Setting Voltage Limits With VVR

[dorrisant]

I realize that changing the lower limit with VVR (cathode bias) is accomplished by changing the value of the resistor that ground references the pot. That said, is it just as simple as inserting a resistor between the pot and B+ in order to limit the upper limit? I have thought of doing this for a while, just don't recall seeing it anywhere. I'm wondering if there isn't another resistor between the MOSFET and B+.

VVR_Schematic-grey.png

[sluckey]

is it just as simple as inserting a resistor between the pot and B+ in order to limit the upper limit?

Yes.

[davent]

I've done it in the amps i've built with 6AQ5 output tubes

dave

[dorrisant]

Sweet! Thanks for the replies.

[R.G.]

Even less widely known is that you can add a bipolar transistor and a couple of other parts and make that circuit also have a current limit.

This can be useful when you want to reduce current surges into a filter cap at power on or when changing off standby.

[roberto]

Something like this?
http://www.eleccircuit.com/wp-content/u ... 0-300V.jpg

[Ten Over]

Even less widely known is that you can add a bipolar transistor and a couple of other parts and make that circuit also have a current limit.

This can be useful when you want to reduce current surges into a filter cap at power on or when changing off standby.

Doesn't the 10R resistor in conjunction with the 12V zener already constitute a current limiting scheme?

[R.G.]

Something like this?
...

Yep, that's the general idea.

Doesn't the 10R resistor in conjunction with the 12V zener already constitute a current limiting scheme?

It does. It's a little sloppy, and depends on the threshold voltage and transconductance of the MOSFET. But it does limit and can be made to work.

The advantage of using a separate device like a bipolar is that the bipolar's threshold is both much smaller and more definite than the MOSFET's Vt. You get to use a much smaller sensing resistor, as it only has to generate 0.6V or so at the cutoff current, instead of the difference between the 12V zener and the Vt of the MOSFET. That's likely to be about 6-8V. A 10 ohm resistor will give a cutoff of about 700ma and dissipate 4.9W when that happens. A bipolar sense resistor would need to generate about 0.6V at that same 700ma, and dissipate 0.42W.

[davent]

Even less widely known is that you can add a bipolar transistor and a couple of other parts and make that circuit also have a current limit.

This can be useful when you want to reduce current surges into a filter cap at power on or when changing off standby.

London Power's Power Scaling circuit as shown in TUT5 uses that set-up. O'Connor had also posted the circuit at 18watt many, many years ago.

dave

[R.G.]

Yep, it's old. There was a version that used a bipolar pass device instead of a MOSFET in an old ARRK handbook I saw once. The MOSFET makes it easy to extend to high voltages and low drive currents.