Capcitor discharge problem

If you want to discharge the capacitor, why not connect the resistor and RY2 contacts in series, directly across the capacitor? Make the resistor small, just large enough to protect RY2 contacts and the capacitor from excessive current. If at the time the capacitor is being discharged, you also want to insure that the load current is zero, use SPDT contacts for RY2, connect the moveable contact to the output end of the capacitor (now goes to RY1 moveable contact), the normally-closed side to the RY1 moveable contact, and the normally-open side to the resistor, the other side of which goes back to the capacitor's input. I suppose ten ohms would be OK, though considering the activation time of any mechanical relay and the dielectric absorption in the capacitor, a hundred ohms would probably be fine:

Now just WHY you have a capacitor in there is a bit of a puzzle to me, since ultimately it will just limit the maximum time that a current of a particular polarity can be delivered to the load. But I'll leave that puzzle to you.

Cheers, Tom

Hi,

Thanks for the reply! I am using the capacitor for AC coupling. Can I use another method to do it? John

The point is that the capacitor at that point in the circuit won't behave like an AC coupling capacitor driven from a voltage source, driving a resistive load. Why? Because if the current source has a DC component, that DC component will flow into the capacitor (and out the other end of the capacitor, through the load), charging the capacitor until the current source can no longer supply enough voltage. Then the current source will no longer supply the requested current. If there is a higher-frequency AC component in addition, it will become clipped as the source approaches the limit of the voltage it can supply. In contrast, a source with constant DC voltage just puts a charge of that voltage on the capacitor, and after the initial transient, the load is unaffected by the source's DC component. If you had a DC current source with infinite voltage compliance and a coupling capacitor with infinite voltage capability, the load would see a DC current through the capacitor.

Another way to look at it (in the frequency domain instead of the time domain) is that the capacitor's AC impedance is infinite at DC and is inversely proportional to frequency. If the source driving it has infinite impedance (a perfect current source), the capacitor will look relatively like a short-circuit down to very low frequency. In the limit as the frequency goes to zero, the current remains the same, so the load continues to see the same current. That model assumes linearity, which will fail as the current source output DC voltage approaches the supply rails.

One way to insure that you don't need to reset the capacitor is to monitor the DC voltage at the output of the current source and use feedback to insure that it stays within bounds. Another way is to insure that the DAC output does not have a DC component, or at least that any DC component of its output is negligible. Then you should not need a coupling capacitor at all, assuming low DC offset in the DAC and op amp circuits. Since I don't know what it is you are trying ultimately to accomplish, it's a bit difficult for me to give recommendations.

Cheers, Tom

Hi,

I am trying to provide constant current with no DC component to tissue via electrodes. So, the circuit's load is tissue. I thought that the capacitor will not let DC component pass through it and will let only AC pass through it. you mentioned that DC component can be reomoved at the DAC's output. Can you advice me how? Will a capacitor between the output of the DAC and the input of the INA133 ( non-inverting input ) remove the DC component?

John

John, if this is going to HUMAN tissue, I suggest you stop right now.

You don't appear to know basic electronics well enough to come up with a safe circuit.

Any circuit hooked up to a human needs to be very carefully designed and tested.

Same goes for animals, unless you want the PETA types to get on your case.

Unless you are _cooking_ them ;-)

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

Hi,

Its not going to humans or animals. the tissues will be extracted from rats and will be in the dish under the microscope. Now, yes i do not know about electronics that much and i have not tested the design as you can see that I am seeking advices and analyzing. So, if you have advice for me then you are most welcome though you do not have to be scared and tell me not to go for it.

Thanks

Jim Thomps> >

I like my PETA activists medium-rare, served on a bed of rice :)

Tim

John,

How much current do you want? Specifically, what is the instantaneous peak value of current you want to achieve? What is the expected maximum and minimum impedance represented between the electrodes in the tissue? I suppose "constant current" means current with a constant RMS value, or current essentially independent of the impedance of the tissue. Clearly if there is no DC component, it's not constant in an instantaneous sense.

If you just AC couple the signal from the DAC to the voltage-to-current converter, then the output current (neglecting any small DC offsets in the op amps) must also have no DC component. If it does, you could easily null that out with a small current fed through a high-value resistor from a potentiometer (variable resistor; "trim pot") connected between the supply voltages. So the circuit would place a capacitor between the DAC and the input to the current to voltage converter, and omit the capacitor at the output. If the DAC is driven from a digital source that can filter out any DC component in the signal digitally, you could omit the capacitor. And you could (perhaps) even monitor the current in the tissue with an integrator, fed back to some earlier point in the circuit, to automatically insure that the DC component of the current in the tissue is zero, or as nearly so as the integrator is able to detect.

If the desired maximum current is small enough (a few microamps), it should work fine to just use a voltage source, with a high-value resistor between the voltage source and the tissue. If the resistor can be 100 times as large as any variation in the tissue resistance, you know the current will be stable over that variation to within a percent.

Cheers, Tom

There is really no way to apply an AC current to any sort of load without having some DC component, at least during the first several cycles. However, it can be made arbitrarily small if you can delay the application of the final value of AC current.

The DC component is really the average of the waveform over the period of time from first application to the present. If you simply apply a sine wave current, starting at zero crossing, the average current will first increase until it reaches the peak, and then decrease as it goes through zero crossing, and continue decreasing until the second zero crossing, at which it will be zero. As continually more excursions occur, the true average over time will decay toward zero.

This is why it is not good to use a zero crossing solid state relay to an inductive load, as there will almost always be a net DC component that can saturate or magnetize the inductor. Applying the waveform at the peak causes the DC component to swing positive and negative as a damped oscillation.

One way to minimize DC offset is to start the signal at a very low level, and gradually increase its amplitude until the desired value is reached. If this is acceptable, it can be done easily with the DAC by using a voltage ramp as the reference, or by providing the correct digital signal to the input.

If this is for a scientific study then it is imperative to have a full understanding of the physiology of the electric current through the tissue, and the effects of various DC levels over short periods of time, as well as the magnitude, frequency, and length of time of application of AC signals. It would be helpful to know the reason for the need to block DC offset, and determine if it can be tolerated for a short time at the start of the test.

Paul

Gee, and i always thought that transformers would not transport DC. Then again transient conditions is not DC by definition. Then you wander and return to near topic.

Now if OP is having problems with digging the desired out of the startup transient that is a different problem, and appropriate methods for controlling the transient are to the point.