Making DIY Electronic DC Load (problem with op amp driving the mosfets) - Page 1
Forum section: electronics / projects

author: Dzoro
author_rank: Contributor
timestamp: April 10, 2023, 12:30:22 am
content: Hi everyone it is me again with the brain fart DIY Electronic DC Load from Scullcom Hobby Electronics youtube channel which has problems since i started building it. At the previous topic i posted about it, i had problems with the arduino code which i fixed thanks to a member from this forum @pqass. So now i have 2 new problems with the circuit, but before that i need to tell you about the ic's changed on my circuit (down bellow are the two schematics, mine and Scullcom one).I changed those components because in my stupid country we don't have anything. All the electronics suppliers still sell ic's from 1960, and they don't have simple op amps that are used this days, but they have crt microprocessors that no one buys and they still sell them,but what can i do. So i bought what i could find the closest to Scullcom circuit.Components changed:ADC MCP3426 to ADS1115Op Amp AD8630 to TLC2274CNRTC module MCP79410 to DS13074.096 reference voltage REF5040 to MCP1541Op Amp for the voltage sense circuit OPA277PA to OP27GLogic Level Mosfets BUK956R1-100E to BUK9508the other component that are changed are not important and they will not change the working principle of the circuit.So the first problem is that on the lcd screen voltage measurement when nothing is connected on the sense input is reading 450mV, but when i connect voltage on the sense input the voltage is spot on, i put 5v i read 5v i put 15v i read 15v and those 450mV don't add to the measured voltage. So i don't know if the problem is in the circuit or the code.And the other more important problem is that the op amp doesn't drive the mosfets.I will explain this as clear as i can.So because it is more readable, look at the Scullcom schematic, when i measure the output of the DAC and i increase the current set on the display the voltage measured goes from 0 to 4.1V which is at maximum supply voltage of 4.096. Than it goes to a buffer or a voltage follower (in the schematic U7A) on the output of that buffer i should get the same voltage, but when the DAC puts 3V in the buffer, the output of it has 700mV and the chip is brand new, than it goes to to a resistor divider which divides it by 10 so if i have 1V i will have 0.1V go in the (U7B in the schematic) but no matter the voltage on the input of U7B, on the output of it i don't have voltage. So can someone take a look at Scullcom schematic and see why is that happening on mineThanks Dzoro

author: TopQuark
author_rank: Frequent Contributor
timestamp: April 10, 2023, 03:54:04 am
content: For the first issue where you read 450mV when the sense pin is floating (not driven by a source), it is probably reading whatever charge that has built up at the pin, or more likely a voltage formed by internal voltage sources and parasitic impedances. As long as the sense pin reads correctly when connected to a source, and it will always be connected to a voltage source, it should be fine for what you are doing.Second point, your TLC2274CNG4 positive power supply pin doesn't seem to be connected to your voltage regulators.

author: pqass
author_rank: Super Contributor
timestamp: April 10, 2023, 03:58:47 am
content: Quote from: Dzoro on April 10, 2023, 12:30:22 amLeaving the opamp -input (P7) unconnected should just turn the op amp into a buffer with output close to 0V (given P8 is grounded through R31, 20K, and R13, also 20K, negative feedback). Maybe long wires are picking up noise? Check with an oscilloscope; a multimeter would just show an average voltage.See attached for how sense inputs are handled in an HP 6060B load; they are connected to the input terminals (+/-IN) via resistor+cap (4.7R, 47nF). Also, the input terminals are connected together via C25+R100||R88, 2.2uF, 3.9R||3.9R, cap+resistors and C37,C34,R156 (22nF, 22nF, 10M) via the chassis.HP 6060B service manual:http://ridl.cfd.rit.edu/products/manuals/Agilent/power%20supplies/CD1/Service/6060ser.pdfQuoteConfirm that U7 power pins have 5V across them. Confirm DAC voltage is present on the +input and the -input to output feedback isn't broken. Measure the output of U7A without it being connected to R6; ie. lift the leg/cut the trace. If output != +input then you have a bad opamp. Use a breadboard to confirm all four opamps in the package work prior to placing in-circuit; ie. connect -input to output and test that output follows input via function generator+oscilloscope or pot+multimeter. If output == +input then you have a short/problem downstream. Re-check connections.

author: Dzoro
author_rank: Contributor
timestamp: April 10, 2023, 04:30:56 am
content: Quote from: TopQuark on April 10, 2023, 03:54:04 amIn the youtube video there shouldn't be 450mV on the screen but never the lessI cheked and i actually forgot to put 5v power line on the schematic and the chip was nor powered, but the interesting thing is that right now when i powered it it measured 34 volts and when i connected 5v to the chip it went down to 26 volts and i didn’t touch the circuit from the last time when i measured 450mVAlso one more thing is that now when i apply 15 v to the sense pins doesn't stay like that and doesn't add to it, it subtracts 2v which is completely crazyI am assuming that the op amp is bad even before i bought it.Also one more thingMy op amp is cmosAnd for the AD8630 i cant find information to see if it is bipolar or cmos, and i don't know if that would make a change

author: Dzoro
author_rank: Contributor
timestamp: April 10, 2023, 04:39:48 am
content: Quote from: pqass on April 10, 2023, 03:58:47 amThanks for the advice i will check everything and i will give you feedback

author: Dzoro
author_rank: Contributor
timestamp: April 10, 2023, 11:04:50 pm
content: Thanks for the replies i fixed it finally.The problem was the chip, it was bad from the factory and also i forgot to put 5v power to it (thanks for noticing that), i used a different one MCP6024. It is not the best one, but it will do the job until i get something more precise in my hands. The only thing left to fix is the code for the RTC module because i wasn't able to fix that, so if you can help about that i would be really grateful.ThanksDzoro