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irmanao
Joined: 08 Apr 2015
Posts: 77
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| zero crossing thyristor code problem |
 Posted: Thu Sep 17, 2015 9:56 am |
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dsPIC33EP512MU810 (dspic33e usb starter kit)
ccs version 5.008
I would like to do zero crossing detection on a 50Hz signal (0.1V-3.1V) to fire thyristors.
I do get a pulse at the zero crossing with my program (tested it with oscilloscope) but i only need the pulses to be fired when the signal enters the positive half cycle.
Here is the working program (I added to the code the _adprevious_ to subtract it from _ad_ to get the pulse only when the signal enters the positive half cycle). (it still doesn't work though).
| Code: | #include <33EP512MU810.h>
#device adc=12
#fuses PR_PLL,NOWDT,NOPROTECT
#use delay(internal=80000000)
#pin_select INT1=PIN_D6
#pin_select INT2=PIN_D7
void init_all (void);
int16 dms=9;
int16 ad,;
int16 adprevious=0;
void main()
{
init_all();
WHILE(TRUE)
{
ad=read_adc();
if ((ad-adprevious)>0 && ad<2011 && ad>1959){
delay_ms(dms);
output_high(PIN_F13);
delay_us(60);
output_low(PIN_F13);
}
adprevious=ad;
}
}
#INT_EXT1
void ext_int1(void)
{
dms-=1;
if(dms<1){dms=9;}
}
#INT_EXT2
void ext_int2(void)
{
dms+=1;
if(dms>9){dms=1;}
}
void init_all (void){
ext_int_edge(1, H_TO_L);
ext_int_edge(2, H_TO_L);
enable_interrupts(GLOBAL);
enable_interrupts(INT_EXT1);
enable_interrupts(INT_EXT2);
setup_adc_ports( sAN0 | VSS_VDD );
setup_adc ( ADC_CLOCK_INTERNAL );
set_adc_channel ( 0 );
}
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Thanks for your help |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19466
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| Posted: Thu Sep 17, 2015 2:36 pm |
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For what you describe, use the comparator.
Just program this to use a reference voltage just on the voltage corresponding to the 'zero' point, and set it up to trigger when the signal goes above this point. When it triggers you have your crossing.
However in both cases, you need to be aware of the nature of typical AC. Put an oscilloscope of the voltage, and be prepared for just how erratic any attempt to detect crossing using a direct measurement of voltage will be. Unfortunately, AC in this sort of environment, has very little resemblance to 'sine'....
This is why accurate commercial zero crossing detectors do_not_ in general directly measure the voltage. Instead the commonest approach is to use a phase locked loop, synchronised to the AC, and trigger off this. If your waveform is reasonable clean, this may not be needed, but you need to be sure before starting. |
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temtronic
Joined: 01 Jul 2010
Posts: 9207
Location: Greensville,Ontario
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| Posted: Thu Sep 17, 2015 5:43 pm |
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also...
be sure NOT to send AC to ANY PIC pin !!! It will destroy the PIC !!
Jay |
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drolleman
Joined: 03 Feb 2011
Posts: 116
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| Posted: Thu Sep 17, 2015 9:58 pm |
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| Another way is to use optical isolator on the input. This will tell you that we have a positive cycle. Then on the output use another optical isolator that has zero crossing, then hook up your thyristors to the zero crossing optical isolator. This will be much easier to code. |
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irmanao
Joined: 08 Apr 2015
Posts: 77
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| Posted: Fri Sep 18, 2015 4:47 am |
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If i use the comparator to trigger when the signal is above the 'zero' point won't i still have the same problem?
I found an example here
http://www.ccsinfo.com/forum/viewtopic.php?p=134432
but i can't understand how to apply it.
Thanks for your time and help... |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19466
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| Posted: Fri Sep 18, 2015 8:06 am |
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Setup correctly, the comparator will generate an interrupt when the signal passes 'above' it's threshold. This is what you are asking for.
Means your code can then have time to do other things....
Try changing your test to:
| Code: |
if ((ad>adprevious) && ad>1959 && ad<2011){
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Takes less instructions, and will work whatever the type of the variables. |
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