My constant current load - 90W , 3A 30V

[Spikee]

Is there any advantage in using an one ohm 100W power resistor instead of an 0.1 ohm 10W resistor ?
I did some research in some other designs and youtube video's but i am not sure yet.

In the ohmic region of the mosfet the resistor will only dissipate 9W (30v 3A -> 3v drop * 3A = 9w in power resistor 1ohm ) but when the mosfet is in full saturation the 90w will need to be dissipated almost fully in the load resistor.

I'm not sure if im thinking correctly but if i use a 0.1ohm load resistor it will take in ohms region 0.9W (30v 3A -> 0.3 v drop *3 A = 0.9W )
but in full saturation it still needs to take almost the full load of 90 W ?

thanks

spikee

[T4P]

Let's say you are measuring 1amp, 1amp = 1 volt so therefore you get more resolution and accuracy at lower voltages but the dropout voltage is higher though at higher currents
Unless you use a specific DMM chip you know will work for sure with milliohms then with lower currents just go with a 1ohm shunt resistor

[LEECH666]

Not really, you will only be able to dissipate more power in that specific resistor. If you want to parallel A LOT of FETs and only use a single shunt resistor is the only application I can think of where this matters. For example in one my dummy load units I want to be able to draw 1.25A.

Power dissipated in 0.1 Ohm shunt resistor is:
P= I² * R = (1,25A)² * 0.1R = 0,15625W

I'll be using a 3W resistor that I'm having around, so that should more than do for my application.

The maximum current that 0.1 Ohm / 3W resistor should be able to handle is:
I_max = sqrt(P/R) = sqrt(3W/0.1) = sqrt(30) = 5.477A

A 0.1 Ohm / 10W resistor should be able to handle:
I_max = sqrt(P/R) = sqrt(10W/0.1) = sqrt(100) = 10A

For a 0,1 Ohm / 100W resitor that would be:
I_max = sqrt(P/R) = sqrt(100W/0.1) = sqrt(1000) = 31.622A

So unless you want to draw more than 30 Amps, no it probably won't make sense to use a 100W resistor.


//EDIT: Sorry I misread again. I thought you where talking about 0.1 Ohm / 100W vs. 0.1 Ohm / 10W.

Cheers,
Florian

[blackdog]

Hi

Here are some pictures off my Active Load.
He is really finished :-)

Inside picture


Kicking Ash! YES almost 300Watt's for 5 minutes, temp heatsing 85C.


Now I just have to find the screws to close the case...

Kind regarts,
Blackdog

[SeanB]

I think using a 100A plus stud mount diode is a little overkill for 20A........, and the heatsink on it is a little small anyhow.

[blackdog]

Hi SeanB,

Uhm, the diode is only for reverse protection...
It is Not Mounted on a heatsink, just my way to mount it.
The diode has only to blow the fuse.

Regarts,
Blackdog

[Spikee]

Leech , you were almost correct . Whit a 1 ohm resistor of 100w i will be able to pull 10A max.
My mosfet and heatsink can only handle around 120W max. At 12 volt i can utilize the resistors rating fully at 10 A.
In my application i dont need to test high currents at low voltage so the 1 ohm 100W resistor is perfect.

Thanks for the info , tomorrow i will recieve my parts and i can start testing the pc heatsink.

Gr. Spikee

[Spikee]

So i just mounted the 100W resistor and the mosfet on the heatsink. I mixed the artic silver stuff , put the two TO247 package opposite side of each other (to utilize the heatpipes fully) . Then i clamped them on the heatsink and applied ~10W on the 1ohm resistor so the curing would go faster.
it was ambient temperature of 18 deg C here and now the heatsink is stable at 39 deg. C (no fan , upside down ) .
You immediately see the advantages of heatpipes. the whole heatsink is at the same temperature and the fins got hot before the mounting plate.
After a few mins the mounting plate and the fins get around the same temperature.

Pic:

Whole assembly:
https://www.dropbox.com/s/3dse7kpoq3tft5k/2012-12-09%2023.05.08.jpg

Mosfet and power resistor:
https://www.dropbox.com/s/v9hmkshzxbjd6d1/2012-12-09%2023.06.18.jpg

[Spikee]

So i just picked up my components at college  .
The 1ohm 100w resistors are huge , do i need to put them on an external heatsink ? or can they handle the 100W just mounted on the metal case.
Item number: WH100-1R0JI  maybe i can fit them on the top of the processor side.

pic:

50K ohm 10 turn pot
2x 1ohm 100W resistors
10x 0.1 ohm resistor 3W

[T4P]

Check the given thermal specs (Tj-a and Tj-c) of the resistors as well as calculate roughly how much temperature rise you can tolerate on a heatsink (I would recommend not going over 85C if possible so)
Think of the voltage drop (1V in your case so if you're pulling a amp it's 1W but surely it can handle it right? Actually such a huge resistor is somewhat overkill)
If you're just pulling 3A, seriously it's only 9W
I recommend using a fan or a better fan because those heatsinks IMO optimum-ly can't do more than 115W at 80C (50C rise i think?) at least

[Spikee]

The resistor wattage is calculated for a max current of 10A.
P = i^2 * R
p = 10^2 * 1
p= 100W

I probably wont use it fully 90% of the time . But i would like to have the possibility to do it . In price it did not matter that much.

gr. spikee

[T4P]

I guess if you would dare to use a low-priced but totally awesome heatsink from china it's called the Deepcool ice blade GS (In china it's translated as frostsaber player edition) 1kg raw weight and lots of cooling power  Simply because the artic cooling HSF is far too small for beyond 115W
Aside from that,
At 10A with the resistors dissipation i doubt you even need the mosfets anymore if you want about 100W max 
If you want to continue using the shunts at >5A i think you'll have to use 0.1ohm resistors or switch them in and out with a comparator and a SSR (Might swamp the resistance of the resistor)

[Spikee]

I have it running at 30.92V and 3.02A stable.
Mosfet case temp: 53 deg C.
heatsink temp (processor bottom) : 28 deg C.
resistor temp : ~ 28 deg C.
Fan: 12V DC

These cpu fans are a giant improvement , i ran into the limit of my power supply else i would have pushed it to 110 / 120 W .

The only thing that is acting up is the LM324N , it worked fine the first hour but it will only shoot up to the top rail now.

Things i did:
Replace the LM324N (several times)
check all the wiring (several times)
added pull down resistor at output of op amp
measured the output of the pot meter (no problems there)

Result: still a non working voltage follower opamp ...

Is the mosfet capacitance or something like that to high for the low power mosfet ?? i really don't know whats wrong :S .

I connected the opamp now in this weird way and it works. It only changes a a couple of 10 mA from 0-> 30V so it is still constant current.

pic:



Why is it working ?
if i pull out the wire of the feedback , the regulation wont work anymore and the mosfet will jump into saturation.

i also made a youtube video for you guys:


after the video i disconnected the fan to see how much it affects the heatsink temperature.
Within 2 mins the heatsink went to 40 deg. C. (almost the same as mosfet temp)  and kept rising.
So i connected tge fan again and in 2-3 mins the heatsink went back to 29 deg. C. and the mosfet still at 40 deg. C. (normal) .

I did some calculations and the Artic freezer 7 pro rev 2 is : 0.21 deg C. / W  (mosfet 0.75 + 0.25 deg c /w ) which makes it a total of :
 1.21 deg C. /W


So i just tested a lead acid battery. I put my mosfet into saturation and you could immediatly see the disadvantage of using a 1 Ohm resistor at low voltages (10V) . I was limited to 7A @10.4A so i used one of my 3W 0.1 Ohm resistors i had laying around and i could get 20A + until the 10A fuse gave in my Rigol DM3058 (max 10A ) . i used my current clamp to measure the 20A + currents. I may make a relay or ssr in the final build so you can change from 1 Ohm 100W load resistor to a 0.1 Ohm 100W one. @ 20A i also saw some smoke coming from some of my leads , they were a bit thin.

yet another day has ended 52 minutes ago ... guess i'll get some sleep now.

gr. spikee

... still not asleep but i checked the fuse of my rigol dm3058 and it has blown indeed. Seems it will be my current clamp for now on.

[LEECH666]

Looks to me like you're directly driving the MOSFET from the Pot now. The OP is basically redundant now.

But I am not expert. Struggeling with OPs too atm.

Cheers and good night,
Florian

[Harvs]

What exactly are you trying to do by that?  The MOSFET gate should be connected to the op-amp output like in every other circuit that's posted around here. In that circuit the op-amp output will go to the positive rail because your -input is less that you're +input voltage.  Simple as that, the op-amp would normally control the FET gate so that it can increase the current level until the - and + inputs match.

[Jay_Diddy_B]

Hi,

I have been following this thread for a while. It is time to jump in and offer some advice that will lead to successful loads.

Here is a simple load:



The circuit is based on the LT1635. This is a combination of a 200mV reference and an op-amp.

The circuit use a 0.1 Ohm resistor in the source of the MOSFET to give a maximum current of 2A.

The datasheet for the LT1635 can be found here: http://www.linear.com/product/LT1635

MOSFET selection

Some MOSFETs that have been optimized for switch mode power supplies will not work well in the Linear Region. We can tell this by looking at the Safe Operating Area curve on the datasheet. Here is a Vishay IRF530:



This curve shows where it is safe to operate the MOSFET. This MOSFET is only limited by power dissipation.

The second SOA curve shows a Renesas 2SK3481



Note: It sis not safe to operate this MOSFET at high voltages and low currents.

MORE Power, More MOSFETs

To increase the power handling more MOSFETs are required. It is not a good idea to parallel MOSFETs.
Why??

The transfer graph:

shows that at currents below 10A the drain current will increase with temperature for a given gate source voltage. The hot MOSFET will hog the current and get hotter. We now have the possibility for thermal run away.

Matching the MOSFETs may help a little, but the better way is to use an op-amp per MOSFET. This removes need to match the MOSFETs.



This technique is found in most commercial loads.

JDB

[Harvs]

Note: It sis not safe to operate this MOSFET at high voltages and low currents.

Sorry I'm probably missing something here, but how did you come to that conclusion?

For DC loads the mosfet naturally has a Vds limit (the right hand line), a power dissipation limit (as marked on the line sloping down to the right), and a limit of the Rds_(on).  Apart from these things, what's not safe about operating this mosfet at say 80V, 0.15A?

Cheers

[Spikee]

As i stated in my previous post the lm 324n wont regulate at all and the output will jump to the top rail. It looks like the lm324 cant drive the mosfet any more. Ill have to look if i have a higher power mosfet to drive the fet.

[LEECH666]

Hi,

[...]

shows that at currents below 10A the drain current will increase with temperature for a given gate source voltage. The hot MOSFET will hog the current and get hotter. We now have the possibility for thermal run away.

Matching the MOSFETs may help a little, but the better way is to use an op-amp per MOSFET. This removes need to match the MOSFETs.

[...]

This technique is found in most commercial loads.

JDB

Hi JDB,

The Master/Slave design is exactly what I am trying to build in my "own variant" of a dummy load.
I've linked to my thread previously but anyway here is the link again
https://www.eevblog.com/forum/beginners/derpy-load-is-this-dummy-load-design-any-good/

Cheers,
Florian

//EDIT:

Spikee, since the LM324 is a quad OP: Did you properly terminate the unused gates? I read that if you leave the unused gates at undefined levels that this might interfere with the used gates.

For example here: http://www.electro-tech-online.com/general-electronics-chat/42965-beginner-unused-half-dual-opamp.html#post353639

and here

http://twilightrobotics.com/wiki/Properly_Terminating_Unused_Op-Amps

[Spikee]

I will try your suggestion leech. I had them floating all the time. I will try it now.

[LEECH666]

http://www.maximintegrated.com/app-notes/index.mvp/id/1957

This article by maxim is a bit more detailed, but I am not sure how large the impact of an undefined unused gate really is. I guess it's worth a try.

[LEECH666]

This thread is also informative ...

https://www.eevblog.com/forum/beginners/dave's-constant-current-load-traps-for-young-players/

[Spikee]

So i replaced the LM324N whit a L2722 1A opamp and i terminated the unused pins. Now it works perfectly.
I will now continue testing whit a lead acid battery to achieve higher currents.

[Jay_Diddy_B]

Hi Harvs and the group,

Harvs asked:

Note: It sis not safe to operate this MOSFET at high voltages and low currents.

Sorry I'm probably missing something here, but how did you come to that conclusion?

For DC loads the mosfet naturally has a Vds limit (the right hand line), a power dissipation limit (as marked on the line sloping down to the right), and a limit of the Rds_(on).  Apart from these things, what's not safe about operating this mosfet at say 80V, 0.15A?

Cheers

It comes from looking at the SOA curve for the Renesas 2SK3481  MOSFET



If you study the DC line.

At 5V Vds the maximum current is 11A a dissipation of 55W
At 9V Vds the maximum current is 6A a dissipation of 55W

At 20V Vds the maximum current is 1.8A a dissipation of 36W
At 40V Vds the maximum current is 0.65A a dissipation of 26W

You can see how the maximum Dissipation is reduced at higher voltages.

Some MOSFETs do not like being operated in this part of the SOA graph at all.

For further reading on this:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100014777_2010016074.pdf

or

http://powerelectronics.com/power_semiconductors/power_mosfets/power_trench_fets_fragile/


Note Bipolar transistor have similar de-ratings, but the reason for the failure is very different. Google 'Second Breakdown BJT'

JDB

[Harvs]

Thanks JDB, I understand what you're talking about now.

Cheers