cupoftea
Advanced Member level 5
Hi,
We have a zero cross detector , one on each phase to phase, for a 3 phase delta supply. We are just operating it with one singe 60Hz phase (85-125vac) at the moment. We are using a single phase AC power source. (with settable frequency and amplitude, etc)
The ZCD is a very common one that I have seen before, but I can’t show it here. It is all discrete, using BJTs, diodes, Zener, res and caps.
It gives a zero cross pulse just before the zero cross.
On our other product, the 3 phase star 240VAC phase system, this zero cross detector seems to operate fine, giving a pulse some 400us away from the zero cross.
But when we put in the single phase input at 85-125vac, in the 3 phase delta product, it sometimes goes wrong. The expected (acceptable) operation, which we usually see, involves the rising edge of the zero cross pulse getting nearer to the zero cross as the mains is increased from 85vac to 125vac. This is also the operation that the simulator shows for it. However, sometimes a particular channel will show the reverse of this, ie, the zero cross pulse getting further from the mains zero cross as the mains is incremented from 85vac to 125vac. Also, the rising edge of the zero cross pulse ends up too far from the zero cross when at 125vac (about 600us instead of 300-400us).
So anyway, sometimes the circuit works properly, and sometimes not. On the 240VAC 3 phase star system, we haven’t yet seen it go wrong on that.
We are thinking its possibly one of the following…
1…Something to do with the diff probes leakage current to earth.
2…Something to do with the single phase AC power source
3…Something to do with the input common mode choke , in that with just one phase into it, it will show more inductance, and there is ringing at the zero cross which affects the zero cross detector.
Do you have experience of the same problem?, a zero cross pulse sometimes going wrong like this?
We have a zero cross detector , one on each phase to phase, for a 3 phase delta supply. We are just operating it with one singe 60Hz phase (85-125vac) at the moment. We are using a single phase AC power source. (with settable frequency and amplitude, etc)
The ZCD is a very common one that I have seen before, but I can’t show it here. It is all discrete, using BJTs, diodes, Zener, res and caps.
It gives a zero cross pulse just before the zero cross.
On our other product, the 3 phase star 240VAC phase system, this zero cross detector seems to operate fine, giving a pulse some 400us away from the zero cross.
But when we put in the single phase input at 85-125vac, in the 3 phase delta product, it sometimes goes wrong. The expected (acceptable) operation, which we usually see, involves the rising edge of the zero cross pulse getting nearer to the zero cross as the mains is increased from 85vac to 125vac. This is also the operation that the simulator shows for it. However, sometimes a particular channel will show the reverse of this, ie, the zero cross pulse getting further from the mains zero cross as the mains is incremented from 85vac to 125vac. Also, the rising edge of the zero cross pulse ends up too far from the zero cross when at 125vac (about 600us instead of 300-400us).
So anyway, sometimes the circuit works properly, and sometimes not. On the 240VAC 3 phase star system, we haven’t yet seen it go wrong on that.
We are thinking its possibly one of the following…
1…Something to do with the diff probes leakage current to earth.
2…Something to do with the single phase AC power source
3…Something to do with the input common mode choke , in that with just one phase into it, it will show more inductance, and there is ringing at the zero cross which affects the zero cross detector.
Do you have experience of the same problem?, a zero cross pulse sometimes going wrong like this?
