Need ideas on where to start for a circuit.

Thread Starter

Halsey

Joined Mar 4, 2018
30
I need a DC circuit that has an ability to reliably go from 1mA to 2mA and back to 1mA in a sine-wave fashion and will do this consistently under a varying load. Any tips are appreciated.
 

crutschow

Joined Mar 14, 2008
23,293
What's the sinewave frequency range and the maximum load resistance?
Sounds like a constant current source will do what you want.
Do you have a sinewave generator or do you also need to build that?
 

-live wire-

Joined Dec 22, 2017
883
Here is my understanding: you want to make an sinusoidal AC constant current supply that supplies an adjustable average current of 1-2mA. Your power supply is DC. Is that correct? So now here is what I would like to know. What voltage range would you like? How precise do you need it to be? What kinds of protections are necessary (for momentary overcurrent, etc.)? How good do you need your control system to be? What frequency would you like? More info would be helpful.

So here is my idea based on the given info. You have it be phase-angle fired so that it always measures the current at a certain point in the sine wave. You could use a high-value shunt (with op-amp) hall sensor or something. You just need to figure out when the measured current would be the average current along the sine wave. Then maybe use linear or even better PID control.

But how do you generate such a sine wave from DC? The solution is simple. Make a square wave oscillator and feed the output to some capacitors. The square wave oscillator should use an H-bridge with inputs from a 555. Then have a 555 with inv and non-inv inputs to the h-bridge (both halves). The frequency will determine the sine wave frequency. Then your caps should turn it into a sine wave. You can then use a transformer to step it up or down for your desired voltage range on the output. A transformer

So then how do you make it adjustable? Well, you just need to add a DC-DC buck converter in the beginning. This reduces the voltage into the oscillator and obviously on the transformer output. But how do you make a DC-DC buck converter? Just use a MOSFET with driver, some caps, and then your control system feeds it a high-frequent variable duty cycle PWM. You may want to add a voltage divider to make sure the load changes do not affect the output voltage, but then you are regulating two things. You will probably not need to regulate the voltage if you are regulating the output current though. But one last thing. Keep in mind that there will be voltage drops and other losses. So tiny voltages on the output may be 1 or two on the input.

I am not the most knowledgeable about control systems, but a microcontroller-based system is one way to go if you need more advanced control. For linear there are LM regulators and such.

One thing about the DC-DC converter. Maybe use an I2C PWM generator, VCO, or something similar because you want a few 100kHz and a 10+ bit resolution. That is at least a 100MHz clock for a microcontroller.
 

AnalogKid

Joined Aug 1, 2013
8,100
Here is my understanding: you want to make an sinusoidal AC constant current supply that supplies an adjustable average current of 1-2mA.
Disagree. I think the req is for a sinusoidal alternating current with a fixed +1.5 mA DC offset. The signal has a negative peak value of +1 mA and a positive peak value of +2 mA. Voltage compliance is unknown at this time.

Sinewave oscillator circuit driving a Howland current pump.

ak
 

BR-549

Joined Sep 22, 2013
4,936
I'm just thinking out loud. Never really played with it. But maybe a valve in place, or in series, or in parallel with the set resistance. One ma difference ain't much. Will still need ac control source.

I have never studied them in depth. I would assume that both current and voltage controlled sources are available.

I have never heard of a variable, varying current source.......but sounds pretty neat.
 

-live wire-

Joined Dec 22, 2017
883
And what kind of resistor can be controlled in that fashion?
Actually... what if you did that by operating a MOSFET in the ohmic region? Maybe with some capacitors on the output for smooth sine wave and an H-bridge for + and -? Just a thought here.
 
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