HCPL-7860 datasheet(16/18 Pages) HP

16 / 18 page

Table 3. Isotek (Isabellenhütte) Four-Terminal Shunt Summary.

Note: Values in brackets are with a heatsink for the shunt.

Shunt Resistor

Part Number

Shunt Resistance

Tol.

Maximum RMS

Current

Motor Power Range

120 Vac-440 Vac

Ω

PBV-R050-0.5

0.5

0.8 - 3

0.6 - 2

PBV-R020-0.5

0.5

2 - 7

0.6 - 2

PBV-R010-0.5

0.5

4 - 14

3 - 10

PBV-R005-0.5

0.5

25 [28]

7 - 25 [8 - 28]

5 - 19 [6 - 21]

PBV-R002-0.5

0.5

39 [71]

11 - 39 [19 - 71]

8 - 29 [14 - 53]

For a two-terminal shunt, as

the value of shunt resistance

decreases, the resistance of the

leads becomes a significant

percentage of the total shunt

resistance. This has two

primary effects on shunt

accuracy. First, the effective

resistance of the shunt can

become dependent on factors

such as how long the leads

are, how they are bent, how

far they are inserted into the

board, and how far solder

wicks up the lead during

assembly (these issues will be

discussed in more detail

shortly). Second, the leads are

typically made from a material

such as copper, which has a

much higher tempco than the

material from which the

resistive element itself is

made, resulting in a higher

tempco for the shunt overall.

Both of these effects are

eliminated when a four-

terminal shunt is used. A four-

terminal shunt has two

additional terminals that are

Kelvin-connected directly

across the resistive element

itself; these two terminals are

used to monitor the voltage

across the resistive element

while the other two terminals

are used to carry the load

current. Because of the Kelvin

connection, any voltage drops

across the leads carrying the

load current should have no

impact on the measured

voltage.

Several four-terminal shunts

from Isotek (Isabellenhütte)

suitable for sensing currents in

motor drives up to 71 Arms

(71 hp or 53 kW) are shown

in Table 3; the maximum

current and motor power

range for each of the PBV

series shunts are indicated.

For shunt resistances from 50

Ω down to 10 mΩ, the

maximum current is limited by

the input voltage range of the

isolated modulator. For the 5

Ω and 2 mΩ shunts, a heat

sink may be required due to

the increased power

dissipation at higher currents.

When laying out a PC board

for the shunts, a couple of

points should be kept in mind.

The Kelvin connections to the

shunt should be brought

together under the body of the

shunt and then run very close

to each other to the input of

the isolated modulator; this

minimizes the loop area of the

connection and reduces the

possibility of stray magnetic

fields from interfering with the

measured signal. If the shunt

is not located on the same PC

board as the isolated

modulator circuit, a tightly

twisted pair of wires can

accomplish the same thing.

Also, multiple layers of the PC

board can be used to increase

current carrying capacity.

Numerous plated-through vias

should surround each non-

Kelvin terminal of the shunt to

help distribute the current

between the layers of the PC

board. The PC board should

use 2 or 4 oz. copper for the

layers, resulting in a current

carrying capacity in excess of

20 A. Making the current

carrying traces on the PC

board fairly large can also

improve the shunt’s power

dissipation capability by acting

as a heat sink. Liberal use of

vias where the load current

enters and exits the PC board

is also recommended.

HCPL-7860 Datasheet(PDF) 16 Page - Agilent(Hewlett-Packard)

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