e022032.pdf

TEST &MEASUREMENT

High Power

Adjustable Load

with power limiter

Design by P. Hirschbrich

An adjustable load resistance is indispensable for the realistic testing of

power output stages. This circuit provides an electronic alternative to the

use of inconvenient high-wattage resistors.

IC1.D and the surrounding compo-

nents.

The load circuit consists of a

power FET and resistor R2. An elec-

trolytic capacitor is connected in par-

allel with these, smoothing the volt-

age across the FET and R2 and sup-

pressing spikes. The FET is

controlled by IC1.A. The op-amp

attempts to keep the voltage at its

inverting input (that is, the voltage

across R2) the same as that on its

non-inverting input. The latter is in

turn equal to the voltage on the

wiper of 10-turn potentiometer P1.

We have therefore achieved what we

sought to achieve: the current drawn

is proportional to the wiper setting

of P1. This means, of course, that the

wiper voltage of the potentiometer

must be kept constant, and this is

guaranteed by the 2.5 V voltage ref-

erence D7. At the upper limit of the

potentiometer’s travel we have a

voltage of 0.779 V (because of R3),

assuming that S1 is open.

Only the ﬁrst ﬁve turns of the ten-

turn potentiometer can be used

before the power limiter comes into

effect. If the current sink is to be

used within its power limits, a

smaller range of voltages across R2

is more useful: this is the function of

R12 and trimmer P2. When S1 is

closed these are connected in paral-

lel with P1 and are adjusted so that

the voltage is reduced by a factor of

ten. This allows a current setting in

the range 0 A to more than 5 A.

If S2 is switched to position ‘CR’

(constant resistance), a different

voltage is applied across P1. The

source is no longer provided by the

voltage reference but by the input

voltage. The remainder of the circuit

works as before, meaning that the

input current is proportional to the

input voltage, and the constant of

proportionality is equal to the con-

stant resistance. With R4 being

475 kΩ

Technical

specifications

Input voltage:

0 to +100 V DC

Sink current:

0 to 20 A

Load resistance:

to >100 k

Maximum

power dissipation:

approximately 100 W

Linearity error:

<0.5 %

The circuit described here has two modes of

operation. It can function as an adjustable

current sink or as a variable load resistance.

At a voltage of between 0 V and a maximum

100 V the sink current can be set in the range

0A to 20 A. As a load resistance it can be set

to values from less than 1

load resistances from inﬁnity

to 1

(at the extreme settings of P1)

can be achieved.

to more than

Power Limiter

The remaining three op-amps in the

LM348 form a power limiter to

ensure that the power dissipation of

the circuit remains under control. An

analogue multiplier is not used in

this circuit. R10 and R11 form a volt-

age divider across the input voltage,

which operates linearly as long as

diode D2 does not conduct. This

does not happen abruptly, but rather

follows the characteristic curve of

the diode, providing a gradual tran-

sition as the input voltage rises.

IC1.C buffers this voltage and drives

100 k

. The maximum power dissipation is in

the region of 100 W. The circuit is designed

for operation with DC voltages, but with the

addition of a rectifier can also be used with

(low frequency) AC voltages. A carefully con-

structed and calibrated unit can deliver a lin-

earity of better than 0.5 %.

Current Sink

and Load Resistance

Let us first consider the circuit as a current

sink (S2 in position ‘CC’: constant current) ini-

tially ignoring the part of the circuit consist-

ing of the three op-amps IC1.B, IC1.C and

Elektor Electronics

2/2002

TEST &MEASUREMENT

22k1

+15V

0...+100V

R11

0...20A max.

100W max

2n2

R13

D 6

rot

rood

red

rouge

IRF1310N

10k

IC1.A

1N4148

1k1

IC1.C

250

Ω

IC1.D

2n2

100 µ

200V

R14

10k

IC1.B

D 5

grün

groen

green

vert

10T

20k

R10

R12

1N4148

10W

TL431CLP

000088 - 11

TR1

5V1

160mA T

IC2

IRF1310N

TL431CP

+15V

7 815

IC1

VR11

470 µ

35V

4 µ 7

35V

15V...18V

≥

50mA

IC1 = LM348N

Figure 1. An op-amp, a resistor and a FET form the basis of this item of test equipment.

inverter IC1.D, which inverts the

characteristic curve of the diode and

amplifies it with a gain of ten. The

output voltage of the op-amp is

raised by an amount equal to the

diode forward voltage via D3.

The operating point of D3 is deter-

mined by the reference voltage, R10

and the setting of trimmer P3.

Finally, comparator IC1.B compares

the output voltage of the inverter

with the voltage across R2 and, if the

power limit is exceeded, pulls down

the control voltage to op-amp IC1.A,

turns off the green ‘OK’ LED and

lights the red ‘warning’ LED.

Note that this is only a crude pro-

tection circuit for limiting the power

above about 100 W. It is highly

dependent on device-to-device vari-

ations among diodes and on temper-

ature, but is entirely adequate for

protecting against excessive current

draw from, for example, a 12 V car

battery.

The circuit as a whole is powered

from a mains power supply, which as

usual consists of a mains trans-

former (15 V to 18 V, at least 50 mA),

a bridge rectifier and a 15 V fixed

voltage regulator.

C2 smoothes the rectiﬁed DC volt-

age and C3 suppresses transients.

Since the op-amps are operating

near to the lower input voltage, a

negative supply is required for them

(unless rail-to-rail types are resorted

to). To this end Zener diode D1 pro-

vides a negative supply voltage

about 5 V below the lower input

voltage.

using a large heat sink. The data sheet for the

FET indicates that a heatsink rated at

0.9 K/W or better is required. Alternatively,

the FET can be ﬁtted with a modern CPU fan.

The circuit is intended for hobby use or for

short laboratory tests, and not for continuous

use (for example 24 hour soak tests). If addi-

tional thermal protection for the transistor is

desired, a thermal relay (closing at 105 °C)

can be fitted. The relay is glued to the tran-

sistor using two-part adhesive, and wired so

as to short pin 3 of IC1.A to the lower input

potential.

If you do not wish to fit a fan, but never-

theless want to operate either at high loads

or continuously, then you can connect up to

five FETs in parallel (for example type

BUZ344). The circuit also works well with

150 W to 200 W power Darlingtons such as

the MJ11016 in a TO3 package, but not in a

parallel arrangement and only with input

voltages above 1 V.

R2 also plays its part in dissipating power.

Either a 15 W power resistor should be ﬁtted,

a few millimetres above the surface of the cir-

cuit board, or a 10 W power resistor in a

metal housing with heatsink can be used.

The FET, the power resistor R2 and elec-

Construction

and Calibration

Since we have not shown a printed

circuit board layout for this circuit, a

few words on construction are in

order. The circuit presents no enor-

mous technical difficulties, and sim-

ple prototyping board will suffice for

construction, but a suitably heavy-

duty enclosure must be found. The

FET must be able to dissipate up to

about 85 W under peak conditions,

and the heat must be carried away

2/2002

Elektor Electronics

TEST &MEASUREMENT

trolytic C4 should be placed next to one

another centrally on the rear side of the (hor-

izontally-mounted) circuit board, with the two

high-current terminals (a suitable type is

available from AMP) on either side. The com-

ponents should be connected with thick wire,

taking care not to overheat the components

when soldering.

The control electronics should be placed

immediately next to the FET, and in particu-

lar the connection between the output of the

control op-amp IC1.A and the gate of the FET

should be short.

Since the electronics are ﬁtted at the rear

of the enclosure, the components ﬁtted to the

front panel (S1, S2, P1, the LEDs and the two

banana sockets) must be wired to the circuit

board. For the input we have already sug-

gested AMP connectors, while for the remain-

ing — low power — connections ordinary sig-

nal wire will do.

The power supply can be ﬁtted in

a spare corner of the circuit board.

The mains enters via a switched and

fused input socket, which avoids

having the mains switch on the front

panel. LED D5 serves as a power

indicator.

In the interests of safety, the ‘thin’

wiring should be kept well sepa-

rated from the power supply and

from the thick input wiring.

Check over the circuit once more,

ﬁt it into the enclosure and check the

wiring and that the heatsink is iso-

lated from the FET (!), and switch on

at the mains. Assuming that no

smoke appears, and that the supply

and reference voltages are correct,

the unit can be switched off again

and the LM348 fitted in its socket.

The control circuit can now be cali-

brated.

Turn the unit on again and apply

a voltage of 10 V to the input. The

voltage at pin 13 of the IC should be

the same as that on pin 12 (P3). At

the output of IC1.D a voltage of

0.95 V should be measured, and we

want the current through the FET to

be limited to about 10 A. Adjust P4

to set the voltage on pin 5 to 200 mV.

This corresponds to a current of 10 A

at 10 V (or 100 W). You will ﬁnd that

a little patience is required in adjust-

ing P3 and P4. The adjustment of P2

has been described above. The cali-

bration is now done: ﬁt the lid to the

enclosure and your unit is complete.

(000088-1)

Elektor Electronics

2/2002

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