How would I build a circuit to generate 4VAC?

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

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

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 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...

ROFL!

a DDS driving a tube heater...

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

The operating current at 4v would be 1.0 amp.

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.

Yes RMS numbers. How hard do you think this circuit will be to construct?

The filament to the best of my knowledge is isolated.

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.

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?

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

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

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 | |

Okay- so it's a 4 ohm filament-probably 0.4 or less when cold...I am assuming you are working with RMS numbers.

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

Okay- check back later or tomorrow.

I think I would approach this problem in two steps. The first would be to come up with a well-regulated 4.0 vac for the filament. This could be done in several ways, but an easy and effective method might be to get a ferroresonant regulator with 120 volts output, followed by a filament transformer of somewhat more than 4 vac output. You're asking for just a little more than 1% regulation. A typical ferro usually isn't spec'd for 1% regulation, but their spec is for no load to full load. I suspect that with a fixed load, you might be able to get 1% regulation. It's worth a try. And the fact that the output is distorted (somewhat like a square wave on the uncompensated ferros) won't be a problem for you since you're just heating a filament. Just be sure you measure the voltage at the filament with a true RMS meter. Let's say you get a 120 to 6.3 volt filament transformer for the output of the ferro; you say you have a pot for adjustment in there now. You could do that with the ferro setup. But it might be better to have a small variac before the filament transformer rather than a pot (rheostat?) after it. Then you can adjust the voltage applied to the filament to exactly 4.0 vac, and it won't vary as much if the 1 amp current changes a little with ageing of the filament for an applied 4.0 volts as it would if you use a variable resistance for the final trim.

To verify the performance, you would want to set all this up, trim for 4.0 vac at the filament, and have another variac in front of the ferro and vary the ferro input voltage over the expected line voltage range and see if you get 1% regulation at the filament. (There may be some ferros that have tighter regulation. I wonder what would happen if you cascade two ferros?! You only need 4 watts of power; perhaps two small cheap ferros in cascade would give the regulation you need, if a single one isn't good enough.)

And, if ferros aren't good enough, you'll have to solve this part of the problem some other way, Whatever works.

The second step in this problem is to get the slow turn-on. I would get a couple of low Rds power FETs. Low voltage N-channel FETs with Rds of 10 milliohm are common these days. Connect the sources and gates together. The two drains now form a two-terminal resistance which can control AC; put that arrangement in series with the output of the filament transformer that is feeding your filament. It might be good to match the threshold voltage of the two FETs to avoid DC in the filament transformer. Now all you have to do is turn the FETs on slowly. Put a low leakage film capacitor of perhaps 10 uF between the gates and sources (which are already connected together pairwise from the two FETs). Charge the capacitor through several megohms and you will get the slow turn-on you want. With a high enough resistance here, you should be able to get as slow a turn-on as you could want. The 20 milliohm resistance of the fully turned on FETs won't affect your regulation much, and anyway you can do your final trim of the voltage at the filament with the FETs in circuit, fully turned on.

You will need some mechanism to turn the FETs off when you power down so they're ready for a slow turn-on at the next power-up. I think I would put a reed relay contact in series with a low value resistor (1000 ohms) across the capacitor, arranged so that the contacts are open when power is applied to the circuitry, and closed when the power is off, thereby discharging the cap and turning the FETs off.

How to charge the capacitor, since the FET gate-source circuit is floating? I can think of several ways, but for easy simplicity, how about this. Since you will be charging the capacitor through several megohms, it won't take much current to do the job. Put two 9 volt transistor radio batteries in series, put the several megohms in series with them, and apply to the capacitor which is across the gate-sources (if you use logic level FETs, one battery may suffice). When the power is on, as soon as the capacitor is charged up, the current from the batteries drops to zero (except for leakages, of course). It's only while the reed relay shorts out the capacitor (through 1000 ohms) that the batteries have to supply a constant current of several microamperes. Replace the batteries every few months. And if you want to reduce the battery drain to zero while the power is off, arrange another relay contact in series with the batteries to open when power is off (or maybe use a FET).

This isn't what the OP suggests. To quote him:

"I need it to be very stable (4.0V +/- 0.05v)"

and:

"The pot is adjusted to 4VAC after everything warms up and then that is it."

and:

"...not to mention that it is not a controlled warm up to the 4v."

It's the voltage that he says he wants to control, not the current. Perhaps he *should* control the current, but that isn't what he's asking for. He has been in contact with the manufacturer and says that "The manufacturer now offers this on new equipment", with *this* apparently being what the OP asked for in his original post, namely:

"...very stable (4.0V +/- 0.05v.)"