so i need to use a common rail power supply as in the AC common is common with the DC common. then i need to recieve a 0 to 10vdc varable signal and increase its current while keeping the voltage the same. so i went to use an OP amp set it up with outpuut feed back to the negative input and it works great from 2 VDC to 10VDC but will not go below 2 VDC i am assuming this is because the output has to be 2 volt above the negative rail voltage. unfortunatly as sdtated above i have to use a common rail power supply so the reference signal for the 0 to 10VDC input is the same as the reference for the power supply. are therer any op amps that can work much closer to the negative rail or is there soem way to fool the op amp into thinking its negative rail is 2 volts below the common?

 

There are thousands of different op-amps on the market. You did not say which op-amp you are using.

Yes, there are op-amps that will output voltages close to the supply voltages.

 

so i need to use a common rail power supply as in the AC common is common with the DC common. then i need to recieve a 0 to 10vdc varable signal and increase its current while keeping the voltage the same. so i went to use an OP amp set it up with outpuut feed back to the negative input and it works great from 2 VDC to 10VDC but will not go below 2 VDC i am assuming this is because the output has to be 2 volt above the negative rail voltage. unfortunatly as sdtated above i have to use a common rail power supply so the reference signal for the 0 to 10VDC input is the same as the reference for the power supply. are therer any op amps that can work much closer to the negative rail or is there soem way to fool the op amp into thinking its negative rail is 2 volts below the common?

Search for op amps with "rail to rail" inputs and outputs. I'm not sure they get all the way to zero volts, but they can get very, very close. I don't know op amps very well yet, but I believe the LMC6482 would be one possible solution.

Another option is to use a chip that generates a negative voltage to use for the negative power supply on the op amp. There are devices made for this exact situation.

 

If you need actual 0V inputs and outputs (not just close,) here's an example of a chip that creates a very small negative voltage, just enough to allow a rail to rail op amp to reach 0V:
http://www.ti.com/product/LM7705

You can also use more traditional voltage converters to generate a negative voltage of larger magnitudes:
http://www.ti.com/product/TL7660

These solutions may be overkill, depending on how accurate your 0V signals need to be. In most situations I've dealt with so far (which is fairly limited at this point,) I haven't needed a negative supply.

 

i am using a LM 1458 because i needed a high current output

 

i am using a LM 1458 because i needed a high current output

What is the load that you're driving with the op amp? How much current do you need to get out of it? What is the nature of the signal? How quickly does it change? (trying to determine how much of an issue bandwidth and slew rate are.) Show us a schematic of your circuit so far.

You can probably use a low current op amp in conjunction with a transistor to handle the current, instead of needing a special high current op amp, but we can't say for sure without all the details.

 

I need to boost an analoug 0 to 10vdc input. When I try to use this signal to speed control VFD,S it can only handle 2 before voltage drop due to the resistance the VFD input, so I if use the incoming 0 to 10vdc to a dual op amp it can drive more tandem VFD,S especially if it had a high current output.

 

What is the spec for the input and how many of them do you need to drive?

Bob

 

I need to boost an analoug 0 to 10vdc input. When I try to use this signal to speed control VFD,S it can only handle 2 before voltage drop due to the resistance the VFD input, so I if use the incoming 0 to 10vdc to a dual op amp it can drive more tandem VFD,S especially if it had a high current output.

***EDIT: BobTPH beat me to it!

Ok, do you have a spec sheet or datasheet for the VFD? Can you find out what the input specs are? Also, what's the largest number of VFDs you want to be able to run off of this circuit? (I'm trying to determine the output current requirements for this circuit.)

 

Ebeowulf has already given you several practical solutions that work.

Another trick I've used is to employ a general purpose opamp like a LM358, which will swing to about 200 mV, then use a NPN emitter follower.
Its Vbe drop will consume any of the opamp's remaining Vol voltage, and will provide you with an additional current boost.

 

I can't believe that the antique LM1458 (a copy of an old RC1458 and MC1458) is still available. I think it was the very first dual 741 opamp (designed 52 years ago) ever made.

 

Like followers of Imhotep, they rock down the street going (in a deep voice): "741....741...."

 

The LMC6482 looks to be a good idea however i spoke to my local supplier and they are not sure if they can get them still. I use several 0 to 10vdc output sensors linked together in high signal select method depending on the job any where from 1 to 10, this is fed to up to 6 VFD,S some are better than others but I do not get to choose which are used. The same concept will also be used to control 3 phase EC motors that have built in speed control.

 

The LMC6482 looks to be a good idea however i spoke to my local supplier and they are not sure if they can get them still. I use several 0 to 10vdc output sensors linked together in high signal select method depending on the job any where from 1 to 10, this is fed to up to 6 VFD,S some are better than others but I do not get to choose which are used. The same concept will also be used to control 3 phase EC motors that have built in speed control.

It's still not clear to me what the input requirements of the VFDs are. I'm familiar with them in concept, but haven't designed circuits to interface with them, so it's not like I have their specs memorized.

How much current does a VFD input draw at max voltage? Or, to put it another away, what's the input resistance of the VFD control signal input?

As for the LMC6482, I like it because I've had good experiences with it, but there should be lots and lots of rail to rail op amps to choose from. There's no need to get just that one if it's hard to find (although I'm surprised to hear that it's hard to find - where are you located?)

Regardless of op amp choice, rail to rail only deals with the question of that 2V lower limit. You still have to figure out how much current you need out of it. Very few op amps are designed to deliver significant amounts of current. It's still quite possible that you'll need an amplifier stage of some sort no matter which op amp you choose.

 

All VFD,S are different, theoretically there should only be a high input resistance so I was surprised to discover input voltage drop with E510 drives when linking more than 2 together, the manufacture will not tell and I did not have the time or gear out on the job to check the current draw I just had to find a quick answer to keep client happy. I will try to get the out put current of the sensors as that will be a guide

 

Oh, I also forgot to ask what voltage you have available to power the op amp. If it's high enough, you can use @schmitt trigger's suggestion.

 

Max 24vdc but was thinking of including a 12vdc or 15vdc regulator in circuit, can not use negative rail only o to 15vdc max

 

Only stores that are "essential" like food stores are allowed to be open during this COVID-19 catastrophe.
So maybe if The Dollar Store is open because they sell candies then the electronic parts stores can also sell candies to be open.

 

My local electronics parts store is selling hand sanitizers, alcohol swipes, gloves, safety goggles, as well as the usual electronics components. You can order by phone or on-line and receive goods by delivery or curb-side pick-up.

 

My local pharmacy ran out of those important things.
Some countries have curbside pickup of bodies of people killed by the virus.
Quebec has the Canadian military helping them.