Transformer Calculations? - Page 1
Forum section: electronics / beginners

author: Savetheday
author_rank: Regular Contributor
timestamp: October 08, 2016, 03:09:08 pm
content: HiI'm going to make my own meter same as thishttp://ludens.cl/Electron/esr/esr.html. I need to have ferrite core input 400 output 20 transformer.I have some under my hand some of them I toke it from power supplies and salvaged from other devices.So far for that transformer I thing I need to have henry or resistance units so I could able to compare it with my transformers.How do we calculate henry and frequenz?Thanks.

author: bktemp
author_rank: Super Contributor
timestamp: October 08, 2016, 03:38:31 pm
content: All the yellow cores at the top of the picture are iron powder cores, not suitable for use as a transformer, but great as an inductor for a stepdown or boost regulator.For the ESR tester, you should use a small, low power transformer. There are probably some usable parts in your picture, but it is hard to take them apart without breaking the ferrite core (the core is glued together, so you need a solvent or heat to soften the glue before trying to prize them apart).The transformer at left side at the very bottom of the picture is probably the best one for this circuit, because it is quite easy to take it apart and reassemble it after rewinding because the halves are only clamped together (or maybe glued a little bit). It is probably an RM6 core:https://de.tdk.eu/inf/80/db/fer_13/rm_6.pdfI think I know where you bought the stuff.

author: bktemp
author_rank: Super Contributor
timestamp: October 08, 2016, 04:22:26 pm
content: I did some quick measurements:This one should be usable without rewinding.If you connect it the way shown in the picture you get a turns ratio of about 20:1.The primary has 5.5mH, 1.4ohms, the secondary 13.7uH, 35mohms. There are several other secondary taps giving a higher or lower ratio.The transformer being used in the ESR meter has a EA-77-188 core. The datasheet gives an AL value of 1060mH/1000turns. So 200 turns have 42.4mH if there is no additional gap between the core halves.Because of the low voltage and low power 5.5mH should also work fine.

author: Savetheday
author_rank: Regular Contributor
timestamp: October 08, 2016, 05:11:23 pm
content: Quote from: bktemp on October 08, 2016, 03:38:31 pmdamn you got meQuote from: bktemp on October 08, 2016, 04:22:26 pmI'm measuring like thatPrimarytransformer_1Secondarytransformer_2thanks a lot.

author: bktemp
author_rank: Super Contributor
timestamp: October 08, 2016, 06:13:23 pm
content: Your wires/test connectors seem to have quite some resistance (~1ohms). For correct resistance measurements of smps transformer windings 4 wire measurement is mandatory.I use the same clips on my transistor tester. They are great for connecting transistors because they are small and avoid shorts between the pins, but their contact resistance is rather high. When measuring low resistance values or ESR I prefer alligator clips or 4 wire clips.

author: Savetheday
author_rank: Regular Contributor
timestamp: October 09, 2016, 10:22:35 pm
content: Quote from: bktemp on October 08, 2016, 06:13:23 pmI get it nowI checked my graber resistance with 2 graber clips hooking up together Im getting 0.1 Ohm. I'm not sureI think this small transformar is going to fit good also with size.I'm measuringlike thatPins. Ohm.1-3 1.2 Ohm L=5.67mH Primary4-6 0.1 Ohm - SecondaryWhat do you thing?

author: bktemp
author_rank: Super Contributor
timestamp: October 10, 2016, 05:39:42 am
content: The graber clips are probably made out of copper or some copper alloy. After some time they seem to build up an oxide layer, increasing the contact resistance. When connecting directly together, the sharp edges pierce that layer and form a rather goot contact. But if you connect them to a round wire, the pressure gets distributed evenly keeping the layer intact. You need to rotate them several times to scrape the oxide layer away. That will decrease the resistance considerably. My experience with those clips aren't good, therefore I replaced them with alligator clips for low resistance measurements. Their sharp teeth easily penetrate any oxide layer making a much better contact.Without measuring the inductance of the secondary, it is impossible to say if the turns ratio is correct. If you don't have a function generator, you can use the ESR meter circuit and connect the transformer and measure the output voltage. If the output voltage is about +/-200mV, the turns ratio is ok. You can use the other transformer as a reference for the turns ratio.