Dummy Load II

[ronv]

In the old system (how soon we forget) The sensor was on the pipe and set for 70C. With no water it seemed to trip before the FETs fryed. So I think with it on the FET plastic and set for 80C it will trip much faster. Just my 2 cents.
We can scale the reference to hold the current at 4 amps/FET below 24 or 25 volts - I think. We need to come up with a way to test it before we wire up a bunch of FETs. Maybe 3 or 4 supplies in series.

 

[Mr RB]

Correct me if I am wrong, but I think if we did it that way it would scale a higher voltage to be a higher current... wouldn't it? High reference voltage = high current.
...

Yep. Just like in the old days when you would connect a huge wirewound or watercooled resistor to the PSU output to test a PSU. It's a dummy load.

Then the step up from that was to use a large transistor array, with a pot to bias it on. It's 1960's technology I guess, but works very well and performs something like an adjustable resistor, that's how my big dummy load works it is just a massive adjustable resistor, extremely simple and reliable.

The next step up from that is to put a zener on the pot, so it makes it a simple adjustable constant current load.

Adding all the complexity and closed loop feedback amps etc means it might be a couple percent more accurate as a constant current load (for what that's worth), but along with all the complexity and extra work comes the possibility of things going wrong in a big way.

 

[()blivion]

Yep. Just like in the old days when you would connect a huge wirewound or watercooled resistor to the PSU output to test a PSU. It's a dummy load.

OH OH OH... I see... do it with intent to approximate a resistor. I think I understand what you were getting at now.

Hummm... I don't know if we should do that here or not though. On the one hand, adding more and more PSUs together does add more and more watts output capability. So to be able to fully test the max DUT power, we would need to increase the dummy loads max power drain. And this could possibly be approximated as simply an adjustable resistor. On the other hand, the dummy load we are looking at making now will probably not be able to handle any more than say 2KW, so we definitely want to cap it off some where. However, using just a resistor approximation, both voltage and current are going to increase along with each other, so the wattage increase will be exponential using this configuration. Where as a circuit that lowers the current limit as the voltage increases keeps the relationship of voltage-watts stable.

Thinking about it more, I believe going back to constant current, but increasing the wattage handling capability's of the dummy load is probably going to be best. All the PSUs are getting connected in series, so the current is always only ever going to be equal to one PSUs capability, which is 50A. But with more supply's in series, the voltage is increasing, thus the available wattage is also increasing. And I'm sure we want to be able to test the max wattage of the largest DUT configuration that Jeremy comes up with, otherwise hes not really testing them as much as he should. Now an actively sensed current limit is only good for keeping the dummy load from hitting it's wattage limit. Which, as per above, the load should be made to handle more than any reasonable DUT configuration can supply anyway. And a resistor approximation scheme is going to have serious problems with wild wattage swings at different DUT configurations. So the only viable option remains constant current.

But then again, who knows exactly what draw the real load looks like to the PSU. I believe it's just a buck converter that is charging Li-Po RC batteries. And the batteries are going to have their charging curve. So that, in some form or another, is going to be what the real load looks like I would imagine. I don't think it's going to be nearly as stressful as what we are planning to do to the PSUs with this dummy load. So maybe not fully testing the larger configuration is viable.

 

[ronv]

So Jeremy, I think we are where you need to decide on the auto current adjust and we are almost ready to finalize it. The auto adjust is not much more than the old current read out circuit.
To summarize where I think we are?

15 fets= up from 10 - allows 60 amps improved margin
.2 ohm higher wattage sense resistors. Either 10 watt or pipe mounted. Your call based on mounting.
Improved op amp - ability to get closer to 0 current.
100 amp current shunt - for direct current measurement
4 amp fast blow fuses (15) - oops protection
Integrator method - for noise/oscillation
logic voltage regulator = eliminate 12 volt supply, allow power sequencing for integrator.
temperature sensing on FET plastic - 80C . Faster response
operation 12 to 50 volts 1500 watts max.

What did I forget?

 

[Dusey52]

But then again, who knows exactly what draw the real load looks like to the PSU. I believe it's just a buck converter that is charging Li-Po RC batteries. And the batteries are going to have their charging curve. So that, in some form or another, is going to be what the real load looks like I would imagine. I don't think it's going to be nearly as stressful as what we are planning to do to the PSUs with this dummy load. So maybe not fully testing the larger configuration is viable.

The typical charger that Jeremy is catering to is a microcomputer controlled buck-boost arrangement in the 1000 to 2000 watt range with up to 65 amp input. Depending upon the unit they can charge 6, 8 or 10 cell LiPolys at 30 to 40 amps. Some are even dual channel and can do two at a time with different settings. They can most definitely make these 50 amp power supplies beg for mercy.
Here’s a few if you want to read more about them (look at the $150 to $350 range): **broken link removed**

 

[Dusey52]

So Jeremy, I think we are where you need to decide on the auto current adjust and we are almost ready to finalize it. The auto adjust is not much more than the old current read out circuit.
To summarize where I think we are?

15 fets= up from 10 - allows 60 amps improved margin
.2 ohm higher wattage sense resistors. Either 10 watt or pipe mounted. Your call based on mounting.
Improved op amp - ability to get closer to 0 current.
100 amp current shunt - for direct current measurement
4 amp fast blow fuses (15) - oops protection
Integrator method - for noise/oscillation
logic voltage regulator = eliminate 12 volt supply, allow power sequencing for integrator.
temperature sensing on FET plastic - 80C . Faster response
operation 12 to 50 volts 1500 watts max.

What did I forget?

That looks pretty complete to me. At < $1 for the auto limit/adjust circuit that will save replacing 15 fuses if 4 power supplies are connected with the knob turned up too high I can't see a reason not to include it. Just need to decide on the limit vs adjust method. Jeremy when you order the logic regulator, make sure it's HV model, a couple of letters in the part number makes all the difference!

 

[ronv]

Yea, don't buy any parts yet. I'll try to wrap all the ideas together and post the spice file and picture so everyone can play with it. Ive been switching computers so each schematic may not be complete.

 

[ronv]

Shame you don't have all the parts. With the storm you might have a lot of time to put it together.

 

[Mr RB]

...
logic voltage regulator = eliminate 12 volt supply, allow power sequencing for integrator.
...
What did I forget?

Just out of curiosity, what would happen if the main PSU (DUT) is cutting on/off? Is that still going to be safe since the 12v PSU for the controller will be losing power too?

I remember you saying something about finding a solution for a startup issue but will that still suffice in a situation of DUT dropouts?

If you could include some extra large caps on the 12v rail, that would allow the control circuit to remain running even with repeated short dropouts of the DUT. Provided that does not cause problems with your startup timings solution.

 

[()blivion]

If the dummy load circuit has a low enough power draw, which I'm sure it does, we could do as RB suggests. Also we could put a diode just in front of the caps such that power can't leak back out of them to the DUT or the FETs, which may or may not be shorts at such time. Might get a mild voltage boosting effect if the DUT is cutting in and out, and even if not any power in the caps will end up being retained better and still carry over during cut outs as per RBs suggestion. Most three terminal regulators can handle up to something like 40 Volts. Probably want to put in some kind of transient protection in case this gets exceeded. Maybe an inductor->diode->cap circuit to filter and boost power during odd input conditions.

Just throwing out some quick ideas... Now then, back to doing other crap.

 

[jocanon]

Hi guys...sorry, it's been a pretty crazy week. I've been recruited by my former employer, a big four accounting firm, and may be taking an offer from them which would mean I will be moving across country to sunny California...anyway, that's a story for another thread.

I am not sure I understand how much more complicated it would be to make the current auto adjust, I just as soon keep it simple. If it's easy to do then I guess it would be nice. But probably just keep things simple and keep it constant current.

 

[ronv]

Just out of curiosity, what would happen if the main PSU (DUT) is cutting on/off? Is that still going to be safe since the 12v PSU for the controller will be losing power too?

I remember you saying something about finding a solution for a startup issue but will that still suffice in a situation of DUT dropouts?

If you could include some extra large caps on the 12v rail, that would allow the control circuit to remain running even with repeated short dropouts of the DUT. Provided that does not cause problems with your startup timings solution.

I think we are ok. The problem with power up was the 12 volts had been on but no 24, so the fet was turned on fully because there was no current. When the 24 came on the long time constant of the integrator kept the current very high for quite some time. Now the 12 comes on after the 24 so that problem is gone.
I have played with power down, but not since we said we would go up to 50 volts. There might be a problem there.
When it was just the 24 the maximum current was limited to 4 amps by the pot running off of +12. I have a diode from +12 to +24 to keep current from going backwards thru the regulator so the 12 volts follows the 24 down. It did look like there were some timings (power on/off glitches) that would get the current a little higher than the set point but not higher than 4 amps/FET.

Jeremy, I'm not sure the auto tune circuit is so complicated. But it is often not as simple as it seems. It may complicate this power up/down stuff, but it is more a matter of trying to plug all the possible holes.

Where would you go in Calif. San Fran probably? I lived up that way for a while in San Ramon. Also spent some time in So. Cal. Really liked it, but make sure you get some $. Most companies out there are ready to help with housing and stuff. Or they used to be.

 

[jocanon]

Ok ronv, if its not that complicated then let's go for it...but if it gets to be too buggy we can just bag it.

I am looking to move out west, either Phoenix, AZ (I was born and raised in Mesa and still have family there) or Sacramento, CA area (wife's family is out there). I contacted my former employer since they are an accounting firm serving many clients to see if they had any clients that needed someone but then they said they might need me so I am looking into that now, waiting to see what they offer..so if anyone knows any good corp tax manager positions heavy in ASC 740 tax provision reporting in Phoenix (or even Tucson) or Sacramento let me know...I know this is an electronics forum but hey, I don't want I leave any rock unturned !

 

[ronv]

Gives me a headache just thinking about it. The taxes I mean.

Might as well go first class. We will take a look at the auto range.

 

[ronv]

Dl2.1

Wasn't quite as easy as I thought it might be.

Here is a first pass with auto range.

Here is what I did. I let the op amp go to the positive rail at about 24 volts so that is the highest current. It's not a well speced place but might be ok. What'ca think?
Couldn't do the alarm the same way with the variable reference voltage so I flipped the comparator and added a couple of little FETs to turn the current off with high temperature and turn on the alarm. I haven't tried to test all the power up/down and ripple stuff, so if you have some time give it a go.

**broken link removed**

 

[()blivion]

How do you do the potentiometer thing again? That is the only thing stopping me from being able to play with the *.asc file. "Model not defined" or some such thing...

 

[ronv]

Hi ()

Here is the link where Eric helped me with it.

https://www.electro-tech-online.com/threads/dummy-load-ii.132379/#post1103417

 

[Dusey52]

In case anyone's interested, Linear released an updated version of LTSpice a few days ago (v4.18a).

https://www.linear.com/designtools/software/

 

[Dusey52]

Seems like this thread has gone quiet so I'm not sure anyone is still working on this project but I've come across a FET that may be worth looking at. It doesn't have a logic level gate but everything else about it very good. If you want to drive them hard you might get by with as few as two, although I'd probably go with three or four to be safe.

https://www.fairchildsemi.com/ds/FD/FDL100N50F.pdf

 

[jocanon]

Hi Dean, thanks for finding that. I will be working on this project again at some point...some things happend in my personal life to really turn things upside down for me a few weeks ago, and then I got a great job offer from a CPA firm in Phoenix, AZ...to make a long story short, we are packing up and moving across country to good old AZ right now (yeah, I can get out of the snow!) So, as you can imagine, moving a family of 6 from Buffalo, NY to Phoenix, AZ is no small undertaking, it has been keeping me very very busy lately. Once I get settled in AZ and life gets pretty much back to normal I will be picking it up again. I still have at least 125 more power supplies to sell. If I can find a supplier of inventory to get more used DPS-600PBs in bulk (they seem to have dried up on eBay), or find a different power supply I can buy in bulk then I will likely be selling more than just the 125 I have left...so I definatley will be needing a dummy load to test them.