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constant current source problems

Hi,

Please go the following link and then figure 16 page # 13.

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I can not find a way to post the schematic on the web so i am explaining that I what did I do with the circuit. The INA133 is getting the input from a voltage ouput DAC ( span is +/- 2.5 volts) . DAC is connected to pin# 3 of the INA133 and pin#2 in grounded. A capacitor of

47uf and is connected between the non-inverting pin of the OPA131 and a relay1 ( which connects the output of the capacitor to two different loads ). The capacitor provides AC coupling. The relay1 connects the current source to one laod at one time. The power supply of the circuit is +/- 16 volts. Inorder to discharge the capacitor, I connected a relay2 in series with a resistor at the input of the capacitor or at the node where the OPA131 and the capacitor connects together. So, when the capacitor gets charged I just turn on the relay2 and connect the node to zero volts.

Problems:

  1. My application needs to switch between the loads, So when I turn on the circuit, the circuit is connected to the load A, I get AC signal across my load with no DC offset. But when I switch to load B then I get DC offset with AC signal across the second load. I do not know why, may be the current source does not like to be floating and it hits the maximum power supplies during the switching time from one load to another load? Is it true or is there some other explanation.

  1. Am I discharging the capacitor in a rigth way. I a resistor necessary to disharge it or I can just short it with the ground. The capacitor charges upto 14 volts.

Thanks John

Reply to
john
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If I'm interpreting your description correctly, the only way that a load can get current is through the capacitor -- correct? If that is so, then there are only two possibilities for getting a DC current into a load: One, you're switching things around fast enough that an induced bias on the capacitor gets dumped into load B, and two, your capacitor isn't working like a capacitor.

You can test the first theory by just holding the output on load B for a while, and see if the bias dies out.

The second theory would easily prove correct if your capacitor is electrolytic and you're running it backwards. Electrolytics are polar devices, and act like crappy diodes when you use them backwards -- could this be your difficulty?

Define "right". I would connect the resistor across the cap, rather than grounding one end of the thing, and I would give it sufficient time to discharge (5x RC at least, more if you need a great deal of precision). Shorting the cap directly is manly, but hard on both cap and relay.

--

Tim Wescott
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Reply to
Tim Wescott

Hi, Thanks very much for ur reply

"Define "right". I would connect the resistor across the cap, rather than grounding one end of the thing, and I would give it sufficient time to discharge (5x RC at least, more if you need a great deal of precision). Shorting the cap directly is manly, but hard on both cap and relay".

If I connect the resistor across the capacitor then the current ( DC) can flow into the load. My application can not afford it.

I am not using the electrolytic capacitor. I am using the polarity less capacitor. If I discharge the capacitor and during the disharging I just switch the load, I think then the load B will not get any thing. What do u advice?

regards John

Reply to
john

Hello,

  1. What if I just turn off and on the +/- 16 volt supply of the INA2133 by using two relays simulataneously. Now the DAC is ON ( but generating only zero at the output ). Will this scheme hurt the INA2133 OR OPA131 in any way? If not then I think this will solve the problem. Turn OFF the power supply, swith the relay to load B and then turn ON the power supply again?

  1. If the constant current source have no load then how does it behave. Does it output hits the rail voltage?

Please advice John

Reply to
john

Forget it. Does not exist two relays having the same response times so depending on sensitivity of your cirquit to supply unballance (momentary!) you are for nice head scratching. Do everything KISS.

Stanislaw

Reply to
Stanislaw Flatto

Hi,

anybody else can advice me.

John

Reply to
john 

--
Yes. It has to. 

Consider:  If the source is supposed to pump current into a load by
increasing the voltage across the load until the desired current is
in the load then, if the load\'s resistance is so high that the
source\'s voltage can\'t push that current through the load, the
source\'s output voltage will go to the rail(s) in its attempt to
supply the required current into the load.
Reply to
John Fields

Hi ,

Thanks for the reply. Would you please advice me on the point#1 that if I trun OFF or ON the power supply. Will it hurt the INA 133 in any way?

Thanks John

Reply to
john

Reply to
john 

--
I don\'t know.
Reply to
John Fields

Hi,

Anybody who knows!

John John Fields wrote:

Reply to
john

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