op-amp for my power supply?

[Sajjadkhan]

pufffffff, ok after checking the whole market i found AD623,, its rail to rail and 100uV of offset. what do you thing about it?

secondly is it possible to sense current with an op-amp which is not rail to rail, any reason? as i found AD620 also. I would be having negative supply in my circuit though.

 

[Sajjadkhan]

OK I didn’t go for AD623 as I found a better solution MAX472. It was expensive but I am building something for myself. One problem I have come across is that input voltage on sense pin is up to 36V and my toridal transformer voltage is 18,24,36 VRMS, so to achieve 30v on my supply I would have to switch to 36V at regulator input, as 24VRMS gives 33.9V after passing through rectifier and capacitor but subtract 10% for ripples and 2V for regulator so I can not use 24V.

So I am thinking to connect Rsense after regulator as it wont be going above 30V. Rsense will be before the load so I can say its still on the high side. Problem is that if Rsense value is large than there will be a voltage drop on it and that will be compromised from load.

I have already bought 10 resistors of 1 ohms, 0.5W and 0.1% tolerance (pretty expensive). I will connect them in parallel to make 0.1 ohm of Rsense. So I will get Max voltage drop of 0.3V at 3 Amps Max.

I can compensate for this 0.3V drop if I connect Rsense before the Rref resistor. Look at the schematic.

As voltage is exceeding 40V so I am using TL783, its pretty much same as lm317 but for higher voltages.

Schematic is for understanding purpose only!
Any suggestions?

 

[crutschow]

You just need to use a low input offset op amp, of which there are many. Just Google "low offset op amp". The OP27 is a common example.

 

[Sajjadkhan]

You just need to use a low input offset op amp, of which there are many. Just Google "low offset op amp". The OP27 is a common example.

Its not rail to rail or have single supply operation, its not mentioned in the data sheet.

 

[goldsmith]

What do you mean by rail to rail ?
By the way , if you want design a simple PSU for your self , why not a simple SMPS ? or if you want linear , why not a simple LDO ?
Best Wishes
Goldsmith

 

[Sajjadkhan]

Rail means supply like positive rail and ground rail. Op-amps which usualy operates at positive and negative voltage and dosent need ground,those are not rail to rail. In a datasheet if op-amp supply voltage is mentioned as +- 3v to +- 18V then its not rail to rail like in AD620. but if its written as like +-1.5v tp +-16v or 3v to 32V as in lm358 then its rail to rail. only rail to rail op-amps can operate in single supply circuit, again by single supply i mean that you dont have negative voltage in your circuit.

Frankly speaking i have completed computer system engineering and i haven't done power electronics. so i don't know much about those SMPS, buck and boost kinda things. any ways people usually say that use a linear power supply to avoid noise in your circuit. Else every where on net i have seen people building power supplies using linear transformers.

 

[alexan_e]

Rail means supply like positive rail and ground rail. Op-amps which usualy operates at positive and negative voltage and dosent need ground,those are not rail to rail. In a datasheet if op-amp supply voltage is mentioned as +- 3v to +- 18V then its not rail to rail like in AD620. but if its written as like +-1.5v tp +-16v or 3v to 32V as in lm358 then its rail to rail. only rail to rail op-amps can operate in single supply circuit, again by single supply i mean that you dont have negative voltage in your circuit.

That is not what a rail to rail opamp means, what you describe is an opamp which can operate with a single supply and doesn't require a symmetric one.

**broken link removed**

Rail-to-rail op amps
Designers often request rail-to-rail capability when selecting an op amp. It would seem an obvious choice since many applications benefit from maximum signal swing. But a true rail-to-rail op amp may not be required and furthermore, may have disadvantages in the application.

Rail-to-rail implies that the op amp has a rail-to-rail input capability and rail-to-rail output. Rail-to-rail output is merely a relative term, as there is no industry standard to define the term. Depending on load conditions, rail-to-rail output amps may swing to within millivolts from the rail to hundreds of millivolts.

Alex

 

[Sajjadkhan]

Are you saying that a true rail to rail means that voltage goes down to ground exactly or lets say touches the rails, if so then i knew it, sorry i dint mentioned it..

---------- Post added at 16:59 ---------- Previous post was at 16:52 ----------

Thanks for the doc, its quite useful.

 

[alexan_e]

A rail to rail opamp has to do with the range that it can accept or output.

With your definition LM358 is supposed to be rail to rail but if you check the datasheet you will see that the Input Common-Mode Voltage Range is 0 to (V+) -1.5v

Now compare that to a rail to rail opamp like OP279 , with a power supply of 5v the Input Common-Mode Voltage Range is 0 to 5v (0 to V+)

Alex

 

[goldsmith]

linear power supply to avoid noise in your circuit

Hi Again
About this i'm pretty disagree with you ! if you learn how to design an SMPS correctly , you won't have any problem !
I suggest you take a look at datasheet of LM2576 ( for start at SMPS it will be a nice idea ! )
Best Wishes
Goldsmith