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Metal bar on DMM PCB

Is this a cheap Chinese fuse or a current shunt?Are these two Arduino Pro Micros jumpered correctly?What is this component with metal tabs?What are these metal plates covering parts of PCBs called?What are those small metal bars going across a PCB for?What is this connector? Round 8 pins, 5 notches on metal shellHow to control this 72-LED light bar PCBIdentification of SMD componentWhat is up with this fuse?Need help identifying replacement components on PCBAdding SMA conectors to a device whose metal casing is connected to earth ground through at a single point

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12

1

$begingroup$

What is this large metal bar inside my DMMs? One of them labeled it ST. They appear to be connected to the COM port or fuse. Is this just a big jumper for the 10 A ammeter?

pcb identification

share|improve this question

asked May 30 at 19:14

EricEric

6311 silver badge55 bronze badges

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Yet another picture of a shunt in a DMM.
  $endgroup$
  – Nick Alexeev♦
  May 31 at 20:35
  

  
  
  
  

add a comment
 | 

12

1

$begingroup$

What is this large metal bar inside my DMMs? One of them labeled it ST. They appear to be connected to the COM port or fuse. Is this just a big jumper for the 10 A ammeter?

pcb identification

share|improve this question

asked May 30 at 19:14

EricEric

6311 silver badge55 bronze badges

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Yet another picture of a shunt in a DMM.
  $endgroup$
  – Nick Alexeev♦
  May 31 at 20:35
  

  
  
  
  

add a comment
 | 

$begingroup$

What is this large metal bar inside my DMMs? One of them labeled it ST. They appear to be connected to the COM port or fuse. Is this just a big jumper for the 10 A ammeter?

pcb identification

share|improve this question

asked May 30 at 19:14

EricEric

6311 silver badge55 bronze badges

$endgroup$

share|improve this question

asked May 30 at 19:14

EricEric

6311 silver badge55 bronze badges

share|improve this question

asked May 30 at 19:14

EricEric

6311 silver badge55 bronze badges

asked May 30 at 19:14

EricEric

6311 silver badge55 bronze badges

asked May 30 at 19:14

EricEric

6311 silver badge55 bronze badges

3 Answers
3

active

oldest

votes

28

$begingroup$

That is not a simple jumper.

That is the precision resistor used to measure the current. This is also known as a "shunt" - hence the designation ST for shunt.

You measure current by passing it through a known resistance and measuring the voltage across that resistor. Using Ohm's law, you can calculate the current from the voltage and the resistance.

If you look closely, you will see that one of them has been trimmed by making nicks in the wire. That changes the resistance slightly. You measure a known current with a new meter, then whack on the shunt to make your new meter display the known current.

The thick ones like that are usually for the 10A range. The lower current shunts are usually small, precision resistors on the board.

share|improve this answer

edited May 30 at 19:57

answered May 30 at 19:19

JREJRE

28.9k77 gold badges5353 silver badges9292 bronze badges

$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  +1 Looks like they might have added a bit of solder to reduce the resistance on the left one, perhaps overshot with the nick. They're usually solid Constantan wire, which is solderable and has a reasonably low tempco.
  $endgroup$
  – Spehro Pefhany
  May 31 at 18:32
  
  

  
  
  
  

add a comment
 | 

8

$begingroup$

It is the current shunt.

Your meter probably has a 200 mV full scale range and will read 10.00 A with 100 mV voltage drop across the shunt. From Ohm's Law we can calculate that the shunt resistance = V/I = 0.1/10 = 0.01 Ω.

A decent meter will have a proper fuse protecting the shunt. The fuses in your photo look too small so be very careful.

share|improve this answer

answered May 30 at 19:20

TransistorTransistor

101k99 gold badges104104 silver badges226226 bronze badges

$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  The cheap Micronta 22-181B meter I own has an unfused shunt on a separate input jack. The jack is clearly labelled 10A MAX UNFUSED on meter to make you think twice about what circuit you're going to put it in series with. (There's also a separate jack for the 400/40mA current input, on a different fuse from the 4mA / volatage / resistance / capacitance / diode-test input.) Anyway, probably the OP's meter has an unfused 10A shunt.
  $endgroup$
  – Peter Cordes
  May 31 at 11:07
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Both the OPs meters appear to have positions for two fuses, suggesting both current inputs are fused. In the right hand picture you can even see the fuseholder labeled as 10A. Whether those fuses (especially the glass ones) are adequate to protect the meter and it's user in the event of reasonablly forseeable misuse (i.e. forgetting the probe is plugged into the current socket and then trying to measure mains voltage) is highly questionable though.
  $endgroup$
  – Peter Green
  May 31 at 13:49
  
  

  
  
  
  

add a comment
 | 

2

$begingroup$

It is a hollow copper tubing or a "cheap & dirty" 1% current shunt resistor for measuring current on the 10A using voltage (specifically mV).

Here's a Murata 0.25% current shunt:

It costs $20.
See the difference?

Due to the Positive Transfer Coefficient (PTC) characteristic of metal conductors, heat causes the resistance to increase and yield a false rise in voltage sensed as a current. Generally, voltage drops for current sensing are limited to 50mV for this reason. Additional heatsinks may increase this limit.

share|improve this answer

edited May 31 at 17:10

nabulator

44211 gold badge33 silver badges1515 bronze badges

answered May 30 at 20:53

Tony Stewart Sunnyskyguy EE75Tony Stewart Sunnyskyguy EE75

82k22 gold badges3131 silver badges119119 bronze badges

$endgroup$

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  $begingroup$
  Proper current shunts are made of alloys like Constantan or Manganin that have near zero temperature coefficient over their normal working temperature range, though they can suffer resistance changes if overheated. riedon.com/media/pdf/RS.pdf
  $endgroup$
  – Phil G
  May 31 at 13:56
  

  
  
  
  

add a comment
 | 

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28

$begingroup$

That is not a simple jumper.

That is the precision resistor used to measure the current. This is also known as a "shunt" - hence the designation ST for shunt.

You measure current by passing it through a known resistance and measuring the voltage across that resistor. Using Ohm's law, you can calculate the current from the voltage and the resistance.

If you look closely, you will see that one of them has been trimmed by making nicks in the wire. That changes the resistance slightly. You measure a known current with a new meter, then whack on the shunt to make your new meter display the known current.

The thick ones like that are usually for the 10A range. The lower current shunts are usually small, precision resistors on the board.

share|improve this answer

edited May 30 at 19:57

answered May 30 at 19:19

JREJRE

28.9k77 gold badges5353 silver badges9292 bronze badges

$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  +1 Looks like they might have added a bit of solder to reduce the resistance on the left one, perhaps overshot with the nick. They're usually solid Constantan wire, which is solderable and has a reasonably low tempco.
  $endgroup$
  – Spehro Pefhany
  May 31 at 18:32
  
  

  
  
  
  

add a comment
 | 

28

$begingroup$

That is not a simple jumper.

That is the precision resistor used to measure the current. This is also known as a "shunt" - hence the designation ST for shunt.

You measure current by passing it through a known resistance and measuring the voltage across that resistor. Using Ohm's law, you can calculate the current from the voltage and the resistance.

If you look closely, you will see that one of them has been trimmed by making nicks in the wire. That changes the resistance slightly. You measure a known current with a new meter, then whack on the shunt to make your new meter display the known current.

The thick ones like that are usually for the 10A range. The lower current shunts are usually small, precision resistors on the board.

share|improve this answer

edited May 30 at 19:57

answered May 30 at 19:19

JREJRE

28.9k77 gold badges5353 silver badges9292 bronze badges

$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  +1 Looks like they might have added a bit of solder to reduce the resistance on the left one, perhaps overshot with the nick. They're usually solid Constantan wire, which is solderable and has a reasonably low tempco.
  $endgroup$
  – Spehro Pefhany
  May 31 at 18:32
  
  

  
  
  
  

add a comment
 | 

$begingroup$

That is not a simple jumper.

That is the precision resistor used to measure the current. This is also known as a "shunt" - hence the designation ST for shunt.

You measure current by passing it through a known resistance and measuring the voltage across that resistor. Using Ohm's law, you can calculate the current from the voltage and the resistance.

If you look closely, you will see that one of them has been trimmed by making nicks in the wire. That changes the resistance slightly. You measure a known current with a new meter, then whack on the shunt to make your new meter display the known current.

The thick ones like that are usually for the 10A range. The lower current shunts are usually small, precision resistors on the board.

share|improve this answer

edited May 30 at 19:57

answered May 30 at 19:19

JREJRE

28.9k77 gold badges5353 silver badges9292 bronze badges

$endgroup$

share|improve this answer

edited May 30 at 19:57

answered May 30 at 19:19

JREJRE

28.9k77 gold badges5353 silver badges9292 bronze badges

answered May 30 at 19:19

JREJRE

28.9k77 gold badges5353 silver badges9292 bronze badges

answered May 30 at 19:19

JREJRE

28.9k77 gold badges5353 silver badges9292 bronze badges

8

$begingroup$

It is the current shunt.

Your meter probably has a 200 mV full scale range and will read 10.00 A with 100 mV voltage drop across the shunt. From Ohm's Law we can calculate that the shunt resistance = V/I = 0.1/10 = 0.01 Ω.

A decent meter will have a proper fuse protecting the shunt. The fuses in your photo look too small so be very careful.

share|improve this answer

answered May 30 at 19:20

TransistorTransistor

101k99 gold badges104104 silver badges226226 bronze badges

$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  The cheap Micronta 22-181B meter I own has an unfused shunt on a separate input jack. The jack is clearly labelled 10A MAX UNFUSED on meter to make you think twice about what circuit you're going to put it in series with. (There's also a separate jack for the 400/40mA current input, on a different fuse from the 4mA / volatage / resistance / capacitance / diode-test input.) Anyway, probably the OP's meter has an unfused 10A shunt.
  $endgroup$
  – Peter Cordes
  May 31 at 11:07
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Both the OPs meters appear to have positions for two fuses, suggesting both current inputs are fused. In the right hand picture you can even see the fuseholder labeled as 10A. Whether those fuses (especially the glass ones) are adequate to protect the meter and it's user in the event of reasonablly forseeable misuse (i.e. forgetting the probe is plugged into the current socket and then trying to measure mains voltage) is highly questionable though.
  $endgroup$
  – Peter Green
  May 31 at 13:49
  
  

  
  
  
  

add a comment
 | 

8

$begingroup$

It is the current shunt.

Your meter probably has a 200 mV full scale range and will read 10.00 A with 100 mV voltage drop across the shunt. From Ohm's Law we can calculate that the shunt resistance = V/I = 0.1/10 = 0.01 Ω.

A decent meter will have a proper fuse protecting the shunt. The fuses in your photo look too small so be very careful.

share|improve this answer

answered May 30 at 19:20

TransistorTransistor

101k99 gold badges104104 silver badges226226 bronze badges

$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  The cheap Micronta 22-181B meter I own has an unfused shunt on a separate input jack. The jack is clearly labelled 10A MAX UNFUSED on meter to make you think twice about what circuit you're going to put it in series with. (There's also a separate jack for the 400/40mA current input, on a different fuse from the 4mA / volatage / resistance / capacitance / diode-test input.) Anyway, probably the OP's meter has an unfused 10A shunt.
  $endgroup$
  – Peter Cordes
  May 31 at 11:07
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Both the OPs meters appear to have positions for two fuses, suggesting both current inputs are fused. In the right hand picture you can even see the fuseholder labeled as 10A. Whether those fuses (especially the glass ones) are adequate to protect the meter and it's user in the event of reasonablly forseeable misuse (i.e. forgetting the probe is plugged into the current socket and then trying to measure mains voltage) is highly questionable though.
  $endgroup$
  – Peter Green
  May 31 at 13:49
  
  

  
  
  
  

add a comment
 | 

$begingroup$

It is the current shunt.

Your meter probably has a 200 mV full scale range and will read 10.00 A with 100 mV voltage drop across the shunt. From Ohm's Law we can calculate that the shunt resistance = V/I = 0.1/10 = 0.01 Ω.

A decent meter will have a proper fuse protecting the shunt. The fuses in your photo look too small so be very careful.

share|improve this answer

answered May 30 at 19:20

TransistorTransistor

101k99 gold badges104104 silver badges226226 bronze badges

$endgroup$

share|improve this answer

answered May 30 at 19:20

TransistorTransistor

101k99 gold badges104104 silver badges226226 bronze badges

answered May 30 at 19:20

TransistorTransistor

101k99 gold badges104104 silver badges226226 bronze badges

answered May 30 at 19:20

TransistorTransistor

101k99 gold badges104104 silver badges226226 bronze badges

2

$begingroup$

It is a hollow copper tubing or a "cheap & dirty" 1% current shunt resistor for measuring current on the 10A using voltage (specifically mV).

Here's a Murata 0.25% current shunt:

It costs $20.
See the difference?

Due to the Positive Transfer Coefficient (PTC) characteristic of metal conductors, heat causes the resistance to increase and yield a false rise in voltage sensed as a current. Generally, voltage drops for current sensing are limited to 50mV for this reason. Additional heatsinks may increase this limit.

share|improve this answer

edited May 31 at 17:10

nabulator

44211 gold badge33 silver badges1515 bronze badges

answered May 30 at 20:53

Tony Stewart Sunnyskyguy EE75Tony Stewart Sunnyskyguy EE75

82k22 gold badges3131 silver badges119119 bronze badges

$endgroup$

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  $begingroup$
  Proper current shunts are made of alloys like Constantan or Manganin that have near zero temperature coefficient over their normal working temperature range, though they can suffer resistance changes if overheated. riedon.com/media/pdf/RS.pdf
  $endgroup$
  – Phil G
  May 31 at 13:56
  

  
  
  
  

add a comment
 | 

2

$begingroup$

It is a hollow copper tubing or a "cheap & dirty" 1% current shunt resistor for measuring current on the 10A using voltage (specifically mV).

Here's a Murata 0.25% current shunt:

It costs $20.
See the difference?

Due to the Positive Transfer Coefficient (PTC) characteristic of metal conductors, heat causes the resistance to increase and yield a false rise in voltage sensed as a current. Generally, voltage drops for current sensing are limited to 50mV for this reason. Additional heatsinks may increase this limit.

share|improve this answer

edited May 31 at 17:10

nabulator

44211 gold badge33 silver badges1515 bronze badges

answered May 30 at 20:53

Tony Stewart Sunnyskyguy EE75Tony Stewart Sunnyskyguy EE75

82k22 gold badges3131 silver badges119119 bronze badges

$endgroup$

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  $begingroup$
  Proper current shunts are made of alloys like Constantan or Manganin that have near zero temperature coefficient over their normal working temperature range, though they can suffer resistance changes if overheated. riedon.com/media/pdf/RS.pdf
  $endgroup$
  – Phil G
  May 31 at 13:56
  

  
  
  
  

add a comment
 | 

$begingroup$

It is a hollow copper tubing or a "cheap & dirty" 1% current shunt resistor for measuring current on the 10A using voltage (specifically mV).

Here's a Murata 0.25% current shunt:

It costs $20.
See the difference?

Due to the Positive Transfer Coefficient (PTC) characteristic of metal conductors, heat causes the resistance to increase and yield a false rise in voltage sensed as a current. Generally, voltage drops for current sensing are limited to 50mV for this reason. Additional heatsinks may increase this limit.

share|improve this answer

edited May 31 at 17:10

nabulator

44211 gold badge33 silver badges1515 bronze badges

answered May 30 at 20:53

Tony Stewart Sunnyskyguy EE75Tony Stewart Sunnyskyguy EE75

82k22 gold badges3131 silver badges119119 bronze badges

$endgroup$

share|improve this answer

edited May 31 at 17:10

nabulator

44211 gold badge33 silver badges1515 bronze badges

answered May 30 at 20:53

Tony Stewart Sunnyskyguy EE75Tony Stewart Sunnyskyguy EE75

82k22 gold badges3131 silver badges119119 bronze badges

edited May 31 at 17:10

nabulator

44211 gold badge33 silver badges1515 bronze badges

edited May 31 at 17:10

nabulator

44211 gold badge33 silver badges1515 bronze badges

answered May 30 at 20:53

Tony Stewart Sunnyskyguy EE75Tony Stewart Sunnyskyguy EE75

82k22 gold badges3131 silver badges119119 bronze badges

answered May 30 at 20:53

Tony Stewart Sunnyskyguy EE75Tony Stewart Sunnyskyguy EE75

82k22 gold badges3131 silver badges119119 bronze badges