MOSFET Amplifier Circuit Issues

Thread Starter

DocQBN

Joined Jan 2, 2017
32
I'm trying to problem solve this circuit. I know this amplifier was working correctly and originally it took 0-5V and output 0-125V. The particular type of circuit I am unsure of, but I think it is a cascading amplifier circuit. The problem is that when I input ~2.5V the output is 109V (should be ~50V) and the maximum input it will take is 0-3V, so the ratio is incorrect. I believe the issue stemmed from when I put 5.28V into one of the input channels. I have another board that I mapped out to compare voltages at different points. At the LF147 IC I wasn't able to see any changes in voltage between the good board and the bad board; I attached a picture of the bad board with voltages mapped. Additionally, I attached a schematic for one of the channels. The only difference I found was at the drain of the MOSFETs (109V), but due to the way they are connected, I am having difficulty pinpointing the problem. I'm finding it odd that all four channels of this circuit are affected. Any help would be appreciated, let me know if I can clarify or include more information to provide a better sense of the situation.

I attached the datasheets for both the amplifier IC and the MOSFETs.
 

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Thread Starter

DocQBN

Joined Jan 2, 2017
32
What is the model number of your amplifier?
I'm not sure if you are talking about the overall circuit or the amp IC. I attached a picture of the circuit board that has the IC name and number as well as the company that made the board. The actual board was pulled from a piece of lab equipment so this is all the information I have.
 

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JoeJester

Joined Apr 26, 2005
4,390
Since it came from some LAB equipment, you must have the schematic, not your hand drawn one.

The image in the previous comment only show half the story. How about a full image of the foil side.

What is the input and what do you expect as the output or are you saying your input is 0V to 5V DC?

As part of a LASER, I would have thought the input and output were pulses.

It appears you have four identical circuits. Are all of them defective or just one?
 
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Thread Starter

DocQBN

Joined Jan 2, 2017
32
Since it came from some LAB equipment, you must have the schematic, not your hand drawn one.

The image in the previous comment only show half the story. How about a full image of the foil side.

What is the input and what do you expect as the output or are you saying your input is 0V to 5V DC?

As part of a LASER, I would have thought the input and output were pulses.

It appears you have four identical circuits. Are all of them defective or just one?
Hey thanks for the speedy reply, I took a picture of the back (attached) and redrew some portions of the schematic to provide a better image (reuploaded). I realize the hand-drawn schematic isn't preferred, but I am having a difficult time finding an actual company released schematic with the information I have.

It is supposed to take in a variable 0-5 volts and there should be an equal ratio output of 0-120 volts.

This is not part of a laser per se, it's a piezo driver that works very closely with the laser haha. Are you familiar with the device brand?

Yes, the two ICs have two inputs each thus a total of 4 outputs all with identical circuitry. The only points where the circuits mix are the 12V, -12V, 120V, and GND traces.
 

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JoeJester

Joined Apr 26, 2005
4,390
You have four independent circuits there. How many are defective? Have you checked them out independently?

If you have not, make a test plan and do some voltage measurements on each of them. Record each measurement. You can organize the info easily. If you have questions, let me know, I'll give you some ideas
 

Thread Starter

DocQBN

Joined Jan 2, 2017
32
I applied the 2.58V to each of the inputs independently and each output displayed the same 109V respectively.

I cannot figure out if one circuit is damaged but possibly feeding something through one of the shared traces and thus affecting the others or if they are all equally damaged. This is part of my confusion, I don't understand if I was working with one channel how did they all become equally damaged?

I have not gone further to voltage map the other circuits (similar to the picture). Would you like me to put a chart together that shows the in/out while varying 0-5 for one channel? All channels?

What tips or content should I read to better understand how to solve this?
 

JoeJester

Joined Apr 26, 2005
4,390
First we will clean up that diagram.

The next set of pictures you take will be without a flash and without direct lighting obscuring things with glare.

I used your values in the diagram except for the one capacitor marked CC-1 and the zener diode. I would like you to identify them properly.

The second picture gives you an idea of what I see on the board .... I would like you to verify the values on the components. There were some in the picture I could not verify by the codes on the part.

Other than that, a testing plan would be measuring various portions of the circuits. You already know the outputs, all of them, are not consistent with the input signal, as specified. Once we have confirmed those part values, we can continue.

Right now, using your values, I get about 96.2V at the output, but then I'm using a 1k load. Without the load, I get 105.8
 

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JoeJester

Joined Apr 26, 2005
4,390
A couple of other points ...

Do you know the load the device that connects to the output jacks put on the circuit? Right now, if you don't have a comparable load your reading could be exactly as you see it.

The signals that come into this amplifier, has an output impedance as well. Do you know that or do you have the drawing of the output of the source as well as the input of the load?

How are you testing them? Just a DC voltage at the input jacks and nothing on the output jacks? I'll assume you have the +12, -12, and 120 set properly.
 

Thread Starter

DocQBN

Joined Jan 2, 2017
32
A couple of other points ...

Do you know the load the device that connects to the output jacks put on the circuit? Right now, if you don't have a comparable load your reading could be exactly as you see it.

The signals that come into this amplifier, has an output impedance as well. Do you know that or do you have the drawing of the output of the source as well as the input of the load?

How are you testing them? Just a DC voltage at the input jacks and nothing on the output jacks? I'll assume you have the +12, -12, and 120 set properly.
I will get those measurements and confirm those values for you. I'll take another picture at a different angle.

I will record the measurements with a load attached (piezo motor - I do not know the resistance of the load, but I will attempt to find out). Last time I measured with a load attached I did not recall a significant difference in voltage when it was not attached.

I do not know the impedance of the circuit I made to drive it. I made a circuit using the MCP4725 IC.

All my testing is using a Fluke 289; I currently do not have another accurate multimeter to have an input and output setup. All other voltages are fixed and are correct.
 

JoeJester

Joined Apr 26, 2005
4,390
Using the schematic I drew ... here are two results.

One is only the meter as a load and one is a load used for maximum power transfer, paralled by the meter. The 289 has a 10 M ohm input impedance. The AM1 is an ameter in series with the load (and meter)

meter.jpg

maxpower.jpg

I'll await for your confirmation on the values.
 
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Thread Starter

DocQBN

Joined Jan 2, 2017
32
@JoeJester thank you for your recent help, I have to temporarily put this project on hold. I will get back to it a few weeks from now with the information you requested!
 

JoeJester

Joined Apr 26, 2005
4,390
@DocQBN no problem. If I'm on the sunny side of mother earth, I'll still be in this forum. I found it a number of years ago and thought it could be something grand.
 

Thread Starter

DocQBN

Joined Jan 2, 2017
32
@JoeJester Hey! I've been able to come back to this, thank you for your patience. Going back through our conversation I've measured capacitor labeled C1 on my drawing (0.107uF) and the diode looks like a 1n914 zener measuring 600mV when using the multimeter.

I've uploaded multiple pictures from different angles if you need more or different angles just let me know.

As for testing impedance I need to solder an SMA connector together so I can put the multimeter in series with the load and I'll get those numbers asap!
 

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JoeJester

Joined Apr 26, 2005
4,390
What did you use for a load?
Where did you measure those voltages?

On the two graphs, the x axis is the 0 to 5V input voltage to the specific amplifier. Vo is the voltage across the load.

Can you verify against the actual components, the values used on my drawing? I'd hate to continue till I am looking at the circuit you are viewing.

i.e. R8 is a 100 ohm resistor 0.25W ...

You will need to update your testing plan identifying where you took those measurements and the input signal level at the beginning of the signal path.
 

Thread Starter

DocQBN

Joined Jan 2, 2017
32
I connected the 289 DMM in series at the SMA output connector on the board with the mirror(piezo motor) to measure the current. I took voltage measurements at the same location with a different DMM.

I made a list of all the color codes on the resistors and their respective values (see attached pic). I looked at the schematic you uploaded and we need to make a few adjustments, I found differences with C4:10uF, R5:24.3 Ohm, R8:10 Ohm, and C3:47pF. The colored bands on R8 are brown, black, black, and gold so 10 ohm; I measured this with the DMM and between all 4 circuits they measured differently, quite significantly actually. I measured the resistor across all four circuits and got 429ohm(next to the silk marking J1 on the board), 3.5K near J2, 38.8k near J3, and 82ohm near J4.


I will conduct another test, with your new guidance I will have a DMM measuring at the input, DMM at the output (right at the SMA connection), and finally for the DMM measuring current, should I disconnect the mirror and only use the 289 as a load or would you like me to use the 289 and the mirror in series, or try both?

Thank you!
 

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JoeJester

Joined Apr 26, 2005
4,390
If you are measuring the resistors in circuit, you can have the other circuit components influencing the readings. I will adjust the values.

Gold in the fourth band indicated 5% tolerance. I'll let you confirm whether or not they are in tolerance based on your measurements.

There are four individual circuits on that board. Some circuits could work and some may not. That is why we will be testing them individually.

R5 I believe is a 24k Resistor, and the fifth band is a temperature coefficient.

Resistor-color-code-chart.png

On edit .....

Here is the revised schematic. DO NOT subject the circuit to a continuous signal (DC). Look at R8 and it's size. It is approximately 1/4 W. 6A would produce 360W and you would be the magician that made R8 disappear.

Do you have an pulse generator?

Test-Circuit-Pulse.jpg

The duty cycle should be 0.5% so you can get the full current AND not blow up R8.

Oscope.jpg
 
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Thread Starter

DocQBN

Joined Jan 2, 2017
32
@JoeJester

Please pardon my absence, I've had time to revisit this project recently.

The DMM confirms R5 is 25K, thanks for that information I didn't think of a temperature band. I do not have a pulse generator but I just received another DMM.

In regards to conducting more tests, should I not try the original method of varying 0-5Vinput? I'm not sure where the 6A came from.
 

JoeJester

Joined Apr 26, 2005
4,390
Here is the problem, your working with a system that want's a pulse input and designed for a pulse input. You could use a square wave for testing, with the square wave going from 0 to 5 volts.

I suspect you will get this type of a response using a DC input.....vice the pulse input as once the fet is turned on, it's on, till you make it turn off.

DCtest.jpg

The trigger you have going into the input is a specific pulse, probably about 50 uS on with a longer off period. Do you know the model number of the device connected to this amplifier? Maybe we can discern some specifications from that.
 
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