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Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
wrote ...
If you want an accurate current measurement you want to put the burden resistor after the rectifier. Otherwise the voltage drop of the diode(s) subtracts from the voltage developed across the burden. As Luca said two diode voltage drops can easily be significant because you usually choose a low resistance burden to keep the CT's volt-second product as low as possible.
I'll third that if you are trying to quantitatively measure the current with high accuracy, but for the given application of overcurrent detection, its not really needed.
If you're actually going to quantitatively measure current, then you might want something more accurate, but for a general OCD circuit thats works on the order of 500-1000A, its not necessary.
If your circuit works out to be 1V per 100A, then even if the Vfwd (total contribution) varies by 0.5V, thats only 50A which certaintly isn't going to make much of a difference unless your already on the hairy edge. But if you set your OCDs conservatively, which is what i do and recommend, its not going to matter much.
Plus, i use a sample-n-hold circuit for my OCD circuits which is why i have the before the diode bridge, as if the burden is after the bridge, it will just negate the RC filter. Its all on what your preference is.
wrote ...
Moreover, if you use a RC with a time constant comparable (or greater) to the RF period you will miss the *peak* value of the current and you will measure the avreage value of the cutrrent within the ON period... But I think that the purpose of the OCD protection it is to stop the driver if the peak current exceed the threshold. So, i would use a RC with a time constant at least 10 times smaller than the RF period so that you can filter the fast spikes (noise) but you keep the correct image of the instantaneous current of the primary.
Thats not true. The RC constant basically determines the discharge time of the filter, not the charge time. Charge time is dependent on the impedance of the CT circuit feeding into that RC circuit.
I use an RC filter on my OCD circuits where its more of a peak-and-hold circuit. It will capture all the peaks with no problem, and bleed time will reset in between pulse bursts.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Well I got the top circuit working just fine I just needed a lower burden resistor went from 10ohms to 5 ohms so for like 2.4V into the comparator thats is 480A in the primary circuit. I Just have to work on the tuneing some more. Every one thats has replyed to this thread has been a big help, thanks.
Registered Member #154
Joined: Sun Feb 12 2006, 04:28PM
Location: Westmidlands, UK
Posts: 260
Hi Eric,
Make sure you read Steve's notes on why you shouldn't adjust the OCD past 3.75v if your comparator supply is 5 volt due to reliabilty issues. All the best,
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Bennem wrote ...
Hi Eric,
Make sure you read Steve's notes on why you shouldn't adjust the OCD past 3.75v if your comparator supply is 5 volt due to reliabilty issues. All the best,
Mel
Yes thats why I had to lower the burden resistor to 5 ohms so that I could have it set to a lower voltage.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
You can always put more voltage into the comparator (Vcc) up to the datasheet ratings. The output is open collector and can be tied up to anything you want - i.e. interfacing with 5V logic(within datasheet specifications) I think there was a 12V rail on that circuit.
Registered Member #2481
Joined: Mon Nov 23 2009, 03:07PM
Location: ITALY
Posts: 134
EastVoltResearch wrote ...
wrote ...
Moreover, if you use a RC with a time constant comparable (or greater) to the RF period you will miss the *peak* value of the current and you will measure the avreage value of the cutrrent within the ON period... But I think that the purpose of the OCD protection it is to stop the driver if the peak current exceed the threshold. So, i would use a RC with a time constant at least 10 times smaller than the RF period so that you can filter the fast spikes (noise) but you keep the correct image of the instantaneous current of the primary.
Thats not true. The RC constant basically determines the discharge time of the filter, not the charge time. Charge time is dependent on the impedance of the CT circuit feeding into that RC circuit.
I use an RC filter on my OCD circuits where its more of a peak-and-hold circuit. It will capture all the peaks with no problem, and bleed time will reset in between pulse bursts.
Ok, that's true if you put the capacitor after the rectification bridge, not in parallel with the burden resistor... But then you need a second resistor to realize the RC constant you are talking about...
EDIT: I have just realized that you already wrote what I have written...
Anyway, I don't understand the reason of using a peak-hold detector... If the comparator is fast enough it will trip the protection even without the "hold" function...
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Luca wrote ...
EastVoltResearch wrote ...
wrote ...
Moreover, if you use a RC with a time constant comparable (or greater) to the RF period you will miss the *peak* value of the current and you will measure the avreage value of the cutrrent within the ON period... But I think that the purpose of the OCD protection it is to stop the driver if the peak current exceed the threshold. So, i would use a RC with a time constant at least 10 times smaller than the RF period so that you can filter the fast spikes (noise) but you keep the correct image of the instantaneous current of the primary.
Thats not true. The RC constant basically determines the discharge time of the filter, not the charge time. Charge time is dependent on the impedance of the CT circuit feeding into that RC circuit.
I use an RC filter on my OCD circuits where its more of a peak-and-hold circuit. It will capture all the peaks with no problem, and bleed time will reset in between pulse bursts.
Ok, that's true if you put the capacitor after the rectification bridge, not in parallel with the burden resistor... But then you need a second resistor to realize the RC constant you are talking about...
EDIT: I have just realized that you already wrote what I have written...
Anyway, I don't understand the reason of using a peak-hold detector... If the comparator is fast enough it will trip the protection even without the "hold" function...
Luca
In practice, especially with military transmitter systems, i've seen instances where pulse-by-pulse overcurrent circuits actually fail to trigger comparators - even in so called "high speed" comparators. The top of a sine wave can be very narrow, especially if your bandwidth of your sensing circuit is not optimized and you end up with a peaky looking triangle wave vs. a sine wave. So as a practice, i've always incorporated peak-n-hold circuits to ensure the peak value is held long enough to trigger a comparator.
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DRSSTC OCD trouble
I Just have to work on the tuneing some more. Every one thats has replyed to this thread has been a big help, thanks.
