negative rail for low side current sense?...

[DJ Delorie]

Mikko OH2HVJ <mikko.syrjalahti@nospam.fi> writes:

A typo from me - I meant 40 (or 400) ohms to shunt non-GND-end and 3k3
(or 33k) to 3.3V so you\'re running roughly constant 1mA through. This
causes a small gain/offset error, but that\'s easy to compensate.

I understood. I don\'t want the high voltage side connected to the 5v
(not 3v3) bus at all, even through a 33k resistor, for both safety and
noise reasons.

 

[DJ Delorie]

Mikko OH2HVJ <mikko.syrjalahti@nospam.fi> writes:

A typo from me - I meant 40 (or 400) ohms to shunt non-GND-end and 3k3
(or 33k) to 3.3V so you\'re running roughly constant 1mA through. This
causes a small gain/offset error, but that\'s easy to compensate.

And yes, I know the opamp\'s circuitry effectively connects that to 5v.
I also know that board no longer works due to a spike on the high
voltage side.

 

[Lasse Langwadt Christensen]

mandag den 21. marts 2022 kl. 00.21.17 UTC+1 skrev DJ Delorie:

Mikko OH2HVJ <mikko.sy...@nospam.fi> writes:
A typo from me - I meant 40 (or 400) ohms to shunt non-GND-end and 3k3
(or 33k) to 3.3V so you\'re running roughly constant 1mA through. This
causes a small gain/offset error, but that\'s easy to compensate.
And yes, I know the opamp\'s circuitry effectively connects that to 5v.
I also know that board no longer works due to a spike on the high
voltage side.

stick a diode clamp on the sense resistor

 

[whit3rd]

If you know the +15 supply comes up before motor current, and the sense resistor (s)
are near ground, you can power a few current sources (PNP transistor, emitters thru matched
current-program resistors to +15, bases tied together and driven by an op amp for half milliamp each).
Those, will pull up end B of a few level-translation resistors with their ends A connected
to a current-sense resistor terminal.

At half a milliamp, a + 2.5V offset (into a high impedance) happens when the resistors
are 5000 ohms. It\'s accurate enough if the sense resistors are much under 5000 ohms, of course.
1% resistors means that\'s 2.5 +/- .07V or so, it\'ll have to be calibrated for \'zero\'.
15V gives you enough leeway that the PNPs can just get their bases connected to (filtered) +5
if you don\'t need to regulate better than the +15/+5 rail difference voltage.

 

[Klaus Vestergaard Kragelund]

On 19/03/2022 02.03, DJ Delorie wrote:

Klaus Vestergaard Kragelund <klauskvik@hotmail.com> writes:
You offset both the input and the reference, they should both be
around 50% VDD, with a little difference due to the gain and the
needed 50% VDD on the ADC

That sounds like multiple op amps per leg... either both signals need an
opamp to offset them, plus a third to difference them, or if you try to
reduce the opamps, you can\'t offset them enough *and* amplify them at
the same time (which is what I\'m seeing now).
You add small offset to the non inverting input (VCC/(gain*2)

Like seen in page 6 here:

https://www.st.com/en/evaluation-tools/steval-ihm028v1.html

Cheers

Klaus

 

[Piotr Wyderski]

Lasse Langwadt Christensen wrote:

> https://www.maximintegrated.com/en/design/technical-documents/tutorials/3/3740.html

Flybuck requires additional windings and this alone might suffice to
cause panic.

Best regards, Piotr

 

[server]

On Thu, 24 Mar 2022 17:50:00 +0100, Piotr Wyderski
<bombald@protonmail.com> wrote:

Lasse Langwadt Christensen wrote:

https://www.maximintegrated.com/en/design/technical-documents/tutorials/3/3740.html

Flybuck requires additional windings and this alone might suffice to
cause panic.

Best regards, Piotr

The DRQ74 family of dual-winding inductors are great, and cheap.

An autotransformer flyback makes twice the voltage as a single
inductor version. Add in the input voltage too!



--

I yam what I yam - Popeye

 

[Piotr Wyderski]

jlarkin@highlandsniptechnology.com wrote:

> The DRQ74 family of dual-winding inductors are great, and cheap.

Yes, I am using these for 20W SEPICs. Another benefit is that you can
one more diode footprint and one capacitor to the PCB and populate it
for SEPIC mode or a plain boost mode at the P&P stage. But when you need
more insulation strength or just more windings, you quickly run out of
options and a custom design is needed.

Best regards, Piotr