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I am looking for a supplier of a particular specification small transformer.

I work for a charity which sends donated school science apparatus to schools in developing countries.  We carry out basic repair work where possible.  I am looking for a transformer to convert between 3 and 24 volts.   RS item 504773 has the correct turns ratio and is the correct size but it doesn't work in my application because the coil resistances are too large.  The coil resistances needed (measured as d.c. rather than a.c. impedance) are about 0.7 and 14 ohms.  Is such a transformer still made or was it a special for this apparatus which is no longer made ?

0 Votes


April 19, 2021 08:43

Hi David, I keep coming back to your post, but I think we need more details to help.
When you say to convert between 3 and 24V does this mean you want any transformer with a fixed 3Vac to 24Vac output?
You mention RS 504-773 which is a 15Vac transformer. This provides maximum output when the two secondaries are wired in parallel and you will achieve the 6VA rating. Is this proving an inadequate rating?
You also mention dc resistances as 0.7 and 14 ohms. I presume this is the secondary resistance and then the primary resistance of an example working transformer?
If a low impedance secondary is required you could choose a transformer of the same VA but with a lower secondary voltage i.e. closer to the 3V you mention, such as the 5Vac transformer in the same range RS Stock No.:504-694.

If you are saying the VA rating is not enough then you will have to select a higher rated VA transformer but this will be larger as the physical dimensions tend to be in proportion to the power rating.

0 Votes

April 19, 2021 08:43

Thank you for your interest and response. The short reply is that another line of enquiry has produced a recycled transformer which just about does the job and so I have used this. The longer reply (if you are interested) is that many years ago a company by the name of Irwin produced various kits for school Physics teaching. One of these kits was to show students why power is transmitted over long distances at high voltages. The kit had 2 modules - module 1 was connected to a 3V bench power supply and consisted of a step up transformer (to 24V) and a customer represented by a 3.5V MES bulb. Module 2 represented distant customers and consisted of a step down transformer and a number of identical lamps. This step down transformer was designed to produce 3V output from 22V input, thereby allowing for the small voltage drop along the transmission line which consisted of 2 lengths of constantan wire (or very thin copper wire). The transmission line could be accessed either through the transformers or by bipassing them, thereby allowing the increased efficiency of high voltage transmission to be seen by direct comparison. We have received 2 of these kits, one is complete and has been despatched to a school in a developing country, the other had a damaged module 2 which is why I have been looking for a replacement transformer. Low resistance windings seemed to be crucial so that the current through the MES lamps was not limited by the coil resistance. As an aside, having several remote customers allowed the further demonstration of the effect on the output voltage of power demand. If you have got this far then again thanks for your contribution. The charity is

April 20, 2021 14:53

@DavidHall39 Hi David, actually a very interesting project, both the equipment and the charity! Pleased you have resolved it.

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