Safest way to test unkown transformers with wire taps - Page 1
Forum section: electronics / beginners

author: queennikki1972
author_rank: Regular Contributor
timestamp: May 12, 2019, 02:08:15 pm
content: Aside from a suicide cable plugged directly into outlet with gator clips, what is the ideal safest setup for powering on 120vac transformers for the purpose of identifying the secondary voltages of unkonw transformers.? Should I build a box with banana jacks and a switch? Just use a short extension cord with inline switch?This is not for the large transformer I previously asked about.I would of course use gator clips to the multi-meter before powering on rather than loosely probing.This is all of course if I know for a fact its 120vac primary.Was thinking about buying a variac from Amazon and using the suicide cable plugged into the variac transformer..

author: radiolistener
author_rank: Super Contributor
timestamp: May 12, 2019, 02:52:31 pm
content: Use multimeter to measure coil resistance. Maximum resistance coil is primary for high voltage.

author: capt bullshot
author_rank: Super Contributor
timestamp: May 12, 2019, 02:57:29 pm
content: A variac won't isolate your transformer under test from the mains. So no added safety here.Extension cord with switch is my preferred choice, since it's easily available. Just ensure the switch is double-pole (disconnects live and neutral).

author: radiolistener
author_rank: Super Contributor
timestamp: May 12, 2019, 03:01:53 pm
content: Use 1-2 A fuse if you will try to connect it to mains. It helps to avoid big bada bum

author: queennikki1972
author_rank: Regular Contributor
timestamp: May 12, 2019, 03:26:06 pm
content: So Ill get a plastic outlet box and plastic cover, install a DPST switch with rubber boot and a panel mount fuse 1-2 amp and add alligator clips on the hot end with some shielding around the gator clips. Maybe even throw in a AC powered red light. Gives me something to do.

author: bob91343
author_rank: Super Contributor
timestamp: May 12, 2019, 04:21:01 pm
content: You weren't clear about what you are trying to learn about the transformer. If you need to identify the primary connections it might be wise to measure the inductance of each winding before applying power. If the transformer is a step up transformer for instance, the highest resistance winding probably isn't the primary.I like to measure the winding resistances and draw a diagram showing each of them. The weight of the transformer will be a good guide as to how much power it can handle. I assume each winding has about the same efficiency and so the voltage and winding resistances will go hand in hand. If it has several secondaries, then the primary resistance will be lower due to the combined loads.When you finally apply power to what you think is the primary, measure the current while increasing the voltage from a variac. If the current starts to increase rapidly, you are applying excssive voltage and likely are not energizing the primary winding.Years ago I used some high voltage transformers in reverse, by energizing the secondary, to use it as a source for low voltage needs. It worked well. Still, I had to respect the current rating so as to not overstress the windings.

author: bdunham7
author_rank: Super Contributor
timestamp: May 12, 2019, 04:52:10 pm
content: First, if you are going to do this sort of thing, you should at least have SOME idea what the transformer characteristics are--you don't want to find out the hard way that your transformer is from a neon sign and has 18,000 VAC output. Since you mention being sure of which is the 120VAC primary, I'll assume you are playing with reasonable transformers.Next, if you can, buy one of these:https://www.ebay.com/itm/BK-Precision-1655A-AC-Power-Supply-Variable-120v-600VA-with-leakage-probe-USED/254178053357?epid=1106286010&hash=item3b2e314ced:g:NYYAAOSwMoZcmh-cOr its smaller cousin the BK 1653, or something similar.Last, you need to learn about transformer characteristics to determine what you have. Transformer characteristics are actually quite complicated and go far beyond winding count and thermally-limited maximum current. For example, you have a characteristic known as "Z", which is related to output impedance and can be measured using the above mentioned variac, shorting the output with an AC ammeter and then observing how much input voltage it takes to drive the ammeter to the transformers rated output current. This tells you how much the voltage will sag under load. You also have primary-secondary leakage, which can be measured directly with the the 1655. And so on--there's a lot to it. But to start, a fused, protected iso-variac like the ones I've shown here are an important first step in doing this without human or transformer casualties.

author: bsdphk
author_rank: Regular Contributor
timestamp: May 12, 2019, 05:53:10 pm
content: Locate the winding with the highest DC resistance.Power that winding with low voltage AC from another transformer, a few watt should be enough.Measure the AC voltages on all the other windings.Calculate the winding ratios.Use that to deduce likely primary and secondary voltages.If the primary consists of two windings (for 200+V serial 100+V parallel), you can find the right connection using the same basic method: If the secondary voltages are all zero, you need to swap the wires on one of the primaries.

author: MarkF
author_rank: Super Contributor
timestamp: May 12, 2019, 05:59:11 pm
content: Remember the series light bulb we talked about before.If the light stays OFF, you are okay. If the light comes ON, you have a short.The bulb wattage will determine the current limit.This assumes that the device-under-test can handle the current.You are still at some risk but not near as much.Assuming you're testing a 120VAC circuit, you need a 120VAC light buib.TheQuote from: bsdphk on May 12, 2019, 05:53:10 pmThe winding with the highest resistance may NOT be the primary.I have an old transformer that I believe came from a TV. One of the secondary windings is 150VAC.

author: rstofer
author_rank: Super Contributor
timestamp: May 12, 2019, 06:13:01 pm
content: Maybe the leads follow the color code 'standard':http://www.rfcafe.com/references/electrical/xfr-lead-color-code.htm

author: bsdphk
author_rank: Regular Contributor
timestamp: May 12, 2019, 06:23:42 pm
content: "I have an old transformer that I believe came from a TV. One of the secondary windings is 150VAC."Did you overlook "Power that winding with low voltage AC from another transformer, a few watt should be enough." ?When you feed 12VAC and no more than a few watt into the winding with the highest DC resistance, it would have to be a very strange transformer indeed, if any of the other windings can hurt you.

author: queennikki1972
author_rank: Regular Contributor
timestamp: May 12, 2019, 06:44:32 pm
content: I guess I had not thought that far. I have a few small transformers from alarm clocks and some of what i think are 12v and 24 v transformers. I realize now i should be looking at all of the advice above because i never know what size i might run into. I just thought on the very small transformers (LIGHT WEIGHT) it would be as easy as measuring across the secondary to see the output voltage. But you are all right, i first need to know the transformers primary and secondary and so fourth before lighting anything up and be more cautious. I will hold off on testing unknown things until i understand more clearly. I dove straight into building power supplies without even knowing all my symbols yet, lol.Cart before the horse.I am about the order a set of books called encylopedia of electronic components and already have the getting started in electronics by Forrest M Mims III. It would be great if i could find books specifically on transformers.MarKF Im paying attention, I will be making notes as you guys explain.

author: queennikki1972
author_rank: Regular Contributor
timestamp: May 12, 2019, 06:58:59 pm
content: EXAMPLETiny transformer from alarm clock2x red wires primary, I know because i clipped the wire myself.2x black secondary unknown voltage unknown currentred to red 1.479k ohmblack to black 12.7 ohmblack secondary to red primary 11M ohm.Made in china no markings plastic coreWant to know output voltage and possible current

author: bob91343
author_rank: Super Contributor
timestamp: May 12, 2019, 07:19:56 pm
content: Back in college we used a big book on 'magnetic circuits and transformers' to work through some of the characteristics of practical inductive components. (I have a copy.) While I don't recommend this for the layman, I want to give a heads up that these things are anything but simple.First you need to realize that they are not linear. That is, as you increase the voltage applied, the current doesn't follow in proportion. Further, when you raise the input, there is a point at which the core saturates and the incremental inductance drops markedly, causing a rapid increase in current. That's why one needs to respect the ratings. This not only depends on applied voltage but on the frequency of that voltage.The heat generated comes from two main sources - the core and the windings. Heat is the worst enemy, as it can melt copper and cause smoke to come from insulation, etc.So if you are seeking a more profound education, realize that there is a lot to assimilate to appreciate these components.

author: MarkF
author_rank: Super Contributor
timestamp: May 12, 2019, 07:22:04 pm
content: Quote from: queennikki1972 on May 12, 2019, 06:58:59 pmIf you can identify the primary winding, you can just plug it in and measure the secondary voltage.Current is harder. If it doesn't say, all you can do is guess based on transformer size and secondary wire gauge.OrIf you can locate a fuse, you can go with its value as long as it hasn't been replaced with a bogus value.OrWiring diagrams as in you giant transformer where each secondary had a fuse.

author: ptricks
author_rank: Frequent Contributor
timestamp: May 12, 2019, 07:42:50 pm
content: Quote from: MarkF on May 12, 2019, 05:59:11 pmThis !I did tv repair in the 1980's and this was great for troubleshooting power supplies that would blow the fuse often. One could also use resistors but the light bulb (incandescent not LED or any others) was so much easier and you can change the limit so easily with a bulb swap.

author: Jwillis
author_rank: Super Contributor
timestamp: May 12, 2019, 08:49:08 pm
content: Checking the resistance of the winding s won't guaranty the identification of the primary winding s because a step up transformer like a MOT (Micro wave Transformer ),can have over a hundred ohms at the secondary as compared to a fraction of an ohm at the primary.Even Some power transformers with multiple secondary winding s can have one or more taps with higher resistance than the primary.If you know, for a fact, the identification of the primary using the lamp method WILL NOT GUARANTY SAFETY EVER. Because the secondary may have several thousand volts that a multi meter will not handle. DO NOT USE THE LAMP METHOD to identify an unknown secondary voltage EVER!Look at the winding s .If any winding is connected to the core DO NOT connect the core to ground.If your not sure if the winding is connected to the core do not connect core to ground.Draw a diagram of of the transformer and identify the coils with continuity and orientation.Always check for continuity between any winding and the core.Use LOW voltage AC at the primary and then measure the voltage at the secondary. The use of a variac is safe at low voltage.You only need at most 24 VAC but 10VAC is plenty to produce a magnetic field .Turn off the variac . Set Variac at the lowest setting and connect to the primary and connect a volt meter across primary.The markings on the variac may not be accurate.monitor the voltage.Set multi meter to the highest AC setting and connect to secondary you want to identify.Turn on Variac and VERY SLOWLY increase voltage until you get a reading on the multi meter.Very slowly means just the slightest movement.Reading the secondary voltage of a MOT can be as high as several hundred volts even at very low voltages at the primary.Record the voltage on the primary and the voltage on your secondary Volt meter.Divide Mains voltage (120) by the reading on the primary . This number will be your multiplier for the next step .Multiply the reading on the secondary by the multiplier . This is the approximate voltage of the secondary at 120VAC.Again these will be approximate because as you increase the voltage at the primary the field gets stronger and the secondary will become closer to to its turn value.example 1 : If the variac is set at 10 VAC and you read 5 volts at the secondary. 120VAC / 10VAC = 12 . 12 X 5 = 60. So at 120VAC the transformer secondary will be approximately 60VAC.Remember if you get higher voltages at the secondary relative to a low voltage at the primary you may have the secondary identified as a primary.Reverse the transformer and retest.If you get very high voltages relative to the primary .I would advise to shut down and discontinue testing.We can assume that its a step up transformer and further experience is required.You can test high voltage transformers in reverse .But at this point it's best just avoid them until you have more experience.This is what happens to a multimeter connected to an "unknown transformer" using the lamp method.I sacrificed this meter to show what can happen because I knew the transformer had 3000VAC at the secondary.Do not assume the secondary winding will be lower than the primary.Always use very low primary voltages to test unknown transformers.

author: bsdphk
author_rank: Regular Contributor
timestamp: May 13, 2019, 05:32:52 am
content: "Checking the resistance of the winding s won't guaranty the identification of the primary winding"That is not the point really... you use the highest DC resistance winding to minimize the amount of power while you measure the ratios.And I fully agree on not using the "lamp method" until one is 100% sure how to connect the primary/ies for local grid voltage (or higher).

author: stj
author_rank: Super Contributor
timestamp: May 13, 2019, 08:05:49 am
content: it's actually very simple.use the output of a 12v transformer to feed the unit you want to test,then multiply the output by 10 to get the true value.even if you get the primary and secondary mixed up your still going to only see 3digit values and wont kill yourself or damage the meter.and it's isolated.

author: soldar
author_rank: Super Contributor
timestamp: May 13, 2019, 09:16:32 am
content: Quote from: MarkF on May 12, 2019, 05:59:11 pmYou can get a lot of information about a transformer or inductance with very simple tools. And you can learn and practice a lot too which may be more valuable. You do not need variacs or other fancy equipment. Just a multimeter.Take a winding of the transformer and feed it through a lightbulb. Measure the voltage and current at the winding. Now calculate the impedance of the winding. Now measure the DC resistance of the winding. Now calculate the inductance of the winding. Do that for all windings.Now you have a good first order approximation of that transformer and you can calculate currents and voltages for inputs and outputs.

author: David Hess
author_rank: Super Contributor
timestamp: May 13, 2019, 04:10:27 pm
content: I start by using a function generator to produce a 60 Hz sine wave for excitation so I can measure the transformer's turn ratios using an AC voltmeter. If necessary, an oscilloscope can be used to measure the phasing although it is possible to do this with an AC voltmeter if you are clever.Then I use calipers to measure the wire sizes which gives some idea of the current rating for each winding.

author: bob91343
author_rank: Super Contributor
timestamp: May 13, 2019, 05:05:25 pm
content: Yes the current rating can be deduced from wire gauge if you can measure it. Figure perhaps 400 to 500 circular mils per Ampere. (A circular mil is the square of the diameter of a round wire, measured in thousandths of an inch.) So a 0.001 inch diameter wire can safely handle something around a half Ampere. Transformers that run hot will run on the low side, maybe even as low as 300 CM/A and conservative units maybe 700. These are steady state ratings; for units intended for low duty cycle you can push it. Of course a limiting factor is winding resistance.In cases of very low rated currents, the wire size may be larger due to cost considerations involved with winding very small wire.The total power rating of a transformer can be deduced from its weight; look in transformer catalogs for other units of similar construction and frequency and see what comes close. A transformer with many secondaries will give less power for a given weight due to insulation space overhead, etc. High end (say, military or medical applications) will be more conservative. Cheap commercial stuff the opposite.

author: rstofer
author_rank: Super Contributor
timestamp: May 13, 2019, 07:40:35 pm
content: Quote from: David Hess on May 13, 2019, 04:10:27 pmOr it can be done with a 'kick test'. Section 7.2https://nptel.ac.in/courses/108106071/pdfs/1_7.pdfA small(ish) batter and an analog voltmeter. I guess analog V-O-Ms are out of style...I wonder if it would show up on the bargraph of a DMM.