Electronics > Projects, Designs, and Technical Stuff
My constant current load - 90W , 3A 30V
Spikee:
Is there any advantage in using an one ohm 100W power resistor instead of an 0.1 ohm 10W resistor ?
I did some research in some other designs and youtube video's but i am not sure yet.
In the ohmic region of the mosfet the resistor will only dissipate 9W (30v 3A -> 3v drop * 3A = 9w in power resistor 1ohm ) but when the mosfet is in full saturation the 90w will need to be dissipated almost fully in the load resistor.
I'm not sure if im thinking correctly but if i use a 0.1ohm load resistor it will take in ohms region 0.9W (30v 3A -> 0.3 v drop *3 A = 0.9W )
but in full saturation it still needs to take almost the full load of 90 W ?
thanks
spikee
T4P:
Let's say you are measuring 1amp, 1amp = 1 volt so therefore you get more resolution and accuracy at lower voltages but the dropout voltage is higher though at higher currents
Unless you use a specific DMM chip you know will work for sure with milliohms then with lower currents just go with a 1ohm shunt resistor
LEECH666:
Not really, you will only be able to dissipate more power in that specific resistor. If you want to parallel A LOT of FETs and only use a single shunt resistor is the only application I can think of where this matters. For example in one my dummy load units I want to be able to draw 1.25A.
Power dissipated in 0.1 Ohm shunt resistor is:
P= I² * R = (1,25A)² * 0.1R = 0,15625W
I'll be using a 3W resistor that I'm having around, so that should more than do for my application.
The maximum current that 0.1 Ohm / 3W resistor should be able to handle is:
I_max = sqrt(P/R) = sqrt(3W/0.1) = sqrt(30) = 5.477A
A 0.1 Ohm / 10W resistor should be able to handle:
I_max = sqrt(P/R) = sqrt(10W/0.1) = sqrt(100) = 10A
For a 0,1 Ohm / 100W resitor that would be:
I_max = sqrt(P/R) = sqrt(100W/0.1) = sqrt(1000) = 31.622A
So unless you want to draw more than 30 Amps, no it probably won't make sense to use a 100W resistor.
//EDIT: Sorry I misread again. I thought you where talking about 0.1 Ohm / 100W vs. 0.1 Ohm / 10W.
Cheers,
Florian
blackdog:
Hi
Here are some pictures off my Active Load.
He is really finished :-)
Inside picture
Kicking Ash! YES almost 300Watt's for 5 minutes, temp heatsing 85C.
Now I just have to find the screws to close the case...
Kind regarts,
Blackdog
SeanB:
I think using a 100A plus stud mount diode is a little overkill for 20A........, and the heatsink on it is a little small anyhow.
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