Bridge rectifiers IC with AC signals faster than line rate (50 / 60 Hz)... Ok? - Page 1
Forum section: electronics / projects

author: TVHeadedRobots
author_rank: Contributor
timestamp: October 20, 2014, 02:09:49 pm
content: Hi,I'm curious to get some input on a circuit I'm working on. I've attached a schematic below. Basically I have an application that requires me to use a current transformer to sense whether a brushed DC motor is running or not. To do this I've used a full-wave bridge rectifier to rectify the signal I then do some signal conditioning and run that into a comparator (the application requires the weird solution.) Simple enough, but here is the question. I've tested the bridge rectifier using 4x SS24 diodes and it seems to work fine. I used the SS24s because they were in my BOM but they have since been eliminated (replaced with double diodes.) I was thinking about replacing the 4 SS24s with a bridge rectifier like the DF005S-E3/45 (http://www.mouser.com/ds/2/427/dfs-240405.pdf). Looking at the datasheet I can't really determine how this is going to work with all of the high frequency content present in my signal when they are really designed to work at 50 / 60 Hz. Maybe this is a dumb question and it doesn't really matter. I also have some parts on the way to test, but I was curious to get some input on this as well. Thanks for the help.Skip

author: MK
author_rank: Regular Contributor
timestamp: October 20, 2014, 02:51:54 pm
content: those look to be 1n4001 equivalent diodes in a bridge, would not want to use them in an rf bridge, but plenty fast enough for some serious ultrasonic frequencies.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 20, 2014, 03:22:48 pm
content: This might be wrong, but can't you just bias the transformer to get your unipolar output?

author: TVHeadedRobots
author_rank: Contributor
timestamp: October 22, 2014, 07:42:24 pm
content: MK, def not RF, it is just motor brush noise essentially. It ranges from several hundred Hz to very low kHz. There is more higher freq garbage, but I don't care about it. The 4001s did work OK for the application.Alex, I'm not sure that applying a DC bias voltage is going to work in this situation. The CT could be installed in several different locations on many different motors so I don't know the amplitude of the signal coming out o the CT to being with. I also only have 12V max to work with without using a boost converter. MAybe I'm missing something, but my initial thought is I don' think I could do that in this situation.My real concern is can I use something like the bridge rectifier part mentioned above (DF005S-E3/45) to replace the 4001s I was using. Considering I really don't care about HF data I think I'm fine, but I just want to make sure that I'm not missing anything. I have some parts on the way for testing.Thanks.

author: eneuro
author_rank: Super Contributor
timestamp: October 23, 2014, 06:18:43 am
content: Quote from: TVHeadedRobots on October 20, 2014, 02:09:49 pmIt is not possible to add some kind of rotor position encoders and detect its movement simply using IR transmiter & receiver ?Easy to implement and assign to MCU pin change interrupts to make RPM counters etc. Sometimes a few black & white paints needed, but maybe such customization could solve other problems too if possible.

author: TVHeadedRobots
author_rank: Contributor
timestamp: October 23, 2014, 03:49:31 pm
content: Eneuro, I wish it was. I'd have a lot more info about what is going on with the pump if I could. Unfortunately that isn't an option. This is actually used to sense whether or not a bilge pump on a boat is running or not. It needs to work with many different off the shelf pumps without requiring modification of the pump itself. The install also needs to be fairly simple. Disconnecting power and slipping a CT over the power cable is pretty easy to do for most people. I'm really just curious whether anyone thinks that I'll see any problems replacing the 4001 diode bridge with a bridge rectifier IC. I don't think there will be any issue, but I was curious to get someone elses input as well.Thanks.

author: retrolefty
author_rank: Super Contributor
timestamp: October 23, 2014, 05:32:06 pm
content: QuoteCorrect me if I'm wrong, but trying to pass and measure DC motor current through a CT doesn't work? CTs pass only AC over their rated frequency and current ratings.

author: TVHeadedRobots
author_rank: Contributor
timestamp: October 24, 2014, 02:58:42 pm
content: Hi retrolefty, It does actually work. I believe that the reason this works is because the current changes direction through each winding as the motor spins and this change is picked up through the brushes. That being said, I may be off base on this and if I am, please correct me. None-the-less, it does work. I've also attached a couple of scope captures so you can see what the signal looks like on the CT as well as after it has been processed by the circuit from my original post.Any thoughts on the bridge rectifier IC replacing the 4001 circuit?

author: calexanian
author_rank: Super Contributor
timestamp: October 25, 2014, 06:16:42 am
content: Everything else aside 1N4148 is the de facto signal steering diode in low voltage low current applications. Low junction capacitance and high speed. Also VERY cheap. in most digital circuits, that mystery glass diode is usually a 1N4148, or a SD103 for the low voltage drop.

author: TonyStewart
author_rank: Contributor
timestamp: October 25, 2014, 09:02:58 am
content: The factor for choosing a diode for high speed switching is called the reverse recovery time. Since this is not sub-nanosecond speeds, the 1n4148 is best suited.I would be concerned about the burden load for the CT to ensure the cap does not cause false readings. CT's are design for a fixed Resistive burden load, not nonlinear bridge rectifiers. It would be better to use a precision OA rectifier..

author: Circlotron
author_rank: Super Contributor
timestamp: October 25, 2014, 12:29:29 pm
content: Quote from: TonyStewart on October 25, 2014, 09:02:58 amBut if you are passing noisy DC through the CT then linearity would be a secondary concern.If you do uses a burden resistor, make sure you put it after the diodes. The CT output voltage may not be very high and the diode voltage drop will vary with temperature. Putting the burden resistor after the diodes cancels this effect because the CT secondary is a current source. And yeah, my vote is also for 1N4148.

author: Zero999
author_rank: Super Contributor
timestamp: October 25, 2014, 08:47:05 pm
content: I don't see why you need the diodes.Bias one end of the CT secondary to half the supply voltage and then connect it to a Schmitt trigger to give a nice square wave.

author: Richard Head
author_rank: Frequent Contributor
timestamp: October 26, 2014, 01:04:43 pm
content: I'm not sure why the circuit is actually working as passing DC through a CT is going to push the core to one side of the B-H loop and eventually saturate it. Even if the current waveform is pulsed DC the core will flux-walk and core saturation result. I suspect you have been lucky and the core hasn't saturated fully due to a lowish primary current.I would be inclined to go for a Hall effect device but unfortunately they are pricey.

author: SeanB
author_rank: Super Contributor
timestamp: October 26, 2014, 02:59:59 pm
content: Basically it relies on the CT being run very conservatively. Even a current through the CT of rated still is very low so as not to drive the core into a non linear region, so it will still work as this application does not really care. The diode bridge will work here, as it basically is detecting the brush and commutator noise off the motor while it rides on a large DC offset. Non linear sure and if you got a pump made on a good day with low brush noise, or a built in noise filter, it will not work. But for most motors and placed close to it it will work. 25pF capacitance for the diodes says they are small fastish junctions, so will work up to a few kHz at low current without too much probem. Might not do 1A at 10kHz, but will do 50mA no problem. What are the resistor and capacitor values, it seems to be working well to do the voltage conversion. With the frequency it is a largish multipole motor probably doing around 7000RPM.

author: TVHeadedRobots
author_rank: Contributor
timestamp: October 27, 2014, 09:02:57 pm
content: Excellent. Thanks for all of the input. SeanB, thanks for summing things up and nice point about a pump with a noise filter, that hadn't even crossed my mind. I'm going to have to dig into that one. The values on the resistors and cap are as follows; R48 150, R49 100k & C33 is 4.7uF. It took a bit of fiddling to choose them. The CT that I'm using is the CR8448-2000-F. It is rated for 20Hz - 200kHz. I'm not running the CT more than about 18" from the pump. The pump used for the scope caps was a Sureflo 1500 bilge pump (http://www.shurflo.com/marine-products/bilge-pumps/shurflo-1500-2000-gph-bilge-pumps/29-shurflo-1500gph-bilge-pump.) I'm also including a slightly better version of the pic with the scope captures that includes a few more captures and some notes incase anyone is interested.Richard, thanks for the suggestions I'll look into them. That said, I'm curious, even if the CT is saturated why does it matter in this particular application? The only thing I really need to do here (perhaps this wasn't explained well) is to determine if the pump is running or not. I don't need to accurately sense the current consumed by the pump and I'm not concerned about distortion of the transformed signal. In reality I'm running that CT permanently in saturation. Maybe I'm missing something. If I am please let me know. I'm actually not familiar with flux walk. I'll be asking Google about it as soon as I'm done here.To address all of the suggestions for alternate solutions such as hall sensors (which happened to be my first choice) I had considered many of them but they were either going to be more costly or require a more complex circuit to implement considering what I had to work with. Also since this is going on a boat you can only count on a 12V DC supply. With that in mind, options like an offset voltage isn't all that practical (unless I'm missing something,) especially when looking at the signal coming out of the CT. None-the-less I appreciate the input.NOTE: The scope captures are showing comparator output from an LM339. That comparator isn't present in the circuit because I'm using a built-in comparator on the MCU to handle those duties in in the actual application circuit.

author: TVHeadedRobots
author_rank: Contributor
timestamp: October 27, 2014, 09:21:42 pm
content: Hero999, I just gave your suggestion more thought and that probably would work. It does mean adding an IC to the BOM that is more expensive than the bridge rectifier, still it is interesting and something I hadn't considered. I'll knock it up on a breadboard and see what I get. Thanks.

author: TonyStewart
author_rank: Contributor
timestamp: October 27, 2014, 09:42:31 pm
content: I'm not clear on your overall issues and design details on detection reliability with ringing waveforms, which are not supressed yet.You should like to have at least 20dB SNR with any decent detector for low error rate. ... Unlike the waves shared so far. short probe gnd measurements are critical and diode CV^2 is fairly constant per diode selected for reverse voltage capacitance will resonate with CT inductance where Q is determined by burden.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 27, 2014, 11:54:03 pm
content: Quote from: TVHeadedRobots on October 27, 2014, 09:21:42 pmYou know, I think it's a wonderful idea...