Dummy Load II

[schmitt trigger]

Sounds a good strategy.
What are Ron's changes? I seem to have missed them.

 

[jocanon]

post #20...he added 4.7K resistors from gate/op-amp output to ground, and a couple caps and resistors going to the voltmeters.

 

[ronv]

Good point () about them going like pop corn over time. Maybe some more thought will come up with something. We could also try to turn it off instead of shorting out the big power supply. Maybe a big OR gate with each fet to a comparitor. Not sure if it would be fast enough, but maybe. Might also need a bigger sense resistor (more power). I did find some big fuse blocks we could use if you want to try fuses, but come to think about it the fuses have the same problem as the resistors that () brought up. We might be able to get an indicating fuse block I suspose. More thinking.

 

[ronv]

Jeremy,
You did say your regular voltmeter showed the voltages to be stable?

 

[jocanon]

Jeremy,
You did say your regular voltmeter showed the voltages to be stable?

Well, yes...sort of...it was stable with my watt metter I purchased from a hobby store that is used to read current in lipos when they are hooked up to RC planes...so I am not sure if it is exactly the same as a regulare VM. I don't know if it has some sort of internal smoothing built into it maybe that's why it doesn't fluxuate as much??? I do have a regular VM I could also test it with when I get home tonight.

This is what I am talking about that shows it as stable (I mean, it still fluctuates a very little amount but not very much at all). I've been using this to read the current when I test PSU's on the 8 FETs since I had to disconnect the damaged FET which had the effect of messing up the current reading on the dummy load:

https://www.hobbyking.com/hobbyking/store/__10080__Turnigy_130A_Watt_Meter_and_Power_Analyzer.html

 

[jocanon]

You know, now that I think about it, I wonder if Mr RB was really onto something with the whole TO-220s not being able to withstand the heat... because what started this whole thing was it seemed that I had to turn the pot a little more to get the same level of current one day, so I wondered if all the FETs were working properly and that is why I decided to test them. So I was trying to test the output voltage of each op-amp when I accidentally shorted it to Vcc... I ended up testing each one and the output voltage from the op-amps all seemed right, but I wonder if the FETs are getting degraded over time? However, it also could have just been my imagination, I wasn't sure because it wasn't like I had to turn the POT a lot more to get up to 47 amps, it just seemed like MAYBE I was turning it a little more, I couldn't tell for sure so I wanted to check. I guess what I am getting at, is it might be a good precaution to increase the number of FETs to give it more head room.

 

[ronv]

I don't think so but maybe Mr. RB could recheck my math. It is very close if the over temperature kicks in, so since it runs so cool we should probably lower where that shuts it off. Since you are going to add more for higher power anyway we can throw in a few extras to increase the margin. It's not like we are selling a million of them. I just don't like to add a lot of parts since everyone has a failure rate. At some point it becomes self defeating.

 

[jocanon]

Good points, plus 10 is a nice round number for doing the math

I think it would be good to lower the over temperature cut off, come to think if it, if I ever damaged them it probably was when I was testing the over temp. We could lower it all the way to 50C and be good to go.

 

[()blivion]

I was thinking I would build it and then IF it is still unstable with the mods that ronv made I will then look into getting a scope. I think the only risk in that strategy is I may have to do some re-work...but if it is stable I may be able to say the cost of scope.

Should also test if it is not an artifact of the meter, rather than genuine noise. You can do this by putting a RC lowpass filter inline with the meters sense wire. If it is true nois, the meter will calm down with the filter inserted. If it is fake noise, made by the meter it's self, then nothing much will change.

Again, 90% chance the problem is Op-Amp feedback oscillation.


Edit: Guess I was a bit late on this one.

 

[()blivion]

We could also try to turn it off instead of shorting out the big power supply.

The problem with open circuiting all the FETs if one goes south on us is that this won't un-short the bad FET. Thus all the current that was going through the other FETs will be redirected to the bad one, exasperating the problem. On the other hand, shorting all the FETs has the opposite effect, it would steal all the current from the bad FET, causing it less stress. Finally, a large fuse or circuit-breaker in line with the DUT will see the over current condition we just created and open the circuit at a safe limit. This will also let us explore the possibility of an over voltage safety by reusing the same circuit with a different trigger mechanism.

Bottom line.
If we want to make the system fail open circuit (which is the best thing to do), there is no way around the need to have another switching element added inline with the main power rail. The device FETs are too week by themselves to conform to this task reliably. Either a large main circuit-breaker or fuse, or something like a very large normally open relay that is only powered when the system self check passes. Any of these methods could be used as the corner stone of a comprehensive overvoltage + overcurrent protection system.

The last method allows the possibility for us to power the dummy load off the DUT, which could be nice.

 

[Dusey52]

Guys,

I have a couple of questions that may seem silly but I’ll go ahead and ask them anyway for my own enlightenment. Here goes:

- Why not use a precision voltage reference instead of a voltage divider hung off Vcc to derive Vref?

- Why not ditch the x10 amplifier that generates a guesstimate of the output current reading and replace it with a summing amp that gives the actual current reading regardless of how many FETs are working?

 

[jocanon]

...The last method allows the possibility for us to power the dummy load off the DUT, which could be nice.

I know when we first started we thought about powering from the PSU under test, but I am not sure I think that is a good idea anymore. I like the idea of have a separate reliable power supply for the logic circuits, I don't know, it just seems cleaner to me. Also, the voltage decreases slightly when the PSU is under load and I wonder if that would mess with the logic circuitry, maybe not. Also, I don't know if this would be a problem or not, but sometimes I test 12v power supplies and sometimes I test 24v.

 

[jocanon]

Guys,

I have a couple of questions that may seem silly but I’ll go ahead and ask them anyway for my own enlightenment. Here goes:

- Why not use a precision voltage reference instead of a voltage divider hung off Vcc to derive Vref?

- Why not ditch the x10 amplifier that generates a guesstimate of the output current reading and replace it with a summing amp that gives the actual current reading regardless of how many FETs are working?

Hm, those seem like excellent questions to me. Isn't there a way to measure the entire current, the way my watt meter does, I mean, I guess I could just stick a watt meter on there if there isn't an easy way to make my own (btw - I know there is an easy way to make my own, I have just never done it...I think you need a shunt or something like that, I remember seeing that somewhere).

 

[ronv]

Guys,

I have a couple of questions that may seem silly but I’ll go ahead and ask them anyway for my own enlightenment. Here goes:

- Why not use a precision voltage reference instead of a voltage divider hung off Vcc to derive Vref?

We could do that I guess, but the reference doesn't need to be precise since the current is dialed in. It is a regulated supply so it should be stable.

-

Why not ditch the x10 amplifier that generates a guesstimate of the output current reading and replace it with a summing amp that gives the actual current reading regardless of how many FETs are working?

That's a good idea, but Jeremy's about just putting in a real ammeter is the best. Then it doesn't matter how many FETs or what value sense resistor we use.

**broken link removed**

 

[jocanon]

...That's a good idea, but Jeremy's about just putting in a real ammeter is the best. Then it doesn't matter how many FETs or what value sense resistor we use.

**broken link removed**

I just ordered two, one for this dummy load and one for the old one.

 

[ronv]

Here is an electronic circuit breaker that might work. It won't be easy to debug if doesn't. What do you guys think.

https://www.electro-tech-online.com/custompdfs/2013/01/mic5031.pdf

 

[Dusey52]

We could do that I guess, but the reference doesn't need to be precise since the current is dialed in. It is a regulated supply so it should be stable.

True, I just thought that since one of Jeremy's goals was to improve stability it couldn't hurt (and they're relatively cheap). If my math is right, 10mv on the reference controls about an amp of load so it doesn't take much noise or drift to swing the load around.

That's a good idea, but Jeremy's about just putting in a real ammeter is the best. Then it doesn't matter how many FETs or what value sense resistor we use.

Agreed that the 'real' ammeter should be the best however those inexpensive digital meters often require an isolated independent supply which can make them a pain to use.

 

[Dusey52]

Here is an electronic circuit breaker that might work. It won't be easy to debug if doesn't. What do you guys think.

https://www.electro-tech-online.com/custompdfs/2013/01/mic5031.pdf

That thing is slick! I wonder if it works as well as it reads?

 

[ronv]

It is -I think. I never tried one.

The way the circuit is now it is .13 volts per amp. We did add a larger filter cap to the +12 in this one. Jeremy also has .1 ufd on each op amp that isn't shown in the schematic.
I'm not sure how much attention we paid to the grounding. That could be a problem we should take care with in this one.

 

[Dusey52]

I might be reading it wrong but isn't it .13 volt per amp for each of the 10 FETs, so .013 per amp total? And the new one is a little more sensitive due to the smaller sense resistor.