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Topic: Switching ON/OFF High Current Load (Read 1 time) previous topic - next topic

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?

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?



larryd

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.





No technical PMs.
The last thing you did is where you should start looking.

Paul_KD7HB

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

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?

Paul_KD7HB


Coding Badly


nuggetchris

#9
Jul 14, 2018, 06:40 am Last Edit: Jul 14, 2018, 06:42 am by nuggetchris
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...

larryd

#10
Jul 14, 2018, 07:03 am Last Edit: Jul 14, 2018, 07:14 am by larryd
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.




No technical PMs.
The last thing you did is where you should start looking.

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.

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?

Coding Badly

#12
Jul 14, 2018, 07:53 am Last Edit: Jul 14, 2018, 07:54 am by Coding Badly

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


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.

Paul__B

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. :smiley-roll:  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.

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