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simf14
Joined: 29 Jul 2007
Posts: 4
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 29 Jul 2007 19:42 isolator amplifier |
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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.
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amitpjag
Joined: 29 Jul 2007
Posts: 7
Helped: 1
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30 Jul 2007 10:06 Re: Isolator/Amplifier |
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I think you must shift to +12v and -12v supply.
Differential supply will also be helpful in case of input noise,spike.
Try it.
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IanP
Joined: 05 Oct 2004
Posts: 6490
Helped: 1542
Location: West Coast
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30 Jul 2007 12:33 Re: Isolator/Amplifier |
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The LM338 voltage regulator can be used to generate output voltage from 1.25-to-MAX ..
If you need the output to be as close to 0V, you have to "shift" the output level using, for example, diodes, that's first ..
Second, your current circuit can not properly control the LM338 ..
Take a look at the attached picture (replace 317 with 338 and 741 with 358 or 324) ..
The voltage error amplifier (U1A) has direct control over the ADJ pin, and also has feedback from the output (R7-R9) ..
This circuit may need some small modifications, but should produce linear output voltage @ the rated current ..
If you don't need ON/OFF option - remove Q2 and R6 ..
Regards,
IanP
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simf14
Joined: 29 Jul 2007
Posts: 4
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30 Jul 2007 15:14 Re: Isolator/Amplifier |
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Thank you Ian. Sorry for my level of knowledge. I don't fully understand why there are three leads for the DA labeled Blank, DRVO1 and AN0. I have two output leads on the DA labeled AnalogInput0 and GND...
I bet the lead labeled Blank is for the on/off command, could the one labeled Analog Input be the ground lead??? Also is there sufficient isolation from the DA to prevent me from somehow blowing the expensive thing up???
Thanks so much!!!
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simf14
Joined: 29 Jul 2007
Posts: 4
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31 Jul 2007 5:37 Re: Isolator/Amplifier |
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On Ian's drawing can anyone tell me the difference between what Vcc is and V_sup??
Thanks
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31 Jul 2007 5:37 Ads |
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IanP
Joined: 05 Oct 2004
Posts: 6490
Helped: 1542
Location: West Coast
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31 Jul 2007 6:15 Re: Isolator/Amplifier |
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Vcc is the microcontroler's supply (+5V) and Vsup is the main external supply voltage, in your case +12V ..
If you don't want (need) to independently monitor the output voltage you can omit part of the circuit that feeds back the output voltage to the microcontroller (U1B) ..
Regards,
IanP
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simf14
Joined: 29 Jul 2007
Posts: 4
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31 Jul 2007 15:12 Re: Isolator/Amplifier |
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Thank you Ian I will give it a try this week. I appreciate so much your help. It has been a very discouraging trek with my level of knowledge.
Your circuit appears to be perfect for me, I'll try to build it this week. I hope I may further impose upon you to possibly answer a couple more questions later...
Cheers! 
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