Recent content by saad87

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    0-1A Bidirectional Current Source

    I'm trying to design a bidirectional current source for the purpose of measuring very small (0.0001 Ohm) resistances. I've come across this circuit: I can see that the transistors are there to 'boost' the current but I don't understand the purpose of the 1K resistors and the capacitor. I...
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    A Simple Ohmmeter - Design Dillema

    So after looking at both PNPs and P-Channel MOSFETs, I think I'll go with the MOSFET variation. But I've run into another problem here and am looking for advice. The cable that I'm measuring can be upto 500m long. This in itself isn't a problem but the cable picks up all sorts of noise. There...
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    A Simple Ohmmeter - Design Dillema

    Thanks uart and averagesupernova. Im going to try and build this circuit a bit before I commit to the design. One question about the MOSFET: you state I need one with low threshold voltage and that I completely understand. However, normally, a MOSFET's source is tied to ground and the gate...
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    A Simple Ohmmeter - Design Dillema

    Here we go... sorry I didn't supply a schematic earlier. Note, Q1 is now a generic PNP transistor. In the LTspice sim. This circuit works when there is a zero ohm test resistor but when there is an open circuit present at the test resistor the current flows through R1, through the emitter...
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    A Simple Ohmmeter - Design Dillema

    uart and averagesupernova, Thank you very much for that explanation. I simulated the circuit in LTSpice and you were right - the ckt doesn't work. I think I missed the fact that an NPN would cause the feedback to be positive instead of negative. One question, however, does this handle the case...
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    A Simple Ohmmeter - Design Dillema

    Got it: Replace Q1 with a darlington. Makes sense. Your second point, regarding changing the swapping the inputs of the op-amp confuses me. This is my first analogue circuit (I have designed and implemented digital circuits before). My question is, why would the circuit be better if I simply...
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    A Simple Ohmmeter - Design Dillema

    I've been design a simple ohm-meter which can measure resistances in the range 0.001 to 100 Ohms. I am aiming for an accuracy of 3%. Here's the circuit (so far): The circuit is basically a programmable current source which can source a current of 100, 10 or 1 mA. At junctions Va and Vb...
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    Terminating and Routing SPI

    I'm looking for some advice regarding how to terminate SPI signals which are going to an external PCB. I know that SPI isn't meant for board to board communication but in this case the connecting ribbon cable is short - about 6" max length. My frequency isn't high, just 2 MHz but I have...
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    Pulse Skipping - Calculating Frequency Limits

    Thanks! Sorry about the late reply - I didn't notice anyone replied. The input voltage is just 7V. Actually, I can make it anything I want as it's in my hands, so I'm going with 7V. The 5V and 3.3V rail have the highest current requirement (0.5A and 1A respectively). My initial estimates...
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    Pulse Skipping - Calculating Frequency Limits

    I'm trying to design my first switching regulator circuit. I require three voltage rails, 5V, 3.3V and 1.8V. But to get started, I'm just focusing on 1.8V. I've decided to use the LT3507 as it seems to be commonly available here. It also has 3 buck regulator outputs, so it's a one-chip solution...
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    External Power Supply or Put Regulator on Main PCB?

    Thanks. I've already found some that I like. One question I had though - is it worth it to integrate the power circuitry on the PCB? What is the disadvantage of keeping the power supply external? If I keep the PSU external, I don't need ventilation for the main circuitry and it can be...
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    External Power Supply or Put Regulator on Main PCB?

    Any regulator that you could recommend? I would prefer a easy to use one as this would be the first time I implement switchers!
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    External Power Supply or Put Regulator on Main PCB?

    I'm close to finishing my first 4-layer board. Initially, we felt that keeping the power supply as a separate unit would be better. This was because we wanted to avoid having heat on the main PCB - mostly because the area where the device is intended to be used is very hot and also quite dusty...
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    TVS Diodes for ESD Protection?

    I had a small question about ESD. I have implemented TVS diodes on my project at the input side. However, the output is still unprotected. It's a bit like this: CPLD 1 -> Logic-Level MOSFET -> WIRE UNDER TEST -> TVS DIODE -> Pull up -> CPLD 2. So the input side is protected from ESD...
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    Puzzled by MOSFET switch

    I have the following circuit wired up on a protoboard. The datasheet for the BSS138 MOSFET is here. I'm puzzled regarding what I'm seeing happen with this circuit - when I apply 3.3V at the gate resistor, the MOSFET turns on fully and I see 3mV at the output. This, of course, is expected...
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