Active rectifier/peak detector @ up to 150kHz...
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From: Nicko on 5 Apr 2010 04:27 Hi, I've got a current transformer looking at AC pulses to an inductor - these can get quite big due a feedback mechanism that tunes the circuit for resonance & maximum current. In order not to detonate the driving bridge, I want to inhibit the bridge very quickly (in the next cycle) should the peak (not RMS) current start getting too high - we are talking a range of 20-200A depending on the drivers. Frequency is up to 150kHz. My current transformer produces 100mV/A (its a Pearson 410) and is pretty linear. I was thinking of using an active rectifier followed by a comparator, but am so out of touch with "current" thinking, I was wondering if there is any other approach I should look at - a simple bridge and comparator doesn't work at the lower currents due to the forward drop of the diodes... Thanks
From: Wimpie on 5 Apr 2010 06:13 On 5 abr, 10:27, Nicko <n...(a)desmith.net> wrote: > Hi, > > I've got a current transformer looking at AC pulses to an inductor - > these can get quite big due a feedback mechanism that tunes the > circuit for resonance & maximum current. In order not to detonate the > driving bridge, I want to inhibit the bridge very quickly (in the next > cycle) should the peak (not RMS) current start getting too high - we > are talking a range of 20-200A depending on the drivers. Frequency is > up to 150kHz. > > My current transformer produces 100mV/A (its a Pearson 410) and is > pretty linear. I was thinking of using an active rectifier followed by > a comparator, but am so out of touch with "current" thinking, I was > wondering if there is any other approach I should look at - a simple > bridge and comparator doesn't work at the lower currents due to the > forward drop of the diodes... > > Thanks Hello Nicko, I looked into the datasheet and it is a real current transformer (that has many secondary turns and few primary). The open load voltage will be factors above the loaded voltage. This is a property of current transformers. The 100mV/A is valid with 50 Ohms load. This high open load voltage is sufficient to overcome the diode voltage drop of a bridge rectifier. It increases the core flux somewhat as the transformer has to supply 2*(diode drop) on top of the rectified output voltage. Connect (in at test setup) a 4-diode bridge (low voltage small signal schottky diodes, not 1N400X series) directly at the output of the current transformer and then load the rectifier with 50 Ohms. This will probably give you useful results. Of course, do not operate the transformer (with rectifier) without 50 Ohms load. Best regards, Wim PA3DJS www.tetech.nl without abc, PM will reach me.
From: Tim Williams on 5 Apr 2010 11:33 Inductors don't resonate. What are you really doing? Sounds similar to my induction heater. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms "Nicko" <nick(a)desmith.net> wrote in message news:26ee95f3-c4db-4161-a097-4bd9e745b44d(a)z11g2000yqz.googlegroups.com... > Hi, > > I've got a current transformer looking at AC pulses to an inductor - > these can get quite big due a feedback mechanism that tunes the > circuit for resonance & maximum current. In order not to detonate the > driving bridge, I want to inhibit the bridge very quickly (in the next > cycle) should the peak (not RMS) current start getting too high - we > are talking a range of 20-200A depending on the drivers. Frequency is > up to 150kHz. > > My current transformer produces 100mV/A (its a Pearson 410) and is > pretty linear. I was thinking of using an active rectifier followed by > a comparator, but am so out of touch with "current" thinking, I was > wondering if there is any other approach I should look at - a simple > bridge and comparator doesn't work at the lower currents due to the > forward drop of the diodes... > > Thanks
From: John Larkin on 5 Apr 2010 11:52 On Mon, 5 Apr 2010 01:27:49 -0700 (PDT), Nicko <nick(a)desmith.net> wrote: >Hi, > >I've got a current transformer looking at AC pulses to an inductor - >these can get quite big due a feedback mechanism that tunes the >circuit for resonance & maximum current. In order not to detonate the >driving bridge, I want to inhibit the bridge very quickly (in the next >cycle) should the peak (not RMS) current start getting too high - we >are talking a range of 20-200A depending on the drivers. Frequency is >up to 150kHz. > >My current transformer produces 100mV/A (its a Pearson 410) and is >pretty linear. I was thinking of using an active rectifier followed by >a comparator, but am so out of touch with "current" thinking, I was >wondering if there is any other approach I should look at - a simple >bridge and comparator doesn't work at the lower currents due to the >forward drop of the diodes... > >Thanks Why not use two comparators, one for positive swing and one for negative? John
From: Nicko on 5 Apr 2010 12:34
On 5 Apr, 16:33, "Tim Williams" <tmoran...(a)charter.net> wrote: > Inductors don't resonate. What are you really doing? Sounds similar to my > induction heater. Its a resonant RC tank circuit that can be detuned by external influences, so I track it using a uP - similar to an induction heater where you want the circuit to resonate but where you might change your work coils and thus move the resonant frequency - either way you don't want your bridge FETs/IGBTs to over cook... |