#217

episode 217
my design of a DC load

[@button-puncher]

Nice! BTW, laminate (like for a countertop) is a nice/quick/easy way to have a clean look on repurposed cases. You can get a sheet for relatively cheap at your local home improvement store. Just use contact cement to stick it to the case. Put on two layers if you've got some big holes to cover for more rigidity. If you do the iron-transfer laser printed paper trick, it can look pretty darn professional for graphics.

[@timbelson9522]

The builduo of messy cables was not what you initially think of 😅 , very nice and tidy"mess", my Rf teacher always reminds the class that most RF is handmand as well as unique so imagine the $$$$ tag it accompanies for the equipment we learn from. Ive learned a lot of nice tips and tricks and really enjoy the nano vna vida amd I'm now thinking of a tiny sa to add to with it :) , when i complete my trade couse at start of April 2021. When i do do you accept Post or ship Us-Can

[@Irish-Death]

I'm a relative beginner here, so bear with me please. A few days ago I stumbled upon your channel when looking for ideas on a dummy load that was effective yet simpler than many "newfangled" ones that dominate today. I love what you've done and it was the perfect inspiration to start towards my own. Hook it up, turn a knob, accurately see voltage and current, nothing more, no arduino, no programming, and that's why it is so great. My question is about the voltage divider on your amperage display op-amp, keeping in mind that I just started learned about op-amps a few days ago. I thought the other commenter here caught it, but I'm still confused. Assuming I'm understanding the schematic correctly, the op amp "+" input would equal the voltage drop across the resistor, let's say 15 amps through the 0.05 ohm resistor so +0.75V relative to ground? Is it correct (in a simplistic way) that with negative feedback like this the op amp is "trying" to make the two inputs equal? To do both that and have a 15V output for the display, the resistive divider has to be set up for the "-" input to have 1/20th the output voltage (to "reverse" the 1/20th ohm shunt)? Wouldn't that mean it should go: (op-amp output) -> (19k resistor or 20k pot) -> (op-amp "-" input tap) -> (1k resistor) -> (ground) It seems to me that the other commenter, @dominikworkshop6007, was halfway right in that the "-" input needs to center tap the two resistors. Though, I think they missed that the 20k pot needs to be the one between output and inverting input, while the 1k needs to be between inverting input and ground. I only typed all that out for the sake of analyzing my own logic. Obviously you have a vastly larger understanding of all this and have a working unit, so I'm left wondering whether I'm misunderstanding something or if you built it right and miswrote on the schematic. If you somehow see this and make it this far through my wall of text, I'm appreciative of your time and honored that you spent it here. After discovering your channel a few days ago I can clearly see that your channel is a goldmine of years of knowledge and I'm thankful to have found it. Thank you!

[@IMSAIGuy]

this should help: www.scullcom.uk/design-build-an-electronic-dc-load-part-1/

[@Irish-Death]

@@IMSAIGuy Thanks for such a quick reply, but I don't quite understand how that resource addresses the question I posed. I'm wondering if you didn't see the whole comment? I know on my devices I often have to hit a button or something to expand longer comments.

[@IMSAIGuy]

sorry, I don't have the time, the resource is done well and should help@@Irish-Death

[@Irish-Death]

@@IMSAIGuy gotcha, no worries. It was a long winded way of asking if the amp display op-amp's resistive divider was wonky, but still not corrected by the "correction" of the other commenter. No obligation to respond further, thank you for your time.

[@68CHUCKLES]

A few seconds B-4 you turned over the cover and revealed the schematic, I was thinking to myself I should make a comment about how you should put the schematic under the cover ! DOH !!!!!

[@Croccydile]

Nice use of an old HP case like that

[@ydonl]

Just a note... I was sort of baffled by the schematic, which doesn't show the ground connection of the lower leg of the "output". You did say "ground" at one point while pointing at it, so I assume it actually was!

[@IMSAIGuy]

you are right, the bottom end of the 0.05 resistor should have a ground connection on the schematic

[@johndevires5911]

Just what I wanted. How do you know what is the maximum current range it is set to and that 20k trim POT, is it to "calibrate" the current display meter?

[@IMSAIGuy]

yes the 20k is to calibrate the meter

[@bayareapianist]

Just search for "adjustable constant current load". I bought a couple to test my rechargeable batteries. They are very cheap now

[@33samogo]

What op-amp you are using? Great device!

[@IMSAIGuy]

probably a TL072

[@33samogo]

@@IMSAIGuy for such a precise device, I was expected a more precise op-amp, do you think you could achieve the same accuracy with LM358?

[@IMSAIGuy]

@@33samogo the 072 is much better than a 358. but the 358 would probably work ok

[@Irish-Death]

​@@IMSAIGuy At the top of your diagram I see "LM324". Isn't that the op-amp you are using?

[@dominikworkshop6007]

nice build, but shouldn't the 1k resistor be between output and inverting in? xd (10:10)

[@IMSAIGuy]

it is a non-inverting amp.

[@dominikworkshop6007]

IMSAI Guy oh, sorry I meant inverting input XD

[@IMSAIGuy]

oh, you're right. wonder if I wired it right and just messed up the schematic. seems to work :)

[@dominikworkshop6007]

@@IMSAIGuy I'm pretty sure you wired it up correctly, sometimes things like that happen.