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Question, Current Comparator

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Audiobahn

New Member
I am looking for current comparator to compare two currents in the magnitude of mA and output a logic one or zero if the reference current is exceeded or not. Is a current comparator a simple voltage comparator with dropping the two currents over same value resistors? I checked in RS and Farnell but don't seem to be able to find any current comparators or equivalent circuits.

Thank you,
Chris

PS How long has this forum been up?
 

MikeMl

Well-Known Member
Most Helpful Member
Two ZXCT1009s, each sensing one of the two currents to be compared, connected to a normal IC comparitor would do it elegantly
 

Audiobahn

New Member
I read a dedicated comparator switch is much faster and better suited for the job vs op amps. I find voltage comparators Im just wondering is current comparator is simply a layout with a voltage comparator. There's nothing regarding this on the internet :(

Shame I didnt find this forum earlier...
 

MikeMl

Well-Known Member
Most Helpful Member
You are forgetting that there is a 0.001Ω shunt connected between the inputs to the ZXCT1009. It only responds to the differential voltage between the inputs; not the common-mode PWM.

btw: I'm assuming that you want to compare the AVERAGE of the two currents, not the instantaneous current. You can filter the outputs of the two channels with an appropriate time constant, and then compare those.
 

Audiobahn

New Member
Thank you for your reply Mike. Well I am actually trying to spot any instantaneous overcurrent on the PMW signal. I am trying to implement regerative breaking on an electric vehicle. The PWM signal will be sampled using a transducer so I expect a saw-tooth waveform for the current tbh. Whatever the case I want the peak not the average. Thats the reason I am investigating into current comparators.

Please note I am not an electronics expert so go easy on me ;)
 

MikeMl

Well-Known Member
Most Helpful Member
Check the spec sheet on the high-side current monitor to see what its frequency response is. On my battery charging application, I wasn't concerned about high-frequency response; I low-pass filtered the output of the ZXCT1009 with a one second time constant.
 

Audiobahn

New Member
Thank you both for your comments. I am worried about the instantanious current to protect both gate driver and batteries. The current to the comparator will be in the range of milivolts but that will represent the normal current which will depend on the force of the breaking really. I want to limit the current at 200A tops. A simulation showed that currents exceeding 300A would be possible with max speed and breaking.
 

Audiobahn

New Member
Right so this is sort of fixed. Instead of comparing currents I will be comparing voltages which represent currents (Cheers to Ohms Law). I purchaced a simple voltage comparator (LM393P) from TI. Datasheet here: LM393P pdf, LM393P description, LM393P datasheets, LM393P view ::: ALLDATASHEET :::

It has Vcc, GND, IN-, IN+ and OUT. Vcc is obviously the supply voltage (up to 36V). GND (No comment), OUT should switch to Vcc and GND depending on IN. Now regarding IN- and IN+, Say I want to compare +5V vs an unknown input (Say +7V). Do I just supply +5V on one side and +7V on other side? OR do I need to put -7V somehow... (If yes how?).

In the datasheet it says "The voltage at either input or common-mode should not be allowed to go negative by more than 0.3 V. The upper end of the common-mode voltage
range is VCC+ − 1.5 V, but either or both inputs can go to 30 V without damage." so I suppose its +VE voltage on both terminals.

But the chip doesnt work. Is there something I am missing? Too high Icc maybe? I appreciate your help.
 

OutToLunch

New Member
What is the device that is being controlled by the PWM signal? Is it MOSFETs? IGBT? Whatever the switch is, you can predict the drop across it when it is turned on. Assuming it is a MOSFET, you can set up current source that pulls current through a resistor that is tied to the drain of the FETs. You can then set up a comparator to look at the voltage drop across the resistor versus the voltage drop across the FETs. If the drop across the FETs exceeds the drop across the resistor, then you have an OC condition.

Iocp = Iset*R/Rds(on)
where
Iset = current source current
R = resistor value tied to current source and FET drain
Rds(on) = Rds(on) of FETs
You would have to take into consideration the increase in Rds(on) due to the increase in temp of the FETs from ambient temp increases and self heating. If the switching is inductive, there is likely to be a peak and a valley, so that would likely need to be taken into account as well.
 

Audiobahn

New Member
Thank you for your message. Here's a bit more how the system works. First thing to note is that its a regenerative breaking system on a go-kart. Hence the Iset will vary depending on the speed of the actual go-kart. An ecu controls the PMW switch to switch a MOSFET. This is then fed into the batteries. The ECU calculates the appropriate duty cycle depending on the level of braking. The comparator will compare the current generated vs a threshold current. We already have a circuit to translate current in voltage (range 0-5V) using a current transducer. This voltage will then be compared through the comparator with a threshold voltage corresponding to the threshold current.

I just don't know what I'm doing wrong with the comparator...
 
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