There is a way to modify a VVR as to obtain something like a Brimistor ?

[kagliostro]

Is possible to modify the schematic of a VVR to obtain a progressive conduction from a minimum to the max value of B+ ?

I heard about circuits using the charge of a cap to "drive" the conduction of a mosfet, but I didn't find a schematic with values of component as to be practically used



Thanks

Kagliostro

EDIT: a nice pdf about Brimistors   http://www.domino405.co.uk/documents/Brimistors.pdf

[kagliostro]

I've find this, but I'm not sure about it

Someone can confirm that this is usable ?

Thanks

Kagliostro

[HotBluePlates]

That looks essentially right. You should wait for someone knowledgeable to confirm that.

Assuming only the 33k and 22uF determine the timing of the circuit, it looks like you'll rise to full d.c. voltage in ~3.6 seconds.

[Tiny_Daddy]

I would use a higher voltage FET. The old VOX I have worked on were running 525V, important when switching on with SS rectifier.

[kagliostro]

Many thanks HotBluePlates & Tiny Daddy

Assuming only the 33k and 22uF determine the timing of the circuit, it looks like you'll rise to full d.c. voltage in ~3.6 seconds.

As to have an (initial and definitive) fine tuning of the time to rise voltage is better to work with the resistor or with the cap value ? Should a trimmer in series with a lower value resistor do the job the right way ?

I would use a higher voltage FET. The old VOX I have worked on were running 525V, important when switching on with SS rectifier.

I'll follow your advice

EDIT: will this be a good choice for this circuit and for the VVR circuit ??

http://docs-europe.electrocomponents.com/webdocs/0db2/0900766b80db2783.pdf

Kagliostro

[kagliostro]

I've read about Capacitor Multipliers and I'm a bit confused

isn't the "Mosfet Brimistor sostitute" a Capacitor Multiplier ?

someone can clarify ?

Thanks

Kagliostro

[HotBluePlates]

The time to charge a capacitor to an applied voltage is proportional to the value of the capacitance and the resistance in the circuit.

A cap will charge to about the full applied voltage in 5 time constants; 1 time constant is equal to R (in ohms) * C (in Farads). See RC Time Constant.

The bigger the R or C, the slower the rise time from 0v to the full applied voltage. Increasing either (or both) makes each time constant bigger, and it still takes 5 of them to reach approximately full voltage.

You capacitance multiplier is essentially making C bigger. Therefore, it makes R*C (time constant) bigger, so the rise to full voltage takes longer. So yes, a capacitance multiplier and "MOSFET brimistor" are two names for the same circuit.

[kagliostro]

Thanks for the infos HotBluePlates

I'll read with attention the link you posted

as to obtain the slow B+ increase and a good ripple rejection
(given by the 470k resistor paralleled with the cap in the capacitor multiplier)

do you think that to add a cap between the 470k resistor and ground will give the wanted effect ?

Kagliostro

[HotBluePlates]

If you read the text that accompanies the figures you posted, I bet it will say that the 470k resistor was added to solidly define the bias of the MOSFET. If so, I wouldn't add the extra cap.