How would I build a circuit to generate 4VAC?

A

abekas67

Guest
Dear Electronic experts! I need to build a circuit that will output
4VAC at 1 amp from a 120VAC supply. The trick is that I need it to be
very stable (4.0V +/- 0.05v. I also need it to slowly and evenly ramp
up from 0v to 4v over a period of about 1 to 3 minutes. The ramp up
time is not too critical - so the easiest design is fine. This project
is to slowly warm up the heater in a crt so care has to be taken not to
exceed 1 amp and not to go above the 4v. A conservative approach would
be best. Any assistance would be appreciated! Thank you very much!
 
abekas67 wrote:
Dear Electronic experts! I need to build a circuit that will output
4VAC at 1 amp from a 120VAC supply. The trick is that I need it to be
very stable (4.0V +/- 0.05v. I also need it to slowly and evenly ramp
up from 0v to 4v over a period of about 1 to 3 minutes. The ramp up
time is not too critical - so the easiest design is fine. This project
is to slowly warm up the heater in a crt so care has to be taken not to
exceed 1 amp and not to go above the 4v. A conservative approach would
be best. Any assistance would be appreciated! Thank you very much!
Click to expand...
If this is just to heat a filament, use DC. The circuit is probably
simpler. If it must be from power outside the TV, you can use an
unregulated AC to DC adapter, followed by a voltage regulator that you
ramp up over the 3 minutes. Is this circuit to extend the CRT life? I
think that will depend upon the CRT.

Thanks,

Frank
 
The last time I came across this sort of requirement, it was for an
X-ray generator, rather than an ordinary cathode ray tube.

If the OP really is working with an indirectly heated cathode, DC
heating would be by far the easiest solution, but it would not be a
good idea with a directly heated cathode.

-----------
Bill Sloman, Nijmegen
 
abekas67 wrote:
Dear Electronic experts! I need to build a circuit that will output
4VAC at 1 amp from a 120VAC supply. The trick is that I need it to
be very stable (4.0V +/- 0.05v. I also need it to slowly and evenly
ramp up from 0v to 4v over a period of about 1 to 3 minutes. The ramp
up time is not too critical - so the easiest design is fine. This
project is to slowly warm up the heater in a crt so care has to be
taken not to exceed 1 amp and not to go above the 4v. A conservative
approach would be best. Any assistance would be appreciated! Thank
you very much!

The conservative approach is to initially drive the filament with a
Click to expand...
bipolar 60Hz voltage controlled current source well under 1A, until the
operating voltage is reached, and then use filament voltage feedback to
the current source to maintain the output at 4.00V +/-1%. What is the
operating current at 4V? That might give you a clue as to setting the
current source at ramp up...
 
On Sun, 21 Aug 2005 17:41:14 -0700, abekas67 wrote:

Dear Electronic experts! I need to build a circuit that will output
4VAC at 1 amp from a 120VAC supply. The trick is that I need it to be
very stable (4.0V +/- 0.05v. I also need it to slowly and evenly ramp
up from 0v to 4v over a period of about 1 to 3 minutes. The ramp up
time is not too critical - so the easiest design is fine. This project
is to slowly warm up the heater in a crt so care has to be taken not to
exceed 1 amp and not to go above the 4v. A conservative approach would
be best. Any assistance would be appreciated! Thank you very much!
Click to expand...
How much power do you need?

One approach I can think of off-hand is to use a DDS to generate the sine
wave, and supply the DDS DAC from an ADC whose voltage would ramp up
slowly by way of a counter. This makes everything nice and digital. A BAC
can be used to filter out any high-frequency noise from the DAC output.

If you need a lot of power, you can still use this approach to generate
the signal, and buffer the output with a suitable transistor. Luckily, you
don't need much bandwidth.

Obviously I'm leaving out a lot of details. If what I say is baffling to
you, then just ask again, or maybe try sci.electronics.basics.

--Mac
 
"Mac" <foo@bar.net> a écrit dans le message de
news:pan.2005.08.23.03.24.12.276692@bar.net...
On Sun, 21 Aug 2005 17:41:14 -0700, abekas67 wrote:

Dear Electronic experts! I need to build a circuit that will output
4VAC at 1 amp from a 120VAC supply. The trick is that I need it to be
very stable (4.0V +/- 0.05v. I also need it to slowly and evenly ramp
up from 0v to 4v over a period of about 1 to 3 minutes. The ramp up
time is not too critical - so the easiest design is fine. This project
is to slowly warm up the heater in a crt so care has to be taken not to
exceed 1 amp and not to go above the 4v. A conservative approach would
be best. Any assistance would be appreciated! Thank you very much!

How much power do you need?

One approach I can think of off-hand is to use a DDS to generate the sine
wave, and supply the DDS DAC from an ADC whose voltage would ramp up
slowly by way of a counter. This makes everything nice and digital. A BAC
can be used to filter out any high-frequency noise from the DAC output.
Click to expand...
ROFL!

a DDS driving a tube heater...

and also a relay CPU to program the DDS and a card reader to enter the
program...


--
Thanks,
Fred.
 
This circuit is to extend the life of the CRT. The manufacturer now
offers this on new equipment but will not sell an upgrade or provide a
drawing or any additional information on how to do it. I think we have
to stay with AC not DC - even in the new designs they stayed with AC
and it would have been much easier to do DC for them.
 
abekas67 wrote:
The operating current at 4v would be 1.0 amp.
Click to expand...
Okay- so it's a 4 ohm filament-probably 0.4 or less when cold...I am
assuming you are working with RMS numbers.
 
Yes RMS numbers. How hard do you think this circuit will be to
construct?
 
abekas67 wrote:
Yes RMS numbers. How hard do you think this circuit will be to
construct?
Click to expand...
Well- you will need a floating power supply to avoid transformer or
capacitor coupling into the filament, a power element to provide the 1A
operating current, and then some as yet to be decided nickel-dime linear
things to regulate the power element. Using something like the LM675
from http://www.national.com would be likely. And your isolation
step-down transformer to power the circuit will be on the order of 20VA.
Is that filament at a high voltage with respect to chassis GND, or isolated?
 
On 21 Aug 2005 17:41:14 -0700, "abekas67" <stuart@usaor.net> wrote:

Dear Electronic experts! I need to build a circuit that will output
4VAC at 1 amp from a 120VAC supply. The trick is that I need it to be
very stable (4.0V +/- 0.05v. I also need it to slowly and evenly ramp
up from 0v to 4v over a period of about 1 to 3 minutes. The ramp up
time is not too critical - so the easiest design is fine. This project
is to slowly warm up the heater in a crt so care has to be taken not to
exceed 1 amp and not to go above the 4v. A conservative approach would
be best. Any assistance would be appreciated! Thank you very much!
Click to expand...
In another post you say this is to extend the life of a CRT
filament. Is this a piece of equipment you already have? If so, what
heats the filament now? Is the voltage applied to the filament now as
stable as what you are asking for?
 
This is for equipment that I have now. Right now it is a very simple
circuit that just has a transformer and a variable pot. The pot is
adjusted to 4VAC after everything warms up and then that is it. But
what really happens is that i goes to 4v too fast and the precision is
very poor. When the machine is turned on the next day, after being off
all night, for example the voltage could be between 3.7 and 4.2v. That
is just too wide a range, not to mention that it is not a controlled
warm up to the 4v. I guess that someone might be wondering why i care
about 4.0V at 1 amp so much? The crt is in a film scanner and the crt
design limits the number of power on/off cycles that can be happen
before the cathode source marterial is depleated (I hope I explained
that correctly). On new designs of this film scanner they did design
the slow gradual warm up that does not deplete the cathode quickly.
The other issue is that a new crt costs over $8,000.
 
On 27 Aug 2005 17:12:14 -0700, "abekas67" <stuart@usaor.net> wrote:

This is for equipment that I have now. Right now it is a very simple
circuit that just has a transformer and a variable pot. The pot is
adjusted to 4VAC after everything warms up and then that is it. But
what really happens is that i goes to 4v too fast and the precision is
very poor. When the machine is turned on the next day, after being off
all night, for example the voltage could be between 3.7 and 4.2v. That
is just too wide a range, not to mention that it is not a controlled
warm up to the 4v. I guess that someone might be wondering why i care
about 4.0V at 1 amp so much? The crt is in a film scanner and the crt
design limits the number of power on/off cycles that can be happen
before the cathode source marterial is depleated (I hope I explained
that correctly). On new designs of this film scanner they did design
the slow gradual warm up that does not deplete the cathode quickly.
The other issue is that a new crt costs over $8,000.
Click to expand...
Puzzling! Maybe the "cathode" is actually the filament?

I'd simply use an audio power amplifier and ramp-up; or maybe even
just use a 1 Amp AC current source ??

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
This solution sounds easier to do, but how would I ramp the voltage up
automatically?
 
On 27 Aug 2005 20:45:33 -0700, "abekas67" <stuart@usaor.net> wrote:

This solution sounds easier to do, but how would I ramp the voltage up
automatically?
Click to expand...
Multiplying DAC would be one way.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
I read in sci.electronics.design that abekas67 <stuart@usaor.net> wrote
(in <1125187934.797276.204850@g43g2000cwa.googlegroups.com>) about 'How
would I build a circuit to generate 4VAC?', on Sat, 27 Aug 2005:
This is for equipment that I have now. Right now it is a very simple
circuit that just has a transformer and a variable pot. The pot is
adjusted to 4VAC after everything warms up and then that is it. But
what really happens is that i goes to 4v too fast and the precision is
very poor. When the machine is turned on the next day, after being off
all night, for example the voltage could be between 3.7 and 4.2v. That
is just too wide a range, not to mention that it is not a controlled
warm up to the 4v. I guess that someone might be wondering why i care
about 4.0V at 1 amp so much? The crt is in a film scanner and the crt
design limits the number of power on/off cycles that can be happen
before the cathode source marterial is depleated (I hope I explained
that correctly). On new designs of this film scanner they did design
the slow gradual warm up that does not deplete the cathode quickly.
The other issue is that a new crt costs over $8,000.

The filament resistance varies with temperature and it's the 1 A that
Click to expand...
matters, not the 4 V. At switch-on, you need a lot less than 4 V, as you
know.

The traditional method of doing what you want is to use a negative
temperature coefficient (NTC) thermistor in series with the filament.
This is extremely simple and works with an AC supply. Other methods of
producing a time-varying AC supply can be complicated.

If you can't find an NTC thermistor that is suitable (cold resistance
just under 4 ohms and hot resistance around 0.1 ohm or less, rated
current 1 A or a little more), consider using an audio power IC, fed
with a 50 Hz signal through a simple voltage-controlled amplifier (even
a simple shunt FET attenuator) with a ramp control signal of the
duration you require. Although not elegant (not Bloggsian), this is
simple and uses no exotic parts.
--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
I read in sci.electronics.design that Jim Thompson
<thegreatone@example.com> wrote (in
<lod2h1ljo7sqhk8a33cijbrsj5o43egj5f@4ax.com>) about 'How would I build a
circuit to generate 4VAC?', on Sat, 27 Aug 2005:
On 27 Aug 2005 20:45:33 -0700, "abekas67" <stuart@usaor.net> wrote:

This solution sounds easier to do, but how would I ramp the voltage up
automatically?

Multiplying DAC would be one way.
Click to expand...
Plenty of analogue ramp or sawtooth generator circuits in op-amp
application notes and more revealed by a Google search.
--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
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