Questions in regard to design and build of a dim bulb tester with Autotransforme - Page 1
Forum section: electronics / projects

author: Geronimo
author_rank: Contributor
timestamp: December 23, 2020, 12:32:22 am
content: Hello to the community,I am planning to build a so called dim bulb tester with two coupled transformers (of the same type/size) for isolation, followed by a variable transformer for voltage setting.The two transformers are secondarily coupled so that at the end of the chain the full line voltage (except for a 1-2 voltage loss according to my meassurement) is generated again.So basically it is a poor mans 1:1 isolation transformer. The variable transformer allows the voltage to be set/limited between a few volts up to full line voltage level.The question is, where should I put the 5 intended incandescent light bulbs?I plan to use two 40 and three 60 watt incandescent lamps so that I get a maximum of 260 watts or around 1.1 amps.The incandescent lamps normally run on full mains voltage.If I put the lamps in front of the variable transformer, they will always be at around full mains voltage level, if I put them behind the variable transformer they might only get a few volts and,as I understand it, would be a much higher limitation of the maximum current with unchanged power.Hence the question: Should I put the incandescent lamps at the input of the variable transformer or better behind where the voltage is regulated down if necessary?Another question:Assuming I put a suitable incandescent lamp between the two transformers that I use for isolation, would this have the same effect (assuming I use lamps of the same wattage at the right voltage (9V or 18V))? I think so, don't I?The second transformer would be limited in its maximum power and thus the entire structure behind it. Am I wrong with this assumption?Last question:The two transformers for isolation have a center tap (each 0V <-> 9V <-> 18V with a maximum of 18.15A in total).So far I have been connecting the outer ends of the secondary winding (0V <-> 18V) assuming that it would not be good to also connect the center taps to one another at the same time.Is that so and would the transformers be damaged if I also permanently connect the center taps?Assuming the windings of the two transformer are nearly the same and wound the same way; does the coil of the secondary winding benefit (e.g. split the load) or will those be harmed if I connected the center tap too?I would be happy if someone who has (better/more) experience with this matter gives me a tip or two.I'd also be happy to read counter-proposals or additions to the little project described.For example, I'm still not sure whether I need any noise/line filter due to the lot of inductance or maybe should put several fuses at the line input and output for safety.The two joint transformers for isolation and the variable transformer (Autotransformer/Variac) are working as intended so far.Anyhow I'd like to get it done right as possible.Cheers,Geronimo

author: Benta
author_rank: Super Contributor
timestamp: December 23, 2020, 12:50:33 am
content: Leave the center taps alone. Your connections are correct as they are.One thing you don't seem to have inyour calculations: 230 V bulbs have a much lower resistance when running "cold", so your power budget might not fit. They're not just resistors.

author: ledtester
author_rank: Super Contributor
timestamp: December 23, 2020, 12:50:57 am
content: QuoteA dim bulb tester can be viewed as an independent mains device -- i.e. it is still useful if you run it off of mains.Consider building a "variable AC power supply" with your isolation transformer and variac. The topology would be:mains input --- variac --(+)-- iso. transformer --(+)where (+) means there is a mains outlet. This allows you to produce variable mains voltage as well as a variable isolated output. You can plug your dim bulb tester into either (+) outlet as needed.For ideas here's the Heathkit variable AC power supply:http://www.byan-roper.org/steve/steve-at-play/antique-electronics-and-2/heathkit-ip-5220-variable.html

author: Geronimo
author_rank: Contributor
timestamp: December 23, 2020, 01:24:05 am
content: Thanks for your comments a suggestions, those are really of good use for me.Maybe I'm trying to build to much "at once" and within one caseIm following the idea of M Caldeira who has a nice build of the dim bulb tester on his bench but would also like to get the Variac included.I'm basically using stuff which I have lying around, maybe with some additions like digital voltage and amp meters.The idea of having the two different outlets is actually good, I'll see if I can get this included in my design.My intention however is to have the dim bulbs within the variable power supply and isolation transformer "all-in-one".Puting the Variac in front of the isolation transformer sounds like a good idea. Now I need to understand what this design will cause to the isolation tranformers.Does it matter to provide them with lower voltage and will they deliver the same power in case?So far I've been thinking of isolation first but it should work for me the other way around as well.I understood the center tap shall not be used, I'll leave it as is and will only use the outer ends of the secondary windings.I plan to have a bypass switch for the dim bulbs and will jointly use them as appropriate and according to the needs of the device under test.It happen to me that I have blown a full set of fuses and transitors at the power stage a few timeswhilst working on the repair of an old (70s) amplifier.As possible, next time I'd like to know in advance if the device is drawing more current than intended. Thus it would save me from having to order new parts again and again.Within the next days I will provide more details and some pictures of the final build.So far thanks for your support, I really appreciate your ideas and suggestions.Cheers,Geronimo

author: trobbins
author_rank: Frequent Contributor
timestamp: December 23, 2020, 02:09:23 am
content: I'd recommend repairing transistor amps with a variac only, as it allows faulty parts to be identified and a good circuit's operation to be set up for bias and tested in the lowest stress manner. A light bulb tester may save parts from damage but is still a stressful means of testing.You may need to learn more about isolation techniques, and if you need an isolation transformer, for testing purposes, as often an isolation transformer is not needed, and it may divert your mind to thinking all is now safe, whereas there are other safety methods to deploy before even contemplating an isolation transformer.A light bulb tester aims to let normal operating current through, but to constrain current if it gets higher than wanted (due to a fault), plus it provides some 'soft-start' action. Not all equipment operates at the same normal current level, so any test set up needs to be able to easily change the combined 'wattage' of bulbs being used in series with the DUT - either by manually changing bulbs or by switching them in/out of circuit.Back-to-back transformers typically don't provide the same output voltage as input voltage. A variac before such an arrangement allows the initial transformer primary voltage to be a bit above mains voltage, such that the output voltage can reach nominal mains if needed.I'd recommend a cheap 'watt meter' be part of any such setup - the modern plug in types measure V, A, VA, W etc, and although not super accurate are a safe way to measure nominal mains voltage. and appreciate what magnetising current is being pulled by a transformer.

author: Geronimo
author_rank: Contributor
timestamp: December 24, 2020, 01:16:50 am
content: Quote from: trobbins on December 23, 2020, 02:09:23 amThus in combination with the recommendation from LEDTESTER and BENTA, I will change my design slightly and have the two outlets and the watt meter in place.The "on" and "off" switching of the corresponding power according to the requirements of the DUT is part of my planning.I regularly buy interesting but broken gears that are mostly on the lower end of power consumption. In addition to the simple test for short circuits using an ohm meter, the use of a variac should allow me to take a first look at such "unknown" devices. Some of the devices lying around here are powered by vacuum tubes such as some FM radios from the 50s or some VTOM. The mentioned build is intended to be used for this type of candidates. In fact I can deal much better with those than with the transitor devices from 70s and above.Thanks a lot for your support, I wish you all a peaceful and mery christmas and a very happy new year.Stay safe and healthy.Cheers,GEronimo