I need help with Diff Amp Cct (see attached image). Im a bit rusty with my electronics but ive jumped back into the deep end.
Im building a Energy Management System for a Lithium battery pack. Im using an off the self BMS.
The battery pack will swing between 27v and 29.5v. I have voltage divided (VbattIn) down to swing of 3.5V to 4.1v. I have DC-DC down to 5v (Vcc) for the control circuits.
I want to compare Vcc to VbattIn (which should be 0v - 1v max ). Problem is when connect R6 to GND it shorts out via the 2k path to GND. My guess ts the selection of the 1k resistors around the Op Amp.
Any feedback / comments or suggestions would greatly help.
Background;
1, i will Gain up the out put once i get Ctt working. So Vout swings between 1V and 5V.
2, The EMS will be for a battery Mower, i will control the Battery during discharge to the motor and charging via PV .
Can see that.
Nobody is using an LM741 any more for at least thirty years
They don't work properly on a single 5volt supply, so forget about them.
"Voltage loupes" are generally speaking a waste of time.
Get an ADS1115, and divide the pack down to 4.096volt (PGA1) or 2.048volt (PGA2).
High enough resolution for the voltage window,
and build-in internal reference (not the potentially unstable 5volt supply).
Leo..
I need help with Diff Amp Cct (see attached image). Im a bit rusty with my electronics but ive jumped back into the deep end.
...
I want to compare Vcc to VbattIn (which should be 0v - 1v max ). Problem is when connect R6 to GND it shorts out via the 2k path to GND. My guess ts the selection of the 1k resistors around the Op Amp.
Firstly a voltage divider must never be loaded - so you'd need a buffer after the divider to drive a diff-amp circuit.
However that's completely unneeded because the gain/attenuation of a diff-amp circuit is completely adjustable
by the resistor ratios. It can do the division by attenuation itself.
You have chosen a unity gain diff. amp circuit, that's a special case. You have chosen 1k resistors in
your diff. amp circuit, which is pretty low for an opamp circuit - the '741 can't actually drive 1k, it's
good only for loads of 2k or more. 10k is a much more common value to see in opamp circuits unless
low-noise is an overriding issue.
If you are trying to compare voltages, use a comparator, not an opamp.
µA741's are pretty hopeless by modern standards, and here I think you need a fully rail-to-rail opamp anyway -
or a comparator perhaps...
The diff amp equation is basically V2-V1.
You have your batt voltage connected to V2 instead of V1.
Your 5V is >than your batt input so obviously V2-V1 will be <0 | Where V1 >V2
Also, FYI, use an Analog Devices (formerly Linear Technology) LT1215. As already mentioned,
if you're using a 741 you're either on a remote island in the pacific or on the moon. Nobody
uses them any more and there is really NO EXCUSE to use one NOW. There are any number
of better op amps available for less than the cost of a piece of candy.
Last but not least:
Get an ADS1115
Excellent suggestion. It has differential mode.
Don't forget to change the default setup in the example sketch to ADS1115 and comment out
the ADS1050.
Thanks all for your comments. The LM741 was what i had lying around and i have also played around with LM324N. Its quite a while since i worked on the bench so im doing this project as a bit fun to get back on the horse so to speak.
I will definitely check out the ADS1115 . i wll have to work out what is meant by "divide the pack down to 4.096volt (PGA1) or 2.048volt (PGA2)." I assume PGA1 and PGA2 are set points on the ADS1115.
Ill start reading up on it.
Either way this whole project has got me back into electronics and old things like divider networks and opamps are starting drift back into my mind.
'Thanks for pointing out my circuit was backwards'
'Your welcome'
You could simply ASK "What does PGA mean?"
ANSWER: Programable Gain Amp setting.
Have you ever heard of a datasheet ?
Do you know why they exist ?
PS: "Welcome to the 21st Century".
Take your 741, and flush it down the toilet and never speak of it again on this forum"
(no one has used them since the invention
of fire)
DaveDave:
i wll have to work out what is meant by "divide the pack down to 4.096volt (PGA1) or 2.048volt (PGA2)." I assume PGA1 and PGA2 are set points on the ADS1115.
The ADS1115 can be set to have a full-scale readout at user-selectable voltages.
If you set it to PGA2 (examples have options for that),
then 0-2.048volt input produces A/D values of 0-32767 (15-bit).
Just convert that A/D range to the voltage range you want with a suitable multiplication factor.
Leo..
The ADS1115 can be set to have a full-scale readout at user-selectable voltages.
If you set it to PGA2 (examples have options for that),
then 0-2.048volt input produces A/D values of 0-32767 (15-bit).
