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Solenoid driver design
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MagicSmoker:

--- Quote from: mbless on November 26, 2019, 09:28:06 pm ---...
Speaking of noise, do you have any suggestions for a noise-free low-side current sense circuit?
--- End quote ---

As Tim mentioned, you can compensate the L/R time constant of the shunt with an RC filter, but at risk of pointing out the obvious, note that this slows down the response speed/reduces the bandwidth. Wirewound resistors - even the non-inductive kind - are a poor choice here, as a result.

The attached LTSpice circuit is one I whipped up for another post here awhile ago showing how to do RC compensation of a shunt's inductance (and get a bit of voltage gain to boot) with a standard differential amplifier circuit.

T3sl4co1l:
Well that's the point, the shunt has excess bandwidth, in fact rising steadily above Fc = R / (2*pi*L).  The RC, as given, exactly compensates this to give flat bandwidth. :)

You can also make a bit of a transformer to null the EMF, which is kinda sorta practical for metal-link style shunts.  Hard to know exactly how much to couple, though.

Edit: your circuit left off the other leg -- the output is still peaked; this should be compensated like so.  Ideally, R5*C2 is the same time constant, but it's actually less (and there's some other funny business, hence R7) due to the particular characteristics of the amp used.  So, in general it would be 100pF and no ESR, but you can tweak for a given amp.

Tim
MagicSmoker:

--- Quote from: T3sl4co1l on November 27, 2019, 08:38:31 pm ---...
You can also make a bit of a transformer to null the EMF, which is kinda sorta practical for metal-link style shunts.  Hard to know exactly how much to couple, though.
--- End quote ---

 ???


--- Quote from: T3sl4co1l on November 27, 2019, 08:38:31 pm ---Edit: your circuit left off the other leg -- the output is still peaked; this should be compensated like so.  Ideally, R5*C2 is the same time constant, but it's actually less (and there's some other funny business, hence R7) due to the particular characteristics of the amp used.  So, in general it would be 100pF and no ESR, but you can tweak for a given amp.
--- End quote ---

You added a pole, basically, to complement the RC integrator. Blindingly obvious in retrospect, but not something I had thought of before so yet another thanks to you. Don't spend them all in one place!

EDIT - wrong terminology.
T3sl4co1l:
You're welcome.  Yeah, the key to a differential amp is to do everything on both sides, which again, seems obvious in retrospect.  Also check the CMRR of the two arrangements. :-+




The transformer thing looks like this -- notice the shunt is more or less a single turn, so the field/EMF that it drops (its ESL) appears in the same space.  Say we tapped some of that EMF with a turn or two, and put it in series with the sense line (blue)...

The flux linkage is imperfect, so a single turn cannot be enough.  The key is knowing how much of a second turn is needed.  Would be doable with a planar transformer -- assuming you could get nichrome foil on your PCB. :-DD  Not very manufacturable otherwise, but neat for testing and one-offs.  (Adjustment is simple: apply a square wave and bend the loop until the peaking/rounding goes away.)

Tim
mbless:
As an update, I made three driver versions: low-side, high-side, and combination low- and high-side driver previously shown. Here's a brief schematic of them.


Upon testing them, I found that all three drivers are capable of producing the same solenoid current decay. This image shows the current vs. time after falling edge of the on signal.

The reference is the broken equipment I spoke of earlier in the thread. The reference and three driver versions lie on top of each other when I soldered in the bi-directional TVS diode (D3). Without the TVS diode, the current decay is much longer, i.e. takes longer for the solenoid to close. (D1 appears to do nothing despite being in the reference equipment.)

Given they are all the same, I will continue with the low-side driver since the n-channel mosfet is easier to drive and has lower resistance. I will post an update when I get around to testing the suggested current shunt filters.
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