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HV Shunt Regulator PCB

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#183
Hi Jan,

I believe the circuit is pretty much as stock in the manual in the first page.
However, SB had a little adjustment to the power supply pins to use opa656 instead of the obsolete opa655. I wonder that's the reason why opa655 didn't work.
 
#184
pchw said:
Hi Jan,

I believe the circuit is pretty much as stock in the manual in the first page.
However, SB had a little adjustment to the power supply pins to use opa656 instead of the obsolete opa655. I wonder that's the reason why opa655 didn't work.
Click to expand...

I noted that the 655 has a max supply voltage of 11V, which may be an issue (not having seen the circuit, but knowing that it works with 36V supply amps like 741). It may also oscillate, but 'it doesn't work' doesn't really help us further.

jd
 
#185
Hi Jan

When fitted with 741, the voltage at pin 2 and 3 is 7.0V and o/p at pin 6 is 3.40V, all rock steady
When switched to OPA655, the voltage at pin 2 and pin 3 drop to about 5V to 6V and o/p to 2.6V .All the 3 voltages are drifting and seen on the digital meter.
Hope these info help you to narrow down the problem
Thanks

Goh
 
#186
When fitted with OPA655, the following are the voltages:

Pin 2 6.9V
Pin 3 6.5V
Pin 6 2.1V

Supply voltage 10.5V

Original Schematic


Goh
 

Attachments

  • HPHVreg_Sch.jpg
    HPHVreg_Sch.jpg
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#188
Audiohifi said:
When fitted with OPA655, the following are the voltages:

Pin 2 6.9V
Pin 3 6.5V
Pin 6 2.1V

Supply voltage 10.5V

Original Schematic


Goh
Click to expand...

Looks OK. -in is higher than +in so the output will drive down. 2.1V at Vo is about as low as it will go, but apparently it is not low enough to close the shunt. I assume that the HV output is too low?
Are you sure there is no other change and that it works when you just plug in a 741? Did you also change the shunt fet type?

Edit: what's the Vo with a 741?

jd
 
#189
Looks OK. -in is higher than +in so the output will drive down. 2.1V at Vo is about as low as it will go, but apparently it is not low enough to close the shunt. I assume that the HV output is too low?
Are you sure there is no other change and that it works when you just plug in a 741? Did you also change the shunt fet type?
Edit: what's the Vo with a 741]

Yes, no changes, plug in 741 or 351 and it works. The o/p at pin 6 is 3.4V , pin 2 and pin 3 are 7.0V with 741.
I think the o/p needs to be 3.4V to turn on the shunt Mosfet
According to the manual, the check for opamp is OK , ie pin 2 and pin 3 7.0V
pin 6 about 1.0 V befor the HV is turn on

See if you can hear your circuit on the radio, as a test for oscillation. Place the radio near by and turn your circuit on and off, you might hear it.
Yes, there is transient noise when HV is turn on

Goh
 
#191
Audiohifi said:
Looks OK. -in is higher than +in so the output will drive down. 2.1V at Vo is about as low as it will go, but apparently it is not low enough to close the shunt. I assume that the HV output is too low?
Are you sure there is no other change and that it works when you just plug in a 741? Did you also change the shunt fet type?
Edit: what's the Vo with a 741]

Yes, no changes, plug in 741 or 351 and it works. The o/p at pin 6 is 3.4V , pin 2 and pin 3 are 7.0V with 741.
I think the o/p needs to be 3.4V to turn on the shunt Mosfet
According to the manual, the check for opamp is OK , ie pin 2 and pin 3 7.0V
pin 6 about 1.0 V befor the HV is turn on

See if you can hear your circuit on the radio, as a test for oscillation. Place the radio near by and turn your circuit on and off, you might hear it.
Yes, there is transient noise when HV is turn on

Goh
Click to expand...

There is something fishy. If the HV output is too low, then the voltage at +vin is also too low, below the 7V ref at -vin. Therefore, Vo should go down to turn down the shunt so that the HV rises again, until +vin=-vin. Yet, Vo is already lower than with the 741, so there's something wrong here. I only can think about oscillation, and you never know what your DC multimeter indicates with a (hf) oscillation signal. That 2.1V Vo is probably much higher, higher than with the 741, but with an oscillation ac imposed. What does tan ac meter show on pin 6? (Probably not much, the freq is probably too high).

Try a brute force to kill the oscillations, increas the feedback cap across the opamp to 10nF or something (C13).

jd
 
#192
ikoflexer said:
The same type/amplitude noise with both the 741 and opa655?
Click to expand...

The transient noise is from nearby FM radio. 741 don't seem to have the transient noise
I understand from other posting(not this thread) that many high bandwidth opamp struggle with output. Does this meant that the output is not enough to turn on the Mosfet?

Just get pin 2 and pin 3 at 7V, with output pin 6 at 3.4V and the Mosfet will turn on.How to do it?
 
#193
janneman said:
There is something fishy. If the HV output is too low, then the voltage at +vin is also too low, below the 7V ref at -vin. Therefore, Vo should go down to turn down the shunt so that the HV rises again, until +vin=-vin. Yet, Vo is already lower than with the 741, so there's something wrong here. I only can think about oscillation, and you never know what your DC multimeter indicates with a (hf) oscillation signal. That 2.1V Vo is probably much higher, higher than with the 741, but with an oscillation ac imposed. What does tan ac meter show on pin 6? (Probably not much, the freq is probably too high).

Try a brute force to kill the oscillations, increas the feedback cap across the opamp to 10nF or something (C13).

jd
Click to expand...

So if there is oscillations, the mosfet will not turn on?
Before, HV is supplied, the voltages are as those in the Schematic
HV rise slowly if the regulator is working
Now it shoot up to the raw voltage almost immediately
Sorry, just tell me how to check oscillation correctly with a scope

Goh
 
#194
Hook the scope signal probe to the output and the ground probe to the ground. Set the scope to AC, vertical amplifier to about 5V/div, horizontal amplifier to about 1mS/div, and then turn on your circuit. You can play with the vertical/horizontal settings after you started the circuit. If you don't see a straight line (such as when you set the scope to GND) you got oscillation. Hope this helps.
 
#195
Audiohifi said:
So if there is oscillations, the mosfet will not turn on?
Before, HV is supplied, the voltages are as those in the Schematic
HV rise slowly if the regulator is working
Now it shoot up to the raw voltage almost immediately
Sorry, just tell me how to check oscillation correctly with a scope

Goh
Click to expand...

I made an error in my previous post. The Vo must be lower than with the 741 to keep the shunt off.
But with oscillations, the DC value measured with a multimeter has no validity. So if you have a scope, check it as described by ikoflexer.

jd
 
#196
janneman said:
I made an error in my previous post. The Vo must be lower than with the 741 to keep the shunt off.
But with oscillations, the DC value measured with a multimeter has no validity. So if you have a scope, check it as described by ikoflexer.

jd
Click to expand...

Jan

You are right. It is oscillating like mad. I put a AM radio close by and the tone of the oscillation is loud and clear.

Following Ikoflexer setting on the scope, a shot of the screen is attached

What do I do now to make the regulator work properly

Thanks for the troubleshooting

Goh
 

Attachments

  • P1010676.JPG
    P1010676.JPG
    638 KB · Views: 831
#197
Audiohifi said:
Jan

You are right. It is oscillating like mad. I put a AM radio close by and the tone of the oscillation is loud and clear.

Following Ikoflexer setting on the scope, a shot of the screen is attached

What do I do now to make the regulator work properly

Thanks for the troubleshooting

Goh
Click to expand...

You got it!

Try to increase C3 (I think; its the one from output to -in) until the oscillations stop. Depending on the value, it may be acceptable. Or try a more docile opamp, one that doesn't have such a very wide bandwidth (but not a 741 🙂 ). Did you try a 5534?

jd
 
#200
janneman said:
You got it!

Try to increase C3 (I think; its the one from output to -in) until the oscillations stop. Depending on the value, it may be acceptable. Or try a more docile opamp, one that doesn't have such a very wide bandwidth (but not a 741 🙂 ). Did you try a 5534?

jd
Click to expand...

Hi Jan

I will try 5534 some day when I can get it. I tried AD817AN and it is not oscillating.
Because you came to my reply and help, there is much interest and a GB is on

Goh
 
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