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Profile: Velly
Name: Velly Offline
Joined: Tuesday, October 25, 2016(UTC)
Last Visit: Sunday, March 5, 2017 9:54:07 PM(UTC)
Number of Posts: 2
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Topic: Questions about comparator LM393   Go to last post
Posted: Saturday, March 4, 2017 4:14:18 AM(UTC)
I'm trying to measure the voltage at a fuel gauge and determine that when the voltage is 3V (representing 1/4 tank) I need a 12V light to go on representing low fuel. Looking at the LM393 on the attached diagram, V+ would be the 12V input from the car battery.

I'm unsure how to bring in the input from the fuel guage so when the voltage at the fuel guage drops to the 3V the output of IC 1A goes high.

On the attached diagram would I need to connect the fuel gauge voltage at the + input of IC 1A (not requiring R1 and R2 as shown) and then adjust RV to 3V? So in theory when the + input of IC 1A drops to 3V, it matches the input of 3V controlled by RV at the - input of IC 1A which makes the output go high?

As far as the 12V light, can I just connect it directly to the output or would I need to consider a transistor & relay configuration?

Since LM393 has 4 comparators and I will be using 1 of them, do any of the other comparators need to be tied to voltage as well to prevent "floating" ?


Topic: A DIY circuit of low voltage audio amplifier   Go to last post
Posted: Wednesday, October 26, 2016 3:49:28 AM(UTC)
Hello,One could call me a freshman in electronics and I just designed my first little audio amplifier for headphones.

(Pocket design, I want to be able to drive 4 headphone pairs from a single source) I want to post my project here .I hope you can correct me if there are mistakes. And I hope my experience can give you a little help .

Exposition time:

Here's the schematic, so you guys know what I'm talking about:

(Without power supply (9V bat) and single channel)


Ofc, 4 headphones draw a lot of current, which is why I need a proper output stage. Up to 200mA to be precise. The current design I made uses 2N5551 and 2N5401 transistors but their DC current gain is horrible (<10) which causes the OPA to overdrive. I don't want to much load on the OPA as well (max 20mA). A higher input voltage is out of question.

Therefor I need better output transistors (Q3 & Q4) which provide a higher current gain but also are capable of switching fast enough for audio signals. They should be cheap and small. I don't know that much components and the market out there is just a jungle of different parts.

The question:

Is it possible to compensate for the clipping with different output transistors? The circuit behaves great on 32 ohm but a lower load impedance wrecks it. I'm also open for darlington array solutions.

*EDIT: I can't use SMD parts. I need through hole packages.

**EDIT: Juuuust figured! This OPA is not even able to supply 20mA, because R9 & R17 are to large. In that case, a darlington setup with the current transistors could work, I just have to lower those resistors. Will keep you guys updated. ;)


Solution: There's more than one solution to this problem:

1. Use a different OP amp which can drive the transistors rail to rail (see accepted answer). This only works if the 2N5551 and 2N5401 output transistors are configured in a darlington setup. However, the OPA used in this circuit almost drives rail to rail, already. (+-4V on the output)

2. Other output transistors with a better gain. 8550 (PNP) and 8050 (NPN) behave greatly for this application, they still need a darlington setup, though. (Special thanks to Spehro Pefhany)

3. Increase the supply voltage. This works like a treat, IF you're actually able to do so. Sadly, I am not in my setup.

4. Replace the output stage with a "diamond buffer circuit". It performs well in the simulation, did not test it on the bread board, yet. But it's nature of a voltage follower driving the push/pull transistors decreases the total swing of the op-amp, which results in better slew rates and therefor also frequency response of the circuit. Thanks to ioplex for adding this in.

My Solution:

I kept the OPA and used a darlington setup on the output. On my previous tests I forgot to modify the voltage divider to adjust the offset voltage between both transistors. I fixed that and now the circuit works. No clipping even on the worst input signals, that's what I wanted. Tests with my bench supply show that it actually operates down to +-3V which is great if you consider the voltage drop on the battery as it decharges over time. This is just the first design, I will change some things in the future. It is pretty clear that this is not the best setup and that I'm not using the best components for it. Design goals, however, were low component costs and supply voltage. And these are met with a decent performance of the circuit. It does sound gorgeous, feel free to try it yourself! :)

Thanks to everyone for the help I learned a lot through this project.

Here is the new schematic (2N5550 is a Transistor 2N5551):


PS: What's no in this schematic is the compensation cap on the COMP - COMP/BAL pins of this particular op-amp. It's 47pF but LT-Spice has no proper symbol for the NE5534P.