Dummy Load II

[ronv]

Hmmm. 100 ohms should have been plenty low enough. I'm guessing your regular meter read the 24 volts??
Maybe 2 100 ohm in parallel and well see if it goes up to 23.5.

 

[jocanon]

Hmmm. 100 ohms should have been plenty low enough. I'm guessing your regular meter read the 24 volts??
Maybe 2 100 ohm in parallel and well see if it goes up to 23.5.

I think my regular VM was reading the same average number as the little one, but I don't recall for sure...will check tonight.

 

[ronv]

That would make more sense.

 

[jocanon]

I mean the same average as the little one without the resistor, but again, I need to double check that tonight.

 

[Mr RB]

...
(re putting RC delay on the FET gate)...
I think you could do that on your load because it is open loop. But with the closed loop it needs to be pretty fast or it will oscillate. If we were using BJT we could run it open loop with just some emitter resistors to even up the Vbe drop of the parallel transistors, but with the wide range of threshold voltage using FETs we can't use the same method.

It's just the opposite, based on my experience. If the FET Vgs is held fixed, the FETs essentially behave like a resistor (Rds) which is the ideal dummy load. Basically it's a massive variable resistor that you set with the pot.

Re the closed loop control it will be incredibly stable with the big integration added. That's because the amp control loop is still fast (if any +- error on the current sense, the opamp output will respond at full speed). But because the output (after the amp) is damped it has a finite rate of change, basically you have something equivalent to an accurate high speed control loop, controlling a nice big stable flywheel.

It will maintain an accurate average, and be very stable, the down side is that its dynamic response will be slow which does not matter in this case as you are only changing the control signal slowly (pot) and if the PSU voltage changes suddenly the dummy load will behave like a resistor in response to the PSU transients which are faster than it's ability to change the FET Rds.

 

[Dusey52]

ronv, where did you get the LTSPICE models/symbols for the LM358 & LM339?

 

[()blivion]

Most people sign up to this Yahoo group, then download any models they want.

**broken link removed**

 

[Dusey52]

Yeah, I've been there looking around. Couldn't find any that worked with ronv's simulation (or at least I couldn't figure out how to make them work).

 

[ronv]

I'm tied up today, but I will try to zip up my models and attach them. There may be a few %$* ups but most are ok.

() how do you mix your pictures in with the text. Is it just a paste? What kind of file?

 

[()blivion]

() how do you mix your pictures in with the text. Is it just a paste? What kind of file?

Mix pictures in with text? Sorry, I don't quite follow, on which image? Which post?

If you mean "how do I have the schematic next to the waveform like that", it's because I have three monitors.

 

[ronv]

Like post 117.

No attachment just a picture in with the text.

 

[()blivion]

I'm pretty sure someone already mentioned this before, (I'm too lazy to go and look) but there could be serious problems when using an Op-Amp with "normal" offset.

In simulation I was having problems when the reference voltage was really low, and I think this was why, because I was just using the most generic OP-Amp I could find. If we haven't done so already, (can't remember) It might be worth it to invest in low offset + rail-to-rail type Op-Amps. A low offset can't possibly have negative side effects as far as I know, so there is no reason not to do this other than to save like $5. And a rail-to-rail Op-Amp lowers the minimum current we can make the system draw. Also good.

Here is a simulation with a more or less "normal" Op-Amp. The green line is current, the blue is our control voltage. Notice that even when the control is telling it to, the current can't go below ~5 Amps. Not good at all.


And here is the same simulation, the same control ramp, this time with a better Op-Amp. Note that you can't even see the control voltage this time because it exactly follows the current. Which is what we want.




The more I think about it, the more I think we have gone over all this already. If so, disregard this post.

 

[()blivion]

Like post 117.

No attachment just a picture in with the text.

Oh Oh Oh!!!... "That". OK.

The button at the top of the text box that looks like a picture of a tree, you click that and then it lets you look through your computer for a picture, upload it, then it puts it in the the post.

 

[Dusey52]

I'm tied up today, but I will try to zip up my models and attach them. There may be a few %$* ups but most are ok.

() how do you mix your pictures in with the text. Is it just a paste? What kind of file?

That would be fantastic, thanks!

 

[ronv]

Spice files

Hmm Dusey this may work.

Here are 2 files. The one named NEW is the .sub files. The one named New sym are the matching symbol files.
I think you can just unzip NEW and save it as a folder named NEW in the Lib/sub folder in spice.
The NEW sym file you will need unzip it and save into the lib/sym file in spice. But you will need to rename is as just plain NEW.
Hope it works! I think I can try it on my other computer.
If not we may need to ask for help.

If all goes well when you open the componet thing in spice you will see a folder named NEW and the you can click it and select what part you want.

 

[ronv]

Mr. RB,
OK I finally see what you are saying. It is stable with respect to the 24 volts, just not with respect to the reference. The problem we have in this case is the power sequence. The 12 volts is already on when the 24 volt supply comes up. With the fast loop the current pulse is pretty short, but with the big integrator it gets pretty big. I don't know the exact power up time of the 24 volts supply. If it's real slow it may not matter so much.
I have played around a bit with the loop and can make it better, but right now it is 12 db down when it gets to 180 degrees.

 

[ronv]

Hi (),

Thanks for the tip. A tree - I should have known...

I don't think it's offset so much as the LT1097 doesn't go to ground. Might also be the generic FET. I'll have to give it a try.

 

[Dusey52]

Hi (),

Thanks for the tip ...

... Might also be the generic FET. ...

STP80NF12 model from the STMicroelectronics website (click the 'Design Support' tab then look near the bottom of the page).

Or put the .sub & .asy files from the attached zip file into the appropriate 'New' folder. The .sub file is identical to the .lib file on the STMicroelectronics website. The .asy is a re-worked generic N-channel Mosfet.

 

[()blivion]

So... Remind me which Op-Amp we are using? I have the STP80NF12 model and it appears to be working.

Here is the graph of control voltage vs current.


As you can see, things are quite stable with this configuration.

I don't think it's offset so much as the LT1097 doesn't go to ground.

Yeah, that was indeed what was happening. However, in the future if we want to lower the current sense resistor down to say... 50m Ohms or lower to reduce heat, we will want the lowest offset parts we can get for best stability and accuracy.

Might also be the generic FET. I'll have to give it a try.

I tried the above simulation with all sorts of the in-built MOSFET models and most worked fine for me. The only ones that didn't really work were the last two with the highest RdsON, which is understandable. The above simulation with the STP80NF12 also works. Keep in mind though it's still not the exact right Op-Amp.

 

[jocanon]

So... Remind me which Op-Amp we are using? I have the STP80NF12 model and it appears to be working...

We're using the LM358AN, this one:

https://www.mouser.com/Search/ProductDetail.aspx?R=LM358ANvirtualkey51210000virtualkey512-LM358AN