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Old 14th Jun 2019, 6:43 am   #1
Diabolical Artificer
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Default Shunt regulation.

Had a look online about this but couldn't find much on the subject. As far I can see you use a zener or BJT to keep V within a range, so as a result had a bit of a play with a 2N3055 as in this circuit but without a lot of the paralleled R's and caps.

First off used one 6.3v 3.4A winding and after trying various zeners got it to "regulate" at about 5.9v, but anything over a 300mA load and the tfmr complains. Next tried two 6.3v wndgs in parallel: result Q got stinking hot, no regulation and a small pile of dead zeners.

So my findings is that for regulating DC htr supplies it's a waste of bl**dy time, it stresses the tfmr, drops available I dramatically and would need at least 3 2N3055's and a huge HS to regulate 6.3v at around 1A. From what I read shunt regulation is supposed to have better OP Z than series regulation, but at what price?

Another interesting thing I noted was that when checking for Q hottness I felt "drag" when my finger touched the HS, it felt like the HS was vibrating which means I guess the Q was oscillating.

Thoughts? Wisdom?

Andy.
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Old 14th Jun 2019, 7:05 am   #2
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Default Re: Shunt regulation.

Shunt regulation is normally used for very small currents. Its very accurate if properly designed. Using shunt regulation for a valve heater supply is just plain silly.
A low drop regulator will most likely have much better regulation, lower noise and freedom of oscillations.
Trying to regulate 6.3 volts from a 6.3 volt heater winding is also a bit dodgy unless you have a 6.3 winding that is very robust. By that I mean a 5 amp winding "may" supply 2 amps before the in/out voltage of the regulator drops below its safe operating point. This is caused by the resistance of the 6.3 volt winding.

Some American test equipment used DC heating with nowt more than a rectifier, a big cap and a pot to set the correct voltage.
When you rectify AC to DC the rating if the transformer also changes. The charging currents for the capacitor essentially takes 35 or 40 % of the available current.

Using one or two 2N3055's by them selves have no gain, so the base current will exceed the zener current ratings. You will need EITHER a op-amp in front or a couple of extra transistors to boost the base current required.
Transistors DO NOT have the wonderful zero current requirements of a valve grid which is the equivalent of base of the transistor. Transistors are current controlled devices, valve are voltage controlled devices.

I am speaking very broadly here of course, but unless you use a "super beta transistor, AKA A Darlington pair, or even a "super" transistor, which consists of three, four or even five transistors in Darlington mode, to make the base accessible to control. Ideally it will also have feedback to compensate for mains variations.

Cheers Andy
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Old 14th Jun 2019, 7:53 am   #3
Craig Sawyers
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Default Re: Shunt regulation.

There are advantages to using shunt regulators over series ones, but they need very careful design to minimise power dissipation in the shunt element. The circuit above is a very naive design, and it does not surprise me that it misbehaves.

For an efficient heater regulator wrapped around a chinese switching regulator, this takes a bit of beating
http://www.tronola.com/html/tube_ana...ices.html#AppB

A shunt regulator design by Walt Jung is here https://refsnregs.waltjung.org/UnivReg_122714.pdf

But that has a typical supply maximum of 100mA (not a limit though).

In general high quality shunt regulators are fed by a constant current source, and then a shunt element with a decent control loop. And you carefully partition the current between the regulator and the load. So suppose you need a load current of 1A. The constant current source would deliver 1.2A, with 1A going to the load and 0.2A in the shunt element. So for example if you need 12.6V, you would have a rectified and smoothed raw input of say 16V from a 12V Tx, drop 3.4V@1.2A in the constant current source (4W) and 0.2A@12.6V in the shunt (2.5W). So the total regulator dissipation is 6.5W to deliver 1A at 12.6V (12.6W). An efficiency of 12.6/(12.6 + 6.5) = 0.66

Stability needs to be carefully considered, as it does with all regulators, series or shunt, IC or discrete, and needs to be checked with a scope. A snubber on the output is usually necessary with low noise discrete designs.

Craig
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Old 14th Jun 2019, 8:22 am   #4
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Default Re: Shunt regulation.

Shunt regulation is critically dependent on there being both a series and a shunt element. It is the shunt element which is controlled and together the two elements form a 'two resistor' voltage divider.

If the series element is very low impedance, the shunt element must pull vast current to get any significant voltage reduction. The efficiency is lousy, the shunt device gets very hot and the series element too.

If you just clapped a shunt transistor across a heater supply, you were relying on the impedance of the heater winding to drop the volts. This means that the total load was taken to or above the transformer's rating.

Shunt regulators are very common. The simple series resistor into a shunt zener circuit is one. Sometimes a bandgap reference IC replaces the zener. One of these is the reference for most other types of regulator, linear or switching. These are low power cases where the inefficiency is not a problem... and the series resistance is controlled and obvious to the eye.

The second good use for a shunt reg is in regulating the screen voltage of those Eimac high-perveance tetrodes like the 4CX250 and larger families. (the 250 in that type number means 250W anode dissipation rating) These are high power gain RF valves and under some common circumstances can source screen current! sticking in a screen resistor isn't a good choice, they need regulated screen volts for good linearity. A series reg would give nowhere for the negative screen current to go. A shunt reg fits the needs.
Early Philips colour TVs had a shunt reg triode pulling down the EHT (And sourcing X-rays!) but the EHT supply impedance was high enough for it to work, but it was wasteful of expensively produced EHT and the method was soon dropped.

I wonder what people would think "high perveance" meant if they came across the phrase out of context...

Most class-A amplifiers can be viewed as shunt regulators. The anode resistor is the series element, the valve or transistor is the shunt element. The inefficiency is tolerated in a low power stage, is certainly noticed in a power amp stage, and a power supply would be even less tolerable unless a very low power one.

David

Edit: I'm a slow typist Craig.
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Old 14th Jun 2019, 8:30 am   #5
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Default Re: Shunt regulation.

Without wishing to teach my grandmother to suck eggs...
The point about regulation is to maintain the voltage at a constant level with changing load current.
If the load current is likely to vary widely then series regulation is the only practical option.
Shunt regulation comes into its own when the range of current change in the load is small. Valve heaters are ideal.
Shunt regulation does add an extra load to the transformer, usually the circuit is designed to work with the current in the shunt device at between 10 and 25% of the load current.
Also the dc source should be sufficiently above the output voltage (refered to as headroom) to make sure that variations in mains voltage or changes due to the resistance of the transformer windings do not drop the dc source so low that the current in the shunt device drops to near or below 0A

This means in fact that 6.3V ac of a normal valve mains transformer is loo low.
The peak of the 6.3V ac is 8.9V. From this we have to subtract 1.2 to 1.4 volts for the bridge rectifier. We now have say 7.7V available.
The smoothing, reservoir capacitor will not give a perfect dc, the ripple volts will also have be subtracted.
A quick and dirty rule of thumb is that the peak to peak ripple voltage of full wave rectification at 50Hz equal 10XI /C where C is in thousands of microfarads and I is load current in amps. (20XI /C for half wave)
Taking say a 2A load current a 10,000uF capacitor would give a ripple voltage of 2V which means the source voltage will dip to 7.7 - 2 = 5.7V, no headroom at all.
If we accept a headroom of just 1V (which makes no allowance for fluctuations due to mains or load current) then the max ripple can be 0.7V which requires a capacitor of 28,000uf (nearest value 33,000uF)
I used to design power supplies and would not consider trying to convert an existing 6.3V winding to provide 6.3V dc regulated. I have however built a valve preamp with dc supplies but started with a winding of 12V

Peter

I'm also a slow typist......

Last edited by Electronpusher0; 14th Jun 2019 at 8:53 am. Reason: Bad spelling
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Old 14th Jun 2019, 9:04 am   #6
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Default Re: Shunt regulation.

The old brain cells must be dropping off.
The quick and dirty formula I used above should have been 8 X I /C (16 X I /C for half wave)

Peter
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Old 14th Jun 2019, 1:00 pm   #7
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Default Re: Shunt regulation.

I agree with those rules of thumb.

The actual factor depends on what the fraction of the mains cycle charges the caps. The factor of 8 indicates that the capacitors discharge for 8 milliseconds (and hence charge via the rectifier for 2 milliseconds). That is an entirely reasonable number. Some designers use 7 milliseconds (so 7 x I/C) but the numbers are so close as to make no difference in practical terms, since the actual value depends on the resistance of the mains transformer.

Craig
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Old 14th Jun 2019, 3:02 pm   #8
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Default Re: Shunt regulation.

There is one kind of shunt regulator that is just about 100% efficient when it comes to usable power.
The control circuit is powered by a solar panel and falls zero power consumption in the dark.
There is a diode connecting this to a battery most commonly an SLA.
The voltage monitoring circuit samples the battery and when the sun is bright enough to fully charge the battery the shunt regulator kicks in and dumps the unusable power that would otherwise over charge the battery.
That poor transformer with a baked bean sized capacitor across the rectifier will certainly cause big charging pulses to be taken from the transformer. No wonder it was vibrating.
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Old 15th Jun 2019, 6:21 am   #9
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Default Re: Shunt regulation.

"Using shunt regulation for a valve heater supply is just plain silly" Joe post #2 "Shunt regulation comes into its own when the range of current change in the load is small. Valve heaters are ideal." Peter post #5

"they need very careful design" So it's not as simple as bunging a zener on the base then, didn't think it would be from the way the circuit behaved. I know a little bit about valve circuit design but haven't done much with Q's, this circuit would be challenging. Designing a switching PSU is also out of my league but take your point Craig that an off the shelf one may do the job

" I wonder if simply using one of the common (e.g. Ebay 99p) LM2596 buck-boost regulator modules on the full-wave rectified supply would work OK, or inject too much HF noise. " Lowering hum+N is the whole raison d'etre of a DC htr supply after all, so wondered this myself. A big part of the inefficiency of valve amps is the power wasted on heating the cathodes, a SMPSU would go some way to claw back a few wasted watts. I'd have to do the figures for linear V's SMPSU but would think tfmr AC htr supply is pretty efficient in comparison to a SMPSU.

I thought I'd try the attached circuit in post #1 a go, but suspected the big banks of parallel R's were doing more than the shunt Q to drop V to 6.3v. I've used a LM317 and series Qlinear PSU's before for DC htr supplys but thought I'd have a play.

Thanks for all your IP, Andy.
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Old 15th Jun 2019, 11:24 am   #10
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Default Re: Shunt regulation.

In a shunt regulator all the voltage drop comes from the series element. That is how the shunt regulator works. The shunt part does not drop any voltage at all; it just sucks (and wastes) enough current to get the required voltage drop from the series element. Can be used for heaters, but needs to be carefully designed so that under normal conditions it doesn't use much current. Then on startup with cold heaters the series element as an inrush limiter.

In almost all valve audio circuits there is no need for DC heaters. They just create work and problems.
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Old 15th Jun 2019, 1:02 pm   #11
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Default Re: Shunt regulation.

Quote:
Originally Posted by Electronpusher0 View Post
Shunt regulation comes into its own when the range of current change in the load is small.
And when current needs to be sunk as well as sourced -- as in the case of the 4CX tetrode family when heavily loaded in Class AB1 linear service. Here the screen grid sources current over part of the driving cycle, and inadequate regulation in these circumstances degrades intermod performance.

"Never mind the quality, feel the width".
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Old 15th Jun 2019, 1:36 pm   #12
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Default Re: Shunt regulation.

Shunt regulation also has the advantage that it can relatively easily be made short-circuit-proof just by slightly increasing the power rating of the series-resistor.
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Old 20th Jun 2019, 12:06 am   #13
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Default Re: Shunt regulation.

Here's one I made earlier [over 50 years ago] Still going strong - one rectifier diode changes a couple of years ago. .very handy for charging nicads; one day, I'll reduce the series resistor to 5R6 for 2000mAH cells...

JohnB
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Old 20th Jun 2019, 6:25 am   #14
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Default Re: Shunt regulation.

Thanks for that John.

Andy.
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Old 20th Jun 2019, 11:16 am   #15
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Default Re: Shunt regulation.

For the complaint from the OP about a buzzy heatsink I suggest if a mains powered device check earthing.

Casework at half mains through input Y capacitors with a missing earth feels just like that. The TRIO 9R59DS receiver with a two wire mains lead
is a perfect example.
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Old 20th Jun 2019, 1:20 pm   #16
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Default Re: Shunt regulation.

Quote:
when my finger touched the HS, it felt like the HS was vibrating
Agree with above, this usually indicates leakage from the mains, not necessarily harmful if the source is class II insulated, but wrong if it is supposed to be earthed. You can often feel it if you stroke the metal surface of a class II device with the back of your fingers, if it has a touch leakage current of say 0.1mA or more. Although not harmful when the PSU is properly isolated, it can be annoying on metal-cased laptops for example, so at least one popular brand uses a 1k resistor to discharge the leakage to earth if available, without specifying that an earth is mandatory.

I use / buy / build a lot of DC power supplies and regulators for many purposes, DC-DC, AC-DC, from 1W to 1kW. I must have incorporated thousands, if not tens of thousands of various kinds of regulators and converters since I last used shunt regulation for anything handling more than a watt or so. And that was interfacing to vintage gear that also used shunt regulation for a multi-source redundant control supply.
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