Interesting idea using an opto-coupler but I think it would be too
non-linear to give you a good RMS response especially on low heater
voltages where the turn on threshold of the infrared LED is a
significant part of the heater voltage.
The AD736JNZ is £5.16 +20% tax (~ $10) from RS components in the UK,
somewhat more than you would pay fro a 741, but in terms of other high
cost items in an amp I would not call it expensive.
I had enough of messing around with bulbs and CdS resistors many years
ago when trying to stabilize sine wave oscillators :-) .
John
On 10/07/2011 12:22, TexasRF@aol.com wrote:
> Hsu, are you referring to use of an opto-coupler, such as 4N25? Use of
> these devices will allow the regulator bridge and FET to be connected
> to the transformer primary which will reduce current requirements a
> large amount.
> The frequency response of the couplers appears to be limited by the RC
> time constant of the photo transistor base capacitance and external
> current limiting resistor. It would be interesting to compare the
> effects of the resistor value with RMS drive voltage to the device.
> A failure of the FET in the bridge circuit will cause an over voltage
> to the tube filament/heater. Some means to prevent that needs to be
> added.
> 73,
> Gerald K5GW
> In a message dated 7/10/2011 5:37:54 A.M. Central Daylight Time,
> hsu4qro@gmail.com writes:
>
> Hi John,
> You are correct. But a problem ,AD's true RMS chip is not a
> inexpensive device.
> I have an idea, using a inexpensive CdS photoresistor-LED or
> micro-bulb
> photocoupler( like N110 in ICOM PS-35 power supply) because
> bulb and CdS
> photoresistor with very bad frequency response
> so the sampling voltage will associate with true RMS volt
> (I guesss) ...
> If the noise is not a problem and the tube's cathode is
> oxide-coated ,
> the switching regulator is the best. but monolithic IC maximal
> out put current
> only 10A (ST's L4970) others need external power MOS FET.
> 73!Hsu
>
>
> ----- Original Message -----
> From: "John G3UUT" <amps@grebe.plus.com>
> To: <amps@contesting.com>
> Sent: Sunday, July 10, 2011 5:12 PM
> Subject: Re: [Amps] AC filament voltage regulator
>
>
> > I've also been thinking about this problem and the reason I'm
> not keen
> > on a DC regulator is that the distortion and peak current of the
> mains
> > current waveform is already pretty bad with a capacitor input B+
> supply
> > and making it worse by adding the heater as well will
> undoubtedly make
> > the regulation of the B+ worse. This is particularly a problem
> when
> > using portable generators.
> >
> > I've not taken this down to circuit level yet but my idea was to
> use one
> > of the Analog Devices RMS to DC convertors as its important to
> maintain
> > the RMS of the AC constant - especially when using poor quality
> mains
> > from a generator. Hsu - this is the problem with your circuit. My
> > other idea was to use a biphase rectifier with a centre taped
> > transformer and schottky rectifiers to reduce the diode losses -
> which
> > are quite significant with a 5 V heater.
> >
> > 73 John G3UUT
> >
> > On 10/07/2011 03:03, Dr. David Kirkby wrote:
> >> On 07/10/11 01:12 AM, Hsu wrote:
> >>> I do not think a DC regulator for direct heater tube is a
> good idea,I draw a block diagram for AC filament regulator
> here:http://www.zmdz.com/bbs/incomefiles/20117/1089098780.JPG,
> just a block diagram.any advice? I know we have another
> selection,CVT. a special transformet.
> >>> 73!Hsu
> >> What's wrong with a DC regulator?
> >>
> >
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