Switching ON/OFF High Current Load

Hi! For a project I am designing a system of very high-power electromagnets. Each electromagnet is fed a current of around 75A. To allow them to cool, I alternate between feeding power between one and the other around every 10 seconds or so. The device is controlled by a master raspberry pi or arduino and I am struggling to figure out an effective relay type switching device that won't break the bank. Any help?

Hi! For a project I am designing a system of very high-power electromagnets. Each electromagnet is fed a current of around 75A. To allow them to cool, I alternate between feeding power between one and the other around every 10 seconds or so. The device is controlled by a master raspberry pi or arduino and I am struggling to figure out an effective relay type switching device that won't break the bank. Any help?

Don’t cross post!

Don’t cross post!

Assume these are DC, what voltage?

Might be able to place MOSS FETs in parallel.

75Amps and an inductive load, will have huge kickback that needs to be dealt with.

nuggetchris:
Hi! For a project I am designing a system of very high-power electromagnets. Each electromagnet is fed a current of around 75A. To allow them to cool, I alternate between feeding power between one and the other around every 10 seconds or so. The device is controlled by a master raspberry pi or arduino and I am struggling to figure out an effective relay type switching device that won't break the bank. Any help?

And how many thousands of volts would you be using?

Paul

Hi! For a project I am designing a system of very high-power electromagnets. Each electromagnet is fed a current of around 75A. To allow them to cool, I alternate between feeding power between one and the other around every 10 seconds or so. The device is controlled by a master raspberry pi or arduino and I am struggling to figure out an effective relay type switching device that won't break the bank. Any help?

And even more cross posting????

Merged.

Sorry, wasn't quite sure which topic to post under so put it in the most relevant ones. My bad!

The power is delivered over 12VDC. How would you wire together Mosfets in parallel? I always assumed that would overcomplicate the connections...

MOSFETs can usually be connected in parallel with little problems.
Use the same part number for each transistor.
A proper MOSFET driver might be best as the total effective input capacitance will increase and switching speed will slow.
Since the RDSon of each transistor is in parallel you gain the benefit of less voltage drop across the transistors hence they will run cooler.
Of course, use sufficiently sized conductors.
For kick back, transorbs and shunt resistors might be warranted, should investigate the transients with a scope. At 75 amps some experimentation will be needed.

Examples

An input to GND 'turn off' resistor would be added, about 5.6K to 12K.

What type of MOSFETs would these be? Would they each need to withstand the full load of 75A? Searching around on amazon the highest-power MOSFETs I can find peak at 35A.

The circuit still would include a high power diode in parallel with the inductor such as below, correct?
https://www.amazon.com/Amp-Volt-Stud-Blocking-Diode/dp/B004FGPUE6/ref=sr_1_4?s=industrial&ie=UTF8&qid=1531543971&sr=1-4&keywords=100+amp+diode

Example:

lastly where would the shunt resistor plug in to such a circuit if the diode is already in place?

Amazon? Did you seriously not find something appropriate at Mouser, DigiKey, Avnet, or Arrow?

Just in general: Do you really have to switch them on/off?
I guess if you gradually regulate them on and off that should be easier to handle.
That just came to my mind but I am not an expert.

Edgar1:
Just in general: Do you really have to switch them on/off?
I guess if you gradually regulate them on and off that should be easier to handle.
That just came to my mind but I am not an expert.

Heh, heh, Let's look into the maths of that.

So this solenoid draws 75 A at 12 V. To "regulate" it to half power means that it has 6 V applied and 6 V across the controlling FETs. OK, we now have the FETs dissipating 6 by 37.5 - I calculate that at two hundred and twenty-five watts!

That is basically going to require quite a few FETs and a maybe a water cooled heat-sinking system with a large fan-cooled radiator somewhere. :roll_eyes: OK, they sell those for gaming machines, don't they? Maybe not so bad.

With PWM as discussed so far we have three FETs and perhaps a six by eight inch conventional heatsink.

Paul__B:
Heh, heh, Let's look into the maths of that.

So this solenoid draws 75 A at 12 V. To "regulate" it to half power means that it has 6 V applied and 6 V across the controlling FETs. OK, we now have the FETs dissipating 6 by 37.5 - I calculate that at two hundred and twenty-five watts!

That is basically going to require quite a few FETs and a maybe a water cooled heat-sinking system with a large fan-cooled radiator somewhere. :roll_eyes: OK, they sell those for gaming machines, don't they? Maybe not so bad.

With PWM as discussed so far we have three FETs and perhaps a six by eight inch conventional heatsink.

Ok, I get your idea.

If you don't mind maybe you have a short explanation how ESCs (Electronic Speed Controllers, used in RC hobby planes, boats, etc.) handle that issue without huge cooling elements.
Here is a sample of an 100A ESC for 25V with passive cooling element and total 99g.

I know these ESCs do something different but they also regulate inductors 0 - 25V up to 100A - and 100A is not the maximum. They are available for more than 100A.

Edgar1:
If you don't mind maybe you have a short explanation how ESCs (Electronic Speed Controllers, used in RC hobby planes, boats, etc.) handle that issue without huge cooling elements.

Yes, it's called "PWM" - Pulse Width Modulation. That is what has been discussed previously in this thread.

Instead of switching partially on and the control device being a resistor, it spends a proportion of the time switched completely on so that while a substantial current is being passed, there is negligible voltage across the control device and so the power dissipation is minimal, and a proportion of the time switched fully off so that with no current passing, there is again, no power dissipated in the control device, thus overall it controls the average current proportionally without dissipating much power at all (if the device is of correct specification). The successive on and off phases are repeated at such a rapid rate that a light for example, simply seems to be continuously lit to the human eye and thermal inertia averages out heating effects. The proportion of "on" time - the duty cycle - determines the average power or intensity.

The effect is different for an inductor. The short "on" times are insufficient to cause the current in the inductor to increase substantially nor are the "off" times sufficient to cause the current in the inductor to decrease substantially so that the current fluctuates about a value proportional to the mark-space ratio. In effect, the inductor is a low-pass filter averaging the current. Note however that for this to happen, there must be a "catch" diode across the inductor to continue to pass that decreasing current while the control device is off.

BTW, ESCs for brushless (3 wire) motors work by switching current to the windings to create a field rotating at the speed at which the motor (rotor) is desired to spin. Slightly different process.

nuggetchris:
What type of MOSFETs would these be? Would they each need to withstand the full load of 75A? Searching around on amazon the highest-power MOSFETs I can find peak at 35A.

So you would require three of them in the above circuit.

nuggetchris:
The circuit still would include a high power diode in parallel with the inductor such as below, correct?

Sure would! Lots of smoke otherwise. Perhaps a somewhat more generous current rating but for a 10 second turn-around that should suffice and the recovery time of the diode is unimportant.

nuggetchris:

lastly where would the shunt resistor plug in to such a circuit if the diode is already in place?

No, the diode is the correct solution.

Also, I forgot to mention that within the circuit there is a shunt used for a multimeter in order to measure power flow of the circuit. The shunt can be found here:

It is 100A 75mV.

So I am seeing a few options
A) Parallel Diode//IND with use of three expensive relays
(Does this imply splitting source wire to three strands and wiring each strand to relay?)
B) Parallel Diode//IND with use of Tri-MOSFETs and an enormous cooling system
(Same deal as above)
C) Parallel Diode//IND with use of ESC

Which is the best option, though?

nuggetchris:
Also, I forgot to mention that within the circuit there is a shunt used for a multimeter in order to measure power flow of the circuit. The shunt can be found here:

https://www.amazon.com/gp/product/B013PKYILS/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

It is 100A 75mV.

So I am seeing a few options
A) Parallel Diode//IND with use of three expensive relays
(Does this imply splitting source wire to three strands and wiring each strand to relay?)
B) Parallel Diode//IND with use of Tri-MOSFETs and an enormous cooling system
(Same deal as above)
C) Parallel Diode//IND with use of ESC

Which is the best option, though?

Oh, come on! None of us are going to duplicate your system so we can determine which is the best option. You have been given options and it is up to you to determine for yourself the "best" option.

Paul