Hi

I am working on a constant current load, here are some waveform of my breadboard circuit, I can't seem to get the ripple further down to 1-2 mV.

The ripple of my circuit is 10mV, and that translate to 50mA ripple, which I think is too much.

So is that acceptable, or I have a lot more work to do to improve me circuit.

Thanks

Datasheet:
Op amp - OP07C
MOSFET - IRF540N

This is the ripple with average on - 8 average

This is the ripple without average enable.

here is my circuit: (my LTSpice simulation is also attached)

 

Have you looked closely at the power supplies for ripple?

I remember working on a HP power supply in a metrology lab, it had ripple I thought was unacceptable. Then I noticed the ripple corresponded with a AM radio someone was playing in the background. Them being tuned to the same station was coincidence, but it turned out I was looking at RF interference.

 

Any error/ripple at the reference input (opamp positive input) will be amplified in your system. Can you post a schematic of your current real world circuit. As many details are missing in your simulation setup

 

When a constant current load is performing correctly all ripple from the power supply will be visible across the load resistor (assuming that you are using a resistor for load).

To measure the current you need to do either an inline current measurement of the CCS load or a differential voltage measurement of the voltage across the load resistor. You can also measure the voltage across the sense resistor as it is ground referenced.

I have a power supply load tester that consists of an adjustable CCS DC load with AC load capabilities on my webpage. Look in the Swenson regulator section.

www.pimmlabs.com

 

How you deriving the constant current reference (V3)? That needs to be derived from a stable voltage reference source.

 

Have you looked closely at the power supplies for ripple?

Yes, it looks like the majority of the noise is from my SMPS, as long as I replace the 14V rail with a 9V cell battery, the noise reduce down to ~1mV

my 14V is the Vload and also the VCC of the op amp, and the source of to the LM7805, which serve as the Vref for the current control.

 

Any error/ripple at the reference input (opamp positive input) will be amplified in your system. Can you post a schematic of your current real world circuit. As many details are missing in your simulation setup

here is my real word circuit, the L is just to simulate the wire to the MOSFET

 

To measure the current you need to do either an inline current measurement of the CCS load or a differential voltage measurement of the voltage across the load resistor.

Arrh, that sounds like a good idea, why didn't I think of that?

I have a power supply load tester that consists of an adjustable CCS DC load with AC load capabilities on my webpage. Look in the Swenson regulator section.

www.pimmlabs.com

I have looked at your circuit, but I think I need to learn more before I can understand of your circuit. (or look at your circuit more closely)

 

How you deriving the constant current reference (V3)? That needs to be derived from a stable voltage reference source.

It's from a LM7805 -> 10K pot, so I am guess LM7805 is not good enough?

 

Now I find something weird, I got some noise as soon as I connect my probe to the circuit, (probe at Rsense), the ripple is about 1mV to 2mV, always at the middle of the time scale.

This show up in my scope even my circuit is not power up, where is that from? something wrong with my scope? but when I connect the probe direct to its ground, I don't see that.

the probe in use is a x1 probe.

it looks like this