John said:
Something like this might be made to work. Do you see how
it gives you a lot more voltage gain than the original
divider method? think about what you would have if you
replaced the two collector resistors with a current mirror
(to subtract one collector current from the other). Then
you wouldn't need a subtractor output, but only a current to
voltage converter.
There is also a much simpler way to produce the ground
referenced single ended output with a single opamp that has
a common mode range that includes its positive rail.
http://www.maxim-ic.com/appnotes.cfm/appnote_number/746/
Figure 8 shows the general concept.
Sorry if you just wanted a schematic of a finished high side
current sense amplifier.
I am more interested in exploring
possibilities and circuit concepts with you.
I'm totally on the same page with respect to intellectual curiosity.
I already knew about jfet op amps like the TL082 or LF353 where the
common mode goes all the way to the top rail. In fact I've seen the
very same circuit you pointed out in Figure 8 of the maxim app note in
regard to high side current sensing, but kind of forgot. And well, to
tell you the truth, this thread isn't really about high side current
sensing. I do want to amplify a differential voltage at or very near
the top rail, but there will be more impedance behind it. As a matter
of fact, this is about the self-regulating heater we discussed in
another thread on s.e.d. I'm still working on it! And here's the
latest concept:
14v
|
,----+---+----,
| | | |
.01 | | |
| | | _
,------+---------1k--------+ 15k / ^
| | | | \ |
| = 1000uF | | / |
| | ,-----+---10k--|----+-->\200k|
| gnd | | | / |
+---------, | =100uF load \ |
| | | | 0R5 | |
\c /+|-' | gnd 25uH | |
|--< | | PTC heating | |
/e \-|---' coil | |
| U1 | | |
| '--------+----+
| |
| ,----------------+-------100k--------+
| | | |
| | = 100n |
| | | |
| | gnd |
| '-|-\ |\U3 |-'
| U2 | >---| >o---------------------||
+-----|+/ |/ |-,
| |
'------------------Rg--------------------+
|
gnd
U1: TL082
U2: LM393
U3: inverting driver
Still using the concept of the resistance bridge to detect the
temperature of the heating element, but in a different way. Before, I
tried to do it going cycle by cycle, using the comparator to turn the
heater off when it reached the setpoint and the big problem was, when
you turn the heater off, everything stops. And even if there's a way
to get around that issue, the heating coil has inductance, and when the
comparator tries to turn the heating element off, it produces a huge
spike of negative feedback.
In the latest version of the circuit the comparator keeps running as a
voltage controlled oscillator that regulates the power through the
heating element. The RC network connected to the heater resitance
bridge filters out ac, smoothes out the voltage across the resistance
bridge and the diff amp sees this mean dc voltage that follows the
actual temperature of the heating element.
The resistance of the heating element changes about 2 or 3 tenths of a
percent for each degree change of centigrade. To keep the heater
within a ten degree window, we want the diff amp to go to the rails
when the voltage across the sense resistor changes about 2 or 3
percent. Now we have 25 amps going through 0.01 ohms for .25 volts,
and 2 percent of that is 5 mV. Amplifying that to 15 volts requires a
gain of 3000. So I guess Rg would need to be... 3 megs?
ic, Eeprom, ADconverter, I2C
