Whether or not you’re conscious of it or not, three-terminal voltage regulators robotically alter energy provide voltages in the whole lot from audio consoles to distribution amplifiers to audio processing to STLs to FM exciters to transmitters and to modulation screens. A circuit designer’s first selection is a single regulator machine that may change a circuit card stuffed with elements that have been typical in previous designs.
It’s true that broadcasting has entered a digital world. Voltage regulators designed into tools right now typically use switching know-how. They’re a bit extra sophisticated within the strategy of turning into extra environment friendly, however many of the gear in use right now employs conventional three-terminal regulators.
What are they?
Three-terminal voltage regulators bodily seem like an influence transistor and act as an automated variable resistor to deliver a provide voltage right down to a desired voltage for circuits they energy.
Usually, they’ve 17 transistors in a small package deal which are a easy reply to get a dependable fastened voltage from energy sources which are topic to energy bumps, sags and energy provide ripple. They may take 24 volts in and regulate it down to fifteen volts. That output voltage stays fixed with a good tolerance so the vital circuits in digital gadgets should not hampered by energy fluctuations.
They’re analog “sequence” gadgets that get heat, as any resistor would, whereas lowering voltage to a desired worth. Ohm’s regulation: 14 volts in and 5 volts out is a 9 volt drop. If the circuit attracts 0.5 amperes, that ends in 4.5 watts of warmth; therefore the regulator must be related to a warmth sink.
Fig. 1 exhibits three regulators, every with three connections, therefore the title three-terminal regulators. Those in bigger circumstances can dissipate extra warmth. There are different variants, however the one on the left is the commonest.
Necessary info is discovered within the half numbers. LM340T5 is often known as a 7805, which is a five-volt optimistic voltage regulator. A 7812 is 12 volts and a 7912 is a 12-volt regulator for unfavourable energy provides. Some are adjustable with a number of exterior parts. (Keep on with me as there are some vital nuggets of knowledge that might make your life simpler.)
Fig. 2 is an influence provide card in a Nineties classic Continental 802B FM Exciter. As you’ll be able to see, there are three regulators. One provides +22 VDC, one other –22 VDC and the bigger one supplies +5 VDC. They’re in every single place; this exciter has no fewer than eight of those cheap, hard-working circuits.
Issues can go unsuitable
Three-terminal voltage regulators are high-gain gadgets to maintain up with fluctuating enter voltages and circuit masses. With excessive achieve comes the potential of breaking into oscillation and making their very own high-frequency alternating present. They may typically begin oscillating at a pair hundred kilohertz. The regulator will proceed to place out its rated voltage, however with a further 200 kHz or so of undesirable rubbish. An oscilloscope will present that. Ouch, the circuits they feed don’t like that. An audio circuit may need plenty of hiss or distortion in it. A inexperienced mild glowing on the facility provide doesn’t point out all is effectively. It simply exhibits that voltage is current, no matter its purity, or lack thereof.
Fig. 3 is a spectrum analyzer view of the RF output of the identical Continental exciter, which got here out of a Continental 816 FM Transmitter. Sure, it’s on the meant 100.7 MHz, however additionally it is on FM frequencies at about 210 kHz intervals up and down the dial. Now we’re taking interference to different stations!
This downside turned out to be a regulator on the exciter’s RF energy amplifier board. All was effectively within the exciter till the RF obtained to that time. Then the sign was “modulated” by 210 kHz from a regulator gone wild. The wrongdoer turned out to be a ten mfd electrolytic capacitor that failed by going open. Electrolytic capacitors have a restricted lifespan, particularly when they’re in a sizzling setting and as years go by.
Fig. 4 exhibits capacitors related to two regulators in that module.
Fig. 5 exhibits the outcome when simply that one capacitor was changed. The undesirable sign spurs have been gone and simply the meant sign on 100.7 MHz got here by. In a case like this, I like to recommend you change ALL the electrolytic capacitors in that module as others are prone to fail quickly.
What you might be seeing was the exciter operating right into a dummy load. In a transmitter setting, the troubled exciter was feeding a broadband stable state 400-watt RF amplifier. The sign popping out of the amplifier was doubtless similar to what we noticed in Fig. 3 as a result of there is no such thing as a bandwidth limiting. The transmitter’s tube enter tuning community, together with tube plate and loading community after which the antenna, doubtless restricted how far up and down the dial the undesirable radiation went. For certain, the station was heard 210 and 420 kHz in each instructions from the licensed frequency.
On this occasion, the engineer reported the 400-watt RF amplifier metering confirmed larger than regular mirrored energy when it was feeding the ultimate tube. He couldn’t tune it out as a result of the tube’s enter tuning was largely permitting the licensed frequency by. A few of the +/- 210 kHz and +/- 420 kHz undesirable spurious alerts have been mirrored again to the amplifier. A spectrum analyzer would have advised the story instantly.
However there’s extra
Fig. 6 exhibits an oscilloscope view of a 1 kHz sine wave tone going by the exciter and being demodulated by a radio. The tone was thick with undesirable 220 kHz using on it. That’s proper, oscillation degrades station audio as effectively. The waveform turned a pure, clear and skinny sine wave after the capacitor was changed.
Capacitors
Fig. 7 exhibits one other pair of regulators, with capacitors close by, on the audio/automated frequency management meeting of the exciter we’re discussing. Circuit designers and producers appear to love cheap electrolytic capacitors in regulator purposes. They work well past the tools guarantee however fail 10 or so years down the street.
Even the modulated oscillator has one, as seen in Fig. 8. They’re in every single place!
When constructing one thing from scratch, I choose poly foil capacitors which have a a lot decrease ESR (equal sequence resistance), particularly at frequencies above 50 kHz. They final ceaselessly too. The worth I select for three-terminal regulators is often 0.39 mfd and I’ve by no means had an issue. These capacitors are sufficiently small to mount on to regulator leads in a point-to-point wiring scheme as proven in Fig. 9.
Realized
The lesson right here was changing one small capacitor saved a $5,000 expense for a brand new FM exciter, to not point out the effort of recycling. This exciter will stay for years to return.
New regulators
Extra environment friendly switching regulators are displaying up in latest tools designs. Extra energy will be dealt with in the identical dimension part package deal, however they arrive with their very own drawbacks. Not like the linear/sequence regulators that we mentioned, these chop up the incoming DC voltage and let the good voltage out. Capacitors are wanted to maintain them clear so the outgoing DC energy doesn’t have noise using on it to trigger circuit issues.
In abstract, be told concerning the know-how so you are able to do the most effective in your broadcast engineering job.
[Read more tech tips from Mark Persons.]
