Transformer good or bad? - Page 1
Forum section: electronics / beginners

author: Crambone
author_rank: Frequent Contributor
timestamp: November 17, 2021, 05:58:56 pm
content: First thank you everyone for answering all my previous questions I appreciate it and I’m learning thanks to you all.Now for my Transformer, it’s a Filament and Bias Transformer from a Heathkit SB-220.I’ve included a data sheet for the Dahl replacement and the SB-220 schematic showing the transformer.It’s Heathkit part #54-238. I took the following reading off it while removed.1st Green to Green Yellow. .025 ohms2nd Green to Green Yellow .026 ohmsGreen to Green .003 ohmsBlack Green to Black Red 3.7 ohmsBlack Yellow to Black 3.3 ohmsRed to Red 11.7 ohmsAny assistance would be appreciated.

author: CaptDon
author_rank: Super Contributor
timestamp: November 18, 2021, 03:43:29 am
content: Only you can tell if it is good or bad. Apply power to the primary, does it smoke or make flames, answer no, then it is probably good. Are the voltages on the secondary nearly the same as specified, answer yes, then it is good. Trying to make decisions about transformer health based on ohms is difficult and usually requires a 4-Wire ohmmeter to make accurate measurements. Why do you suspect it is bad?

author: Crambone
author_rank: Frequent Contributor
timestamp: November 18, 2021, 05:04:48 am
content: I used a 4 wire HP meter to measure resistance. I will apply power to the primary’s tomorrow and check my voltages. I was expecting to see higher resistance on the 2 Greens .003 seems very low?

author: bdunham7
author_rank: Super Contributor
timestamp: November 18, 2021, 05:33:24 am
content: Quote from: Crambone on November 18, 2021, 05:04:48 amThat does seem low, the G-G resistance should be the sum of the two G-GY readings if the diagram is right. This is all with the transformer completely disconnected? And there's no continuity between any of the G windings and the core? If your G windings are shorted, it will likely draw way too much primary current when you power it up unloaded.

author: Crambone
author_rank: Frequent Contributor
timestamp: November 18, 2021, 01:31:55 pm
content: Yes I removed the xfmr completely for testing. Later today I’m going to put some voltage to the primary’s and see what happens.

author: rsjsouza
author_rank: Super Contributor
timestamp: November 18, 2021, 01:45:34 pm
content: Make sure you apply power to the primary either through some series incandescent lightbulbs or slowly increasing the current and using a clamp ammeter. This way, if there is a heavy short on the secondary, the lightbulbs will be fully lit or the ammeterś measurement will jump quite steeply. This will save you from resin/varnish smell in your bedroom (and an eventual fuse or circuit breaker trip).That said, I would expect a very low resistance when measuring a 5V @ 30A secondary. Maybe not 3 mΩ, but still very low.

author: Crambone
author_rank: Frequent Contributor
timestamp: November 18, 2021, 05:23:29 pm
content: Guys I added a video below of how I tested I would appreciate your thoughts and knowledge.ANDREWhttps://youtu.be/7mEznPCrk8Y

author: bdunham7
author_rank: Super Contributor
timestamp: November 18, 2021, 05:30:57 pm
content: OK, that explains a bit.First, your very low G-G reading vs the readings from G to GY (the center tap) are now explained--the center tap wire is very thin and that is where the resistance is. So all good there.The black dots indicate the relative polarity of the windings--they will all swing either positive or negative together. This is important because if you put windings in parallel or series, you need to get the polarity right. For parallel you make sure the dots are connected, in series just the opposite, no different than connecting two batteries.The two 120V primaries are intended to be connected in parallel (dots together) for 120V and in series (top-to-bottom on your diagram) for 240. For your open circuit testing, using one primary is sufficient, but when you install it they should be parallel. It looks like your transformer is good.

author: Crambone
author_rank: Frequent Contributor
timestamp: November 18, 2021, 05:52:35 pm
content: OK thank you!Now if you don’t have a data sheet for a transformer how can you identify the primary?

author: bdunham7
author_rank: Super Contributor
timestamp: November 18, 2021, 06:06:45 pm
content: Quote from: Crambone on November 18, 2021, 05:52:35 pmIf you just have a generic transformer with no markings and no context, it can be very difficult. If it has many windings, all the more so. You can 'ohm it out' and then put signals into the windings to see what the winding ratios are, but you still don't know if it is a step-up, step-down, what the insulation values are and at what point it will saturate--which involves frequency and voltage. So if I had a transformer that was designed to step up from 100VAC 400Hz to 350VAC for a DC rectifier circuit--say in an old avionics component using tubes--it would be very difficult to characterize it well enough to determine what its intended use and operating parameters were.If you have some idea of where the transformer came from and what it does, it gets easier, but still laborious to really do correctly. And some primary configurations are really complex where they are designed for multiple voltages.

author: rsjsouza
author_rank: Super Contributor
timestamp: November 19, 2021, 03:36:33 pm
content: Indeed the short clip helps a lot.Just a few additional tidbits:- 50/60Hz power transformers are quite robust devices; they are usually quite tolerant to minor overcurrents and overvoltages, which can be seen by the abuse that old equipment take with regards to shorted outputs (in case of power amplifiers, for example), shorted rectifiers, leaky filter capacitors and such.- Since you have a Variac (and a nice lab, BTW), there is a formal method to calculate the parameters of the transformer such as self inductance, primary and secondary reactance, etc: short circuit test and open circuit test. That is probably described in several places around the internet, but my google-fu can't find much in English apart fromthis paper.- A common rule of thumb to approximate the total power of a single-phase 60Hz power transformer with EI or UI nucleus is squaring the middle area of a section of the core in cm2. A typical 4x5 cm core, for example, is typically used up to 400VA transformers. In my experience, this only brings avery rough ballparkof power, which is better than nothing on a completely unknown transformer.