Resistance of ammeter caused voltage drop

[Terry Given]

Dummy wrote:

I have a simple setup as below. The ammeter is used to measure the
transmit current of the radio. Power supply has an display of total
current as well.

Power supply (Vin=7.5V)-------ammeter--------- (Vout) radio

When connecting the ammeter, there's a 0.3V voltage drop across the
ammeter before transmitting. During transmit mode, total current as
displayed at ammeter is 1.80A, matched with the power supply current
display. But Vout is measured to be 6.14V only, thus causing the Tx
power to be lower.

Using direct cable connection without ammeter, the current measured is
almost similar, but the Tx power is much more higher. Current is
1.85A. Vout is 7.0V during transmission - a voltage drop of 0.5V.
I guess the ammeter is giving more resistance the the cable.

So I was wondering whether the ammeter should be used to measure any
high current in circuit level if it can cause some voltage drop. The
voltage drop might affect the circuit performance at the subsequent
stage.

Why not stick one of the power supply leads thru the centre of a DCCT
(aka hall effect DC current transducer). Surely everyone has
half-a-dozen of these sitting in their junk box?

Cheers
Terry

 

[Jim Thompson]

On Thu, 17 Mar 2005 14:11:35 +1300, Terry Given <my_name@ieee.org>
wrote:

[snip]

Why not stick one of the power supply leads thru the centre of a DCCT
(aka hall effect DC current transducer). Surely everyone has
half-a-dozen of these sitting in their junk box?

Cheers
Terry

Oh Woe Is ME! I only have three. (For three phase EC motors

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[Terry Given]

Jim Thompson wrote:

On Thu, 17 Mar 2005 14:11:35 +1300, Terry Given <my_name@ieee.org
wrote:

[snip]

Why not stick one of the power supply leads thru the centre of a DCCT
(aka hall effect DC current transducer). Surely everyone has
half-a-dozen of these sitting in their junk box?

Cheers
Terry


Oh Woe Is ME! I only have three. (For three phase EC motors

...Jim Thompson

Bad Jim! :{

Alas, all 12 I have are 300A rated, 3x overload.....

I must buy a few smaller ones

Cheers
Terry

 

[Jim Thompson]

On Thu, 17 Mar 2005 14:26:38 +1300, Terry Given <my_name@ieee.org>
wrote:

Jim Thompson wrote:
On Thu, 17 Mar 2005 14:11:35 +1300, Terry Given <my_name@ieee.org
wrote:

[snip]

Why not stick one of the power supply leads thru the centre of a DCCT
(aka hall effect DC current transducer). Surely everyone has
half-a-dozen of these sitting in their junk box?

Cheers
Terry


Oh Woe Is ME! I only have three. (For three phase EC motors

...Jim Thompson

Bad Jim! :{

Alas, all 12 I have are 300A rated, 3x overload.....

I must buy a few smaller ones

Cheers
Terry

Mine are 50A FS.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[Larry Brasfield]

"Terry Given" <my_name@ieee.org> wrote in message
news:XM4_d.10432$1S4.1095388@news.xtra.co.nz...

Clearly its pick-on-Larry O'clock, so here's my 2 cents worth:

I dont like your hat

Ouch! You guys sure know how to pile on. That's
very close to the most damaging blow so far. <g>

Cheers

Thanks, and likewise.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

 

[JeffM]

Why are you responding to me?
Go up 3 levels in the thread and respond to the OP.
Jeff

(Clueless Google posters are the bane of Usenet.)

oh no...the thread police
Mark

174 posts to Usenet and you still haven't figured out
context
attribition
threads

Congratulations. You hit the dumb-ass trifecta.

 

[Rich Grise]

On Thu, 17 Mar 2005 16:45:16 +1300, Terry Given wrote:

Actually it was probably payback for the time I gave him a dish
containing about 10,000 0603 caps (200 values), and asked him to sort
them out. Half an hour later, there he was painstakingly measuring each
one, until one of the other techs took pity on him and told him the
correct response to such a request was "piss off"

Was he getting paid by the hour? ;-)

Cheers!
Rich
--
Hlade's Law:
If you have a difficult task, give it to a lazy person -- they
will find an easier way to do it.

 

[Ken Taylor]

"Dummy" <ahkit1021@yahoo.com> wrote in message
news:74bb84c0.0503142125.60e5255@posting.google.com...

I have a simple setup as below. The ammeter is used to measure the
transmit current of the radio. Power supply has an display of total
current as well.

Power supply (Vin=7.5V)-------ammeter--------- (Vout) radio

When connecting the ammeter, there's a 0.3V voltage drop across the
ammeter before transmitting. During transmit mode, total current as
displayed at ammeter is 1.80A, matched with the power supply current
display. But Vout is measured to be 6.14V only, thus causing the Tx
power to be lower.

Using direct cable connection without ammeter, the current measured is
almost similar, but the Tx power is much more higher. Current is
1.85A. Vout is 7.0V during transmission - a voltage drop of 0.5V.
I guess the ammeter is giving more resistance the the cable.

So I was wondering whether the ammeter should be used to measure any
high current in circuit level if it can cause some voltage drop. The
voltage drop might affect the circuit performance at the subsequent
stage.

You could buy a shunt and use a voltmeter (on a suitably low range) to
measure the drop across it.

Ken