Author Topic: Dummy load circuit mods - why?  (Read 1391 times)

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Offline Lee697Topic starter

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Dummy load circuit mods - why?
« on: September 27, 2019, 09:31:50 pm »
G'day....
I put this together 5 years ago, based on Daves dummy load circuit (EEVBlog #102). I'm  thinking of rebuilding it, so have been drawing out the circuit. I can't recall why I have the ceramics across the output/inverting input, or why the resistor is in series with the inverting input or output is there. Searching doesn't reveal the source of the mods (and I'm pretty sure I didn't come up with them myself!). Can anyone help out? Why would they be there? The circuit works well between near zero and 2A, although the lone MOSFET on a small heatsink gets a bit 'warm' at high current.....
Cheers!
 

Offline schmitt trigger

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Re: Dummy load circuit mods - why?
« Reply #1 on: September 27, 2019, 09:36:30 pm »
Loop compensation?
 
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Offline Jwillis

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Re: Dummy load circuit mods - why?
« Reply #2 on: September 28, 2019, 12:23:20 am »
The mosfet is acting like a precision variable resistor controlled by the opamp . So it a sense its not so much passing current to a load but sinking current to ground. So the Mosfet becomes the load . So its going to get hot and will require a very large heat sink .The resistors as schmitt trigger are there to compensate any change in the load voltage back into the opamp.The caps are to prevent oscillations. The resistor between  the ouput and the gate I think also dampens any oscillation but I'm not sure about that one. 
 
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Online ledtester

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Re: Dummy load circuit mods - why?
« Reply #3 on: September 28, 2019, 03:01:24 am »
The capacitors are there to prevent the circuit from oscillating. The amount of capacitance you need will depend on the transistor you are using for the load.

If you have a scope you should observe the voltage at the sense resistor as you increase the current. With no capacitance you probably will see oscillations beginning at 500 mA to 1 A of load current when using a MOSFET.


 

Offline Jwillis

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Re: Dummy load circuit mods - why?
« Reply #4 on: September 28, 2019, 03:57:06 am »
The capacitors are there to prevent the circuit from oscillating. The amount of capacitance you need will depend on the transistor you are using for the load.

If you have a scope you should observe the voltage at the sense resistor as you increase the current. With no capacitance you probably will see oscillations beginning at 500 mA to 1 A of load current when using a MOSFET.
 

But what is the resistor between the output and gate for?
 

Online ledtester

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Re: Dummy load circuit mods - why?
« Reply #5 on: September 28, 2019, 04:24:37 am »
But what is the resistor between the output and gate for?

To prevent ringing caused by the MOSFET's gate capacitance. More details here:

https://electronics.stackexchange.com/a/68754/95488

1K is probably on the high side. I've been advised that 100R is good enough.
 
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Offline MarkF

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Re: Dummy load circuit mods - why?
« Reply #6 on: September 28, 2019, 05:24:16 am »
 
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Offline Lee697Topic starter

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Re: Dummy load circuit mods - why?
« Reply #7 on: September 29, 2019, 10:54:58 am »
Thanks everyone.... I think I'll probe this with the scope - oscillation - does that mean the current load is oscillating? - or something else.  The load is quite stable, I was testing some batteries through the week, and it was drawing 102mA constantly for 48hr.
 

Online ledtester

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Re: Dummy load circuit mods - why?
« Reply #8 on: September 29, 2019, 11:18:34 am »
Yes, it means the load current oscillates. The frequency is usually in the kHz range so it won't be picked up on a multimeter.

Here's an example of oscillations:

https://youtu.be/eN5kKsJ8Qx8?t=30s

Multimeter with blue backlight shows the current in amps. Power is supplied by the Topward power supply on the right.

Initially the load voltage is 5V and the load is stable until 2.5 A is drawn by the load.
Then the load voltage is adjusted to 10V and the load is stable until around 5 A is drawn.
A final test is done at 15V and the load is stable up through 6A which is the limit of the power supply.
« Last Edit: September 29, 2019, 11:20:29 am by ledtester »
 
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Offline Lee697Topic starter

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Re: Dummy load circuit mods - why?
« Reply #9 on: September 29, 2019, 11:20:35 am »
Thanks - but the link just returns a 404... :(
 

Online ledtester

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Re: Dummy load circuit mods - why?
« Reply #10 on: September 29, 2019, 11:22:59 am »
I fixed the link.
 
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