Dummy Load - Page 1
Forum section: electronics / beginners

author: ElectroNub
author_rank: Contributor
timestamp: March 02, 2015, 06:38:52 am
content: I believe I've learned enough to start getting dangerous. I want to check and make sure this schematic looks right before I start plugging in voltages and building it up. Am I doing it right so far? (I think a schematic is attached).This is my first real schematic and project build from scratch. I feel like my head is going to explode from figuring out the magic in op-amps...I'm using parts from a harvested PC power supply. The enclosure will make a nice project box as well. I will be happy to post pics and a build log after I get it working.

author: max666
author_rank: Frequent Contributor
timestamp: March 02, 2015, 07:21:26 am
content: At first glance I see no reason why it shouldn't work.In case it oscillates you could add a series resistor to the gate of the mosfet.

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 02, 2015, 08:09:57 am
content: C1 is connected wrong. In this configururation, it is not stable and will oscillate.C1 should be between th output of the opamp, and the minus of the opamp.

author: codeboy2k
author_rank: Super Contributor
timestamp: March 02, 2015, 10:34:25 am
content: There's at least 10 dummy loads on the eevblog forum already, with lots of discussion and suggestions for how to make it right. Rather than start another, why not read those first?

author: ElectroNub
author_rank: Contributor
timestamp: March 02, 2015, 11:09:46 am
content: I did read the other threads. That is where I started from and this schematic is based on those. But none of those are to my specs and I don't have any of those parts in my inventory. My parts box is not full of everything. Please don't beat me up sir.Thanks for the other suggestions. I moved that filter. Will R4 be enough to help stop incoming noise from the DUT or should I add another cap there?I also used up another channel on the LM324 for a fan control. The pot I have has 2 wipers. Don't need the fan on turbo if it isn't doing anything right?

author: Seekonk
author_rank: Super Contributor
timestamp: March 02, 2015, 02:37:45 pm
content: Close but no cigar. If you are going to have an opamp for the fan, use it to turn it on when it gets hot. Use the 431 as a reference voltage, sense the forward voltage of two diodes in series (voltage lowers as they get hot) and use that to turn on a fan. A common bridge rectifier with a hole in it is easy to mount on a heat sink, use the + - terminals. Use a 4.7K resistor from the 431 reference for the bias current.

author: rdl
author_rank: Super Contributor
timestamp: March 02, 2015, 02:52:16 pm
content: I hope you have an oscilloscope so that you can observe any oscillation. I recommend that the first time you build it that you make provisions to easily substitute for R4, R9, and C1. You might even consider pots for the resistors, that's how I did it. I also used a capacitor substituter to dial in C1.The version I built that was finally stable used:R9 = 150RR4 = 22RC1 = 330NNote that I was using the MTP3055VL, I built on perfboard, and some of the connections were kind of long (over 1 inch), so this probably had an effect on the values required. R4 in particular seemed a bit odd, it had to be between 5 ohms and 45 ohms - nothing else worked. I tried higher values for R9, but saw little to no difference. C1 was fine anywhere from about 200N on up.

author: German_EE
author_rank: Super Contributor
timestamp: March 02, 2015, 06:59:01 pm
content: "I feel like my head is going to explode from figuring out the magic in op-amps..."No magic, just remember some simple rules.1) The inputs of a opamp do not consume any current2) Assuming a normal feedback path from output to inverting input, the output of an opamp will alter until both the inverting and non-inverting inputs are equal

author: ElectroNub
author_rank: Contributor
timestamp: March 03, 2015, 04:34:33 am
content: I do have a low end scope.I like the idea of the pot in the filter. Makes things a lot easier to tune! I guess a dip socket would be easy enough for quick change on caps.I will dig deeper into the fan control. I played around with some thermistors trying to make a temp control work in the past. Maybe I can get it to work now with some of my new op-amp skills