transformer center trapped rectifier not center common bonded ? - Page 1
Forum section: electronics / beginners

author: tablatronix
author_rank: Regular Contributor
timestamp: July 06, 2017, 04:26:08 pm
content: I just took apart a piece of industrial gear to check voltages ( a sensor board was misbehaving ), the 7805 was outputting 4.7 volts.Everything looks common sense, a small 16v center tapped transformer, a power board with 2 diode rectifier, and a filter cap on CT GND, the 2 diode bridge going right into a 7805.Then I noticed the transformer was not wired CT to CT, one leg and ct was swapped ( they are spade connector jumper ed ), maybe someone hooked it up wrong 20 years ago ?What would the actual effect of this be ?I am mostly a dc digital guy..I attached a crude image of what I think is the functional result and normal , but not sure on the actual function of this in this configuration.Was wondering for some insight on how this would be functioning in this configuration.Thanks in advance.

author: SeanB
author_rank: Super Contributor
timestamp: July 06, 2017, 04:44:29 pm
content: It would still work, but the transformer would be a lot hotter than normal as it will have a DC current flowing through the windings, and this will tend to drive the core heavily into saturation on one half of the mains cycle. The capacitor will have a lot higher voltage on it, around 20V, instead of the expected 10VDC it would have, along with a lot of ripple from the half wave rectified current, as the diode connected to the centre tap will effectively be serving no purpose as it will always be reverse biased. The regulator will have a much higher input voltage on it, and this could cause it to fail from overtemperature.Simply replacing the regulator and the capacitor, and connecting it correctly will fix this problem.

author: tablatronix
author_rank: Regular Contributor
timestamp: July 06, 2017, 04:53:39 pm
content: Thanks, that is exactly what I was figuring, terrible efficiency, half wave like ( I just modeled in in falstad to get a better understanding )The capacitor was HOT, probably bad now.The regulator was not 5v.Forgot to check xformer temperature.I did notice the 7805 had about 10v on its input, but no other equipment to test noise or ripple with me.

author: Zero999
author_rank: Super Contributor
timestamp: July 06, 2017, 05:09:06 pm
content: Yes, replace the capacitor and connect it up correctly.1uF is much too smaller value, presumably you just drew it in CAD and didn't change the default value?The transformer is probably rated to give 12V when fully loaded. On light loads it will measure much higher: 16V seems reasonable.You may also want to consider replacing the entire power supply with a switching regulator, if practical.

author: tablatronix
author_rank: Regular Contributor
timestamp: July 06, 2017, 05:10:21 pm
content: Yes, it is a 470uf if I recall, 16v

author: Zero999
author_rank: Super Contributor
timestamp: July 06, 2017, 08:25:18 pm
content: Quote from: tablatronix on July 06, 2017, 05:10:21 pmThen the capacitor would also have been subjected to higher voltages, than its maximum rating rating. The centre tapped configuration would give nearly 11V on the capacitor, yet as it was connected there would be around 22V.