op-amp voltage drop

Wolframore

Joined Jan 21, 2019
2,342
Yeah not a huge fan of linear regulators but they are quiet... so having established the opamp is fine, the real issues is the incredibly Inefficient way of supplying 5v from a linear regulator supplying 23v, audioguru is right about that. Think about it at 10A... eek
 
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sghioto

Joined Dec 31, 2017
2,394
Why the drop?, am I expecting too much accuracy from this circuit?
I suspect as crutschow mentioned internal wire resistance. A 30mv to 60mv drop indicates and average internal resistance of .016 ohms.
A 60mv drop on 14.8 volts is less then .5%. Yes I believe you are expecting too much accuracy.
 

DickCappels

Joined Aug 21, 2008
7,707
I suspect as crutschow mentioned internal wire resistance. A 30mv to 60mv drop indicates and average internal resistance of .016 ohms.
A 60mv drop on 14.8 volts is less then .5%. Yes I believe you are expecting too much accuracy.
You can choose your own specifications. Better than 0.5% can be obtained, but that is over a limited current range. An IR drop in the wrong circuit path can destroy your regulation. One analog supply I made had the strange problem of increasing its output voltage as current draw increased- dangerous if not carefully applied.
 

Thread Starter

greenisland

Joined Jan 7, 2020
73
You can choose your own specifications. Better than 0.5% can be obtained, but that is over a limited current range. An IR drop in the wrong circuit path can destroy your regulation. One analog supply I made had the strange problem of increasing its output voltage as current draw increased- dangerous if not carefully applied.
Thanks everyone, I'm a first poster and a novice in every sense of the word.

Apology, apology, I have rechecked my voltage a pin 3 reference more carefully by soldering a wire very close to it, I am getting a drop of .03v here, I previously stated no drop at pin 3.
The shunt reference is LM4040CIZ-5.0 (Farnell) it has a tolerance of .5%, I knew this when I bought it, but I did'nt take this into consideration fully, I thought .5% sounds mighty tight considering the next one was 1%.

Please correct me if I'm wrong does this mean the 5v could wander by 25mv up or down on the 5v???, is that inaccuracy then increased by the op-amp??.

I made a second board for testing without the 2n2222 and and ran the ref through a divider to achieve 3v at pin 3,, I added the 100pF capacitors as suggested but still got the drop.
I also powered the board by a 7812 regulator same result.

Is it the reference shunt?
 

Thread Starter

greenisland

Joined Jan 7, 2020
73
I Don't. With that said I would be happy with .5% regulation over 4 amps
No thats at 2amp load

12v at output terminal, with a 2 amp load= 11.96v
12v ,, ,, 4 amp load=11.89v

I have a working Altai unit (very similar design) I am comparing it against, and the Altai does'nt drop at all, I have a lot to learn
 

Thread Starter

greenisland

Joined Jan 7, 2020
73
Earlier you said 60mv at 4 amps. Either way why not just duplicate the Altai circuit.
.
Sorry sghioto the drop at the ref is less, ref at rest 4.42, with a 2 amp load (bulb) pin 3 drops to 4.39v
with a 4 amp load pin 3 drops to 4.35v, the error seems to multiply

I'm trying to learn about op amps, and trying to keep it simple, the altai board is very complex with current limiting
 

Thread Starter

greenisland

Joined Jan 7, 2020
73
Sorry sghioto the drop at the ref is less, ref at rest 4.42, with a 2 amp load (bulb) pin 3 drops to 4.39v
with a 4 amp load pin 3 drops to 4.35v, the error seems to multiply

I'm trying to learn about op amps, and trying to keep it simple, the altai board is very complex with current limiting
The ouput of the op amp is the voltage at pin 3 multiplied by the gain?, Is the drop at pin 3 multiplied in a similar way as the gain?
 

Thread Starter

greenisland

Joined Jan 7, 2020
73
I don't understand why you need Q1. The input impedance of the op amp should not cause any load instability on the shunt.
You are quite correct, audio guru suggested this to me last night along with other help, I removed it today and dropped the ref voltage to @3v.0 with similar results as before, I explained all this in a big paragraph near the top of this page.

I made a silly mistake last night reporting no voltage drop at pin 3
 

Thread Starter

greenisland

Joined Jan 7, 2020
73
You are quite correct, audio guru suggested this to me last night along with other help, I removed it today and dropped the ref voltage to @3v.0 with similar results as before, I explained all this in a big paragraph near the top of this page.

I made a silly mistake last night reporting no voltage drop at pin 3
2 amp load 4.39v, 4 amp load 4.35v
 
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