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Forum Index : Electronics : transformer output
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| irishron40 Senior Member  Joined: 22/09/2014 Location: IrelandPosts: 251 |
Hello all. I wanna build a variable power supply which can handle good amps. I started by rewiring an old microwave transformer. I have about 14 windings of 17 amp cable.sealed secondary winding with varnish and have no humming sound from transformer I connected a bridge rectifier and measure 18vdc with no load. when I connect a window wiper motor which draws about 2,5 amp the voltage on meter goes up to 23vdc why is this? thank you |
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| davef Guru  Joined: 14/05/2006 Location: New ZealandPosts: 499 |
My guess is that it is either due to fact you have pulsating DC on the output of your "power supply" or the inductive nature of the motor is affecting your reading. Try a 18Volt/2.5Amp = ~7 Ohm resistor 20-40Watt and then measure your "DC". Then try putting a BIG 10,000uF 50Volt electro on the output and re-measure no load and with your motor. |
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| irishron40 Senior Member Joined: 22/09/2014 Location: IrelandPosts: 251 |
thx davef. just to claRIFY , all i had was the transformer and bridge rectifier. i connected motor strait to bridge rectifire as i just wanted to see woulkd output voltage drop of transformer when pulling 2.5 amps. thx |
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BobD Guru Joined: 07/12/2011 Location: AustraliaPosts: 935 |
As Dave said, you need a big capacitor on the output of your rectumfire else you are presenting the motor with pulses. You may find your multimeter wasn't good at measuring pulsing voltage also. |
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| irishron40 Senior Member Joined: 22/09/2014 Location: IrelandPosts: 251 |
ok thank you. I must order that large capacitor, In meantime i did try 2 ,3300uf 28v capacitors. with no load it showed 18vdc before capacitors and 29.9 volt after capacitors. with a 2.5 amp load that 29.9vdc dropped to 26.8 vdc I am sure that my transformer is able to supply a lot of amps , just wonder is there a simple way to test what it can supply? I have 1 questions about this. 1) Would I see no voltage drop If i use a 10.000uf 50v capacitor thank you for being patient. Im just a beginner .lol ron |
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| BobD Guru Joined: 07/12/2011 Location: AustraliaPosts: 935 |
Ron Have a look at the image below. 
On the left is an AC voltage. In the middle is the rectifier and on the right is the DC output with NO capacitor. You can see that it consists of a series of pulses. The effective voltage output is somewhat lower than the pulse peaks. Your meter sees a kind of average. Your motor may not like it because it's a bit like a rough road with ups and downs. If you put a capacitor on the rectified output you get a picture similar to the image below. 
That rough road, representing the voltage, just got a bit smoother. How smooth is the next question. With no load then it may be very smooth. The more load then the more ripple (as it's called). You can compensate for load by jacking up the capacitor. There are limits to this and in practice, you always have some ripple voltage. A couple of extra points. The average voltage will now be seen by the meter to be higher and if you load the circuit you will increase the ripple and the measured voltage will be lower. There is also the capabilities of the transformer to be considered. More load and the voltage goes down. |
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| irishron40 Senior Member Joined: 22/09/2014 Location: IrelandPosts: 251 |
thats a great explanation. but is 2.5 amp considered a big load? I just thought 3.1 volt drop was a lot thx ron |
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| BobD Guru Joined: 07/12/2011 Location: AustraliaPosts: 935 |
Ron It's all relative. If your transformer can only output 2.5 amps then it is a big load. If your voltage is 5 volts then you probably don't need as much capacitance as you would if you had 25 volts. There are formulas for these things but I am far too out of practice to remember them. You can look here. but it may be too much to soak up. The way to put a bit of stability in your output voltage is to use a regulator which will all the input voltages to vary and still keep a steady output voltage. See the image for an example of a regulated output. The input to the regulator is about 9 volts and may vary a bit (not too much!) and the output stays fairly steady at 5 volts. The 7805 is a commercially available integrated circuit designed for regulation. These regulators can come in many voltages and load capabilities. 
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
A microwave transformer is probably capable of supplying 1Kw. At 30 Volts that is about 33Amps. I get about 15Amps out of my rewound microwave transformer and it runs quite cool. |
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| irishron40 Senior Member Joined: 22/09/2014 Location: IrelandPosts: 251 |
thank you both . I have near hand 2 (3300 uf 28 voltv) capacitors 1 lm338 regulator 1 tip 3055 1 10k trimpot and plenty of diodes and transistors. my understanding is that the 338 canoutput 5 amp. but read that if you also use a tip3055 that you could double that tried to find a schematic using these parts , but no luck so far. thx for you help |
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