Digital to Analog Converter (DAC) Not Working - Seeking Help (Schematic Attached)

Thread Starter

shaydzmi

Joined Mar 26, 2024
8
Hi everyone,

I'm hoping to get some help with a circuit I'm working on. It's a digital-to-analog converter (DAC) circuit.

Unfortunately, the circuit seems to be stuck! No matter what digital value I give it, the output voltage coming out is always around 11 volts.

I've tried troubleshooting by replacing the op-amp (OP07CP), and a transistor (2N5401), but that didn't fix the issue.

I've been trying to understand the circuit by analyzing its components and how they're connected (reverse engineering). This is my attempt at a schematic based on what I've found (attached).

I would be incredibly grateful if anyone could offer some insights into why the output voltage is always stuck at 11 volts, even though the digital input changes. Any advice would be greatly appreciated!

Schematic_DAC-Molleton_2024-03-26.pngIMG_20240319_150728.jpgIMG_20240319_150744.jpg
 

Papabravo

Joined Feb 24, 2006
21,258
The most OBVIOUS thing is that the diodes for the opto-isolators have no connection to GROUND relative to the power connections for U5

Then next thing is that your schematic is poorly drawn with signal lines crossing the isolation barrier and inputs on the right with outputs on the left. It is also incomplete. U5 has no POWER, Data, Clock or Output Enable connections. Was this schematic drawn by a 3rd grader?
 

Thread Starter

shaydzmi

Joined Mar 26, 2024
8
The most OBVIOUS thing is that the diodes for the opto-isolators have no connection to GROUND relative to the power connections for U5

Then next thing is that your schematic is poorly drawn with signal lines crossing the isolation barrier and inputs on the right with outputs on the left. It is also incomplete. U5 has no POWER, Data, Clock or Output Enable connections. Was this schematic drawn by a 3rd grader?
Thank you for the response. As a beginner, I've drafted the schematic without aiming for a comprehensive diagram. My objective was to convey a basic understanding to gather insights into the circuit's operation.

U5 is operating as expected, along with the optocouplers and the A1015 PNP transistors. It appears that the problem may lie with the operational amplifier, though I'm not entirely sure. Hence, I'm looking forward to your ideas.
 

seanstevens

Joined Sep 22, 2009
256
All your electrolytic caps are connected the wrong way around, your opamp is also being fed with the wrong voltage polarity too, is that part of the draft? What is the point of putting up a circuit that has major errors in it and expecting others to tell you what's wrong?
 

Thread Starter

shaydzmi

Joined Mar 26, 2024
8
All your electrolytic caps are connected the wrong way around, your opamp is also being fed with the wrong voltage polarity too, is that part of the draft? What is the point of putting up a circuit that has major errors in it and expecting others to tell you what's wrong?
Thank you for taking the time to look at my schematic! I appreciate your insights.
In the attached schematic, I tried to accurately reflect the component polarities and op-amp voltage connections based on the actual circuit board. Would you be able to take a look at both the schematic and the board photo (attached) and see if there's anything that still looks off?
 

Papabravo

Joined Feb 24, 2006
21,258
Thank you for the response. As a beginner, I've drafted the schematic without aiming for a comprehensive diagram. My objective was to convey a basic understanding to gather insights into the circuit's operation.

U5 is operating as expected, along with the optocouplers and the A1015 PNP transistors. It appears that the problem may lie with the operational amplifier, though I'm not entirely sure. Hence, I'm looking forward to your ideas.
That my friend is a [deleted by moderator] response. Nobody makes a board without an accurate schematic to guide the layout.
 
Last edited by a moderator:

Thread Starter

shaydzmi

Joined Mar 26, 2024
8
That my friend is a [deleted by moderator] response. Nobody makes a board without an accurate schematic to guide the layout.
I agree, but I'm not creating a board; I just want to understand this circuit. The schematic you see represents days of effort. It may seem insignificant to you, but for me, as a beginner, it's a substantial amount of work.
 
Last edited by a moderator:

Papabravo

Joined Feb 24, 2006
21,258
I agree, but I'm not creating a board; I just want to understand this circuit. The schematic you see represents days of effort. It may seem insignificant to you, but for me, as a beginner, it's a substantial amount of work.
I see. You obtained the board from another source, and you have reverse engineered the schematic. Do you know if the boards ever worked or maybe they were discarded because they were made with obvious errors. You might be better off reverse engineering a known good board.
 

MrChips

Joined Oct 2, 2009
30,925
There are two types of technical drawings used in engineering design.

The first drawing is the design plan or what is sometimes called a block diagram. It shows the design concept without the technical details.

The second drawing is the construction plan. This is called the "blueprint" and in electronics it is the schematic diagram. This diagram has full technical details which would allow someone else to build and complete the project.

In order to trouble-shoot, fix, and arrive at a working system, one needs to see an accurate and working schematic diagram.
 

Thread Starter

shaydzmi

Joined Mar 26, 2024
8
I see. You obtained the board from another source, and you have reverse engineered the schematic. Do you know if the boards ever worked or maybe they were discarded because they were made with obvious errors. You might be better off reverse engineering a known good board.
The board is a component of a machine's control panel, and it functions properly except for the part that controls the VFD frequency, which is this DAC circuit.
 

Thread Starter

shaydzmi

Joined Mar 26, 2024
8
There are two types of technical drawings used in engineering design.

The first drawing is the design plan or what is sometimes called a block diagram. It shows the design concept without the technical details.

The second drawing is the construction plan. This is called the "blueprint" and in electronics it is the schematic diagram. This diagram has full technical details which would allow someone else to build and complete the project.

In order to trouble-shoot, fix, and arrive at a working system, one needs to see an accurate and working schematic diagram.
Thank you for this information.

This is a second attempt at presenting a 'complete' schematic, I hope!
I hope there is no mistakes:
Schematic_DAC-Molleton_2024-03-26.png
 

HasBeen

Joined Jul 28, 2023
29
You say the output is stuck at 11 Volts. With respect to what? Exactly where are you attaching which voltmeter probe, and is the output reading + or -? Since the circuit you have drawn is operating from about -18 Volts w/r/t the voltage regulator 7905 ground pin, it would be helpful to know these things. It might help to measure some other voltages in the circuit as well, to see if they make sense or perhaps point to a fault.
 

Thread Starter

shaydzmi

Joined Mar 26, 2024
8
You say the output is stuck at 11 Volts. With respect to what? Exactly where are you attaching which voltmeter probe, and is the output reading + or -?
Positive probe at (+I), negative probe at (-I) reading about +11v.

Since the circuit you have drawn is operating from about -18 Volts w/r/t the voltage regulator 7905 ground pin,
The ground pin of the voltage regulator is connected to the positive supply voltage.

It might help to measure some other voltages in the circuit as well, to see if they make sense or perhaps point to a fault.
I'm prepared to measure any voltages that you require.
 

HasBeen

Joined Jul 28, 2023
29
Where you say you attach the probes, I can't find those points on your schematic. What am I missing? Also, the green line shown shorting the entire switch network can't really be there. Nothing will work that way.
 

Thread Starter

shaydzmi

Joined Mar 26, 2024
8
Here: M01 +I & M01 -I:

1712474163567.png
Also, the green line shown shorting the entire switch network can't really be there. Nothing will work that way.
It's not shorting, there is a resistor (R8).
 

HasBeen

Joined Jul 28, 2023
29
OK. I am getting up to speed. There needs to be a load across the M output terminals to provide operating current for the 2N5401. You don't say if you are testing the system, which would normally have a load connected, or the bare board, which does not have a load. Without a load, the opamp is having to supply all the current to the 510 ohm network through the base - emitter junction of the transistor, which it might not be capable of. What can you tell me about the load at the M terminals?
 

HasBeen

Joined Jul 28, 2023
29
By the way, I believe what is causing a lot of the confusion is that the circuit is positive-ground, but it is drawn the way you would draw a negative-ground schematic. That's why all the capacitor polarities look backwards, etc. It would be easier to analyze if the schematic were flipped upside down, so positives were up and negatives were down. I think that's the way most minds work.
 
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