L200 and UA741 - Page 1
Forum section: electronics / beginners

author: mike_mike
author_rank: Super Contributor
timestamp: October 18, 2018, 10:08:46 am
content: Dear Forum,I built the attached schematic on a home made PCB. When there is no load on the output, the output voltage varies from about 2.77V to 30V. But when I connect a load (12V, 20W bulb), the output voltage goes down to 0.14V and the current is about 0.18A. If I rotate the 100K potentiometer there nothing is happening.I also noticed that the UA741 is heating up while the load is connected.I also checked the PCB and there are no bridges.What should I do to make this project working ?More details: input voltage about 40Vdc, output varying between 2.77 and about 30V, the voltage on the 0.1ohm resistor while the load is connected is 0.02V.

author: Hopkins
author_rank: Newbie
timestamp: October 18, 2018, 10:35:11 am
content: Do not know how to help me? About the knowledge I'm looking for, I'm following up on sharing this information.

author: mike_mike
author_rank: Super Contributor
timestamp: October 18, 2018, 11:14:19 am
content: Please delete this topic.The project has been abandoned.

author: bson
author_rank: Unknown
timestamp: October 18, 2018, 11:21:15 am
content: What's the input voltage? What happens if you use say a 1k resistor load?

author: mike_mike
author_rank: Super Contributor
timestamp: March 03, 2019, 08:26:40 pm
content: I am using a 24V/160VA power transformer.If I use as load an 1K resistor and if the output voltage is 25.1Vdc, then the voltage does not drop.If I am powering a 24V/40W light bulb, the output current is 0.36A, and the ouput voltage drops to 0.7V.I also obtained the following voltages:1. LM741a.pin 7 to pin 4no load = 31.2Vload = 29.9Vb. pin 2 to 3no load = 1.1mVload -1.172V - with 100k pot completely in one directionc. pin 2 to 3no load = 1.1mVload -43.9mV - with 100k pot completely in other direction2. Voltage across R3no load 0.3mVload 34.8mV3. L200a. pin 5 to GNDno load 25.1Vload 0.8Vb. pin 2 to GNDno load 24.8Vload 2.0VI attached the layout for this power supply.I also checked with a new 741, and the power supply is still not working correctly.

author: ArthurDent
author_rank: Super Contributor
timestamp: March 03, 2019, 11:20:56 pm
content: An incandescent bulb with a cold filament can draw much more than its rating. A 12V/20W bulb might draw enough to put the L200 into current limit mode. If you have about a 20 ohm power resistor you could see how that works for a 12V load.

author: jaycee
author_rank: Regular Contributor
timestamp: March 03, 2019, 11:24:01 pm
content: If the 741 is getting hot, i would suspect it's oscillating.

author: malagas_on_fire
author_rank: Frequent Contributor
timestamp: March 03, 2019, 11:42:08 pm
content: In the Schematic the LM 741 and L200 doesn't have bypass caps ? try some ones if you feel they are getting noisy due to the Input or use some battery

author: mike_mike
author_rank: Super Contributor
timestamp: May 04, 2019, 09:24:42 am
content: Hello,I tried another schematic with L200 and LM741. I used a 4A bridge rectifier and a 15Vac/2A transformer. The rest of the components are corresponding with the schematic. The tab of the L200 is not connected to any ground.The problem is that the -5.6V voltage is decreasing to 0.2-0.4V (masured on the Zener diode) when both L200 and LM741 are connected in circuit. If I remove both L200 and LM741, the voltage on the diode is about 4.10V. I tried with a 5.1V zener diode and with 5.6V zener diode, and the results were the same.This problem is affecting the circuit in such a way because the voltage does not go down to 0V and the current does not go down to 0A.I tried the following:1. I replaced L200 and LM7412. I replaced both 1N4004 diodes3. I replaced the zener diode4. I replaced the 2 capacitor (470uF and 47uF)I also checked the PCB and it seems to be good, no bridges.The voltage on the 680R resistor (R1, current limiting resistor for the zener) seens to be around 6V.Please have a look at the schematic and tell me what should I do in order to make the circuit work ?Later Edit: I connected a capacitor as shown in the schematic last schematic, and it seems to work.