't draw more than 2A for example I can put in a fuse around that size, bring the voltage up and see what happens. Old vacuum tube radios were built back in the 110V mains days. You take a radio that hasn't been powered on for 30 years and bang it on with 120V things tend to blow up.

Variacs have always fascinated me.

Hi Mr Carlson I have all my components bar the isolation transformer for my build. I'm going to build my current limiting supply panel in the same way as yours as I'm comfortable dealing with mains (240v here in the UK) but I was wondering whether you have any suggestions when working with higher voltages such as, perhaps, plate voltages say going up to 600v in tube amplifiers? I imagine there could be the possibility of arcs flashing over things like ceramic sockets which may have been contaminated with carbon dust for example. Thanks for sharing your layout for the power supply and the great video.

After watching this again I am curious about the isolation transformer connections. After double checking it appears you are using a step down transformer in reverse. The H numbered terminals are the primary taps while the X numbered terminals are the secondary taps. Could the mismatch be caused by the connection between the Variac and the X taps? This is not strictly academic for me as I recently acquired a used GE 1.5 KVA dry type isolation transformer that i want to use for my own "Dim Bulb" set up.

Mr. C, I've got all the components to make one and I've been procrastinating. Will also have 240 output to drive some industrial electtonics I repair.

Hi Mr. C. I have a small question. When you wire your current-limiting lightbulbs in series before the variac, won't you increase the output current-limit when you turn the output voltage down? Is this intended?

Cool! I like that Frankenstien look.

Hi Mr Carlson. Your videos are excellent! I completed an Associate Diploma over 20 years ago. Plus I’ve had a couple of years worth of some tech bench experience a number of years ago. You’re videos have inspired me to really get back into this field of work. Thank you. I have a few questions. My country’s power is 240V. I want to repair SMPS for pro Audio equipment and tube/Transistor amps. I understand the current from our mains here would be approximately half of your 120V is the US for the same amount of output power to the device. I’m not 100% sure if what current sharing globes to use. I’m assuming to match 1 amp I would simply use 2 x 120Watt globes in series. Some guitar amps draw 2 or even 3 amps. More for big 1000W pro Audio power amps. Thanks heaps in advance.

can the isolation transformer go before the variac... or does it have to go after it?

Great video, using the variac would this not cause the under/over voltage protection to kick in? Many thanks.

I have a Variac brand autotransformer, a topaz isolation transformer and I’m going to build the light bulb current limiter device this weekend. Is there a difference on where the isolation transformer is placed in the chain? Mr. Carlson has it placed last in this video, but could one place it at the front of the chain - that is, at the wall outlet - and then the variac, light bulbs, and DUT last?

Paul. Why do you put the variac on the line side and the isolation transformer downstream? Why not the other way around? Is there a reason to choose one way or the other?

I was wondering, if those bulbs glow slightly all the time, isn't that good? The filaments are already a bit hot and so you should avoid a sudden inrush of current that can happen when the lamp filaments are cold, right?

I was planning to put my variac and current limiting bulbs, on the output (secondary) side of my transformer, then I saw this video. If this was a normal 1:1 isolation transformer, would there be any benefit to which side (input / output) of the transformer you place the variac?

Hi Mr. Carlson. Channel subscriber here from the Philippines. You talked about improving the efficiency of the variac to isolation transformer coupling by adding a capacitor in parallel. How exactly does this work? By adding a capacitor to the transformer, are you resonating it (LC) to 50hz? Thanks

What voltage do you usually start your vacuum tube radio tests at with this setup?

If the bulbs are in parallel if one burns, will the other one take the blunt as well but harsher ?

I'm sourcing parts myself for a build like this, and an enclosed unit was always my goal. I found a tordoidal 1000VA 220-220V isolation transformer so I will need some kind of soft start or it will trip my breakers. And while I am at it, a bridge rectifier behind the variac too so I can get DC of different voltages. I am in 220V land in europe and big bulbs or lightbuls in general are getting hard to find, and with our voltage we need 2x larger ones so they become ungainly. I was instead thinking of using R7S halogen bulbs instead, a 500W one would give about two amps, and a 250 would give one amp, a 100W half an amp and so on...

Great video in any event

Very helpful video with the best explanation I've seen for the purposes and relationship between the bulbs, variac and isolation transformer. If you were building this solely for 120 volts would you keep the bulbs at the same location in the circuit, or would there be any reason to move them to the secondary side of either the variac or isolation transformer?

Hi if i had a home made dim bulb tester and shop brought isolation transformer and Variac unit, in what order should i have the 3 items thanks

Hey, lets say: if you rectify the current, you can also limit current with out any problems?

Is touching any of the wires after the insulation transformer going to electrocute you, since there is insulation between the grid and the potential of the wires?

Excellent !

Hi, just for interest, as it is impossible to obtain incandescent light bulbs these days, can the light bulbs be replaced with current limiting resistors?

Would it be better to put the fuse before the main power switch?

I just designed my unit and came here to see how I did in relation to Mr Carlson's design... I put the bulbs and by pass switch on the output of the isolation transformer, I wired in the Variac output to the isolation transformer primary and it's secondary to the bulbs and on to the load.. appreciate any thoughts.. Did I miss something?

Hi Mr. Carlson, always looking forward to learning from your videos. I was wondering if the variac could be substituted with a rheostat set up as a voltage divider? I have a 500ohm 250W toroidal rheostat which I'd like to use, but I'm having second thoughts wether such a low-ohm device would just present a constant load and send the whole thing straight to brightly current limit. Any help/comment will be greatly appreciated.

"I am very very careful" . A common inscription on gravestones. I have worked on commercial inverters for communications in the 250 KV genre. That will peak your awareness of your surroundings!

Would you call that a power factor correction with the capacitors?

I recently made one of these with two transformers back to back 120/24 volt This created the isolation. Mine is wired much like yours power switch fuse auto transformer two parallel series lamps then the transformer. The circuit puts out 135 volts. If I put a 1.3 amp 160 watt load the 135 volts drops down to almost 80 volts. The auto transformer and the two step down transformers are rated for 4 amps I’m assuming this is due to the voltage drop created by the light bulbs?

You have a very nice lab. It almost appears like your searching for the elusive proton and Tesla.

Hello, I have questions. With your 2 bulb system you said it limits to about 1 amp. How would this be useful to see if a larger device that works normally at say 4 amps? A higher current device would light your bulbs bright every time making a person think there was a dead short when there wasn’t. How would a person go about calculating what wattage bulb to use to allow a current limit to just a bit below that of say a mainline power fuse wired to a power transformers primary? In other words calculate a wattage of bulb to only limit or light brightly at just under main power fuse blow point with say the device under test using either a delay blow, or norm blow type of fuses. Or would a better approach be to note from device label the input wattage of the device under test, under its normal stated operating voltage, and then somehow select a proper wattage bulb that just starts to glow dimly at this point? Thanks for any insight you can provide.

Hi Paul... Very nice design! It looks really cool too, although a bit scary with the exposed circuitry. I restore vintage radios as a hobby and built a small one for testing them. It's only a five amp system and I designed it a little differently than yours, whereas I have the dim bulb on the output side of the variac rather than on the line the way you have it. Although it appears to work fine, I am wondering if my design is in fact safe? if you are interested in seeing it, I could upload the schematic. Your evaluation would be much appreciated.

So I know this video is quite old, but I have been trying to find a variac in that compact formfactor but best I can locate is Staco brand and those things are stupidly expensive. Where can I find a small variac such as that?

Hey, I have a question. What's the difference between a 1000w variac and a 2000w variac? Like is there a slight variation to it's build other than it's internal fuse? Curious to know as I'll be getting one soon.

My question is where did you get those neat bulbs? I'm presently building mine. But frost bulbs just don't cut the mustard.

Hi, first thanks for providing such a great resource to all of us. My question is that I currently have a variac and a separate isolation transformer (2 units). I would like to install the current limiter (big dim bulb tester) to that system. I'm proposing to add the two light bulbs in series with the variac about line and then plug that into the isolation transformer (with an on/off switch) Does that sound correct?

also, what's a good source for that transformer?

A very useful device to have, and a very pretty build, too! (Though I think as a practical point I would have ended up putting a bit of tape or phenolic over the bare primary screws. You might be amazed how easy it is to reach blindly for the knob while staring in a scope and miss your target by a couple of inches. Guess how I know...) BTW, that is very similar to the design of most tube testers from the 1940s and 1950s. A nice little 6.3V lamp labeled "Fuse" in the primary line.

Nice vid. Thank you for taking the effort to show us. What mounting do the bulb sockets have? Couldn't quite make out that detail. Rt-angle bracket screwed to the board? And the curly filament bulbs--did you source new ones somewhere? Or from the junque box? Cheers.

Hi - I use a Sencore PR57 isolation Transformer that I plug my variac into. What do you think of that procedure?? Thanks, John

Am I mistaken? Are the light bulbs (the current limiter) on the primary side of the transformer? Why not have the light bulbs on the secondary side (the isolated circuit/test side)?

You could use a clear acrylic box to cover the assembly if you want the cool look but don't want the chance of an uncool e-shock from accidentally touching the live contacts. Make some cut-outs for the variac knob, switches, outlets and bulbs.

So the 1-1 ratio windings which take us from 120V to 120V isolated, those windings are physically on the same side of the transformer, sort of stacked on top of each other? I always thought a transformer works by having the windings opposite each other, or sort of facing each other.

My Question for you is, what would be a good watt size for a isolation setup ?

What did you tie the earth ground on the equipment side to?

Mr. C., I have a question about your fuse placement. I have always placed the fuse/circuit breaker before the on/off switch. I noticed in all of the designs you have shared, you seem to consistently place the fuse after the power switch leaving the switch energized in the event the fuse blows. What is your view or reasoning for this? I am genuinely curious as I continue to work my way through your many and extremely informative/insightful videos.

I would like to know on the schematic,where you place the capacitors?

It's my understanding that incadescent bulbs in the US may be become unobtanium soon. If I wanted to build my bench test set today (iso xfmr, variac, "dim bulb limiter") without worry of finding replacement light bulbs, what would you recommend to replace the dim bulb current limiter?

A "must have" on any bench. Almost 40 years ago in my electrical engineering classes, we always used 1:1 iso-xformers when working on AA5 radios and similar to avoid a hot chassis. But recently, I read someone using a couple of 2A step-down xformers wired: 120V||24V => 24V||120V as a "cobbed" iso-xformer. Theoretically, it might work. I'm a bit suspect of the safety of such a set up - current draw on the 2nd xformer; might be dangerous. Agree?

I would recommend you put a fuse on the wiper of the Variac to protect the Variac from over current at lower settings example if you set 1/4 voltage you can draw 12 amps on the output wile only pulling 3 amps on the input it will not like that I damaged one with a 2 amp supply fuse I was using it to get 12 volts I learnt right there to fuse and current limit the output of all of my supply's

Just curious because I don't understand the intricacies of the circuitry, but I have a question. Capacitors (even non-polarized capacitors) in an AC circuit - I guess I've always thought of them in use in a DC circuit. Can you shed any light on this or what am I missing? Thank you for your very informative and well thought-out videos!

I don't know if I missed it but what is the wattage rating on the bulbs. Is there a reason that you used two?

Greetings, I'm interested in purchasing an isolation transformer. What advice can you give me? I see some on ebay that are literally a transformer with a plug in cord on the back of the transformer and the front has the socket connection for a tube radio. I also see isolated transformer what are enclosed with an on/off switch. Do I take in consideration the amperage limitation of an isolation transformer? I work on All American Five radios as well as 10 tube radio consoles. I do have a variac and I realize I need am iso-transformer.

I should know the answer to this but at the moment I'm being a bit lazy. If you connected the two 240v windings in parallel (in phase lol) first most obviously you would dispense with the 480v hazard but how if at all would that effect the power factor compensation that you demonstrated with the capacitors? You might find that standard cap values may get you closer to the ideal config. Or the reverse. Worth a try? It may even serve as a place to actually do your PF comp. Feel free to shoot me down.

Thankyou for another informative video. Some Plexiglas over those exposed terminals would allow people to see the workings and it would isolate any one else from getting Thier fingers in there. Can u tell me the difference between a variac and say using something like a dimmer switch. Just starting to learn this amazing field. Thankyou

I like the "olde world" look as well. Some quadrant style volt/ammeters would really set it off! That's my era ! Ha ha.If it's not too,personal, what is your work please? The HV sounds interesting. Again, many thanks for another interesting and practical video. Saludos

Paul, I am also designing an AC power supply for troubleshooting (120VAC only). I thought perhaps you could comment on equipment AC leakage current tests as part of the repair process (Final outgoing test). Take a look at the B&K 1655A user manual for leakage tests, as proposed by B&K. For 120VAC three prong grounded plug, I plan on using a GFCI self testing receptacle as my first screening device (5mA). Also of note, is in the possible need for low pass filters when testing for leakage on switching supplies.

do you have a schematic for this? I would like to build a safer and more compact version of this.

Did you use different shape sockets for the 120V and 240V plugs?

Wouldn't it be more correct to call this a wattage limiting device instead of current? And I'm not talking about the variac, I mean that the bulbs will absorb the current of a faulty device but they also consume the voltage equally when loaded in series right? So more of a wattage limiter equal to the sum of the two bulbs ie two 60 watt bulbs would be 120 watt limiting right?