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Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
OK so I need a little help from a trained eye. I have 2 DRSSTC driver board that should be identical but there not I have some scope screen shots and I was wondering if some one could tell me which one of the shots looks right. The driver is Steve Ward's UD v1.3b
So one at a time I hooked both drivers up to the same coil and did the same test but got different results I'm hoping that some one here can tell me which result looks correct. The scope probe was hooked up to pin 3 of the OCD comparator (LM311N) the comparator is set to trip at 2.4v or 240A of primary current (because I'm using a 10 ohm burden resistor).
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068
Thats not right. The current sense voltage should be rectified and filtered so you should get an exponential ramp up to some DC level, depending on what the time constant is of your filter.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
EastVoltResearch wrote ...
Thats not right. The current sense voltage should be rectified and filtered so you should get an exponential ramp up to some DC level, depending on what the time constant is of your filter.
That is the output of my rectifier, Steve's OCD circuit dosn't use filtering.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Goodchild wrote ...
EastVoltResearch wrote ...
Thats not right. The current sense voltage should be rectified and filtered so you should get an exponential ramp up to some DC level, depending on what the time constant is of your filter.
That is the output of my rectifier, Steve's OCD circuit dosn't use filtering.
Sorry. Steve's circuit indeed uses filtering. You have an RC filter there although its very small. I'm actually not fond of the way he uses the burden resistor after the rectifier. I like the burden resistor right at the output of the CT and then on the otherside of the rectifier use a larger RC (such as 100 ohm with 1uF cap) to provide you with filtered DC to trip the comparator. The time constant of the circuit being sized so that the filtered sense voltage bleeds down enough to reset for every pulse burst.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
EastVoltResearch wrote ...
Goodchild wrote ...
EastVoltResearch wrote ...
Thats not right. The current sense voltage should be rectified and filtered so you should get an exponential ramp up to some DC level, depending on what the time constant is of your filter.
That is the output of my rectifier, Steve's OCD circuit dosn't use filtering.
Sorry. Steve's circuit indeed uses filtering. You have an RC filter there although its very small. I'm actually not fond of the way he uses the burden resistor after the rectifier. I like the burden resistor right at the output of the CT and then on the otherside of the rectifier use a larger RC (such as 100 ohm with 1uF cap) to provide you with filtered DC to trip the comparator. The time constant of the circuit being sized so that the filtered sense voltage bleeds down enough to reset for every pulse burst.
umm ya but there is no RC filter, there is none if you look in schematic the cap is optional and I don't use the cap.
Registered Member #2481
Joined: Mon Nov 23 2009, 03:07PM
Location: ITALY
Posts: 134
EastVoltResearch wrote ...
I'm actually not fond of the way he uses the burden resistor after the rectifier. I like the burden resistor right at the output of the CT and then on the otherside of the rectifier use a larger RC (such as 100 ohm with 1uF cap) to provide you with filtered DC to trip the comparator. The time constant of the circuit being sized so that the filtered sense voltage bleeds down enough to reset for every pulse burst.
I do not agree with this... I think that it is better to put burden after rectification bridge so that on the burde resistor you can measure a voltage that is exactly proportional to the current (you just feed the burder res with the absolute value of the current). Instead, if you put burden right at the output of the CT, you will have a voltage drop on the rectification bridge and your measure will be not so accurate (usually voltage on burden is few V, so even 0.5 V can be a significative error...)
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.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
I'll second that
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.
Diodes are non-linear and temperature sensitive devices too, so it is hard to know how much of the sense current you're not seeing! But, if you put the burden after the rectifier, then the CT secondary will automatically develope whatever voltage is necessary to forward bias the diode bridge and cause the required current to flow in the burden. The diode voltage drops are essentially removed from the measurement!
Just bare in mind that the voltage developed across the secondary of the CT is greater than the burden voltage alone when there is a rectifier in between. At low frequencies or big currents this can cause a marginal CT to saturate due to excessive volt-second product. Always use schottky diodes if using a bridge-rectifier arrangement because their voltage drop is lower and they have no significant reverse recovery time. You can get away with a low PIV rating because one set of diodes is always conducting when the other set is reverse biased.
However, if you use a single diode rectifier with you CT and burden, then a high PIV rating is better because it resets the CT in a shorter time.
As for the time constant of the current sense filter, you want it to be shorter than one period of the signal being sensed. With full-wave rectification the dominant frequency content is at twice the drive frequency, so putting the pole at 5 times this frequency seems like a good bet. Remember that it is not trying to filter out the drive frequency itself, but rather filter out high-frequency switching spikes to prevent false tripping of the comparator.
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DRSSTC OCD trouble
