Over-current Protection Circuit

[TerryJC]

That's what I'd intended to do unless you came up with anything else. I'll add the comparator to the breadboard and if that works, transfer the components to the Veroboard.

I have breadboarded the following circuit:

Passive_Integrator_Comparator_Circuit_(as_built).png (46.85 KiB) Viewed 213 times

As built, I get a squarish sinusoidal waveform at the output of the comparator, (it isn't at PWM frequency; it is higher) but if I remove the 10 K between pins 2 and 6, the circuit works as advertised. This isn't what I'd expected, but I must say I'm not sure what this resistor is for; I simply followed the Application Notes in the Datasheet.

I should be OK with voltage level because I can increase the value of R Out to get the range I need.

Thinking about this, we wouldn't need an amplification stage, because the ZXCT1107 is a sense amplifier and should provide adequate gain.

I'm in the process of redesigning the Veroboard to realise this circuit in the Over-current Protection Module. I'll report back when I have this working.

[TerryJC]

As built, I get a squarish sinusoidal waveform at the output of the comparator, (it isn't at PWM frequency; it is higher) but if I remove the 10 K between pins 2 and 6, the circuit works as advertised. This isn't what I'd expected, but I must say I'm not sure what this resistor is for; I simply followed the Application Notes in the Datasheet.

I think I know what is going on here. Figure 4.13 in the Datasheet shows an Astable Multivibrator and by adding the 1 uF capacitor between Pin 2 and 0 V, I've effectively created a bastardised version of that.

Penri,

If you can see anything wrong with omitting the feedback resistor, please let me know. Otherwise, I'm going to design the new circuit without it. At the moment I'm quite keen on KISS.

[Penri]

Terry
The only thing I can see the resistor in question doing is adding hysteresis to the circuit, that's not something we need for our application therefore I see no harm in leaving it out.

Penri

[TerryJC]

OK thanks.

[TerryJC]

Sitrep:

I had hoped to have completed the Over-current Protection Circuit build today and that would simply leave me to integrate the board into the full Underground system and tweak the software. However things haven't quite proceeded as planned.

First, I got a permanent high voltage (approx 2 V) on the input to the latch (Pin 6 of the comparator and Pin 9 of the Latch in the circuit below). Nothing showed up when I buzzed everything out for continuity and shorts, but there must have been a high resistance path between tracks somewhere because after I had stripped off all the wire links from the Veroboard the fault cleared.

I now have the comparator working as required and the output goes high when the current is around 1.2 A (adjustable with the pot eventually). However, the Latch trips as soon as I power up, (eg with no current being delivered to the load from the +15 V supply) so clearly there is still something amiss. The power-on reset appears to work with a rise time at Pin 13 of approx 30 ms, so there is a logic failure somewhere.

Hopefully I'll be able to sort this out tomorrow.

Over-current_Protection.png (137.37 KiB) Viewed 196 times

[Penri]

Terry

You can always increase the time constant of the power-on reset RC circuit connected to pin 13 of the latch so that everything has time to settle.

Penri

[TerryJC]

You can always increase the time constant of the power-on reset RC circuit connected to pin 13 of the latch so that everything has time to settle.

Yes. That may well be it. However this design worked perfectly originally and I'm not sure why the TC would need to be longer. 30 ms is a long time WRT to propagation delays in CMOS circuits.

I suspect another build error, although I wasn't able to identify it yesterday. Veroboard lends itself to shorted tracks etc, especially when the eyes aren't what they were.

[TerryJC]

It looks like there may be an error in my Veroboard layout because the power-on reset is easily long enough. In this diagram, the yellow trace is at Pin 13 and the blue one at Pin 8:

TEK0000.JPG (97.29 KiB) Viewed 190 times

I tried slapping a 1 uF capacitor across the 100 nF power-on circuit and the rise time rose to 385 ms. The LED still illuminated.

[Penri]

Hum! That’s unfortunate, form that evidence an issue with the board looks favourite as the cause.

[TerryJC]

I have a blood test shortly, so will be tied up for a while.

I've taken some traces from all of the Pins on the Latch and will analyse the results later. Apart from Pin 9, they all seem to show rising voltages, which doesn't seem right. That may help to locate the problem.