Electronics > Beginners

150w DC Load project (low cost, med difficulty)

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blackdog:
Hi,

The way I test the output section of a power source, for example, is this:
I take a power supply that can provide sufficient voltage / power.

I then connect the output section directly via twisted connection cables to the power supply and measure then over the 0.1 Ohm resistor R17 how the signal looks.

Which signal? connect a function generator to pin-5 of U7B.
Set the DC shift on the function generator so that the desired test current is running.
Now also set the AC signal to the function generator, usually I start with 500Hz and 10% duty cycle block signal.

Even now I measure over R17 and the signal should not have any overshoot!
If the signal has overshoot, the compensation is not correct yet.
And now the difficult part. Now place a resistor in series with the + connection of the power supply.

Test with 1, 5, 10, 22, 100 Ohm, think about the current, at 100 Ohm you usually can not run 2 Ampere.
Make sure you stay in the linear area of the dummy load.

The higher the chosen resistance, the lower the bandwidth, especially with the proposed power mosfets.
This is due to the large feedback capacities of the power fets.
It is easier to get a larger bandwidth with fast audio transistors in the power section.

Designing a good dummy load is not easy and at least 75% of your time goes to getting the output section properly stable and bullit proof  :(

Down hear some links of some older design of mine, there are a lof of measurements, use Google translate becaue i'am Dutch, sorry fore that  :-DD

https://www.circuitsonline.net/forum/view/121338/4#highlight=load+dummy

And one with Audio power transistors
https://www.circuitsonline.net/forum/view/106770/1#highlight=load+dummy

Maybe my designs give you guys some inspiration...

Kind regards,
Blackdog


ArdWar:
My first impression is that there's a lot of unnecessary part. I don't think U7A and U7C is needed.

Also, passively balancing MOSFET in linear operation is kind of risky. I dunno how much his balancing transistor can equalize, but I'd prefer individual  drive  :-//

Inverted18650:

--- Quote from: ArdWar on March 02, 2018, 05:57:18 pm ---RDS(on) doesn't matter anyway for this application, unless you want to make a high current, very low voltage load.



--- End quote ---

I use it almost exclusively to test my DIY li-ion battery banks and like to run them from 4.2V down to 3.0V (2.75 depending on the manufacturer of the cells) @ 100 A's. I've spent many hours testing the unit and trying to find ways to tailor it to my application.

edit: I want to get it up to 100A's*

Inverted18650:

--- Quote from: blackdog on March 02, 2018, 07:42:39 pm ---
Maybe my designs give you guys some inspiration...


--- End quote ---

For sure brotha, thanks for the link.

Mechatrommer:
using 4X 120A mosfet in a 5A dummy load is kinda nuts imho, its kinda mosfet-toss-phobic design. there will be only 1.25A max running through each mosfet and the 0.1ohm power resistor. that is 0.125V across each of the R22-25, i wish i could see those nodes probed to be certain they are not oscillating during step respond and steady state.. build a 10X or 100X amplifier if DSO is noisy.


--- Quote from: Inverted18650 on March 02, 2018, 03:23:41 pm ---I built V7 and can tell you without a doubt, you can by-pass both the fuse and diodes.

--- End quote ---
until you plug the voltage source backward.

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