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Related Pages
Fault Protection
Arc Protection
Calculating Shunt Values
Meters in Tube
Transmitters
The following is a description of a few basic safe metering systems. These
systems are
designed to be safe for the equipment and safe for the operator.
See page on power supply
design.
Block Diagram
This is a block diagram of a typical metering circuit. The tube’s control grid to grid-meter path is
normally through the chassis. This lets the control grid be directly grounded to
the chassis.
In the event of a tube arc, grounding the grid directly is safer for everything.
Note that ALL plate current must flow through the plate meter,
and all grid current must flow through the grid meter. Provided something is not
either accidentally or intentionally improperly wired, bias voltage or bias
system will not affect meter accuracy.
Equivalent Circuit For Amplifiers
Most high voltage power supplies, to allow high voltage current measurements
(plate current) without having dangerously high voltages on the meter, and to
also allow grid current measurements in grounded grid amplifiers, are
constructed with the negative power supply lead floating from chassis ground.
The typical current metering circuits for plate and grid, used in a floating-negative-rail power supplies, is shown here.
The entire high voltage power supply section, which includes all rectifiers,
bleeder resistors, and filter capacitors is fully contained inside block PS1. The power supply block (PS1)
negative rail floats from
ground, allowing metering of negative supply rail current and (if required) grid
current. The meter shunts, or current meters, are inserted in the negative lead
path to tube cathode and chassis. This keeps the metering system near ground
potential when under normal operating conditions.
Floating the negative supply lead for metering, while not nearly as hazardous
as inserting meters in the HV positive lead, is not entirely worry-free. If the
positive HV terminal shorts to ground, perhaps from a tube arc, wiring, or
component failure, the negative rail will try to rise negative to full
supply voltage. Because of this, a negative rail voltage-limiting clamp should be
added. The clamp should be something that fails safe, presenting a low resistance to
chassis ground. (The ARRL Handbook and many other sources use a high-wattage 10
or 20 ohm resistor, but such methods are not safe. A negative rail
safety resistor is not a good idea for a few reasons, as we will see in the text
below. The clamp should be a hard clamp, like a semiconductor diode.)
Grid current path
The blue dashed line shows the grid current path. Voltage drop across R3 includes only grid current, or any current from
chassis up to the negative HV supply rail. The grid current path does NOT
include any power supply or anode current.
When the grid meter system is properly wired,
grid or cathode bias voltages will not cause an error in grid current
reading.
Plate current path
The entire plate current path is this red loop. R2 fully samples plate
current and nothing else. Bias voltage or circuitry cannot cause false readings
of plate or anode current.
Cathode bias voltage does subtract from anode voltage, but normally bias is
less than 0.5% of high voltage, so any errors are negligible.
In a grounded grid amplifier, drive voltage adds to high voltage. This
actually increases the effective high voltage beyond what HV metering indicates.
While some articles tell us driver power is not measured and feeds through, that
is not factual. The additional current supplied by the drive power is fully
accounted for, only the additional voltage is not measured.
The cathode shares the anode and grid paths. Any meter or shunt inserted in the cathode will indicate
cathode current, which is a combination of
anode and grid currents. Similarly, bias
diode D1 sees the full cathode current, which is the sum of grid and anode currents.
The value of D1 will
not affect current reading accuracy, D1only
affects the cathode to ground voltage (bias). The bias system (D1) must be
isolated from chassis and power supply negative.
Grounded Grid Amplifier Metering with Grid Meter
We really should have a dedicated grid current meter in every medium power or
high power amplifier. The single most important meter in a grounded grid amplifier,
other than an accurate peak-reading power output meter, is the grid meter. If we
watch any single current or voltage in the amplifier to tell us how an amplifier is
driven, tuned, and loaded,
the grid meter is by far the most important thing to watch!! Grid current tells us when the plate
circuit is
resonant, when the loading control is properly adjusted, and when the amplifier
has lost plate voltage or the load. Grid current tells us when an amplifier is being
overdriven, and when tank voltages are excessive.
Grid current will also tell us if an amplifier
is oscillating. If we watch grid current with the amplifier “keyed” and no
drive power, and if grid current varies with setting of the plate tuning
capacitor, a parasitic oscillation is likely.
The plate current meter (in conjunction with a HV meter) only tells us plate input power.
Plate current does not obviously indicate improper loading control settings, or if the amp is being overdriven.
The plate current meter generally won’t tell us if the amp is oscillating, give
us any idea about linearity, or indicate excessive or the potential for
excessive tank voltages.
Let’s look at a typical circuit for a grounded grid amplifier external power
supply.
Note D2 and C2. Both are critical safety components. When properly sized D2
and C2 will protect the meters, both meter shunts, and the operator under any
condition of HV fault.
D2 threshold voltage must exceed the larger voltage of meter full scale R1 or
R2 voltage.
Voltages in R2 and R1 subtract, so far as D2 is concerned. If grid
meter voltage drop is 1 volt, and if plate meter voltage drop is .5 volts, D2’s
conduction threshold (breakdown voltage) must exceed 1 volt (the higher of the two) by a reasonable
margin. This would require two silicon power rectifiers connected in series
(~1.3 volts conduction threshold).
C2 Selection Considerations
This is one of the rare cases where too much headroom is bad. Use the lowest voltage C2 available in a small disc
capacitor, but try to select a component with reasonable physical size. I typically use 50 volts
for C2. The value generally is around .05 to 0.1 µF. This
capacitor serves multiple functions. C2 bypasses D2 for RF and very short
transients. C2 also provides an additional
clamp at 100 volts or so if D2 should ever open. This clamping action, as an
additional failsafe to hold the negative rail to the chassis, is why C2 should
be a low voltage but reasonable physical size component.
D2 Selection Considerations
D2 can be any diode that can
survive maximum fault current (HV over R4 plus
worse-case path fault resistance) without destroying the case and blowing the
diode path open. Fault path resistance would generally be the
sum of filter capacitor ESR and R4’s physical resistance.
R4 typically should be chosen to limit fault current to non-destructive
values. Typically, in small amplifiers (below 5 kilowatts output), R4 should be about 5 ohms
for every 1000 volts. A 4,000 volt supply
would typically require around 20 ohms fault path resistance. Typically that
path resistance would be comprised of 2.5 ohms ESR in the capacitor bank and 2.5
ohms total in wiring, RF chokes, and other resistors. In this case R4 would be
15 ohms, which adds to the other wiring and component resistances for a total
fault path resistance of 20 ohms.
With 4000 volts the system would have 4000/20 = 200 amperes fault current. A
1N4007 would only
handle 30 amperes before becoming unreliable and shorting, but that does not
mean it would burn open. We really have to test diodes to see how much pulse
current blows the case open. Better quality 1N4007’s will handle 100 amperes for
30-50 milliseconds, and 1N5408’s over 200 amperes for the same time period. If
you are unsure, use a 1N540X series diode, like a 1N5408.
D2 may require more than one diode in series. If the grid or plate shunt
resistance drops more than 0.5 volts at maximum current, additional diodes are
connected in series to form D2.
It is best to consider each silicon power rectifier diode about 0.5 volts per diode.
Shunts and Multipliers calculating or selecting metering
resistance values
In the 1980’s, I managed a precision D’Arsonval meter manufacturing group at
a major meter and aftermarket automotive meter supplier.
This provided a valuable lesson in meter calibration and manufacturing.
- For best current measurement accuracy using external meter
shunts, meters should be calibrated with
voltage
This is because a meter across a shunt actually measures the voltage drop
across the shunt. This is why we see things like “30mV 10A shunt” in precision
shunts
- For best voltage measurement accuracy, meters using external multipliers
should be calibrated with current
This is because meters used with multipliers to measure voltage really are
measuring current through a multiplier resistance
There are two resistances important to current metering, the meter series
resistance and the shunting resistance. These two resistances control the
current division, and along with the meter FS current, determine the meter
scaling. The meter series resistance and current determine the meter full scale
voltage, and that full scale voltage reads across the shunt. By adding external
resistance we can find combinations that allow use of standard resistor values.
Resistor values are available in many standard sizes, but certain values are
more commonly available. Normally resistors have prefix numbers like 10, 12, 15,
18, 22, 27, 33, 39, 47, 56, 68, 82, and starting over at 100, that are
multiplied by a decade multiplier.
To make things easy, I use Excel spreadsheet for picking resistances
You can download it here:
Spreadsheets/meter
calculator.xlsx
Meters in amplifiers or transmitters are the same. Plate or grid
current meters almost always read voltage developed across a shunt
resistance, while
high voltage is measured by measuring current flowing though a series-connected multiplier
resistance.
In
the circuit to the left, shunt resistor R1 sets the voltage applied to the meter
for a given current.
R5 is sometimes necessary to dial the
meter in when meter voltage sensitivity does not lend itself to a common
off-the- shelf value for
shunt R1.
By selecting the proper combination of R5 and R1, we can use almost any meter to
accurately measure power supply DC output current flowing through PA tube anodes. This applies to transmitters and all other systems,
not just external amplifiers.
The same idea applies to grid current shunt R2 and meter multiplier R6. By
selecting the proper multiplier resistance for R6, and the proper current-shunt
for R2, we can almost always find standard values. The R2/R6 combination is for
measuring grid current (shown
above for
typical grounded grid amplifiers).
Picking The Correct Resistance Values
Here’s how to determine resistors for full-scale current:
What is the FS voltage of the meter?
Full-scale meter voltage sensitivity, important in any current meter
used with an external shunt, is meter current
times meter resistance. If the meter is 1 mA and has 50 ohms resistance, full-scale
sensitivity is .001 amperes*50 ohms = .050 volts, or 50 millivolts FS. 50
millivolts is a very common meter movement value.
Another case might be a
meter like Ameritron uses in the AL80A. The AL80A used a 1 mA 450-ohm meter. Full
scale sensitivity is
thus .001 * 450 = .45 volts, or 450 mV. This was not a common meter movement;
when we manufactured the meter a 200-ohm resistor replaced each wire meter lead
inside the meter. This allowed use of a standard 1mA 50mV movement. For
home builders the resistors could be placed outside the meter housing, but at
Prime Instruments we placed multipliers inside the meter housings.
I generally do not like to use 50 mV meters, or other very sensitive meters,
directly with shunts unless there is no other choice. 50 mV meters, or low
voltage sensitivity meters, are more sensitive to ground loop and wiring errors,
and they are more prone to catastrophic damage if a shunt opens.
Consequentially, I almost always use a multiplier resistance combined into
current measurement systems using shunts. A case where a multiplier could not be
used would be a high current shunt in a low voltage supply. We would not want a
200 mV shunt in a 5 volt supply line, because it would reduce voltage to 4.8
volts. We also would not want a high voltage drop with high current, because of
shunt heating. A 200 mV shunt with 20 amps would dissipate .2*20 = 4 watts.
What do we want to measure?
Let’s work through a few problems.
We need to pick a current-shunt that
matches the meter’s FS (full scale) voltage. We also must remember a portion of
shunt current flows
through the meter.
The correct formula for the shunt is meter voltage-sensitivity divided by desired current
minus meter current. Where E = meter sensitivity in volts, I=current being
measured, and i=meter FS current, we have: E /
I – i = shunt resistance
Let’s say we want 750 mA full scale and a standard 50 mV 1 mA
meter. We want 750-1 = 749 mA through the shunt to produce 50 mV across the shunt. The
“missing” 1 mA from the shunt flows through the meter, for a total of 750 mA. The shunt
resistance is .05 divided by .749 = .0668 ohms. That’s a problem for a few
reasons:
- First, people cannot go out and buy .0668 ohm resistors from inventory,
they would be custom manufactured
- Second, tiny resistances in foil traces, wires, or solder joints
can be a large percentage of .06 ohms, and significantly change the effective shunt resistance
- Third, any ground loop voltages or switch contact imperfections can upset
the 50 mV meter voltage. 50 mV is not very much voltage
- Fourth, if the shunt opens, we don’t have much current limiting in
the meter
What can we do to make things less critical? We can increase meter voltage!
What if we made the meter 450 mV by making total meter resistance 450
ohms?
Adding R5, a multiplier of 400-ohms, in series with a 50-ohm meter solves the problem.
Meter current sensitivity is still 1 mA, but meter resistance increased from 50 to 450 ohms.
Now meter FS (full scale) sensitivity
is 450 mV (.45 volts).
In this case, with the meter resistance increased by a multiplier, the meter shunt
must develop .45 volts. The shunt becomes .45 volts divided by .749 amperes = .6 ohms. This
is a standard precision resistance. This is
why Ameritron used a 450-ohm, 1 mA, 450 mV meter. Ameritron could purchase standard .6-ohm
shunts, rather than special custom wound shunts, and save time and money.
Additionally, customers could find standard replacement resistors from many
suppliers.
Grid current, or any other current, is handled in a similar manner. To read 400 mA grid current with
a 1.5 ohm shunt, the meter must have a sensitivity of 1.5*.4 = .6 or 600
millivolts.
Conversely if we have a 450-millivolt meter, the shunt would be
.45/.400 = 1.125 ohms. Closest standard values are 1 or 1.2 ohms. This is the
beauty of using R5 or R6. We can use almost any shunt handy, just by adjusting R5! If we use a very common 1.5 ohm resistor, we would have
1.5 ohms *.4 amps = .6 volts. By using 600-millivolts FS, a standard shunt resistor of 1.5 ohms, along with a suitable
R6 multiplier of 150 ohms in series with a 450-ohm 1 mA meter, still reads 400 mA.
The AL1500
uses a single 1.5 ohm grid resistor. To read full scale 200 mA, the AL1500 grid meter is
.2 * 1.5 = 300 mV full scale. This allows the op-amp that limits grid
current to be set to trigger around 225 mV or so, taking the amp off-line with 150 mA grid
current. The AL1200 has a 400 mA grid meter, so it uses two 1.5 ohms in
parallel for .75 ohms. Once again FS voltage is 300 mV, the same movement is
used for both amps, but the scale is changed and one extra 1.5 ohm resistor is
paralleled in the AL1200.
Shunt dissipation is FS (full scale) shunt millivolts times measured FS
current. With a 400mA FS meter and 300 mV, the shunt dissipation is .12 watts
total. Overkill that dissipation when selecting the part to ensure reliability
and stability.
Meters in Old Tube
Transmitters
Meters in old transmitters generally read grid current and grid voltage of the PA
stage. This always involves switching both meter leads.
Let’s look at a sample transmitter circuit from a Heathkit transmitter:
Notice both leads of the meter float, and the meter is a 1 mA 47 ohm meter.
This is a .001 A * 47 ohm = .047 V or 47 millivolt movement. Most likely
it is actually a standard 50 mV movement, and there was a simple tolerance error
in manufacturer measurements. Perhaps they measured the meter in circuit.
For the purpose of this discussion, the numbers will be taken literally.
This meter is then two things:
For voltage measurements in the transmitter, the
meter is a CURRENT meter. As long as the measured voltage is more than
~100 times the millivolt FS sensitivity of the meter, we can just consider it a
pure current meter for voltage measurement functions.
For current measurements in the transmitter, the
meter is a VOLTAGE meter. As long as the measured current is more than
~100 times the FS current sensitivity of the meter, we can just consider the
meter a
pure voltage meter for current measurement functions.
This is the grid current function. The 5.55 ohm shunt resistor at B and C is
in the grid path. The grid becomes more negative than the grid bias supply, and
forces current through the shunt back into the bias supply.
5.55 ohms will develop 5.55*.001 = .00555 volts, or 5.55 millivolts, per
milliampere of grid current through the resistor.
With 47 mV full scale and no meter current, this will be around 47/5.55 =
8.46 mA full scale. We have to be careful, because 8.46 mA is close to the 1 mA
meter current. We can just add the meter current back in, so current is 9.46 mA
full scale.
The meter is 47 ohms, the shunt is 5.55 ohms. This actually makes true shunt
resistance 4.96 ohms. That’s 4.96 * .001 = .005 volts or about 5
millivolts per milliampere. 47/5 = 9.4 milliamperes full scale.
Why is is calibrated at 10 mA full scale? Because Heathkit used a low meter
voltage (low meter resistance). Switch and wire resistances will make the meter
read lower, and it probably is about 10 mA full scale!
A higher resistance meter would have reduced the effects of hidden switch
contact and wiring resistance, which would make metering more repeatable and
accurate. Although Heathkit did not do that, this is why we should really use a multiplier resistor on the meter or a
higher resistance meter, if possible.
Tube Transmitter Plate Current
This transmitter used a safe plate current system. Like most amplifiers, they
floated the negative rail of the power supply.
The .1 ohm resistor by D (lower left corner) is the plate current meter.
Plate current is measured between the chassis and center tap of the plate
transformer, with the filter caps and bleeders floated back to the center tap.
Modulator cathode current, that includes plate and grid current from all four
modulator grids, is through the .1 shunt at F and G.
The sensitivity across .1 ohms is .1*.001 = .0001 volts or .1 mV per
milliampere.
With a 47 mV meter, we have 47/.1 = 470 mA full scale. We can see how
unimportant meter current now is, because who cares if we are 1 mA off from 470?
We also can see, 47 ohms must not be the true meter resistance. Most likely,
as mentioned at the start, this is a standard 50 mV meter system when switch and
wiring resistances
are all accounted for.
I hope this helps people learn how to use any meter they have with
minimal fuss and bother. Now you see why meter manufacturers, at least
internally, work with millivolts and current for all meters, and why commercial
shunts are defined as mV per ampere. When a meter reads voltage with an external
multiplier, it is normally worked with and calibrated using current. When a
meter reads current with and external shunt, it is normally worked with and
calibrated by using voltage.
Again shunt heat is FS (full scale) meter voltage times maximum scale
current, or I^2*R through the shunt resistor.
Adapting a Junk Box Meter
A recent e-Ham thread had a person replacing a 200 ohm, 10 mA Johnson
Navigator meter with a higher quality D’Arsonval meter. This resulted in a
pinning meter, it was out of calibration even though he used a 10 mA meter.
The problem was the new meter had a FS sensitivity of someplace around 30 mV
sensitivity. He measured 3 ohms across the meter terminal (out of circuit) , so
it was around 3*.01 = 30 mV FS.
The Johnson had a 10-ohm meter shunt, and had a 10 mA meter that was scaled
to read 200 mA full scale. This means the shunt developed .2*10 = 2 volts to
drive the original meter. To be perfectly accurate we should deduct the 10 mA
from the shunt current, so the actual meter shunt voltage was .19*10 = 190 mV
with a 10 mA meter. He needed a 190 mV FS meter at 10 mA.
The most simple solution was to add a ~200 ohms in series with his 10 mA
meter. He had a 180 resistor handy. This gave 183 ohms or so total, and made the
meter 1.83 volts FS. This was close enough to the required ideal 190 ohms, about
5% error.
With just two resistors, and often just one resistor, any equal or more
sensitive meter will generally work with existing shunts.
Metering Without Grid Meter
Since the grid meter is the single most important meter for properly tuning
or operating a class AB2 or class C grounded grid amplifier, it really should
always be included.
A generally proper system, but less grid meter, is shown below. The supply doesn’t need to
be this complex, but I included inrush limiting and critical safety components.
In my opinion, it is better to spend an extra dollar and be safe by adding C2,
D2. C2 and D2 protect the meter, and they protect the operator. It might also be
wise to spend an extra $20 or so and have inrush limiting (RLY1, R7). R4 should
always be included if you use oil filled
capacitors. R4 protects the tubes in the event of an arc. Unless you included
enough resistance in the RF deck plate choke and in the filter capacitors, R4 is
required to protect PA tubes!!
Filter caps are isolated from chassis ground
Current for bleeders is in a closed path across the bridge rectifier
Filtered DC output from the negative capacitor and rectifier terminal goes
through the meter shunt
Meter Wiring and Layout
Wiring meters to shunts can be critical. Most external shunt current meters
operate between 50 mV and 500 mV full scale. As a general rule higher
currents demand lower the meter full scale voltage. Shunt dissipation is
directly proportional to voltage drop across the shunt for a given current.
Where a 500 mV 50 amp shunt would generate 25 watts of heat, and require a shunt
stable with resistance over wide temperature ranges, a 50 mV 50 amp shunt
dissipates only 2.5 watts.
Proper shunt connections require thought and common sense, and a feel for
what the meter and shunt are doing. Here is a correct shunt wiring method:
A and B are shunt termination points. This is where the main current flows
through the shunt.
M1 and M2 are the meter terminals. This is the rule even if one end of
the shunt is grounded!!! In other words if we have a shunt going to ground, the
ground connection would be at A or B. The meter would have two wires returning
to M1 and M2, even if one meter terminal eventually goes to the chassis. NEVER
depend on a chassis path, a shared foil path, or a shared wire path from a meter
terminal to the grounded end of a grounded shunt.
Let’s consider the large strip or bar shunt (?). If the meter was bolted
across the shunt at terminals A and B, full load current would flow though the
bolted terminals. If the bolts loosened or developed a high resistance, current
readings would go up. If the shunt unbolted or lost connections (or even had a
slight resistance in connections), the meter itself would burn out!! When we
connect properly to M1 and M2, any bad connection simply causes the meter to
read low or have no reading. There is nearly zero risk of damaging the meter,
plus connection resistance doesn’t change meter accuracy!
The same connection problem occurs inside an amplifier or transmitter, except
the problem is exaggerated because the shunt is normally a two-wire standard
resistor. With a standard shunt, none of the connections cause ground loops or
excessively high readings. A properly wired meter floats across the shunt tap
points; loose shunt connections never cause a high reading or damage the shunt.
In an amplifier or transmitter, wiring can have multiple problems. If the meter
is grounded to the chassis, or to some other ground point independent of the shunt, a
loose screw, broken wire, or cold joint could throw readings way off. Many
connection failures take the shunt out-of-circuit while the meter remains
connected. This can cause a high reading, or
destroy the meter.
Nearly all meter failures from overload, or oddly behaving meters, are caused
by equipment wiring or design errors.
In the resistor shunt shown above, meter leads must be returned to the shunt
connection pads at M1 and M2.
Capture of circuit layout area for shunt resistor.
The meter is connected across the resistor, not to the current-path
traces to the resistor. Foil trace resistance cannot affect shunt resistance.
Notice both meter leads float.
Circuits to the left show proper and improper meter wiring techniques.
Electrically they appear identical in function, but the lower circuit has
needless safety, reliability, and
ground loop problems. Lack of one cheap diode puts the meters and operator at
risk if HV faults to ground or a connection fails, and lack of one single
additional wire from the meter to shunt allows voltage potential differences
along the ground path to affect the meter. With a 50 mV meter, just one
millivolt of ground loop bias can produce 2% error!
It doesn’t matter if the meter and shunt are on the same chassis unit or not,
so long as BOTH meter wires travel directly to the shunt. With proper wiring,
the shunt can be located near the meter or remote from the meter on another
chassis.
Be sure to RF and transient bypass the protect diode, a .1 uF 50-100 volt
large disc capacitor is ideal. Remember to use
more than one diode in series if shunt voltage is over 0.5 volts.
The protect diode must always be located at or very near the filter capacitor
bank. It is permissible to have an extra clamp or protection diode near or in
the RF deck, but the primary protection diode must always be located at the B-
end of the filter capacitor stack.
Power Supply Design soon
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