Author Topic: AC readings on DC supply  (Read 1184 times)

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Offline DudeGibbs

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AC readings on DC supply
« on: March 25, 2015, 01:54:29 am »
Hello,
Please see attached image. The goal of this project is to achieve roughly 64 volts or more from a 12volt battery.
First start with 12volt battery powering L298 board. This is an Hbridge module that powers an arduino board also. The arduino connects pins 12 &13 to inputs 1 and 2 of the Hbridge. It is then coded to generate a pulse on each pin similar to a 60hz pulse. Here is code :
Code: [Select]

void loop() {
      digitalWrite(MOSFET1, HIGH);   // MOSFET1 on
      delayMicroseconds(360);
      delay(8);  // wait for 8.3 mS
      digitalWrite(MOSFET1, LOW);  // MOSFET1 off

      digitalWrite(MOSFET2, HIGH);   // MOSFET2 on
      delayMicroseconds(360);
      delay(8);  // wait for 8.3 mS
      digitalWrite(MOSFET2, LOW);  // MOSFET2 off }

The output of the hbridge gives steady 12vac. This is then fed to the secondary leads of a 18v transformer(110v primary/18v secondary). The output on the primary is stepped up to roughly 57v then rectified and filtered using the circuit provided in the attached image. The DC output is a steady 64.2vdc but Im concerned about the AC voltage I also measure on the output. When I switch the multimeter to AC, the voltage is doubled or more.

Why am I seeing this ac voltage?

Thank you for reading O0
 

Offline bobcat

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Re: AC readings on DC supply
« Reply #1 on: March 25, 2015, 02:05:49 am »
You are feeding the transformer with square wave pulses. This creates lots of AC harmonics to appear on the secondary. Try installing an LC lowpass PI filter between the bridge rectifier and the DC filter caps. Also- watch for transformer primary saturation due to the long duty cycle primary pulses. Fast transitions on transformers result in fast voltage spikes on the secondary instead of a smoothly changing voltage.
« Last Edit: March 25, 2015, 02:26:05 am by bobcat »
 

Offline electr_peter

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Re: AC readings on DC supply
« Reply #2 on: March 25, 2015, 02:12:54 am »
You probably have a crap quality multimeter. Cheapest DMMs measure AC voltage in very strange fashion - diode+capacitor+scaling coefficient (DC can be interpreted as AC!!!). It works for true AC(but how do you know that in the first place?), but for anything else it is hopeless.

Do this experiment - put meter in AC, put leads on 12V battery and check reading. If you see 0V, switch leads. If you see voltage, meter is a crap. In such case, try measuring your circuit voltage with switched leads, it should remove DC offset error. And buy a better DMM
 

Offline DudeGibbs

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Re: AC readings on DC supply
« Reply #3 on: March 25, 2015, 11:32:02 am »
Quote
You probably have a crap quality multimeter. Cheapest DMMs measure AC voltage in very strange fashion - diode+capacitor+scaling coefficient (DC can be interpreted as AC!!!). It works for true AC(but how do you know that in the first place?), but for anything else it is hopeless.

That was it. I get an AC reading on a battery with my multimeter.
Thanks person.
 


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