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TECH LETTER #3
MEASUREMENT OF LINE AND LOAD REGULATION OF DC POWER SUPPLIES
HARRISON LABORATORIES
DIVISION OF HEWLETT-PACKARD COMPANY
(F
100 Locust Avenue
Berkeley H e i g h t s , New J e r s e y 0 7 9 2 2
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TECH LETTER # 3
December 16, 1964
MEASUREMENT OF LINE AND LOAD REGULATICNOF DC POWER SUPPLIES
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I NTRODTJCTIO N
T h e measurement of the regulation characteristics of a highly regulated DC
power supply involves several subtle procedures and precautions which, i f disregarded,
may completely invalidate the measurement r e s u l t s .
T h i s tech l e t t e r will present two suggested measurement setups--one for measuring the s t a t i c regulation c h a r a c t e r i s t i c s of constant voltage power s u p p l i e s , the other
for the measruement of constant current supplies.
THEORY
Load regulation for a constant voltage supply is defined a s t h e change of output
voltage AVoutwith a change in load c u r r e n t A Iload. Load regulation for a c o n s t a n t
current supply is defined a s the change i n output c u r r e n t A I o u t with a change in load
voltage A V l o a d . Line regulation is defined a s the change in output voltageAVout
( A Iout for c o n s t a n t current supplies) for a change in l i n e v o l t a g e A V l i n e .
Regulation c a n b e expressed either a s a n absolute quantity
or as a percentage.
Vout or
A
Iout
The DC output r e s i s t a n c e RO c a n e a s i l y be found by using t h e equations:
for constant voltage and
nvOUt
%=
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AIload
PROCEDURE
I.
Requlation Measurement of Constant Voltage Power Supplies. Figure i
shows a s e t u p s u i t a b l e for measuring both load and l i n e regulation of a constant voltage
power supply. The following precautions and procedures should b e observed.
(1)
I t is e s s e n t i a l t h a t the l e a d s connecting t h e load d e v i c e to the
power supply and the monitoring device to the power supply be completely s e p a r a t e .
This cannot b e overemphasized. Connecting the monitoring d e v i c e to points AA' (see
Figure 1) i n s t e a d of the power supply output terminals will r e s u l t i n a measurement not
of the regulation characteristics of t h e power supply, but of the power supply plus the
r e s i s t a n c e of the l e a d s between the output terminals and points A and A ' . Even connecting the load path by means of c l i p l e a d s to the power supply and then connecting
t h e monitoring path by means of c l i p l e a d s t o the load c l i p l e a d s will r e s u l t in a measurement error. Remember that the power supply being measured may have a n output impeda n c e of 1 milliohm or less, and contact r e s i s t a n c e between c l i p l e a d s and power supply
terminals may be considerably greater than the specified output impedance of the power
supply itself.
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Because this is undoubtedly the m o s t common error which occurs in the measurement of static regulation of a power supply, Figures 2 and 3 have been included to s h o w
the proper method of connecting the monitoring and load l e a d s t o the power supply terminals. If the measurement is being made a t the front terminals (Figure 2 ) , the load
should be plugged into t h e front of the terminal (at B) while t h e monitoring d e v i c e is
connected by means of a l e a d which is inserted through the hole in the neck of t h e binding p o s t s (at A ) . In t h i s way the only r e s i s t a n c e which is common to both the l o a d a n d
monitoring paths is located from t h e neck of the binding post back. When the s u p p l y is
monitored a t the rear terminal barrier s t r i p (Figure 3 ) , the load device should b e connected t o t h e plus and minus output terminals while the monitoring device should be connected directly to the plus and minus s e n s i n g terminals. S i n c e there is only a s m a l l
current flowing i n the l e a d s connecting the sensing terminals to the output terminals,
the monitoring device connected to the s e n s i n g terminals obtains a valid indication of
output performance. In fact, the monitoring device sees the s a m e performance a s t h e
feedback amplifier within the power supply which i s s e n s i n g and correcting t h e output.
If a n ammeter is not included on the front panel of the power supply b e i n g
(2)
measured, then one should b e inserted i n s e r i e s with t h e load r e s i s t a n c e so t h a t t h e
full load current which is being imposed on the power supply c a n be verified to b e t h e
same as t h e full load current rating of t h e supply.
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The load r e s i s t o r should be chosen t o draw the full rated current of t h e
(3)
supply a t the voltage of measurement. S i n c e the performance of the supply with regard
to both load regulation and l i n e regulation will be t o a large degree independent of t h e
output voltage s e t t i n g , the output voltage may be adjusted to a v a l u e which, together
with the load r e s i s t o r s readily available, will yield the rated v a l u e of load current.
The current l i m i t setting of t h e power supply should be set t o its m a x i (4)
mum v a l u e , well above the v a l u e of current actually being drawn from t h e supply.
Failure to observe t h i s precaution may r e s u l t i n the output voltage dropping a t full
load due to current limiting action.
The monitoring device to b e used c a n b e a differential voltmeter, or
(5)
s e n s i t i v e digital voltmeter, o r ' e v e n a battery or variable power supply in s e r i e s with
a s e n s i t i v e multimeter or DC oscilloscope. The meter to b e u s e d should b e plugged
into a n A C power l i n e which is reasonably isolated from the AC power line feeding t h e
power supply, s i n c e large current changes in the power supply may r e s u l t i n l i n e volta g e changes which would c a u s e the reading of the meter to change independent of a n y
output change of the power supply.
The Variac to be used to v a r y the input AC l i n e voltage should h a v e a n
adequate current rating: a l i n e regulator should not be u s e d anywhere i n the l i n e feeding the power supply. J'ailure t o observe t h e s e precautions may r e s u l t i n waveform
distortion of the input l i n e voltage t o the power supply with resulting degradation of
the regulating performance or, in the case of SCR regulated or preregulated s u p p l i e s ,
serious malfunctioning of the firing circuit.
(6)
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(7) The AC Voltmeter should be known to have a n a c c u r a c y over t h e
s p a n from 105-125 volts of 1% or better. Wherever possible t h i s voltmeter should
be connected directly to the power supply input terminals rather than to t h e output
terminals of the Variac--particularly in t h e case of high current power s u p p l i e s ,
s i n c e the input voltage a t the power supply terminals may differ significantly from
the output voltage of the Variac due to the IR drop in the power l i n e s connecting t h e
Variac to t h e power supply.
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(8) Measurement of l i n e regulation should be accomplished by changing
the input l i n e voltage from 105-125 and 125 -105 volts a t the desired output voltage
and current combination.
(9) Load regulation should b e measured by throwing t h e s w i t c h S-1 and
noting the change in t h e output voltage while the supply is being energized with a n y
l i n e voltage within its input rating. Notice should b e taken of t h e direction in which
the output voltage changes. An output voltage drop concurrent with a n i n c r e a s e i n
load current is indicative of a positive output r e s i s t a n c e a t DC, whereas a n output
voltage rise concurrent with a n i n c r e a s e in load current is indicative of a negative
output r e s i s t a n c e .
Measurement of Load and Line Regulation of C o n s t a n t Current Supplies.
Figure 4 shows a s u i t a b l e s e t u p for measurement of a constant current supply. Comments Nos. 6 , 7 , and 8 under Measurement of a Constant Voltage Regulation apply
to the measurement of constant current regulation. In addition, t h e following precautions should b e followed.
11.
(1) The series monitoring r e s i s t o r should have a reasonably s m a l l v a l u e
s o that t h e voltage drop a c r o s s it is only a small fraction of t h e total output voltage
of t h e power supply. I t should be a wirewound r e s i s t o r using 20 ppm/oC wire (or
better) and should b e operated at a power level of less than 1/10 of its rating. This
last precaution is n e c e s s a r y so that the surface temperature of the power r e s i s t o r w i l l
be only slightly higher than the ambient temperature. Failure to observe t h i s precaution will r e s u l t in a surface temperature "bobble" of the power r e s i s t o r with a
corresponding change in the value of RI. This c a u s e s short-term variations i n the
measured voltage drop a c r o s s t h i s r e s i s t o r which a r e not the r e s u l t of output current
c h a n g e s of the power supply.
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(2) The load r e s i s t a n c e should be c h o s e n so t h a t the voltage drop a c r o s s
it a t the current level of measurement is equal to the voltage rating of t h e power supply
minus the drop a c r o s s the sensing resistor.
(3) Switch S-2 should be placed i n parallel with the load so t h a t the curr e n t regulation of t h e power supply will be effectively measured between a very low
voltage (equal to the voltage drop a c r o s s RI) and t h e maximum rated voltage of the
supply
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(4) The voltage a d j u s t control should be s e t well above the maximum rated
voltage of t h e supply so that voltage limiting will not occur under t h e condition of
maximum load r e s i s t a n c e . If t h e voltage control is s e t a t too low a value, voltage
limiting may occur with the r e s u l t that the output current will drop under the condition when s w i t c h S-2 is opened.
(5)
T h e monitoring d e v i c e should be a differential voltmeter, digital voltmeter,
DS o s c i l l o s c o p e or other voltage monitoring device capable of measuring accurately
the low voltage drop a c r o s s the c h o s e n value of RI.
I t is imperative t h a t no voltmeter be connected a c r o s s the terminals BB'
(6)
of Figure 4. Such a meter will have a constant resistance, and with the change in
voltage which will occur a c r o s s the power supply output terminals a s a result of load
r e s i s t a n c e change, t h e current drawn by the monitoring device connected between BB'
will vary. Even i f the true output current of the power supply were t o remain constant
under t h i s condition, t h e small current change through terminals BB' would be compens a t e d by a n equal current c h a n g e through the monitoring resistor RI, with the result
that t h e voltage drop a c r o s s t h i s r e s i s t o r would change even though the total power
supply current had not changed a t all. It is desirable to u s e a voltmeter i n shunt with
the load r e s i s t o r or u s e the front panel voltmeter of t h e power supply itself a s a n indication of t h e output voltage. Special circuitry has been included in Harrison Laboratories c o n s t a n t current s u p p l i e s so t h a t the presence of the front panel voltmeter does
not degrade the c o n s t a n t current performance.
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Load regulation should b e measured by throwing the switch S-2 and
(7)
noting t h e change i n the output voltage as measured a c r o s s RI while the supply is
operated with a n y l i n e voltage within its rating. Then:
AI
Av
=
RI
is used t o calculate the load regulation.
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LOAD LEAD
MONITOR HERE
FIG 2
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Proper Method of Connecting Monitoring and
Load Lead Connections to Front Panel Terminals.
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F-IG 3
Proper Method of Connecting Monitoring and Load
Lead Connections to Barrier Strip Terminals.
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