Author Topic: What to use for a 45 amp load  (Read 7188 times)

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Offline Mira Rautio

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Re: What to use for a 45 amp load
« Reply #25 on: November 08, 2023, 11:45:51 am »
t's great to see you working on your RC project, and I understand your desire to come up with a unique and cost-effective load solution. Using a CPU heatsink with LED fans is an interesting idea, and it can add a cool visual element to your project.

To determine the size of the heatsink you'll need, you should consider a few factors:

    Heat Dissipation: Calculate the heat dissipation requirement of your load. This will depend on the power you intend to dissipate and the duration of your discharging process. You mentioned 1s/2s LiPo, so you can calculate the power based on the nominal voltage and discharge current.

    Thermal Resistance: Check the thermal resistance of the CPU heatsink. This value indicates how well the heatsink can transfer heat away from the load. The lower the thermal resistance, the better.

    Airflow: Ensure that the fans you're using can provide adequate airflow to cool the heatsink effectively. You may need to consider the CFM (cubic feet per minute) rating of the fans and the specific heat sink design.

    Material and Design: Consider the material of the heatsink. Aluminum is commonly used for its excellent thermal conductivity. The design of the heatsink, including fins and surface area, plays a crucial role in heat dissipation.

    Mounting and Attachment: Make sure that the heatsink can be securely attached to your setup and that there is good thermal contact between the load and the heatsink.

    Budget: As you mentioned, cost is a factor. Keep an eye on your budget and try to find a heatsink and fan combination that fits within your $30 target.

Remember that heat sinks are designed for specific thermal loads, so you'll need to match the heat dissipation requirements of your load to the heatsink's capabilities. If you can find a heatsink with LED fans that meets your needs, it can be a unique and visually appealing solution.

If you provide more specific details about your expected power dissipation and load characteristics, the community can help you with more precise recommendations. Good luck with your project!
 

Offline trilerianTopic starter

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Re: What to use for a 45 amp load
« Reply #26 on: November 09, 2023, 12:01:59 am »
This is not a good way of approaching variable electronic load.

Your aproach of using power resistors as current load and a mosfet to switch the rms current on the load,  doesn't take into consideration:

1) the resistance of power resistors dictates the maximum current sinked in them and also the current spike of the electronic load

2) because of these current spikes, the electronic load can't be used in power supplies/batteries that are not able to source at leas 45 A per your specification (also depended on the supplies voltage) , and the power supply/battery would basically either explode/burn or activate whatever security system in overcurrent scenarios. This means your supply would only be useful for that pack of batteries you have there and would rarely see any use in other projects.

3) your current load would be very bad in constant current at the higher current ratings because the batteries change in voltage based on their capacity. So you can't really maintain something like 40A constant over time. (This will be less of a problem in lower current ratings because you can compensate with your duty cycle)

4) you need to find a method to measure the rms current of your load, either by using a specialised ic or by programming the microcontroller to calculate the rms voltage.


These are just some of the disadvantages that came to my mind and some things you might and might not have considered before starting the project

The superior solution would be to use a completely linear solution (which will be a bit more expensive) but at least will be more reliable/useful of a project.

1.  I did take into consideration the resistance of the power resistors to determine maximum load that could be put on the batteries.
1s battery, 4.2v charged, with 6 0.5Ω resistors in parallel.  So that is 0.083Ω and 4.2v gives a max load of ~50amps, and at 3.8v it is ~47 amps.  Add in tolerance for the resistors, and it should still be fine. 2s lipo could be discharged at 100 amps with this setup, so I see no issue there.  Also, my intent is to start the program at something like 5% duty cycle and increase from there, so any spikes will be just that, momentary spikes. 

2. All batteries that will be used can withstand at least 100 amp spikes, and according to the manufactures, much more than that.  This project is specific to use for RC racing batteries.  Not flight, not bashing, but for 1s and 2s racing applications.

3 & 4. Current will be measured by the ACS772x current sensor.  I have already designed and use a telemetry module that can measure the current and programming required for my microcontroller.  Current will be monitored by the mcu and duty cycle of the pwm will be adjusted to maintain the cc discharge.  Not really sure why this is a bad idea...

A linear solution very well may be the better way to go.  But it is a ground up redesign as I have only a faint idea of where to start based on some of the responses to my posts.

Is there a reason why my project will not work? 

Edit:  Why RMS to measure current?  This is DC current with PWM, wouldn't I want to average?  And if I know my duty cycle and my pwm frequency, I should be able to get pretty accurate results without any filtering.
« Last Edit: November 09, 2023, 12:08:13 am by trilerian »
 

Offline IanB

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Re: What to use for a 45 amp load
« Reply #27 on: November 09, 2023, 05:22:06 pm »
4) you need to find a method to measure the rms current of your load, either by using a specialised ic or by programming the microcontroller to calculate the rms voltage.

Why RMS? You would use an RMS measurement if you are measuring or regulating power. If you want to measure or regulate current, then an average measurement is appropriate.

Edit:  Why RMS to measure current?  This is DC current with PWM, wouldn't I want to average?  And if I know my duty cycle and my pwm frequency, I should be able to get pretty accurate results without any filtering.

Good question. I meant to respond to this at the time when I saw the earlier post.
 

Offline Eraldo

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Re: What to use for a 45 amp load
« Reply #28 on: November 09, 2023, 08:49:17 pm »
4) you need to find a method to measure the rms current of your load, either by using a specialised ic or by programming the microcontroller to calculate the rms voltage.

Why RMS? You would use an RMS measurement if you are measuring or regulating power. If you want to measure or regulate current, then an average measurement is appropriate.

well since we are working with a square wave the rms current would be 'easy' to calculate (you would still need to keep track of the battery voltage so that you would get a more accurate reading)

as for keeping track of the rms current, we would need it if we were to try and keep a constant current on the load. since both the batteries and the power resistors are not really stable with charge/power, you could compensate by varying the duty cycle until the reference RMS current is reached. so if you were to go this route you would want a relatively accurate rms current and that's not easily achieved by just taking into consideration the duty cycle and calculating the rms current with it. but i doubt the precision is a matter of grave importance in this design. a -+ 1 or 2 A wouldn't' be that terrible and just connect a clamp meter to know the exact current

Did OP also say about measuring the capacity ( I'm not sure so I'm just leaving it here) ? in that case at least a 5 % accuracy would be necessary for meaningful measurement.



1.  I did take into consideration the resistance of the power resistors to determine maximum load that could be put on the batteries.
1s battery, 4.2v charged, with 6 0.5Ω resistors in parallel.  So that is 0.083Ω and 4.2v gives a max load of ~50amps, and at 3.8v it is ~47 amps.  Add in tolerance for the resistors, and it should still be fine. 2s lipo could be discharged at 100 amps with this setup, so I see no issue there.  Also, my intent is to start the program at something like 5% duty cycle and increase from there, so any spikes will be just that, momentary spikes. 


What i meant was that this type of electronic load wouldn't be appropriate for general use and as you mentioned it will be only used with 1s and 2s lipo batteries so in that case its fine.


3 & 4. Current will be measured by the ACS772x current sensor.  I have already designed and use a telemetry module that can measure the current and programming required for my microcontroller.  Current will be monitored by the mcu and duty cycle of the pwm will be adjusted to maintain the cc discharge.  Not really sure why this is a bad idea...


as i said in my previous comment it will work fine in 0 to 95% of the current rating of the electronic load. after that there would be nothing to compensate for the resistor drift and based on their temp coefficient they will either increase or decrease in resistance and you would no longer have a constant current load but a load that varies with temperature and the voltage of the battery. Also as i said above by using the duty cycle of the waveform to calculate the rms current only,  the accuracy wouldn't be that great but if it doesn't bother you then its fine.


honestly finishing the project first would be the major goal. and if there are things in which its lacking you could add them with time or even build a V2.0.
I just wanted to make sure you knew the small shortcomings that might have bothered you in the future and it seems you already thought about them.
 

Offline IanB

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Re: What to use for a 45 amp load
« Reply #29 on: November 10, 2023, 03:11:13 pm »
well since we are working with a square wave the rms current would be 'easy' to calculate (you would still need to keep track of the battery voltage so that you would get a more accurate reading)

as for keeping track of the rms current, we would need it if we were to try and keep a constant current on the load. since both the batteries and the power resistors are not really stable with charge/power, you could compensate by varying the duty cycle until the reference RMS current is reached. so if you were to go this route you would want a relatively accurate rms current and that's not easily achieved by just taking into consideration the duty cycle and calculating the rms current with it. but i doubt the precision is a matter of grave importance in this design. a -+ 1 or 2 A wouldn't' be that terrible and just connect a clamp meter to know the exact current

It is not so much that the RMS current would be 'easy' to calculate or not, but rather that the RMS current would be an incorrect measurement. For instance, if you had 10 A pulses with 50% duty cycle, then the average current would be 5 A, which would be the correct value, whereas the RMS current would be 7.07 A, which is clearly not correct.
 

Offline Eraldo

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Re: What to use for a 45 amp load
« Reply #30 on: November 10, 2023, 05:46:24 pm »
well since we are working with a square wave the rms current would be 'easy' to calculate (you would still need to keep track of the battery voltage so that you would get a more accurate reading)

as for keeping track of the rms current, we would need it if we were to try and keep a constant current on the load. since both the batteries and the power resistors are not really stable with charge/power, you could compensate by varying the duty cycle until the reference RMS current is reached. so if you were to go this route you would want a relatively accurate rms current and that's not easily achieved by just taking into consideration the duty cycle and calculating the rms current with it. but i doubt the precision is a matter of grave importance in this design. a -+ 1 or 2 A wouldn't' be that terrible and just connect a clamp meter to know the exact current

It is not so much that the RMS current would be 'easy' to calculate or not, but rather that the RMS current would be an incorrect measurement. For instance, if you had 10 A pulses with 50% duty cycle, then the average current would be 5 A, which would be the correct value, whereas the RMS current would be 7.07 A, which is clearly not correct.

Yeah a blunder on my part. what I meant all along was the True RMS of the signal.
 

Offline trilerianTopic starter

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Re: What to use for a 45 amp load
« Reply #31 on: November 17, 2023, 03:26:49 pm »
New question!
I replaced the mosfets (IRL2910PBF x 2) on the original unit that was given to me for inspiration.  When replacing them I saw that thermal compound was being used between the drain and the heat sink.  Makes perfect sense, need to transfer the heat.  The only problem here is that the tab to the heat sink is the current path as well and all the thermal compound I can find is non electrically conductive.  Since the heat sink is aluminum, I can't use liquid silver either.  Pin 2 on the mosfets are not actually used, only the tab for the drain.  Should I put thermal compound on half of the tab?  I think this maybe the reason the unit was blowing mosfets, because thermal compound was coating the tabs and creating a high IR from the mosfets drain to the battery + side.

I replaced the mosfets without putting any thermal compound on the tabs at all.  I was also curious and ran the unit with my scope probing the gate pin.  Definitely a linear DC signal and not chopped with PWM.  At a 10 amp discharge, the voltage on the gate is ~2.6V. This kind of confused me since the 328p outputs a pwm signal.  I followed the traces from the gate to the 328p and found an RC filter on the 328p to the gate, so this must be filtering the pwm to a DC signal.  Which means the mosfets are operating as variable resistors and sinking the current themselves, right?  Basically how y'all are saying to design it, lol.  But, this is using cheap mosfets and not the more expensive linear fets.

Oh well.  I already ordered some prototype pcbs and components to try my pwm version with an external load.  I'll see how that turns out. 

What a learning experience this is becoming... 
 

Offline shapirus

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Re: What to use for a 45 amp load
« Reply #32 on: November 18, 2023, 12:25:48 am »
Which means the mosfets are operating as variable resistors and sinking the current themselves, right?  Basically how y'all are saying to design it, lol.  But, this is using cheap mosfets and not the more expensive linear fets.
FWIW, those cheap-ish Chinese constant current loads use generic off-the-shelf MOSFETs as power sinking elements (no way they obtain those special MOSFETs designed for linear operation), and they work, as long as heat sinking stays adequate. So at least this is something to keep in mind.

The 45A, however, poses some challenges. First off, heat dissipation. For a 2s lithium battery, worst case 4.2V*2 = 8.4V, this means you'll have to dissipate 8.4V * 45A = ~380W of power. This means a *big* heat sink with a fan (or several fans). To get an idea, check some of the more expensive CPU heatsinks, see their sizes and what levels of TDP they are designed to deal with.

Next, finding a suitable transistor won't be easy, if at all possible, at least to fit your budget with a good margin. In fact, I highly doubt that anything short of a resistor bank in a bucket of water will fit the budget, when you add up all the parts. Budget considerations aside, availability of such a transistor that can sink a continuous 45A and dissipate just below 400W is a big question as well.

Now, what do the CPU makers do when they hit the 5GHz barrier and fail to improve single-core performance any further? Right, they begin to scale performance horizontally by adding more (and more) cores and forcing poor programmers into writing programs that utilize parallelizing algorithms. See the point? You could do the same. Parallelize it. Use *several* cheaper/more easily available transistors. Each of them will have its own current sensing shunt. Each of them can have its own smaller heat sink, or they can share a single big one (good luck finding something suitable though). Since you are going to use a microcontroller, controlling them becomes trivial: read the voltage drop values from each of the shunts (amplifying them before feeding to the ADC, if necessary) and control each of the transistors' gates (or bases, if you use BJTs) individually to: a) make currents flowing through all of them equal; b) make sum of these currents equal to 45A.

If you use, say, 5 transistors, which means only 9A/75W per transistor, you enter the area where those cheap Chinese electronic loads do the job easily, meaning you can solve it using cheap components too.

The only (major) caveat here is that your microcontroller must have a sufficient number of outputs and ADC inputs to read current values and control all the transistors. You might need to use some additional controller(s) to read raw values and send them over a digital line to the main one.

Another caveat: even when current flowing through each transistor is equal, it doesn't necessarily mean that power dissipated on each of them is equal, because their characteristics may vary to some extent. However, it should be close enough, if the transistors you use are the same part number coming from the same batch. You might also read voltage drops across the transistors and control them to make power, instead of current, equal across all of them, but that'll be more difficult and smell like overengineering.

Of course, you'll also need to implement not only total overcurrent protection, but also overcurrent protection for each of the transistors to prevent a cascading magic smoke escape if one of them fails.
 

Offline EPAIII

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Re: What to use for a 45 amp load
« Reply #33 on: November 19, 2023, 03:40:56 am »
The biggest load I have ever seen was at a used generator company. They had to test run generators up to some that occupied a 40 foot semi trailer. The load was a pool of salt water about the size of a van with electrodes in it. When it was in use, the energy went into boiling the water. I am not sure how they controlled the current, perhaps by the depth the electrodes were submerged.

When it got too hot, they had to shut it down and let it cool.

So yes, I would suggest a plastic bucket, some water, some salt, some wire coat hangers for electrodes and an Amp meter and experiment a bit. That should be in your budget.



A bucket of water with wires or resistors in it:

Dummy load in a bucket
mikeselectricstuff



 :-+
Paul A.  -   SE Texas
And if you look REAL close at an analog signal,
You will find that it has discrete steps.
 

Offline trilerianTopic starter

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Re: What to use for a 45 amp load
« Reply #34 on: November 28, 2023, 01:03:33 am »
I cobbled together a start.

6 0.5 \$\Omega\$ 100w resistors in parallel. Thermal glued to the cold side of 3 TEC1-12706 plates which are thermal glued to a 120mmx70mmx27mm heat sink with 2 60mm fans on top.
With a 240w load the resistors hit 50C. The heat sink also got to about 45C. I need a bigger heat sink or fans. Not really sure if the peltier did much.

« Last Edit: November 28, 2023, 01:19:17 am by trilerian »
 

Offline shapirus

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Re: What to use for a 45 amp load
« Reply #35 on: November 28, 2023, 01:32:23 am »
With a 240w load the resistors hit 50C.
That's barely warm for them.
 

Offline trilerianTopic starter

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Re: What to use for a 45 amp load
« Reply #36 on: December 12, 2023, 08:38:44 pm »
So I finally go my prototype pcb in, soldered all my components on the board.  Figured out some code to run, and made some test passes.  First thing... Remember to anlogWrite the pin and not digitalWrite the pin, lol.  Since my resistor bank is low enough, it can pull over 90amps from a fully charged lipo.  I kept running the program and it would instantly trip my safeguards, lol.  Oops...  So analogWrite.  Being that the control is via pwm, the output of the ACS sensor is also pwm, and since the 328pb, I changed my rc filter on the output to a 1µF ceramic capacitor and a 5k1Ω resistor.  This helped the output of the ACS and stabilized the feedback loop a bit.  Working well at 10 amps.  However, my pcb design couldn't handle the stress test at 35 amps, let alone 45 amps.  The load will sink it, lol, but the board couldn't handle it.

However, I am super excited that I got it to work at all, and my next attempt will be better.  Speaking of.  I think I want to go the linear route, yes I know, that's what y'all said.  But you know, stubbornness and all...  I need help choosing the correct mosfet(s) for this design going the linear route. 

Back to the drawing, err, KiCad...
 

Offline Aurgelme

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Re: What to use for a 45 amp load
« Reply #37 on: December 12, 2023, 09:20:40 pm »
I have experimented a lot when it comes to electronic load and linear solutions are what I use.

I learned a lot from this design.
http://www.kerrywong.com/2017/01/15/a-400w-1kw-peak-100a-electronic-load-using-linear-mosfets/
 

Offline trilerianTopic starter

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Re: What to use for a 45 amp load
« Reply #38 on: December 18, 2023, 01:24:03 am »
Another update on this project...

So the first stress test I did after verifying the unit would work, was at 30 amps and a 2s lipo.  No heatsinks on the mosfets since they are just switching.  But, they got so warm that they started to unsolder themselves since I put the drain tab down on the board.  Next test was to flip them over with the drain tab up and rig 2 new mosfets so the legs were soldered where I wanted them.  I was able to get to 10 amps and keep the temps below 100°C.  So the next test was to add 2 more mosfets in parallel to the mosfets already there. With 4 mosfets, all running on the same pwm pin, I did a stress test at 40 amp discharge for 2 minutes.  The mosfets got to 125°C without a heatsink.  So I am getting close here.  Maybe add a couple more and add a heat sink to them...

Beyond that, I think I need to figure out how to do a PID control for the pwm.  Just averaging the current I am fluctuating a few tenths of an amp below to above my target discharge rate.

As to my load, I don't think the peltiers help much.  While the resistors start out around 10°C, the heatsink is already up to 50°C.
« Last Edit: December 18, 2023, 01:27:44 am by trilerian »
 

Offline trilerianTopic starter

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Re: What to use for a 45 amp load
« Reply #39 on: February 13, 2024, 08:14:02 pm »
I have experimented a lot when it comes to electronic load and linear solutions are what I use.

I learned a lot from this design.
http://www.kerrywong.com/2017/01/15/a-400w-1kw-peak-100a-electronic-load-using-linear-mosfets/

I have looked at the writeup multiple times and watched the video. The mosfets being used are beefy and pretty pricey.  Just curious on how much they can really handle independently.  The writeup says due to thermal conditions, they can only handle about half of what they are rated for.  I need a max of about 350W.  But I did order 2 of them to try and test with, but would be nice if I only needed 1 per unit.

Here is a link to the data sheet on the mosfet:  https://www.mouser.com/datasheet/2/240/media-3321496.pdf

Also, do you think this can be controlled via an ESP32 DAC output?
 

Offline Jwillis

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Re: What to use for a 45 amp load
« Reply #40 on: February 13, 2024, 10:46:36 pm »
Start simple first. By taking on to much at one time, things can get messed up pretty fast. Leave the Micro controlling for the last step. Once you have the basic design working, then adding a micro-controller won't be that hard. If the basic electronic load is designed right, then it will manage it's self without the need of a computer. The Micro-computer should only take your input to change the operation of the of the load, monitor to give you a visual readout and log data.If you use it to micromanage something that is capable of managing it's self, you will run into problems.
Do not use PWM to operate the mosfets on an electronic load. PWM will make noise which will introduce errors and takes a lot of extra compilations to remove that noise and the errors it creates. The more complicated you make things means more room for error.
When using MOSFETs in linear designs, pay close attention to how they de-rate with temperature. So you will probably have to parallel more Mosfets than you initially expect to get the desired power.
You don't have to get the most expensive "Linear" mosfets on the market. As long as it has a DC rating for the SOA in the datasheet you can use it for a electronic load. In fact any mosfet could be used but it's better if there is some information about it's use in Linear mode. A  FDL100N50F  for instance is nearly half the price of a IXTK90N25L2.
I can forward my design if you like. There are others here as well that are just as good if not better.
 

Offline pdenisowski

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Re: What to use for a 45 amp load
« Reply #41 on: February 13, 2024, 10:53:30 pm »
I used a standard automotive battery tester to show that one of our supplies really could output 40 amps.  I could only run it for a few seconds at a time, but it did work :)

(And don't try this at home)

« Last Edit: February 14, 2024, 08:02:27 pm by pdenisowski »
Test and Measurement Fundamentals video series on the Rohde & Schwarz YouTube channel:  https://www.youtube.com/playlist?list=PLKxVoO5jUTlvsVtDcqrVn0ybqBVlLj2z8
 


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