constant current source problems

[john]

Hi,

Please go the following link and then figure 16 page # 13.
http://focus.ti.com/lit/ds/symlink/ina2133.pdf

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.

2. 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

 

[Tim Wescott]

Hi,

Please go the following link and then figure 16 page # 13.
http://focus.ti.com/lit/ds/symlink/ina2133.pdf

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.

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?

2. 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.

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
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[john]

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.

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?

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

 

[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?

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

Please advice
John

 

[Stanislaw Flatto]

Hello,

1. What if I just turn off and on the +/- 16 volt supply of the INA2133
by using two relays simulataneously.

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

 

[john]

Hi,

anybody else can advice me.

John

 

[John Fields]

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?

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

---
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.

 

[john]

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

 

[john]

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

 

[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?

 

[john]

Hi,

Anybody who knows!

John