Currently i am using AD8544 for a test circuit to prototype a latching circuit using an Op-Amp. Here is the circuit
I have wired as per above.
Setup: When Vsense is above than Vref, o/p goes high and D2 gets forward biased and this becomes a closed loop so even when Vsense is removed the output is high(LED will glow).
1. When Vref is turned ON first and then Vsense and then Vcc, the circuit works properly and when Vsense is above Vref LED remains ON. working properly.
2. Back to OFF state, I turn ON Vcc first(Vsense and Vref is not turned ON yet) the LED continues to be ON ie O/p goes high.
I am not clearly getting what is the problem.
AD8544 is a quad opamp. I am using only 1 of them. others are left open. will this create such a scenario?
Please help me.
Thanks in advance.
To properly connect unused pins of opamp or channels of opamp not used, here is a good reference.
Where the Vref pin is connected in second case? Way for flowing a bias current is required.
Bias current applied to R4 resistor and cause drop voltage on this. This voltage applied to non-inverting input and amplifying. Output voltage becomes large then drop voltage on diode and circuit is latching
In the second case, only Vcc has been applied. Vref and Vsense is floating.
Yesterday i did some more modifications. please see below diagram. I placed pull down for both inverting and non inverting pins.
Applied only Vcc. Even now also LED is glowing.
Leaving the opamp inputs floating (VREF and Vsense) could cause the output of the opamp to saturate.
Probably the reason why the LED is continuously ON.
Please refer to the link of the reference I mentioned on my first response.
You mean to say output saturates because of input offset voltage phenomenon? I checked datasheet and found voltage gain is 500V/mV and offset voltage is 6mV so technically in a open loop condition with both pins grounded output voltage can come to 500* 6mv = 3V. this is enough for LED and for latching. Is this what happening here?
Please correct me if i am wrong.