So the easy way of doing this is to wire 6 0.5Ω 100watt resistors in parallel as the load. But that is boring...
I'd like to come up with a better idea for a load
There are a bunch of cpu heat sinks with LED fans, I am leaning is this direction, but how do I determine how big of a heat sink I need if it is used as the load?
Why is "boring" a problem? "Boring" is usually a good property for well-engineered solutions. The one thing you don't typically want is "exciting".
How do you define better? Smaller? Cheaper? Lighter? More reliable? To define "better" it helps to define what is unsatisfactory about existing solutions.
This does not make sense. CPU heat sinks are not designed with electrical resistance, which is the primary property that a load should have.
Correct. But the mosfet duty cycle will be controlled by a current feedback from the current sensor. I will program the mcu to use the current draw and adjust the duty cycle as needed to maintain a constant current. My assumption is this will work, because that is how it looks like the unit somebody gave me works.
Correct. But the mosfet duty cycle will be controlled by a current feedback from the current sensor. I will program the mcu to use the current draw and adjust the duty cycle as needed to maintain a constant current. My assumption is this will work, because that is how it looks like the unit somebody gave me works.
Correct. But the mosfet duty cycle will be controlled by a current feedback from the current sensor. I will program the mcu to use the current draw and adjust the duty cycle as needed to maintain a constant current. My assumption is this will work, because that is how it looks like the unit somebody gave me works.
No, this won't work. The goal of a dummy load is to dissipate power, which means the power has to go somewhere safe. The ideal states of a MOSFET used for switching are to be "off" and not conducting, or to be "on" with as close to zero resistance as possible. This keeps the MOSFET cool and prevents it destroying itself, but either way, the power is not going to the MOSFET. If the heat sink does not have significant resistance, then the power is not going into the heat sink either. Which means you don't have a load, you have a short circuit, which is just the "exciting" situation you don't want.
What will happen without proper resistance is that as soon as the MOSFET turns on, it will conduct maybe hundreds of amps through the short circuit (especially if there is a beefy LiPo pack on the other end), and something will quickly blow up. Probably the MOSFET, which will let out the magic smoke and perhaps explode.
Then I am completely at a loss on how the current unit works. Because it uses a 60mm cpu heat sink with the mosfets drain on the heatsink the source pin goes to a 12awg wire which then goes to a low side current sensor, and the positive of the battery is bolted to the heat sink. The mosfets are controlled by a microcontroller.
Edit: Essentially what I am taking from your posts, is that it is a bad design, don't replicate it, use the power resistors I think are boring.
youre not gonna have much luck in designing a reliable 45 A variable electronic load with your budget.
As for the transistors i would go with bjts in parallel. they are more reliable but at these currents the will go 5+ $ a pop and you'll need at least 2 of them in parallel.
The bigger problem for you would be the heatsink which would need to be quite big and would need active cooling (a fan or 2), and the current shunt whic would need to be between 1 and 5 miliohm. in these resistances you would need quite a specialised shunt with 4 terminals, 2 for current and 2 for sensing the voltage ( the resistor itself would set you back half the budget at the very least). you would also need some precision opamps with low offset, noise etc to measure these small voltages accurately.
it,s not as impossible as you might think, but neither as easy as you might think to make this project. Innovation requires efforts.
in the end maybe just a wire in a bucket of water might be the easiest and best solution
My inquiry was for what to use as a load instead of power resistors.
The fets used in the unit given to me are IRL2910PBF
The fets I have in my design are IRLB8721PBF
The unit has the fets hooked to a pin of a 328p with a series resistor in between to the gate. It really is a simple circuit.
Edit:
My assumption was that with a 5v mcu controlling the gate, it would be getting around 4.5-5v.
4.5v gate with a 20-25 amp for current keeps it in the linear section of the graph in the data sheet.
4.5v has an rds on of 13mΩ.
Edit 2:
Of course, the unit that was given to me has a predisposition of burning up fets... But again, I assume other units he build do work.
With all that said, I can still use the same idea with power resistors as the load though, right?
I think what's happening here is a bit of confusion on how the OP's load works. The evidence presented seems to fit.
Indeed the drain of the MOSFET is connected directly to the heat sink through the body of the MOSFET , But the heat sink is acting as a Current Path not as a resistor. Aluminum is a conductor and has very little resistance.The Mosfet is acting as a current Sink producing Heat that is dissipated by the the heat sink and fans. The resistor is a current shunt, likely giving feed back to the Arduino 328p. The Arduino is delivering a PWM pulse to the gate of the MOSFET. Probably 480hz or around 2 millisecond pulse. This depends on the model. The duty cycle will also plays an important role.
It can be done this way, but there is a draw back. Because it is pulsing the current through the MOSFET current sink, it will give an inaccurate measurement of the battery capacity over time because it will be nonlinear. This is why a Linear MOSFET is used with a a DC safe operating range so the battery capacity over time can be measured accurately. Even the cheap loads found on Ebay or Aliexpress work in linear mode and not PWM.The fets used in the unit given to me are IRL2910PBF
The fets I have in my design are IRLB8721PBF
The unit has the fets hooked to a pin of a 328p with a series resistor in between to the gate. It really is a simple circuit.
Edit:
My assumption was that with a 5v mcu controlling the gate, it would be getting around 4.5-5v.
4.5v gate with a 20-25 amp for current keeps it in the linear section of the graph in the data sheet.
4.5v has an rds on of 13mΩ.
Edit 2:
Of course, the unit that was given to me has a predisposition of burning up fets... But again, I assume other units he build do work.
With all that said, I can still use the same idea with power resistors as the load though, right?
The Fets IRL2910PBF that are used in the load giving to you have different safe operating characteristics than the IRLB8721PBF at the 2millisecond pulse of the PWM . The RL2910PBF would indeed sink 20amps safely up to 20 or even 30V drain to source at 2 millisecond pulse if you look at Fig. 8 of the data sheet. https://www.infineon.com/dgdl/irl2910pbf.pdf?fileId=5546d462533600a40153565b9013250b.
The IRLB8721PBF will probably not handle more than 15 Amps at 10V drain to source . And that is even difficult to predict at 2millisecond pulse when you consider what it will do at 10 milliseconds. Look at fig. 8 of data sheet.
https://www.infineon.com/dgdl/irlb8721pbf.pdf?fileId=5546d462533600a40153566056732591
If either the IRL2910PBF or the IRLB8721PBF are operated in linear mode they would most likely fail because the Safe Operating Area in DC cannot be predicted. For either of them, it's is highly unlikely that they can handle more than 5Amps in linear mode depending on drain to source voltage . And most certainly not the expected 10 to 20amps.
Understanding that the unit given to you have probably 5 MOSFETs , the amount of current it can handle is very dependent of the drain to source voltage. The duty cycle of the PWM is also a factor. The higher the Duty cycle the lower the Safe Operating area becomes. This is because with a high duty cycle the MOSFET is kept in the linear region longer.
ALL pulsed Mosfets operate in the linear region but only for short periods of time. https://www.infineon.com/dgdl/Infineon-ApplicationNote_Linear_Mode_Operation_Safe_Operation_Diagram_MOSFETs-ApplicationNotes-v01_00-EN.pdf?fileId=db3a30433e30e4bf013e3646e9381200 https://eepower.com/technical-articles/understanding-linear-mosfets-and-their-applications/#
If you look at a Linear MOSFET designed for that purpose like the IXTH30N60L2,for example, you will see that it has a DC Safe Operating Area. Look at Fig. 14 of data sheet https://www.littelfuse.com/media?resourcetype=datasheets&itemid=5795824c-b355-4cbe-a6c0-e0782d30498c&filename=littelfuse-discrete-mosfets-n-channel-linear-ixt-30n60-datasheet
Many MOSFET will have a DC linear Safe operating area. these are the type that are usually chosen for electronic loads because their linear characteristics can be predicted.
Resister banks can be used as a Load. But unless the load current is monitored, the the measurement of the capacity of a battery will be inaccurate because the resistance changes with temperature. This is why Electronic loads can produce accurate measurements because the current is kept stable in relation to the voltage over time.
There is also maybe this possibility, PSMN1R0-30YLDX. https://www.mouser.com/datasheet/2/916/PSMN1R0_30YLD-2938848.pdf
These are used in a low, race level ESC. Pretty much the most entry level race esc that is used. That ESC uses 12 of these, 4 parallel on each phase of the motor. Supposedly the ESC is rated for 50amps continuous @8.4v. Looking at the data sheet it is a logic level, which is nice. Also smb, which I would like, then I will move to the ACS724 model for the current sensor and get rid of some of my through hole components. Using the DC line in the SOA looks like 7 amps @8v, which as soon as you apply a load you will be down to 8v. So maybe 8 of these in parallel?
These are the mosfets I was considering for another project, simply because I know they work in that application. Being able to use them over multiple projects would be nice.
There is also maybe this possibility, PSMN1R0-30YLDX. https://www.mouser.com/datasheet/2/916/PSMN1R0_30YLD-2938848.pdf
These are used in a low, race level ESC. Pretty much the most entry level race esc that is used. That ESC uses 12 of these, 4 parallel on each phase of the motor. Supposedly the ESC is rated for 50amps continuous @8.4v. Looking at the data sheet it is a logic level, which is nice. Also smb, which I would like, then I will move to the ACS724 model for the current sensor and get rid of some of my through hole components. Using the DC line in the SOA looks like 7 amps @8v, which as soon as you apply a load you will be down to 8v. So maybe 8 of these in parallel?
These are the mosfets I was considering for another project, simply because I know they work in that application. Being able to use them over multiple projects would be nice.
Are you thinking of getting rid of the load resistors then? If you are thinking of operating these at 7A@8V(VDS) then that is the implication. But I don't think so right?
If you are still thinking of doing PWM for the load resistors, then don't look at the DC line and the 8 V point on the IDS vs VDS curve. When the MOSFET is on, the VDS will be very low (determined by IDS * RDSON), so you can push a very large current through this device. The main trick is turning it on and off very quickly. Any time spent in between full on and full off will result in more power dissipation. In fact, it's likely that most of the heat in this FET will result from switching on/off, not from passing current when on. It is a logic level device which can be directly driven by a (5 volt) microcontroller output, but it isn't that simple. The micro's GPIO pins will be able to supply some peak amount of current, which in combination with the gate capacitance, will determine how long the device spends in that transition between on and off. If using more than one FET, try to use a separate pin for each FET, or use a buffer (per FET) to boost current. A logic bus driver usually has higher current output than a generic logic gate. Look for high current drive and fast transition time (e.g. 74AC series, maybe a 74AC245 or 244). If you have a 3.3V microcontroller then you must boost the output to 5 V for the gate; use TTL-input CMOS-output logic like 74ACT245 ("T" for TTL) powered from 5 V; the logic levels for 5V TTL inputs are very similar to and very compatible with 3.3 V CMOS outputs. If you can manage to switch quickly, and use a not-to-high PWM frequency (fewer switching cycles per second), and keep it cool, then you could probably use just one of these. But they are not expensive, so more would give some breathing room. For switching applications like PWM, paralleling FETs is simple; connect Drains together, connect Sources together, then drive all the gates appropriately (maybe connected in parallel, maybe not).
Make sure to use a snubber to absorb the inductive kick from turning off the current flow. This is important! Otherwise you are effectively making a high frequency ignition coil, and it is the FETs that will ignite/burn.