# new member, opened a Sevcon



## patallen (Nov 6, 2012)

Hi, i am a new member, altought i am not decided to make my own ev car yet, i am experimenting theses days with BLDC motors.
I am electronic engineer for automotive market.

I bought for cheap a MARS motor with its Sevcon toasted controller, from a SOLIDNAV conversion.

Controller gaved up, with a code 3, shorted fets.

I opened the controller, not without pain and extensive work since its a sealed and potted unit, to see whats wrong with it, how it is made, and possibily with the goal to fix it.

I reversed the driver outputs and from my experience, is quite marginal and a big missmatch with that motor.

I have some pictures of the inards of that controller, i will post them if there is interest on this thread, otherwise i plan to make my own controller to beef up the %efficiency, considered that about only 50% of the battery juice was properly sent to the BLDC motor...crap.

to be continued !!
cheers !


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## onegreenev (May 18, 2012)

Always an interest. If you learn something then it was worth the effort. We like photos and troubles. Bring it on.


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## smpavlik (Mar 28, 2011)

Welcome to the community!

Very interesting to see Sevcon and compare with other controllers.


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## patallen (Nov 6, 2012)

Here is the poor dead sevcon unit in all its glory on my test bench.


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## patallen (Nov 6, 2012)

Top opened.


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## patallen (Nov 6, 2012)

and what you should not see
the fets are P80NF10P, 8 in parrallel on each outputs/polarity for a total of 48.
RDSon of theses is a mere 0.18 ohm. They could have choosen any other fet than theses ones imho. as a designer for large market/volume, i understand sometimes company have to do tradeoff and cut corners...but hey...c'mon !!
motor resistance alone is 0.038 ohm....go figure.


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## smpavlik (Mar 28, 2011)

What MCU is there?


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## patallen (Nov 6, 2012)

Freescale MC908MRxx series.


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## rgengineer (Sep 11, 2012)

Why dont you just toss the Sevcon and buy a PG controller. They are very reliable and easy to come by.

You can get one at www.electricmotorsport.com


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## patallen (Nov 6, 2012)

rgengineer said:


> Why dont you just toss the Sevcon and buy a PG controller. They are very reliable and easy to come by.
> 
> You can get one at www.electricmotorsport.com


i did that for my personnal knowledge needs and curiosity. Do you know whats is into thoses PG controllers ?

the sevcon is already tossed away, dont worry.


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## PStechPaul (May 1, 2012)

The ST STB80NF10 has 0.015 ohm RdsON, so 8 in parallel should be about 2 mOhm. But they are only 100V and the SOA is only about 6 amps at 50V, for a 10 mSec pulse. So the controller would only be capable of reliable operation at about 3 * 8 * 6 * 50 = 7200 watts. 

I found the datasheet for the Sevcon Millipak and it appears to be rated for a maximum of 48 VDC and 6.5 kW:
http://www.sevcon.com/media/2122/millipak.pdf

Its maximum current output is 300-600 amps, and it does not appear to be designed for BLDC, but rather SEP, Series and PM brush motors. The 300 amps through 2 mOhms would be 180 watts dissipation which seems reasonable, but that may exceed the SOA for switching at 48V. However, at 100 uSec (10 kHz) the SOA current is close to 100A per device, although switching losses may predominate. It seems that the Sevcon controller may be a reasonable design, but apparently may have been overloaded or the internal protection was inadequate.

Welcome aboard!


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## patallen (Nov 6, 2012)

PStechPaul said:


> The ST STB80NF10 has 0.015 ohm RdsON, so 8 in parallel should be about 2 mOhm. But they are only 100V and the SOA is only about 6 amps at 50V, for a 10 mSec pulse. So the controller would only be capable of reliable operation at about 3 * 8 * 6 * 50 = 7200 watts.
> 
> Welcome aboard!


not the same fet. STB and STP are different fets. 38A vs 80A.
sorry i misstype, yes they are 15mohm, still not my preference.


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## patallen (Nov 6, 2012)

its been a while since i posted here, i went ahead and made my own controller.
Note that i am experimenting it is still a very rough prototype that needs a lot of refinement, but it works dam good !!
Not shown yet, full LCD display with current, voltage, % duty cycle, ramp mode, regen brake and such. Microchip app notes and documents are awesome. I am well connected with them.
Thanks for any comments.
http://www.youtube.com/watch?v=x87YHZbrfvU&list=UUC673uaNkUUZovyc5ZeYhDw&index=5


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## PStechPaul (May 1, 2012)

Looks like a good controller. I had not seen that app note before, but my main interest is induction motors and SR motors. Here are links to that app note and a newer one using a dsPIC:

http://ww1.microchip.com/downloads/en/appnotes/00857a.pdf
http://ww1.microchip.com/downloads/en/appnotes/00901a.pdf

Good luck on your project.


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## smpavlik (Mar 28, 2011)

What is power stage voltage/current and what parts do you use?


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## patallen (Nov 6, 2012)

smpavlik said:


> What is power stage voltage/current and what parts do you use?


for now it is 24v
i use the good fets that where left from the dead sevcon unit, 5x for each polarity and bridge so a total of 30 fet, stp80nf10fp iso pack

i have better fets to go in and more to make it more robust (8x and 100A each fets )

for now i limit the current in my software at 100amp but my current test to brake the motor with my hand barely make it reach 60amp, and then i burn my hands even with heavy duty welder gloves. this is not a razor scooter motor !!!


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## smpavlik (Mar 28, 2011)

Video looks impressive. 



patallen said:


> i have better fets to go in and more to make it more robust (8x and 100A each fets )


What current do you expect to have per shoulder? I believe you are aware about SOA.


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## patallen (Nov 6, 2012)

smpavlik said:


> Video looks impressive.
> 
> 
> 
> What current do you expect to have per shoulder? I believe you are aware about SOA.


no, but i guess you realy want to tell me.


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## Tesseract (Sep 27, 2008)

PStechPaul said:


> ...But they are only 100V and the SOA is only about 6 amps at 50V, for a 10 mSec pulse. ...


You keep referring to MOSFETs having a Safe Operating Area (SOA) in the same manner as BJTs, when that is not at all the case.

The SOA curves you see on MOSFET datasheets describe the maximum drain current allowed with a given drain-source voltage *drop*, either from operating in the linear region (as a voltage controlled resistance) or because of the intrinsic drop across RDS[on]. This SOA is strictly a power dissipation problem (more specifically, a power dissipation across a given sum total of thermal resistances from junction to ambient) and NOT at all like the SOA curves for bipolar transistors, which were much smaller in area because of "second breakdown".

When MOSFETs are used in switching applications, however, they only experience "linear" operation during the transitions from on to off or off to on. As long as the time spent in the transition is minimized (facilitated by good gate driver design and a power stage layout that minimizes parasitics) you don't have to worry about violating the SOA.

Thus, please do not refer to SOA w/r/t MOSFETs in switching applications. Stick with static dissipation and thermal resistance and maybe add an extra 5% for switching loss if you don't want to actually calculate it out.


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## patallen (Nov 6, 2012)

Tesseract said:


> You keep referring to MOSFETs having a Safe Operating Area (SOA) in the same manner as BJTs, when that is not at all the case.
> 
> The SOA curves you see on MOSFET datasheets describe the maximum drain current allowed with a given drain-source voltage *drop*, either from operating in the linear region (as a voltage controlled resistance) or because of the intrinsic drop across RDS[on]. This SOA is strictly a power dissipation problem (more specifically, a power dissipation across a given sum total of thermal resistances from junction to ambient) and NOT at all like the SOA curves for bipolar transistors, which were much smaller in area because of "second breakdown".
> 
> ...


Thanks for this clarification, Mr. 

"Operation in this area is limited by max RDS(on)"


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## kennybobby (Aug 10, 2012)

*use rope or a piece of leather*



patallen said:


> ... but my current test to brake the motor with my hand barely make it reach 60amp, and then i burn my hands even with heavy duty welder gloves. this is not a razor scooter motor !!!


wrapped around the shaft and connected to a long piece of wood as a lever arm to apply the torque load --saves hands from burns or getting caught and removing fingers...


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## PStechPaul (May 1, 2012)

One way to test torque is to put a large pulley on the motor shaft and run a belt to a small pulley on a generator (like a 2HP treadmill motor), or a pump, or a fan, or other load. Mostly good for low speed torque, though. You don't want to spin the load 10x the motor RPM! 

For starting torque you can use a torque wrench. For power you can make a dynomometer by putting a resistive load on the DC motor. Or you can get a 120/240 VAC generator head which can put out up to 10 kW, and plug in some heaters or other AC loads.

You can also make a brake load from a disc brake rotor and caliper.


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## patallen (Nov 6, 2012)

PStechPaul said:


> One way to test torque is to put a large pulley on the motor shaft and run a belt to a small pulley on a generator (like a 2HP treadmill motor), or a pump, or a fan, or other load. Mostly good for low speed torque, though. You don't want to spin the load 10x the motor RPM!
> 
> For starting torque you can use a torque wrench. For power you can make a dynomometer by putting a resistive load on the DC motor. Or you can get a 120/240 VAC generator head which can put out up to 10 kW, and plug in some heaters or other AC loads.
> 
> You can also make a brake load from a disc brake rotor and caliper.


good ideas, i have a big 3hp 120v motor (antique) that i can connect with a strap so it will act as a load inertia at least.


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## smpavlik (Mar 28, 2011)

Tesseract said:


> When MOSFETs are used in switching applications, however, they only experience "linear" operation during the transitions from on to off or off to on. As long as the time spent in the transition is minimized (facilitated by good gate driver design and a power stage layout that minimizes parasitics) you don't have to worry about violating the SOA.
> 
> Thus, please do not refer to SOA w/r/t MOSFETs in switching applications. Stick with static dissipation and thermal resistance and maybe add an extra 5% for switching loss if you don't want to actually calculate it out.


This is not totally true. Motor controller is loaded with inductance and you have to respect this. In the most critical and most dangerous OFF part voltage can increase dramatically and causes huge local power dissipation during transition which is his order can break the FET. 

There are some docs which explain why you shouldn't ignore SOA. 

www.fairchildsemi.com/an/AN/AN-9010.pdf
http://www.fairchildsemi.com/an/AN/AN-558.pdf


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## PStechPaul (May 1, 2012)

Reading a bit more deeply I think Tesseract is correct in his assertion that MOSFETs are limited by bulk resistance power dissipation and thermal considerations. But it is important to consider the peak power dissipation that can occur during switching. In my LTspice simulations of my DC-DC boost converter I found peak power spikes well into the tens of kilowatts, although for less than 1 uSec. It is probably best to consider the peak pulse power specification and use 1/2 max voltage and 1/2 max current for SOA. Thus, the STB80NF10 (or STP) is rated at a peak of 15V and 300A at the 100 uSec SOA curve which is 4500W, while the device itself has continuous ratings of 100V and 80A, or 8000W. Of course both would not occur simultaneously, but it is best to limit usage to, say, 50V and 40A, which is 2000W (peak). Total power, according to spec, is 300W (at 25C), and this is further limited by thermal resistance (0.5C/watt and a heat sink of, say 2C/watt), and temperature derating of 2W/C. So at a maximum device temperature of 125C it is limited to about 100W by derating and 40W by the heatsink.

This is safe for resistive loads, but inductive loads can cause high voltage spikes which can cause deterioration and eventual destruction if they exceed the device maximum, and capacitive loads (such as snubbers) can cause brief current spikes into the thousands of amperes. That is where the very high power spikes seem to originate in the simulations, and that is why resistors are added to the snubber. Thus, for instance, this MOSFET used on a 48VDC supply driving an inductive load at 40 amps should have a snubber which can handle the 40 amps at switching and limit the voltage to 100V, so a 2 ohm resistor would be a good choice. The capacitance is chosen so as to absorb the energy of the inductive "kick" during the switching transition, so it depends on the switching time of the MOSFET. You also need to consider the power consumed by the snubber which can be significant at higher frequencies.

I hope this helps to clarify this. Let me know if any of this does not seem correct.


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## Tesseract (Sep 27, 2008)

smpavlik said:


> This is not totally true. Motor controller is loaded with inductance and you have to respect this. In the most critical and most dangerous OFF part voltage can increase dramatically and causes huge local power dissipation during transition which is his order can break the FET.


Inductive loads do _increase_ turn-off dissipation and _decrease_ turn-on dissipation (for a given gate drive time, drain-source voltage and drain current), but this is more properly described as switching loss, and not "SOA", per se. 

The voltage spike which is often observed across the drain-source terminals during turn-off is the result of stray inductance between the input capacitor and the switch (or between the switch and freewheeling diode or the other bridge leg, if an inverter) and it has nothing to do with SOA nor with the inductance of the load! It certainly limits how *fast* you can switch the MOSFET, and having to slow things down excessively to deal with a sloppy layout will result in overheating, but that's because of poor design leading to excessive losses, and, once again, not SOA, per se.




smpavlik said:


> There are some docs which explain why you shouldn't ignore SOA.
> 
> www.fairchildsemi.com/an/AN/AN-9010.pdf
> http://www.fairchildsemi.com/an/AN/AN-558.pdf


You might want to read the very documents you are using to prove your assertions first. On p36 of the first document it basically says the forward biased safe operating area for a MOSFET is a function of junction temperature, thermal resistance from junction to ambient and RDS[on].

Now, where the SOA curves for MOSFETs *can* come in handy is when you want to switch a brief pulse of current that is much higher than the DC rating of the MOSFET, say to discharge a capacitor into a xenon flash lamp or to interrupt a short circuit in a downstream load. This more a case of the thermal impedance effectively decreasing for brief transients, and not really "SOA", per se.


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## smpavlik (Mar 28, 2011)

Tesseract said:


> You might want to read the very documents you are using to prove your assertions first. On p36 of the first document it basically says the forward biased safe operating area for a MOSFET is a function of junction temperature, thermal resistance from junction to ambient and RDS[on].


Thank you for the deep explanation. I will read the doc two times more  



PStechPaul said:


> I think Tesseract is correct in his assertion that MOSFETs are limited by bulk resistance power dissipation and thermal considerations. But it is important to consider the peak power dissipation that can occur during switching.


That is exactly I wanted to say. Junction temperature is a function of power dissipation and practically SOA curve is easier to use for calculation




Tesseract said:


> Thus, please do not refer to SOA w/r/t MOSFETs in switching applications. Stick with static dissipation and thermal resistance and maybe add an extra 5% for switching loss if you don't want to actually calculate it out.


Could you please answer the pure practical question which I believe is interested to anyone who is building its own controller:

Say you have IPP041N12 which is rated 120V, 120A, Rds=4.1mOh
http://www.infineon.com/dgdl/IPP_I_...b02d7&fileId=db3a30432239cccd0122a75b86467ca4
You plan to use Vbat=96V and PWM=0%..100%.
What current is considered to be safe if a bridge has 4 MOSFET in parallel?
Other word, what current(power) the controller should be rated for?


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## PStechPaul (May 1, 2012)

IMO, a 120V part is marginally rated for 96V, especially if that is a nominal battery pack voltage which may well exceed that by 15%. But other than that, an H-bridge consisting of four 120A 4.1mOhm devices in each leg, with a heatsink capable of 2.5C/watt, should be able to handle 40 watts dissipation per device at 25C ambient and 125C maximum junction temperature. For conduction losses, I=sqrt(40/0.0041) = 98A. But there is also a power derating of perhaps 2W/C, (although that applies to the peak power of 300W and at 125C it is still 100W).

I think it would be safe to operate this device at half voltage and half current, or 55V and 60A. Four devices in parallel then could handle a 240A load at 55V or 13.2kW. Device dissipation will vary depending on duty cycle, and at 50% it might do 480A but that will be at 50% load voltage so still 13.2 kW. 

You could also figure switching losses, which can be greater than conduction losses. At 10 kHz, with 1 uSec switching time, the switching losses might be estimated at 55V*60A*0.001*10, or 33W. This would be added to the conduction losses of 60^2*0.0041 = 15W, for a total of 48W per device.

The overall efficiency, then, can be figured by the total losses of 48W * 16 devices = 768W for a load of 55V*240A=13.2kW, or 94.2%.


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## patallen (Nov 6, 2012)

well, i will use my unit at 24v with 8x 100A 100v mosfets. i think it gona be alright.


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## smpavlik (Mar 28, 2011)

PStechPaul said:


> IMO, a 120V part is marginally rated for 96V, especially if that is a nominal battery pack voltage which may well exceed that by 15%. But other than that, an H-bridge consisting of four 120A 4.1mOhm devices in each leg, with a heatsink capable of 2.5C/watt, should be able to handle 40 watts dissipation per device at 25C ambient and 125C maximum junction temperature. For conduction losses, I=sqrt(40/0.0041) = 98A. But there is also a power derating of perhaps 2W/C, (although that applies to the peak power of 300W and at 125C it is still 100W).
> 
> I think it would be safe to operate this device at half voltage and half current, or 55V and 60A. Four devices in parallel then could handle a 240A load at 55V or 13.2kW. Device dissipation will vary depending on duty cycle, and at 50% it might do 480A but that will be at 50% load voltage so still 13.2 kW.
> 
> ...


Actually I've calculated exactly same way and have almost same result -12 kW. As you said, better to utilize 50% of the FET rate. This is a theory.But practice is a bit different. For example I found a "description" of 400A Kelly which has 7x100Ax150V MOSFETS (57A per FET). 
http://endless-sphere.com/forums/viewtopic.php?f=30&t=29720

It looks they uses 50% of the FET... Funny thing is I had Kelly 400Ax96V and it didn't deliver the current! I saw not more than 250A. After heavy fighting with Kelly, I've finally returned the controller back and shortly Kelly de-rated it to 350A in 30 Sec and 140A continuous. So they think only 20A continuous is safe and 50A with restrictions

P.S. I know what dou you want to say about Kelly


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## patallen (Nov 6, 2012)

fwiw, i updated my board with LCD and primitive regen brake.

https://www.youtube.com/watch?v=YdZ4qx_nCWU&list=UUC673uaNkUUZovyc5ZeYhDw&index=1

i plan to draw a real stout PCB for this with some more options and larger mcu as ai want to put more options.
thanks for any comments.


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## PStechPaul (May 1, 2012)

Looks good. I put a comment on the youtube video. It would be good to add a datalogger function and try the controller on a heavier load.


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## Stiive (Nov 22, 2008)

patallen said:


> fwiw, i updated my board with LCD and primitive regen brake.
> 
> https://www.youtube.com/watch?v=YdZ4qx_nCWU&list=UUC673uaNkUUZovyc5ZeYhDw&index=1
> 
> ...


Nice work mate... keep it up!


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## patallen (Nov 6, 2012)

PStechPaul said:


> Looks good. I put a comment on the youtube video. It would be good to add a datalogger function and try the controller on a heavier load.


yeah, thanks for the comments, my goal are very modest with this controller, it is supposed to go, along with the motor, into a toy for my son . Right now i limit the current in software at 100amp, once it is reached the pwm is kept under control to stay around 100amp, or less if the throtle is reduced. I made some test at lower amps and some hand gloved braking to see how it react and it seems to work well.

I plan to make some more changes, right now i have i/o limitations on the pic, i cannot use rs232 AND pwm hardware module, and if i hardcode rs232, the amount of interrupts right now would fu*k up all the timings, so ultimately i want to have both on a larger pic which i already have in stock, and then maybe data logger on its own or even something much much cool, i am working with some Blue Tooth modules for now as a project for one of my clients, so my controller could send/receive some data to PC or my iPhone. How cool that would be...

The motor will go soon into my son's toy (a 1.25 scalled down Willys), that i can sit in too, will try it and post some video of me making slides in the snow


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## patallen (Nov 6, 2012)

fwiw, first test ever with some real load. it hits my 100A software limit quit easy for now, but man this is so cool


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## dgbibook (Oct 29, 2021)

PStechPaul said:


> Reading a bit more deeply I think Tesseract is correct in his assertion that MOSFETs are limited by bulk resistance power dissipation and thermal considerations. But it is important to consider the peak power dissipation that can occur during switching. In my LTspice simulations of my DC-DC boost converter I found peak power spikes well into the tens of kilowatts, although for less than 1 uSec. It is probably best to consider the peak pulse power specification and use 1/2 max voltage and 1/2 max current for SOA. Thus, the STB80NF10 (or STP) is rated at a peak of 15V and 300A at the 100 uSec SOA curve which is 4500W, while the device itself has continuous ratings of 100V and 80A, or 8000W. Of course both would not occur simultaneously, but it is best to limit usage to, say, 50V and 40A, which is 2000W (peak). Total power, according to spec, is 300W (at 25C), and this is further limited by thermal resistance (0.5C/watt and a heat sink of, say 2C/watt), and temperature derating of 2W/C. So at a maximum device temperature of 125C it is limited to about 100W by derating and 40W by the heatsink.
> 
> This is safe for resistive loads, but inductive loads can cause high voltage spikes which can cause deterioration and eventual destruction if they exceed the device maximum, and capacitive loads (such as snubbers) can cause brief current spikes into the thousands of amperes. That is where the very high power spikes seem to originate in the simulations, and that is why resistors are added to the snubber. Thus, for instance, this MOSFET used on a 48VDC supply driving an inductive load at 40 amps should have a snubber which can handle the 40 amps at switching and limit the voltage to 100V, so a 2 ohm resistor would be a good choice. The capacitance is chosen so as to absorb the energy of the inductive "kick" during the switching transition, so it depends on the switching time of the MOSFET. You also need to consider the power consumed by the snubber which can be significant at higher frequencies.
> 
> I hope this helps to clarify this. Let me know if any of this does not seem correct.


can these be tested on the board as I have 49 to test


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