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SERVICE INFORMATION FROM HEWLETT-PACKARD
JANUARY-FEBRUARY 1975
ALL ABOUT
POWER SUPPLIES (1 \
by Dick Gasperini, Editor
I '
.f
. Virttrally every-piece of electronic
. Clear has a power supply so the
probability of having to repair a
failure is rath- high. Therefore,
knowledge of p o w t supply opera-
tion is essential for repair
personnel.
A power supply that may look
familiar is in Figure 1. It will con- This change of output voltage with helps to stabilize the output voltage.
vert the AC line voltage to a DC a change in load ("load regula-
tion") would not be a problem if This technique does have its limita-r
voltage, say 175 volts, for our load, tions. Assume that CR3 is rated at
0 lumped hefe as a single resistor RL. we atways had a fixed load that
dmanded a constant amount of
cbrrent. Most times, though, a load
20 mA. We would select the value
of R2 so that 10 mA of current
is not constant. Therefore, the flowed thru CR3. If the load were
power supply voltage will change reduced by 10 mA, the voltage
and this may be undesirable. would tend to rise and CR3 would
conduct more current, keeping the
Also, what happens if the line volt- voltage at 175V. Similarly, if the
age variets by 2 % In this supply
0? load were to draw more current,
the output voltage will again vary. less current would be drawn by
This supply lacks "line regulation". CR3, again stabilizing the voltage.
To obtain a more stable voltape But we are very limited in the load
we will want an improved supply. variation that can be compen-
sated with a breakdown diode.
This type of power supply has One improvement is to change to
several shortcomings. Since this solid state and to a full wave cir- The output voltage is also chang-
supply can be looked upon as an cuit. This gives a lower internal ing somewhat during this process.
ideal battery with a resistor in impedance and therefore better
series, it becomes clear that the load regulation. (This also has
output voltage will change if the other benefits, like reduced ripple.)
load changes. See Figure 2. A bigger improvement, though,
will result from using a breakdown
diode. See Figure 3.
The breakdown diode will start
drawing current when its break-
down voltage is reached. Thus, as
the output voltage starts to rise
above 175V, the breakdown diode
will conduct heavily, causing more
voltage to be dropped across R2.
If the output voltage were to start
to drop below 175V, the diode
would conduct less, drogping,Ip
voltage across R2. Thus, the dlode
OHmlett-Packard Company 1075
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POWER SUPPLY TUTORIAL
I
I
A breakdown diode has about Redrawing this circuit in the more
10Q of resistance, and this is, in usual way may make it more recog-
essence, the output impedance of nizable. See Figure 6.
our power supply. This may be too
high for our needs. Current limiting has been added
with a series resistor and several
Putting an emitter follower on this diodes. As increased current flows
circuit will reduce the impedance in the load, additional voltage drop Sharper turnoff can be achieved by
by about the gain (a) of the tran- is realized across R3. When the adding a transistor to the current
sistor. See Figure 4. drop across R3 starts to limit circuit. See Figure 7. We still
diode drop, CR4,CR5 and CR6 will will have a high level of power
This circuit operation can be under- dissipation since enough current
stood easily by recognizing that start to conduct, robbing base-
emitter current away from Q1, flowing in R3 to keep Q3
the base o Q1 will have a fairly
f
COnsta tage on it (developed causing it to conduct less. This
across . If the output voltage limits the current flow, and the
output voltage will drop somewhat.
A typical plot of voltage vs current
is shown in Figure 8.
(emitter of Q l ) drops more than
one dlode drop (0.W for silicon,
0.N for germanium) below the
bas& Q1 will tend to turn on
harder, raising the output voltage.
If the output voltage were to rise,
Q1 will be biased off. Thus, the
output voltage is fixed at the
breakdown voltage of CR1 minus
one diode drop (6-E drop of Q ) l.
If this power transistor has a gain
of 1 , the output impedance is
0
now about 1 ohm (that is, 1 m k
change in load current will cause
a 1 mV change in output voltage).
This is sttill too hiah for many clr-
POWER SUPPLY TUTORIAL .-
FOLDBACK RIPPLE dropped across diodes CR7 and
CW8. This limits the B-E current
of Q4, resulting in a constant
Since current limiting comes into Modern electronic circuits require current through CR3.
play when there is a failure (or very clean (ripple-free) power sup-
other abnormality), it may be plies. Looking back at Figure 4, In actuality, the voltage across CR7
advantageous to be able to shut off we see that the output voltage is and CR8 will vary slightly and the
the power supply circuit, rather very dependent on the voltage next evolution would be a constant
than continue to dissipate all that dropped across CR3. But this will current source for them. This can
power. This technique is called vary slightly with the current be done easily with two transistors.
foldback and is shown graphically through it (recall that a breakdown See Figure 12. Transistor Q4
in Figure 9. In the +180V supply diode has a nominal internal resis- receives a constant bias from the
in Figure 10, once the Q2 circuitry tance of about 1052 ). Therefore if drop across Q5. This gives a con-
is turned on, it will continue to the line voltage increases, the stant current through R5, which
conduct, holding Q1 off. voltage on C2 will increase, result- keeps a constant bias on Q5 and
ing in increased current through a constant current through CR3.
CR3 and an increase in the output PROTECTION
voltage. Also the voltage on C2
varies during each rectification Current limiting may not be
cycle. The peak of the rectified sine enough protection for the load.
wave (ripple) will be passed
What happens if there is a failure
through Q1 to some extent. Since in the power supply circuit, such as
even this small variation in the a shorted Q2? The output would
output voltage may be undesirable, rise to the full unregulated voltage.
you may see power supply circuits
with a constant current source If this were the +5 volt supply for
for CR3. See Figure 11. all the IC's in an instrument, most
of the IC's would very likely be
In essence, a constant voltage is destroyed. Therefore, it may be
Notice that the +5V regulator cir-
cuitry is integrated into a TO3
package. While this package looks
like an ordinary power transistor,
it contains the series pass tran-
sistor(s), breakdown reference
diode and associated circuitry
needed for proper regulation.
These fixed voltage regulators can
be purchased for the more com-
monly used power supply voltages
such as 5V and 12V. Some are
available with current limit
capability.
1 * I
* I
i J i -1 1
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SERVICE TIP
c
1
advisable to build in additional age and shorts it to ground if the developing a voltage across R6.
protection in the form of a "crow- voltage rises too high. See Fig. 13. This would cause the SCR to fire,
bar circuit". This is generally an If the. supply voltage rises above shorting out the power supply
SCR that monitors the output volt- 6 volts, CR9 would start to conduct, line (and hopefully blowing a fuse).
ISOLATING POWER
SUPPLY DRIFT
by John Whidden
What is the most effective way to
isolate a drift problem in a power
supply?
To answer that question, let's look
at how a power supply normally
operates.
The programming input to a power
supply consists of a resistive divider
(RR and Rp) connected between
reference zener and -S terminal.
(Refer to figure.) The common point
of RR and R is the input to the
P
amplifier and s called the summing
point. When the power supply is
operating properly, the output
voltage equals the sum of voltage
across Rp plus the offset voltage.
Therefore, variation in either of
these will cause a change in the
output voltage.
Measure and record the following The important thing is not how often
The most effective way to isolate a voltages: the measurements are made but
drift problem is measure and record 1. +s to -s (output) that they are made in the same
the reference supply voltage, the 2. +S to summing (offset) manner and with the same equip-
summing point voltage and the point ment. Usually only 3 or 4 sets of
output voltage at any convenient 3. +S to reference (reference) measurements are necessary to
interval (such as every half hour). zener evaluate the power supply.
-
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SERVICE TIP
I
~ _ _ _ _ _ ..
The following algorithm may be useful to narrow the trouble:
I
Note 1. Clrcult coolant sprayed on Individual 2OPPM resistors will generally cause 0.25 and 0.5% change in the output. A
defective resistor migM cause a 1% to even 25% change.
John Whiddenjoined HP in 1961 and became involved with power supply service and applicationsfor field service and
customer training. John now provides factory back up for field service and prepares service information for instruction
manuals and service notes.
WWW.HPARCHiVE.COM
= 6
ill
I . a 1811A 18 GHz SAMPLER 8Ol2/801SA PULSE GENERATORS @
o
wpplement t 1811A-2. Serial prefix 1309A
Reduced trigger circuit lock-up.
191y VARIABLE TRANSITION
TIME OUTPUT
and below. 8012A-4. All serials. Recommended replace-
ments.
8013A-4. All serials. Recommended replace-
ments.
1915A-12A. %rial prefix 1143 and above. M45A AUTOMATIC PRESELECTOR
Three preferred replacements. Supersedes Replacement YIG filter
8445A-1. All serials.
1915A-12. assembly.
9330AIB AUTOMATIC SYNTHESIZER W l N B SPECTRUM ANALYZER
333ONB-5. All serials. Replacement part RF SECTION
numbers for LED displays. 8551NB-9. All serials. RF input attenuator
3403A/C TRUE RMS VOLTMETER replacements.
3403C-3. Part numbers 5060-9131/32/33. 85928 SPECTRUM ANALYZER, IF SECTION
Replacement part numbers for LED displays. 8552B-9. Serial number 1437A07061 to
NEED ANY SERVICE 346WB MULTIMETER
34698-3. Part numbers 5080-9131/32/33.
1437A07210. Prevention of power supply
failures due t o shorted series regulators 0 2 3
and 024.
NOTES? Replacement part numbers for LED displays.
85951\ SPECTRUM ANALYZER
348OC/D DIGITAL VOLTMETER 8555A-3. Serial number 1434A04235 and below.
348OC/D-1. All serials. Replacement part Improved 500 MHz local oscillator.
numbers for LED displays.
Here's the latest listing of Service 8805A COMMUNICATION
Notes available for Hewlett- 91ooA MULTIMETER SWEEP OSCILLATOR
349OA-9. All serials. Replacement part numbers 8605A-4. Serial prefix 1-A and below.
Packard products. To obtain infor- for LED displays. Replacement of A2 and A3 YIG oscillator
mation for instruments you own, 357M NETWORK ANALYZER
driver assemblies.
remove the order form and mail it 357OA-5. All serials. Replacement part numbers W B SWEEP OSCILLATOR
for LED displays. 8620A-6. Serial prefix 1332A and below. Modi-
to the HP distribution center nearest fication required for 86290A. 2.0-18.0 GHz
you. 357SA GAIN PHASE-METER plu in compatibility.
3575A-2. Part numbers 5060-9131/32/33. 8620g3. All serials. Modification required for
Replacement part numbers for LED displays. 862WA. 2.0-18.0 GHz plug-in compatibility.
389OA INSTRUMENTATION OIOAIB AM-FM SIGNAL GENERATOR
TAPE RECORDERS B64OA-llN884OB-12A. All serials. RF on/off
369OA-24. Serial prefix 1422Aonly. Option 050 switch modification.
2118 SQUARE WAVE GENERATOR (remote control) wiring error.
WdOlvB SYNTHESIZED
2118-3. All serials. Improved reliability of SOSSA DIGITAL RECORDER SIGNAL GENERATOR
600 ohm output. 5055A-2. Serial number 1316AO2635 and below. 868oA-24. Serial numbers below 1145Aoo891.
425AIAR DC MICROVOLT-AMMETER Transistor change to prevent failures in A2 Digital IC. replacement.
driver board assembly. 8680A-25. All serials. Mainframe calibration
425A-7. Elimination of potential shock hazard. quick check.
5477A SYSTEM CONTROL 86608-23. Serial number 1439A00960 and
1208A DISPLAY 54518-7/5477A-1. All serials. Field preventive
H11-1208A-9. All serials. Preferred replace- below. 1820-0450 I.C. replacement.
maintenance procedure. 86806-25. All serials. ROM input assembly
ment for A603. compatibility.
SSOOA/B LASER HEAD
122OA DUAL CHANNEL 5525AlB-3. All serials. Laser tube safety. 86608-26. All serials. Mainframe calibration
OSCILLOSCOPE. 15 MHz quick check.
8940/8941A MULTIPROGRAMMER
122OA-13. All serials. Troubleshooting tips. 694044-1. Serial number 1321A00476 and SMSACALCULATORCARDREADERS
122OA-14. Serial numbers 1416A02656 to below. Added protection on the -12V line. 9869A-5. Serial prefixes below 1434A. New
1416A02760. Sweep problems in the 1 &sec 6941A-1. Serial number 1242A00290 and calculator I/O and mother boards.
to 50 sec range. below. Added protection on the -12V line. 9869A-6. Serial prefixeg below 1444A. -19V
1220A-15. All serials. Insulator caps. regulator change.
1220A-16. All serials. Service kit. 7130A/B and 7131AlB
1220A-17. Serial numbers below 1416A03150. STRIP CHART RECORDERS 10230AIB PROBES
H.V. board modification. 1023ONB-1. Replacement wire kit 10231-
7130NB-2, 7131NB-2. All serials. Disposable 68702. Supersedes 1Wl A-2.
122OA-18. All serials. Antistatic solution. pen conversions. 10231NB-1. Replacement wire kit 10231-
1221A SINGLE CHANNEL 68702. Supersedes 1601A-2.
OSCILLOSCOPE, 15 MHz 7155A PORTABLE
1221A-5. All serials. Troubleshooting tips. STRIP CHART RECORDERS 1057SA STRAIGHTNESS ADAPTER
1221A-6. All serials. Service kit. 7155A-1. Serial prefix 1432 and below. 10579A-1. Serial prefix 1328 only. Improved
1221A-7. All serials. Antistatic solution. Battery charging circuit modification to p r e frequency multiplier board.
vent possible oscillation. 1166lA FREOUENCY EXTENSION MODULE
1308A EIGHT CHANNEL MONITOR 7155A-2. All serials. Modification to Option 005 11661A-5A. Serial prefix 1431A and below.
1308A-9. All serials. Preferred replacements right hand zero. Im roved sum loop locking. Supersedes
for A4Q3. A4Q4. A4Q9, and A4010. 11klA-5.
7200/7202/1203A GRAPHIC PLOTTERS 11661A-6. Serial prefix 143OA and below.
1309A X-Y MONITOR 7200A-12n202A-l1/7203A-11. All seriajs.
1309A-9. All serials. Preferred replacements YIG pretune improvement.
- for A4Q3, A404, A409, and A4010.
EIA RS232C interfacing instructions and 11661A-7. Serial prefix 1433A and below.
information. Improved cooling.
1331AIC X-Y DISPLAY, STORAGE 7200A-13/7202A-12. All serials. Correct charac-
ter register (A3), servo preamplifier (A7), and 29400 SERIES RACK CABINETS
1331NC-12. 1331A serial prefix 1424A through en control assembly (A8) board combinations 2940&2. Instrument slide alignment.
1448A; 1331C serial prefix 1426A and below.
Intermittent erasing. Supersedes 1331C-3.
Por proper plotter operation.
7203A-10. Serial prefixes before 1433A. Com- 34703A DCVIDCAIOHM METER
1331NC-13. 1331A serial prefix 1424A and ponent change on interface board. 34703A-4. Serial numbers below 1251A015.00.
below; 1331C serial prefix 1426A and below. Improvement of reliability when measuring
Modification to add pattern correction adjust- 7210A DIGITAL PLOTTER high volta e inputs.
ment. 721OA-12. All serials. Interface/conversion kit 34703A-5. gerial numbers below 1251A01500.
1331NC-14. All serials. Intermittent blowing for operating 721OA plotters with HP 2100 Elimination of potential intermittent logic
of +50v fuse. series computers. problems.
181M 1 GHz DUAL CHANNEL SAMPLER 72doA OPTICAL MARK READERS 8(1601/888Ou RF SECTION FOR 8660NB
1810A-2. Serial prefix 1308A and below. 7260A-9. Serial Drefixes before 1436A. New A4 86601A-7. Serial number 1443A00781 and
Reduced minimum delay drift. serial interface board. below. Attenuator driver assembly.
1810A-3. Serial prefix 1308A and below. 7280N7261A-10. Serial prefixes before 1444A. 86602A-6. Serial number 1444AO1011 and
Reduced trigger circuit lock-up. -19V regulator change. below. Attenuator driver assembly.
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IEE
1 8 1
articles heve allowed expansion of the use as
now I know WHY it works as it does.
Sincerely,
J.R. Chamberlin
R7 School District
Lee's Summit, Missouri
hfe'.rarely deterip-ate8 as it is primarily set
by t4e doping ratios which do not change
easfly. However, teakage can change which
Causes an operating point shift. Also, hfe
decreases with frequency and this is why the
data sheet value is given at a specific fre-
quency. Any external (or internal) capa-
citance change will thus cause a change in
this,high frequency value of hfe.
Here's your c h a m to share your Ideas and rhm wlth other Bench Brie[? rmclplmts. In. I
Reader's Comer, m wlll print Ietkn to the Edltor, troubkrhooting tips, mqRllcaUon Infob George Stanley
mation, and new took and products that have made your Jobnrkr. In *or$ Redder'sCemer
will feature anything from madem that b of g e n d lntmrt to ektronlc ~ I c p.rrannd.
p More on Transistor Checker
The resistance values for the transistor
If them Is romothlng you have to share with other Bench Briefs checker (Figure
December issue
those shown in
Mr. Stanley: issue. The two circ
cal for all practical purposes. In one case,
The transistor checker (Sept-Oct 7 4 and the switch shorts,out a resistor and in the
Nov-Dec 74) can also be used for impedance other case it ad* a resistor in paritllel.
matching by the following alteration: Incidentally, in the November-December
issue, the output voltage of t& trensfohw
Also by use of AF m RF generator, I assume
the checker can be used to show faulty tran-
(Figure 8) is not shown. It is 6 wdts AC
same as in the September-October issue.
the ---
sistors which instead of bad junctions the htn
has deteriorated. I am going to pursue this We're thinking of runnin
but my is Only OOd to lo M c which ac#t.bdiode), and any other applications
on the interpretation
waveforms. Some are
07
handicaps my results. perhaps you might try vj r p n t i n u i t y is required. especially when doing inc
it.
Aftqir locatin the area of the failure, I imme- an out-of-circuit germanium power tran-
sistor can show an unusual collector-base
Incidentally, the checker obviously can be I diately switca to the checker and appear to waveform because d its relatively high
used to locate bad connections, open PC'. cut Servicing by at least 75% total time. /CEO leakage current.
patterns, open lamps, poor solder connec- I found the circuit in Radio-Electronics
tions (especially if they exhibit resistance or magazine about 3 years ago, but your George Stanley
I I Bulk Rate
U.S. Postage
Addrers Correction Requested Printed in U.S.A.
All rtghtr are rawwed. No part of Bench Brlefa may be reproduced without the express consent of the Edltor. The Edltor may be telephoned at (415) 493-3301, Extenalon 314.
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