Over-current Protection Circuit

[TerryJC]

Penri,

Working on the basis that we should get a clean DC signal out of a device with in-built PWM rejection, it may be what we need. The problem is that my attempts to do that function externally seem doomed to fail.

One odd affect is that, as mentioned, I get a good clean DC signal at the input to the Op-Amp, but only when the Op-Amp is powered Maybe I can find a way to connect the signal from the 10 k / 100 k junction, directly to Pin 6 and not pickup the PWM.

Otherwise, I'll look at the INA240 data sheet.

[Penri]

Terry

The effect you're seeing is the integrator in action, the "smoothing" effect being fed back via the capacitor between the o/p and +ve i/p.


Penri

[TerryJC]

I see what you mean but it's a bit counter-intuitive. I'll try removing the link from Pin 1 to the 10 k resistor on the input to Pin 6 and add a link from the junction of the 10 k /100 k and see what happens.

[TerryJC]

In the end, I had to link Pin 3 to Pin 6, but it seems to have worked.

Tomorrow, I'll add the comparator stage.

[TerryJC]

The PWM breakthrough on Pin 1 appears to be pickup in the wire that was between Pin 1 and the 10 k on Pin 6. When the wire was hanging unterminated from Pin 1, the breakthrough was present. If I touched it with my finger, it increased.

The circuit is now decidedly odd and I need to revisit it to make sure that the comparator will work when that stage is added.

[TerryJC]

It looks like this circuit is a hiding to nothing. Anything I do to the Veroboard seems to allow breakthrough of the PWM pulses and appears to suppress the DC level that we want. This morning I revisited what I did yesterday, realising that I shouldn't (couldn't) safely connect the input of the top Op-Amp to the input of the bottom one without any protection. I therefore moved the link to the junction of the 100 k and 10 k resistors on the top Op Amp and the 10 K resistor on the bottom one. Instant disaster; pulses everywhere. I then restored the circuit to what I had yesterday but I'm not happy with that design.

Unless any other ideas are forthcoming I intend to try the INA240 if it looks viable from the data sheet.

I am now fairly convinced that the use of Veroboard is at least half the problem, the other half being the fact that the ZXCT1107 is really designed for DC loads. I might have got away with it if the circuit had been laid out on a PCB with a proper ground plane, but the sharp edges on the pulses simply treat the stray capacitance in my layout as a path to the 'other world'.

Penri,

Do you have any further thoughts on this?

[TerryJC]

It would appear that the INA240 will do the trick, but they are in short supply at the moment. Both Farnell and RS have them on back order. Mouser seem to have them, but I've never used them before, so I don't know anything about delivery times. I would need to get a mini surface mount adaptor to solder them to if such a thing is available.

It is possible to buy an evaluation module (INA240EVM), from various sources and this has the advantage that the module is supplied as a PCB (or a larger PCB with 2 or 4 modules on it). However, being an evaluation module, these come with a different gain values on each module, rather than a set of four identical devices. Farnell have these at £35 for two modules and RS for the same price for four.

[Penri]

Terry
The evaluation kit sound like a good way to go to me.

Penri

[TerryJC]

OK. I've ordered a PCB with four modules from RS Components. It's on back order, but availability was listed as tomorrow.

I'll keep you posted.

[TerryJC]

RS have just got back with a delivery date of next October.

The order is cancelled, so I need to find another source.