Isolator/Amplifier

[simf14]

I would really appreciate some help from someone kind with a moment to spare for a ignorant novice.

I am designing a circuit that takes a variable 0-5V, 20ma voltage from a D-A converter controlled by my computer called a LabJack and turn it into a proportional 0-8V 2A output to power an hydraulic solenoid for a flight simulator. The catch is the DA signal should be isolated to protect the expensive LabJack and I have a 12V+ regulated power supply to use. (I can get a hold of a 12V+, 12V- if I must).

A very kind gentleman made a circuit up for me (see attached) but I keep blowing the expensive UA741s trying to get it to work...Should I somehow switch the circuit to 12V+ and 12V- ???

Thank you

CP Frustrated in California.

http://img213.imageshack.us/my.php?imag ... ly2ch8.jpg
(post edited, no sign up for photo, thanks Chris)

[MrAl]

Hi there,

In order to view that schematic the surfer needs to log in.
Since many people wont want to log in just to see the circuit,
copy the circuit to another site and post it here so everyone can
see it. We all can then take a look.

[Chris Smith]

Opto isolators are great devices to send signals, isolate, and protect both sides from them self.

[simf14]

what is a good site to copy it to (why don't they allow attachements here like most boards?) I am new to this thing.

thank you

[Chris Smith]

http://www.imageshack.us/

[simf14]

Thank you Chris Smith

here is the picture I hope.

http://img213.imageshack.us/my.php?imag ... ly2ch8.jpg

[Chris Smith]

The opto isolator [not the Darlington/ cascade types] can be an interface between the op amp and the 2n2222 trans.


It really depends on what swing or sensitivity curve is important.

Totally linear?

I sense a feed back that spikes your sensitive op amp, but have you first tried a crude 741 over the more sensitive version you are using?

Also there are many “op ampâ€

[simf14]

What I am being told is that basically the op amp is being driven too high and having the non-inverting input stuck at 6 volts means the 741 is trying to push lots of current into the base of the transistor when no signal is present from the DAC. Also the 2N2222 is trying to act like a variable resistor to control the output from the LM338. There is no feedback or adjustment in the circuit to make the output from the 338 agree with what I want it to be. I still have little comprehension of this and would love to have someone show me on a schematic what should be done properly.

[Chris Smith]

With the FET type op amps, it will just work or just saturate and not work?

I don’t have all my books or charts handy, or your specs, but I would try the replacement FET op amps and watch.

All the values can then be adjusted, with out the loss of the amp.

Also there is a circuit to controll the VR with out the use of the feed back 4000 diodes.

About twenty five years ago I used one preset trans for the feed mechanism to control the LM reg, and no feed back diodes were present.

I have seen this in the books recently.

I know I still have it all but the dust kills me?

BUT,...Its still there on the web.

[Robert Reed]

If for the moment we disregard the Op-Amp, you have a variable regulator circuit with what would normally be a variable resistance replaced by Q1. This is fine if Q1 is driven properly,to varies its current with regards to the job at hand. I don't know what the designer had in mind, but I don't see how this cicuit can work as it stands. Now looking at the Op-Amp, with no signal into the inverting input (0 volts+ 6 Volts Bias),and I am assuming the signals source impedance is very low, the output should be at 6 volts by virtue of the split rail bias at R5,R3 junction ( 6 volts X a noninverting gain of 2 and 6volts X an inverting gain of 1)). Of course the Op-Amps out put would never reach that voltage as it's load (2N2222) would be on the order of 25 0hms which is way too low for it. But still there would be enough drive to put Q1 into saturation. When the signal to the inverting input swings to + 5V plus 6volts bias =11 volts) the Op-Amp would try to output +1 volt - still keeping Q1 in saturation.This would tend to make Q1 look like a very low resistance and a very low voltage division at the junction of Q1, R4 no matter what the input voltage was to the Op-Amp. The regulator output would be a very low and steady voltage throughout. Are you sure you copied this print right? It seems to be lacking the correct base current limiting resistance between Q1 base and Op-Amp out put for the proper Q1 drive. This would now make Q1 a variable resistor in the range desired for proper operation of the LM338. Also D1,D2 have absolutely nothing to do with regulator output voltage - it is there merely for transient and capacitor discharge protection.Most of the time they are completely invisible to it's operation. Any how thats the way I see it unless I have missed something here.

[MrAl]

Hi there,


I have to agree with Robert in that this circuit as is will not work.

Here are some notes to get you started:

The pin labeled 'COM' on the schematic is really the 'ADJ' pin,
but i will call it the COM pin to make these notes simpler.

[1]
An LM358 op amp ic has better specs and it's very cheap too.
[2]
The op amp must have a negative supply voltage of at least minus 2 volts
if using the LM358 (minus 5 volts if using the LM741).
This is necessary because the LM338 can not put out zero volts with
zero volts on the COM pin.
In order to get the output of U2 to zero volts, the COM pin has to
be driven to at least -1.25 volts (not zero).
[3]
The output needs to be loaded by a min of 120 ohms, and that resistor
must connect between the output and the minus supply (not ground).
This is also needed in order to get the output to zero volts.
[4]
The transistor can be eliminated. Not only is it inverting
the signal, it is not needed anyway. The output of the op amp
can control the LM338 COM pin. This means connect the output
of the op amp directly to the ADJ pin.
[5]
The circuit is not connected properly either. It must be
rearranged to get it to work (see below).
[6]
The easiest way to get isolation from the computer is to use
an analog isolation amplifier. These arent real cheap (less
than 10 dollars) but they will save you a lot of time in
troubleshooting. If you cant find one on the web let me know.

New Connections:

First, disconnect everything and get rid of the transistor.
Add a minus supply of at least minus 2 volts. If you dont
do this, the output of the LM338 can only go down to 1.25 volts,
not zero volts.
Connect R4 from output to minus supply.
Connect output of op amp to the COM pin on U2.
Connect 50k resistor from inverting input of op amp to ground.
Connect 30k resistor from inverting input to the output of
the LM338 chip (not the output of the op amp).
Dont use the diodes...they are not needed for outputs less than
15 volts. Dont need the 10uf cap, the COM pin is being driven
by an external low Z source.
Connect input and output caps for the LM338.
Connect the DA +5v output to the non inverting terminal of the
op amp. Connect the other side of DA to ground.
Connect the 12v source to the LM338 and to the plus supply
pin of the op amp. Connect the minus supply pin of the op
amp to the minus supply.
The output of the LM338 now goes from 0 to 8 volts when the
DA converter goes from 0 to 5 volts.

If you want isolation insert a isolation amplifier between
the DA outputs and the op amp and ground.

Turn power on


If you want to build your own isolation amp we'll have to
look up some parts, and it will take you longer to get
this thing up and running.

[simf14]

Al, Robert, Chris, I greatly appreciate your gentlemen's knowledge and help. With my limited experience I can barely solder parts from a schematic. I do need to have maximum isolation in the circuit and need an isolation amp between the DA outputs, Op Amp and ground because the DA is so dang expensive and I am so prone to blow the thing up somehow.

Al I don't care how long it would take to get this thing up and running if you don't mind looking up some parts I downloaded Circuit Maker 6 student edition and will try to draw up a schematic and post today.


I really need the simple and detailed instructions like Al gave me with my abilities. Also a little help locating the proper 12V+and- power supply (mouser?, Jameco???)

Thanks

[haklesup]

First, the UA741 may be expensive but the LM741 is a penny part. Many op amps would work here. D/A can be expensice so that's the one to protect. I suppose that's what you really mean by isolation.

The main problem I see with your schematic (WRT blowing up op amps) is that the design depends on the source Vs1's ground being very well behaved. Notice that ground for Vs1 and ground for the circuit are not the same, only a 100K resistor R3 link the grounds. This is fine as long as they both maintain a true 0V reference but if the source Vs1 is distant and the ground were just a few volts different then you would develop a voltage drop across R3 which will offset the operating point of the op amp causing the output to go to the min or max (rail). While operating a 2A load, ground can fluctuate.

Furthermore, with Vs1 directly connected to the inputs of the op amp, you have a situation where the input protection of the op amp will activate under the same unequal ground condition which could certainly result in blowing the part up (or at least shorting the inputs). THe same can happen in reverse to Vs1 your D/A output.

A simple modification would put equal value resistors in series with the op amp inputs. Since at op amp inputs the current should =0, the resistors provide current limiting to protect the device from overstress in the unequal ground scenario thus providing some isolation. 1M would probably do fine.

The key here is to see the input as a differential pair rather than a voltage source with 0V ground reference. But thats just going on the schematic. In your setup, it is easy to overlook grounding issues. You must remember just because several points are labeled "ground", dosen't mean they all have the same voltage until you short them together. Just put a voltmeter between the two grounds to check.

May be better to find a way to make it a common ground circuit. In this the ground of Vs1 would connect to ground of the output stage rather than the non inverting input. and that input would also be grounded through a series resistor.

An isolation tranformer is a good idea but the input must continue to change at a minimum rate, once it goes to any DC value, the output will just fall to 0V. An opto isolator is mainly for isolating digital inputs/outputs and is difficult to operate as an analog component.

High value series resistors into FET or JFET inputs generally provide the highest impedance isolation for analog. A series resistor at your D/A output would also help prevent overstress current from ruining the part. Since the output is later buffered, the D/A need only be a voltage reference and not supply much current/power. Again 1 M provides a lot of current limiting but may slow response time whicn should not be an issue in a power supply.

No comment on the rest of the circuit, the other guys got that covered.

BTW, you certainly can post images here without using a link from a third party site. Just click on the "add image to post" link just below the text window when you are typing a reply.

[Robert Reed]

Hacklesup
I don't see the "add image to post" option When bringing up the text box, for either replies or posts. Is there something I have to initiate to make that option available?

[Chris Smith]

Profile…….FI Sub Silver

This is right under the Text Box

First a arrow, then a tiny box, followed by.... Add image to post