| Electronics > Beginners |
| Question about a transformer |
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| uk.casmith:
Hi, Everyone, Thanks for your replies. I've attached the photos as requested. I didn't realise but I needed to set my meter up to measure AC. The results are as follows: Lead on far left to far right: 9.4 to 10 volts Lead on far left to middle: 4.8 to 5 volts Lead on far right to middle: 4.8 to 5 volts On the circuit board it labels the black lead in the middle ground but it seems to be giving a center tap value in-between the 9.7 volts, which is what i've read but why name it ground? I hope that helps with more information? I know AC is very dangerous and appreciate everyone's help. I'm going to end up building my project using a AC/DC wall adaptor with a 7805, but I still find this very interesting and would like to do it some time in the future. So any advice is greatly appreciated. Also, I should note I take full responsibility of my own actions. Thanks, Cameron. |
| MarkF:
It's unfortunate that they labelled it Ground. Think of it as a common wire. Most power supplies have a red, black and green terminals. The red and black wires are isolated via the transformer and the green terminal is the ground. This video will help explain them: Your transformer is a center tapped secondary (i.e. the blue wires are the outside of the coil and the black wire is the center of the coil). There are many ways to configure a power supply. Here are four different options. Your picture looks like the top one. |
| Jwillis:
Live and Neutral mains at the primary with no earth ground. At the secondary side ,The black wire is a center tap of the two blue wires .Not an earth ground.The voltages from black to either of the blue wires will be the same approximately4.7VAC .From blue to blue will be approximately double the value at around 9.7VAC .So 4.74 V from black to blue and 9.7 V from blue to blue. After rectification there will be a 3 lines .One will be common ground of zero volts ,one positive of around +4.7volts DC ,and one negative at around -4.7 volts DC. This is because some micro chips require a positive and negative voltage with a common ground to operate. |
| drussell:
--- Quote from: Jwillis on September 17, 2018, 09:29:26 pm ---After rectification there will be a 3 lines .One will be common ground of zero volts ,one positive of around +4.7volts DC ,and one negative at around -4.7 volts DC. This is because some micro chips require a positive and negative voltage with a common ground to operate. --- End quote --- Ordinarily, yes, if you were using a bridge rectifier to get symmetrical positive and negative rails above and below the centre-tap, but in this case the layout is just like the old two-diode full wave rectifiers from the tube era using both sides of the secondary winding with just one diode on each leg of the transformer instead of a four diode bridge arrangement. This configuration is still a full wave rectifier but only one half of the secondary conducts at any given time and the output voltage is only half the total winding voltage, .707 instead of 1.414 times the total secondary voltage, plus you have one less diode drop (which can be an advantage in low voltage, high current supplies, for example.) |
| Brumby:
First - a compliment on the photos, the first one in particular. With the two meter shots, you can see reflections can become a problem - but we can see the readings, so that's all that matters. The last one could be better focussed - but we are just able to make out enough to see how the power is set out. Well done. --- Quote from: uk.casmith on September 17, 2018, 07:08:03 pm ---... but why name it ground? --- End quote --- Unfortunately, this usage appears all too frequently. As mentioned above, it would be better to see this and think of "common" when assessing circuit topology and function. Whether or not it is actually connected to a true ground or "earth" is a separate question. The same can be said for the term "chassis". To complicate matters even more so, even a reference to "earth" should be checked to see if it is actually connected to an earthing point. |
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