Connecting two transformers to mains voltage - linear power supply - Page 1
Forum section: electronics / beginners

author: maor
author_rank: Contributor
timestamp: September 22, 2012, 11:05:15 am
content: I'm working on a basic linear power supply using some random parts I have here and an old kit I found.The kit contains no taps 10:1 transformer going to a bridge rectifier to an lm317 with some filtering.my first plan was to go simple and connect a 7805 to the bridge rectifier and get a constant 5v output, but I struck quite an obvious road bump, there will be about 25 volts dropped on the 7805.So I opened up a onehunglow variable plugpack (which looks horrible, ugly unregulated linear design) where inside I found a small multi tapped transformer.I wicked out the transformer and started measuring the ratios using my iphone as a signal generator and measuring the voltage on my oscilloscope.I came out with these results:the voltage is the simple attenuation ratio calculation for 220 line voltage in israel.Full ratio - 15:1 -> 20.8vpkFirst tap - 20.45:1 -> 15.26vpksecond tap - 27.7:1 -> 11.26vpkthird tap - 34.62:1 -> 9.012vpkfourth tap - 44:1 -> 7.09vpkfinal tap - 75:1 -> 4.16vpkSo I think I can use this transformer on another board where I'll make another bridge rectifier and the 7805 and have a constant 5v regulated.now the question I have is can I do it?, can I simply connect the two primaries in parallel and call these two systems isolated?, is my assumption correct?I added some images of the lm317 circuit and the little multi tapped transformer.

author: maor
author_rank: Contributor
timestamp: September 22, 2012, 03:18:06 pm
content: Can I connect two transformers in parallel?

author: G7PSK
author_rank: Super Contributor
timestamp: September 22, 2012, 03:27:07 pm
content: On the mains side, Yes, there is no difference between fitting two plugs and plugin them side by side into the mains and fitting one plug for both transformers.

author: maor
author_rank: Contributor
timestamp: September 23, 2012, 09:15:55 pm
content: Quote from: Electroalek on September 23, 2012, 04:47:51 pmI was thinking about that going for the obvious answer, but what about the inductance from the two coils?

author: IanB
author_rank: Super Contributor
timestamp: September 23, 2012, 09:21:06 pm
content: Quote from: maor on September 23, 2012, 09:15:55 pmIt depends on the electrical supply to the transformers. Connecting two items in parallel allows them to be considered completely independently and separately from each other as long as the supply is unaffected by either load. However, if the supply should be inadequate, for instance on the end of long thin wires, then one load can affect the supply, which can in turn affect the other load.This is seen most commonly in American homes where things like ovens, air conditioners or electric irons will make the lights dim when they turn on. The lights dim because the wiring is inadequate for the task and experiences a voltage drop due to the current flowing.Normally considerations like this don't apply when connecting small devices in a project box with a proper mains cable and proper internal wiring.

author: ptricks
author_rank: Frequent Contributor
timestamp: September 24, 2012, 08:47:19 pm
content: A few things to consider.Is the transformer from the other supply capable of supplying the current you need ? It will not work if you have a better voltage but not enough amperage.The schematic you posted shows 2200uf capacitors on the outputs of the voltage regulators, that is way too big. With something like a 2200uf the regulator is not going to accurately control the voltage because it takes too long for it to respond with such high capacitance. Something like a 100uf would be better, I even use 10uf sometimes, depends on what it powers. You want the majority of the filtering on the input of the regulators.You can connect transformers in parallel and even series if it is done properly. You can also connect the output of the first transformer to the input of the second transformer to change voltages further. Something like a 1:5 ratio second transformer would be great in your case, stepping the first transformers ~25V down to 5V.Or you can connect two identical transformers so that they share the same secondaries which creates an isolation transformer for mains power. 220VAC converts to 12VAC on the first transformer, 12VAC secondary converts back to 220VAC on the second transformer.