Op-Amp Input Voltage Higher Than + Rail?? - Page 1
Forum section: electronics / beginners

author: Travis
author_rank: Contributor
timestamp: April 01, 2014, 03:20:03 pm
content: Hi all. I have been going through a schematic referenced in episode 397 (EEVblog #397 - Turnigy Accucell 6 Charger Teardown).I have simulated the values for a 6 cell pack at 20 volts (perfectly balanced). The 20 volt input is BAL6. The 16.7 volt input is BAL5. I left out a 620ohm resistor however even with that in the circuit its still over the 5 volt rail.Is this really ok to do?? Maybe I'm not understanding something fundamental about how opamps work? Is the 6.66 volt input to the opamp an issue?Any input appreciated. I'm just getting started so go easy on me.Travis

author: AG6QR
author_rank: Frequent Contributor
timestamp: April 01, 2014, 05:41:07 pm
content: That's generally not OK. To get the authoritative answer, see the datasheet for the particular part you're using. I found a datasheet covering the 324A (not quite exactly your part, but close) herehttp://www.ti.com/lit/ds/symlink/lm324.pdfLook for the Vicr , or common mode input voltage range. The allowable range is something like 0 to Vcc - 2V, depending on the exact part and temperature range. But the important thing is that the top of the range is specified as Vcc minus some value.I don't know the context of that circuit well enough to say where the problem may be, but you're right that the input voltage being higher than the op amp's Vcc is likely to be a problem.

author: Travis
author_rank: Contributor
timestamp: April 01, 2014, 06:02:22 pm
content: AG6QRThanks for the help. The circuit is in the cell balancing circuit of this charger. Its to read the voltage of a single cell in the pack. They claim the unit has a 20V max pack voltage. The only thing I can figure is that they are doing the balancing when the pack is below 15 volts?I have found values that yield a result of 5 volts. However after reading your post it appears that I have to take more into account than just staying below the rails.After looking at a few data sheets it appears that (for the parts I have on hand) the Vicr is VCC - 2V (at full temp range). So that means on 5V Vcc my max voltage should be below 3V?I'm just trying to make sure I fully understand. If this is true it'll make the design a little bit more tricky.

author: Zero999
author_rank: Super Contributor
timestamp: April 01, 2014, 07:22:47 pm
content: Yes, that's gnerally the case.If your supply voltage is 5V and your op-amp's common mode input range is +V-2V then your maximum input voltage is 3V.is there any reason why you can't power the LM324 from the batteries? The maximum supply voltage is 32V.

author: Travis
author_rank: Contributor
timestamp: April 01, 2014, 07:26:55 pm
content: No there is no reason I couldn't power it from the batteries. I was just going by the schematic as presented. I saw that and instantly thought it would be an issue.It appears the easy solution is powering it directly from the batteries.

author: Kryan9
author_rank: Contributor
timestamp: April 01, 2014, 11:07:42 pm
content: Is it okay? Assuming the LM324N, yes. It won't work for the reasons described, but the LM324 has a very rugged input circuit, and the ESD diodes will conduct the voltage to the positive supply (+5V), with the 500k and 250k resistor heavily limiting the current that will flow.If you're using another op-amp, you may have more problems. The supply protection diodes don't exist in all op-amps - instead, some have non-inverting/inverting diodes instead (the LM324 forgoes these). These diodes may or may not provide the same over-voltage protection.