zener on ct...again

[R.G.]

Power in an electronic device is always voltage across it times the current through it. If these are not DC or almost so, you have to take into account the instantaneous values and integrate to get the power. But you're worried about DC.

To wipe off X volts from a voltage of B volts, you need an X volt zener. Or any two zeners which add to X. The power dissipated in the zener is the voltage across it (X) times the current through it, which is the max current drawn from the power supply. Let's say that the current drawn is Y.

So the power needed is X*Y in watts if X is volts and Y is amperes. Example: Dropping 20V from a 500V power supply which supplies 350 ma. You need a 20V zener which can dissipate 20 * 0.35 = 7W. For safety, I would not put less than a 10W zener in there.
You could use two 10V/5W zeners in series. Each drops ten of the 20V and each dissipated 10*0.35 = 3.5W of heat.


If you want to drop 50V off, you need a 50V zener that can dissipate 50*0.35 = 17.5W, and I would use a 20W zener. However, you could also use five 10V/5W zeners. Or three 16V 10W zeners.

The total zener voltage must add up to the voltage you want dropped, and the total zener power must add up to the actual voltage times current to be dissipated plus some safety factor. If you do not allow for getting the heat out of the parts, they will burn up.

You cannot successfully parallel zeners and have them share the load equally with simple circuits. Restrict yourself to simple series connections for more voltage.

You can use a low power zener plus a high power MOSFET or bipolar transistor to make an "amplified zener". In this case the zener sets the voltage to be dropped, and the transistor carries all the power. However, it is easier to get high power transistors and MOSFETs than high power zeners. A 1/2W zener, a resistor, and a power MOSFET can make a 25-50W zener as long as you can get the heat out of the MOSFET.

Remember in any of these that the device dropping the power may or may not be able to be tied to the chassis because of the voltage on it.

[Chuck H]

O.K. I guess I'll come clean hear. Cause I'm really getting some good info, and I could use as much as I can get.

I'm planning to attempt a variable B+ voltage without the use of mosfets (you probably don't need to ask why, but don't anyway...thanks). I plan on using a string of series zeners for this. I'll use a switch to tap ground into the zener string to determine the B+ "setting". What I'm getting now is that heat dissapation is the real enemy to this idea. I can figure the wattage rating for the voltage drop. But what is the best way to dissapate the heat from a string of high current zeners? I'm thinking about a seperate chassis with a big finned aluminum heat sink.?. I'd like to avoid using a fan if possible.

Chuck

[Bruce / Mission Amps]

O.K. I guess I'll come clean hear. Cause I'm really getting some good info, and I could use as much as I can get.

I'm planning to attempt a variable B+ voltage without the use of mosfets (you probably don't need to ask why, but don't anyway...thanks). I plan on using a string of series zeners for this. I'll use a switch to tap ground into the zener string to determine the B+ "setting". ...
Chuck

OK, if you "really" want some good info, don't waste your time with a string of diodes and a rotary switch. Although not my design, just do what you don't want to do.... use a big ass power MOSFET with a heat sink and little circuit that turns it ON, more or less, with a high resistance pot, low wattage zener diode and a couple resistors... etc.
By the way, a switch on the center tap of the high voltage secondary is 100% fine... all it does is act like a standby switch... nothing bad happens if it goes open... the rectifiers in the vacuum tube or the solid state FW rectifier setup just won't turn on.

[jaysg]

mouser carries a selection of 50W zeners in normal and reverse polarity (so you can bolt it to the chassis). 1N3305 through 1N3350. 1N3305RB is cathode to case. Also NT5240A through NTE5296A, where AK designates cathode to case.

[R.G.]

I'm planning to attempt a variable B+ voltage without the use of mosfets (you probably don't need to ask why, but don't anyway...thanks). I plan on using a string of series zeners for this. I'll use a switch to tap ground into the zener string to determine the B+ "setting". What I'm getting now is that heat dissapation is the real enemy to this idea. I can figure the wattage rating for the voltage drop. But what is the best way to dissapate the heat from a string of high current zeners? I'm thinking about a seperate chassis with a big finned aluminum heat sink.?. I'd like to avoid using a fan if possible.

It really is quite difficult to do high power zener strings, at least in a practical sense. Yes, heat is the enemy. Deciding not to use something similar to a MOSFET regulator instantly puts you into some tough mechanical situations. Your choices become one heat sink per zener sufficient for that zener's worst case, or fewer and bigger heat sinks with more than one zener on it. Whether you will need a fan depends on how many diodes, how big the heat sink, and how hot you let the devices get.

It's not much of an electronics challenge, but it is a practical and mechanical challenge.

[Chuck H]

Ok. I give.

One reason I was using this angle is that I do have a mechanical understanding of things and I know nothing about transistors. But the idea seems very impractical as Bruce had suggested. Since I hadn't seen it done before, I thought it might be clever. But I guess it just turns out that even though it's a different way to do it, it's not a good way.

So thank you all, esp. Bruce and R.G., for putting me on the path.

FWIW I think I've come to dissagree with the mechanics behind "power scaling" and will probably go with a built in speaker attenuator type circuit.
(I may need some help with that )

Chuck

[tubeswell]

All this is to say that you can still use a string of low voltage lower power (say 5W) zeners and it will likely be fine (e.g; if you want to drop say 36V for a total dissipation rating of 15W - use 3 x 5W 12V in series - wire them up on a tag terminal and the tags themselves act as extra heat sinks - no pain). In how many instances will you want a zillion zeners to drop voltage?

[R.G.]

mouser carries a selection of 50W zeners in normal and reverse polarity (so you can bolt it to the chassis). 1N3305 through 1N3350. 1N3305RB is cathode to case. Also NT5240A through NTE5296A, where AK designates cathode to case.

The 50W zeners from Mouser cost $8.55 to $9.41 each.

A 180W power MOSFET(512-FQA19N20C) is $1.74.

[R.G.]

It may not seem like it, but we really are trying to help.

One reason I was using this angle is that I do have a mechanical understanding of things and I know nothing about transistors.

Think of a MOSFET as a hydraulic valve. There is a large pressure (voltage) source on one side and a low pressure (voltage) return on the other side. Mechanical pressure on the spool valve lever allows from zero to full flow from the high pressure side to low pressure side.

You can put your load in either the high pressure side or low pressure side. If you put it in the low pressure side, then moving the control lever (gate) allows more pressure (voltage) onto the load from the high pressure side, regulated by the valve (MOSFET)

But the idea seems very impractical as Bruce had suggested. Since I hadn't seen it done before, I thought it might be clever. But I guess it just turns out that even though it's a different way to do it, it's not a good way.

There are places where it's a really good way to do things. But it suffers from some cost and practicality issues; that's why you haven't seen it.

FWIW I think I've come to dissagree with the mechanics behind "power scaling" and will probably go with a built in speaker attenuator type circuit.

What aspects do you disagree with?

It does seem to work.

[Steve Conner]

To be fair, the "180W" MOSFET can only dissipate 180W at an impossible case temperature of 25'C. Even then, it's likely not rated for use as a linear regulator at all. The money is all in switched mode, so that's where the development effort goes.

Having said that, I have used a 50W amp with MOSFET regulated B+ for years. At the time, I went to a fair bit of effort to track down MOSFETs that were specified for linear use. Of course, the likes of IRF740s and IRFP460s will probably work fine. The manufacturer only rates them for use in switched mode circuits, though.

[Chuck H]

RG

I don't know if we need another power scaling thread, but...
If you consider pair of power tubes (lets use EL34s) running at 500V biased at 38ma each for 19 watts idle. Great. Sounds like a plan. Now lets run the same tubes at 250 volts. We would need to bias at 76ma each for the same 19 watts of idle. But instead we'll "scale" the current proportional to the plate volts so we bias at only 18ma (or whatever, I'm sure there's some kind of log relationship or something). But the notion is the same. Now we have our EL34s biased at 4.5 watts each...What? Ok, we don't want crossover distortion so we'll just tank the drive voltage too (adding another cumbersome control to the front panel). So what we have is a radically underbiased pair of tubes with a small input signal. Are the power tubes even really contributing to the distortion at this point, or is the "drive" control (just a master volume) really all we needed in the first place? Like I said on another thread. There's plenty I don't understand. But on the macro view biasing tubes icy cold and just trimming the input signal seems like a recipe for thin crappy tone. Plus, If you plan to use a NFB loop your hosed unless you want to scale that too.

When you consider some of what we do with guitar amps to try and make them sound good, the whole concept seems contrary.

But like I said, I'm not an EE. My concepts of what can or should be done are limited. So any corrections to my thinking are happily accepted.

Thanks

Chuck

[Dave H]

Chuck,

An idle power of 4.5W in the EL34s may be OK. It's not necessarily "radically underbiased" [that's current not voltage Bruce ;0)]. When the amp is power scaled the whole amp becomes "smaller". When it was a 50W amp the EL34s were idling at 19W but now it's a 12W amp with the EL34s idling at 4.5W. The idle dissipation of the EL34s is the same % of the output power for both the 50W and 12W amplifiers so I wouldn't expect the crossover distortion to increase dramatically.

I don't think the preamp drive control was added to prevent crossover distortion. If the whole amp is power scaled it can mess up the biasing of the single ended cathode biased preamp stages so its usually only the output stage that's power scaled. This means that when the power scale knob is turned down the preamp still produces full voltage but the output stage requires a reduced voltage. The "drive" control was added to correct this. Perhaps the drive control could be ganged with the power scaling control then you'd only need one pot?

Dave H.

[R.G.]

Perhaps the drive control could be ganged with the power scaling control then you'd only need one pot?

Good explanation, Dave.

Perhaps this is one place where a small microcontroller would be usefull to read a front panel "Amplifier size" pot and turn down the B+, Bias, and signal level from the preamps to be correct for the desired amplifier size, even if the rates at which these vary are nonlinear. Microcontrollers make great pot-turners.

[Steve Conner]

Argh, silicootie infestation!

[R.G.]

To be fair, the "180W" MOSFET can only dissipate 180W at an impossible case temperature of 25'C. Even then, it's likely not rated for use as a linear regulator at all.

Tee-hee!

Occasionally I like to see if anyone is watching. Trust you to be watching, Steve.

Steve is of course correct. The die in a "180W" MOSFET will dissipate 180W only if you can somehow hold the case down to 25C by something like encasing it in a solid copper casing with forced-circulation chilled water to carry the heat away.

'Course, those 50W zeners have much the same problem.

A practical limit for air cooled transistors (and zeners, and whatever else you have) in natural convection is about 50W. Otherwise the heat sinking requirements get NASA-esque.

However, I was not completely fooling you all. 50W of MOSFET dissipation in a nominally huge-dissipation rated device can be had for $1 to $2. 50W of zener will cost you $8-$9. The economics still hold, just not as eye-poppingly.