Dummy load circuit mods - why? - Page 1
Forum section: electronics / beginners

author: Lee697
author_rank: Regular Contributor
timestamp: September 27, 2019, 09:31:50 pm
content: 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!

author: schmitt trigger
author_rank: Super Contributor
timestamp: September 27, 2019, 09:36:30 pm
content: Loop compensation?

author: Jwillis
author_rank: Super Contributor
timestamp: September 28, 2019, 12:23:20 am
content: 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.

author: ledtester
author_rank: Super Contributor
timestamp: September 28, 2019, 03:01:24 am
content: 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.

author: Jwillis
author_rank: Super Contributor
timestamp: September 28, 2019, 03:57:06 am
content: Quote from: ledtester on September 28, 2019, 03:01:24 amBut what is the resistor between the output and gate for?

author: ledtester
author_rank: Super Contributor
timestamp: September 28, 2019, 04:24:37 am
content: Quote from: Jwillis on September 28, 2019, 03:57:06 amTo prevent ringing caused by the MOSFET's gate capacitance. More details here:https://electronics.stackexchange.com/a/68754/954881K is probably on the high side. I've been advised that 100R is good enough.

author: MarkF
author_rank: Super Contributor
timestamp: September 28, 2019, 05:24:16 am
content: Here is a link to the one I built:https://www.eevblog.com/forum/beginners/struggling-with-dc-load/msg2698044/#msg2698044

author: Lee697
author_rank: Regular Contributor
timestamp: September 29, 2019, 10:54:58 am
content: 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.

author: ledtester
author_rank: Super Contributor
timestamp: September 29, 2019, 11:18:34 am
content: 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=30sMultimeter 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.

author: Lee697
author_rank: Regular Contributor
timestamp: September 29, 2019, 11:20:35 am
content: Thanks - but the link just returns a 404...

author: ledtester
author_rank: Super Contributor
timestamp: September 29, 2019, 11:22:59 am
content: I fixed the link.