DC to DC regulator circuit to handle lots of amps

Thread Starter

SailCS33

Joined Mar 18, 2021
37
I've been looking at high output car alternators (140A and up). What I "need/want" is no more than 40A (be happy with a 30A). Problem is if I get a small alternator like that it would be pushing its max while if I got 140A alternator it would be operating at 1/4 of its rated capacity. Theory is seems good but now what I need is a regulator that would limit the output to the 40A.

I am having a hard time finding any circuits or off-the-shelf DC to DC regulators or buck converters that would be suitable for these amperages. I get it that it could dump out heat but cant it be mitigated with a fan? Any ideas?
 

crutschow

Joined Mar 14, 2008
27,709
Why do you think you need to limit the current?
Devices designed to operate on 12V only take the current they need, they don't care how much current the alternator can deliver.
What is taking this 40A?
 

Thread Starter

SailCS33

Joined Mar 18, 2021
37
Why do you think you need to limit the current?
Devices designed to operate on 12V only take the current they need, they don't care how much current the alternator can deliver.
What is taking this 40A?
Great question. I have 200Ah lifepo4 battery. It will take as much as you give it (over 200A it will start getting damaged). BMS would limit incoming amps by cutting off the source but only as a safety measure. It won't keep it at that amperage.

To go up to 140+ amps I would have to upgrade the pulley to serpentine. Upgrade wires to 3/0 AWG. And with all that, I'd worry about connectors which could corrode overtime and heat up. In my mind, this is not only very expensive but also dangerous when I really have no need in charging my battery at almost full 1C.
 
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MisterBill2

Joined Jan 23, 2018
9,251
How about regulating the alternator field to control the current?That will avoid having to use very high powered semiconductors and at the same time improve the overall efficiency. You will need an ammeter shunt resistor and an amplifier for that to provide a useful voltage. Then a comparator to sense when 40 amps is reached so that it can switch off the alternator field current. That will allow the alternator output to drop, until at some slightly lower current it will switch back on. It will not be perfectly smooth but for battery charging that is not an issue. And the power lost to heat will be very little.
 

Thread Starter

SailCS33

Joined Mar 18, 2021
37
a comparator to sense when 40 amps is reached so that it can switch off the alternator field current.
Ooooh, this sounds interesting. Yes please do tell more! If this is something I can buy please point me to a link or if I need to put it together a diagram would be great.

I already have a shunt and I read amp/volt values very reliably and I made it using INA219 and using it on either arduino or Pi4 but I quoted the part I didnt understand and unclear about.

Thanks again
 

GetDeviceInfo

Joined Jun 7, 2009
1,875
I would think that with a bit of hackery one could apply a chemistry specific charger ic, utilizing it’s functions, sized to handle field current.
.
 

MisterBill2

Joined Jan 23, 2018
9,251
OK, if a current shunt and amplifier feeding a processor board, and probably a display, are already part of the system then you are almost there! The remaining hardware part is an output from that processor to a transistor switch to control the field current of the alternator. Probably there should also be a mechanical relay to disconnect when the system is off. The code to run the output will need to switch on the relay when the system is on, and then switch on the field current when the charging current is below 40 amps, and switch it off when the current reaches 40 amps, or whatever limit has been selected.
What the field current should be will depend on the alternator, often either 3 amps or 5 amps. So the switch will need to be fairly fast to reduce the heat in the controlling transistor while turning on and off.
 

crutschow

Joined Mar 14, 2008
27,709
If i upgrade the alternator I will opt in for the external.
As MB2 noted, you could put a P-MOSFET with a driver transistor in series with the field current wire and have the micro control it based on the current it reads from the INA219.
This would result in it rapidly turning the field current on and off (PWM signal) to achieve the desired current limit.
You would likely want a small amount of hysteresis in the current levels it reads to turn the MOSFET off and then to turn it back on.
 

Thread Starter

SailCS33

Joined Mar 18, 2021
37
Ok so this is the part I'm getting a bit lost in. I am reading the voltage and amperage and I can easily make arduino or Pi4 drive a relay to control the field wire of the alternator but it would seem to me the relay would be doing a lot of on/off clicking. Would something like this do a better job?
https://www.amazon.com/Anmbest-High-Power-Adjustment-Electronic-Brightness/

I also found this. Would this be along the lines of what you are recommending?
https://www.aliexpress.com/item/4000113738564.html?src=google&albch=shopping


Thanks again
 

MisterBill2

Joined Jan 23, 2018
9,251
The constant on/off would be done with a switching transistor. The relay is for shutdown when the system is not running. But consider that before the transistor ellectronic voltage regulators came along, that clicking relay was how automotive voltage regulation worked. So with the right choice of relay it could still work. But I did say the transistor was for the regulating control, at least that is what I intended to say.
 

crutschow

Joined Mar 14, 2008
27,709

MisterBill2

Joined Jan 23, 2018
9,251
They may work, but it's not clear if the can do a high-side switch, which your setup needs.
The one with the opto isolator would be more likely to work.
Given that the only technical information you will get is on that listing that you see, I would avoid both of those products, unless you are able to reverse engineer them and figure out exactly how to use them. The technical support available there is NONE AT ALL!!!
If you know the maximum field current that the alternator can handle, and since you know the volyage available to power it, you can search and find a suitable transistor to control the field. You will need a heat sink, I am sure, but it will not be huge. Here you have the benefit of having some folks available who have interfaced with Arduinos quite a lot. But that is not me.
Quite possibly a power MOSFET device will be the best choice.
We will need to know if both field terminals are available forcontrol, since that varies quite a bit between different models of alternators.
 
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crutschow

Joined Mar 14, 2008
27,709
Below is the LTspice simulation of an example isolated MOSFET driver that should be able to control the alternator field current using the micro output.
I used an opto isolator to help protect the micro from ground transients.

The micro output (green trace) is arbitrarily shown at a 50% duty-cycle, but that will we whatever is required to give the desired output current.

The MOSFET should have a low enough on-resistance so that it doesn't dissipate more than a watt from the maximum field current.
That will avoid having to add a heatsink for it.

You will likely want to put some hysteresis in the current control points from the micro (say a few amps) to keep the switching frequency at a reasonable value.

1617382020029.png
 
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Thread Starter

SailCS33

Joined Mar 18, 2021
37
THANK YOU!
I was educating myself watching this video on the topic of designing a circuit using power MOSFET. This is way over my head for now but I am determined to learn. This stuff is fascinating.
 

MisterBill2

Joined Jan 23, 2018
9,251
certainly some hysteresis will be needed to slow the cycling of and on, that will probably need to be determined experimentally. But since it will be two different parameters in the code it should be quite simple to experiment with.
 

crutschow

Joined Mar 14, 2008
27,709
One note that may help in your understanding is that the P-MOSFET in my schematic requires a negative gate-source voltage to turn on, so in the configuration shown, grounding the gate turns it on (source more positive than the gate) and connecting it to near the supply (source) voltage turns it off.

Thus when the input is high from the Micro, the opto output transistor turns on, pulling its output to ground.
The PNP emitter-follower driver is then also pulled low, putting the MOSFET gate at about one base-emitter drop (0.7V) above ground, turning on the MOSFET.

Do you have an oscilloscope to observe the circuit operation?
 
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Orson_Cart

Joined Jan 1, 2020
54
As mentioned above regulating the field current is by far the best way to arrive at voltage / current limiting.

We make high power buck chopper circuits ( that have to pass FCC RFI regs ) and the chances of a non professional getting a working system going at 140ADC are a bit on the low side - not to mention all the RFI that would be created - any one with a radio in a 500m radius would know when it was going.
 
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