| Author |
Message |
Dummy
Guest
|
 Posted:
Tue Mar 15, 2005 6:10 am Post subject:
Resistance of ammeter caused voltage drop |
|
|
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.
|
|
| Back to top |
|
 |
Tim Wescott
Guest
|
| Posted:
Tue Mar 15, 2005 12:18 pm Post subject:
Re: Resistance of ammeter caused voltage drop |
|
|
Dummy wrote:
| Quote: | 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.
|
I'd look for a better ammeter. My old Heathkit electro-mechanical unit
only drops 1/4 volt at full scale -- I would imagine that a Fluke or
other serious DVM would be better (but I haven't checked, so I can't
guarantee it).
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com |
|
| Back to top |
|
|
Larry Brasfield
Guest
|
| Posted:
Tue Mar 15, 2005 12:50 pm Post subject:
Re: Resistance of ammeter caused voltage drop |
|
|
"Dummy" <ahkit1021@yahoo.com> wrote in message
news:74bb84c0.0503142125.60e5255@posting.google.com...
| Quote: | 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.
|
It certainly looks that way.
| Quote: | 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.
|
That would depend on how badly you want to measure
the current and what effect the voltage drop has. I am
going to guess about those things for now.
Assumption: The transmitter has a linear relationship
between supply voltage and supply current draw.
Assumption: The transmitter does not change its mode
of operation at the reduced voltage you see when the
ammeter is connected. (This is nearly but not quite
tautological with the linearity assumption. But it is
something you can verify independently when the
ammeter is not present.)
Assumption: You have a way, (such as a different cable,
a variable bench supply, or some low Ohm resistors), to
vary the supply with the ammeter connected.
Assumption: Your ammeter is a better instrument than
the meter built into the supply. (Otherwise I do not
know why we would be having this discussion.)
Measure the current with ammeter in at the ordinary
supply voltage. Call this Iao. Measure the voltage
at the transmitter with same lashup. Call this Vao.
Measure the drop across the ammeter, Vad.
Reduce the supply voltage by an amount similar
to what the ammeter drops, leaving the ammeter
in place. Measure current and voltage, to be
called Iar and Var respectively.
Calculate Rt = (Vao - Var) / (Iao - Iar)
This is the slope of the voltage versus current
characteristic for the transmitter.
Calculate Ina = Iao + Rt * Vad
This is an approximation of the current the
transmitter draws when you have no ammeter
to reduce the supply voltage it sees.
| Quote: | The voltage drop might affect the circuit performance at the subsequent
stage.
|
Yes, it probably does. It would help, when deciding
what to do about this, what you are trying to achieve
by measuring the current. Is this a one-time affair,
or will the ammeter become part of the setup? How
accurately do you need to know the current? (My
guess is that this does not matter much.)
There are current meters that impose no DC drop.
You could rent (or buy) one if you believe the
above procedure is too much trouble or not
sufficiently accurate. I do not advise this unless
you have more need for accuracy than I can see.
You could also put a lower shunt resistor across
your ammeter and calibrate the combination.
--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
|
|
| Back to top |
|
|
Robert Baer
Guest
|
| Posted:
Tue Mar 15, 2005 3:32 pm Post subject:
Re: Resistance of ammeter caused voltage drop |
|
|
Larry Brasfield wrote:
| Quote: | "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.
It certainly looks that way.
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.
That would depend on how badly you want to measure
the current and what effect the voltage drop has. I am
going to guess about those things for now.
Assumption: The transmitter has a linear relationship
between supply voltage and supply current draw.
Assumption: The transmitter does not change its mode
of operation at the reduced voltage you see when the
ammeter is connected. (This is nearly but not quite
tautological with the linearity assumption. But it is
something you can verify independently when the
ammeter is not present.)
Assumption: You have a way, (such as a different cable,
a variable bench supply, or some low Ohm resistors), to
vary the supply with the ammeter connected.
Assumption: Your ammeter is a better instrument than
the meter built into the supply. (Otherwise I do not
know why we would be having this discussion.)
Measure the current with ammeter in at the ordinary
supply voltage. Call this Iao. Measure the voltage
at the transmitter with same lashup. Call this Vao.
Measure the drop across the ammeter, Vad.
Reduce the supply voltage by an amount similar
to what the ammeter drops, leaving the ammeter
in place. Measure current and voltage, to be
called Iar and Var respectively.
Calculate Rt = (Vao - Var) / (Iao - Iar)
This is the slope of the voltage versus current
characteristic for the transmitter.
Calculate Ina = Iao + Rt * Vad
This is an approximation of the current the
transmitter draws when you have no ammeter
to reduce the supply voltage it sees.
The voltage drop might affect the circuit performance at the subsequent
stage.
Yes, it probably does. It would help, when deciding
what to do about this, what you are trying to achieve
by measuring the current. Is this a one-time affair,
or will the ammeter become part of the setup? How
accurately do you need to know the current? (My
guess is that this does not matter much.)
There are current meters that impose no DC drop.
You could rent (or buy) one if you believe the
above procedure is too much trouble or not
sufficiently accurate. I do not advise this unless
you have more need for accuracy than I can see.
You could also put a lower shunt resistor across
your ammeter and calibrate the combination.
The shunt scheme would seem to be a good idea. |
Say one gets an 0.1 ohm 1% resistor and uses kelvin connections to
read the voltage drop across it with a DVM (lowest full scale range is
200mV).
Then at one amp, the voltage drop would be 100mV; 3 times better than
the presumed 300mV cited (which is not that much higher than "1/4 V).
BTW, i would put the resistor on the ground side... |
|
| Back to top |
|
|
JeffM
Guest
|
|
| Back to top |
|
|
Mark
Guest
|
| Posted:
Tue Mar 15, 2005 9:15 pm Post subject:
Re: Resistance of ammeter caused voltage drop |
|
|
JeffM wrote:
make sure your connections are all tight and low resistance and you use
heavy wire. It may be the connections to the meter causing the
problem. Or it may be the meter. What kind of meter is it?
Mark |
|
| Back to top |
|
|
Jamie
Guest
|
| Posted:
Wed Mar 16, 2005 12:48 am Post subject:
Re: Resistance of ammeter caused voltage drop |
|
|
Dummy wrote:
| Quote: | 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.
if you want to experiment a bit, you could use an OP-Amp inputs with a |
shunt ( very low value shunt). the Op-Amp can be an Amp for a simple
meter.
of course there is more that you need to do, this is just an idea for
you to ponder with. |
|
| Back to top |
|
|
JeffM
Guest
|
| Posted:
Wed Mar 16, 2005 12:55 am Post subject:
Re: Resistance of ammeter caused voltage drop |
|
|
| Quote: | I was wondering whether the ammeter should be used to measure...
Dummy (the OP)
That would depend on how badly you want to measure the current
You could also put a lower shunt resistor across your ammeter
Larry Brasfield
The shunt scheme would seem to be a good idea.
Robert Baer
Yup. There are even ready-made units.
http://www.google.com/images?&q=shunt+simpson+-edu+-isesurplus+-misstrina
JeffM
make sure your connections are all tight
Mark
|
Why are you responding to me?
Go up 3 levels in the thread and respond to the OP.
(Clueless Google posters are the bane of Usenet.) |
|
| Back to top |
|
|
Ken Taylor
Guest
|
| Posted:
Wed Mar 16, 2005 12:58 am Post subject:
Re: Resistance of ammeter caused voltage drop |
|
|
"Dummy" <ahkit1021@yahoo.com> wrote in message
news:74bb84c0.0503142125.60e5255@posting.google.com...
| Quote: | 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 |
|
| Back to top |
|
|
John Fields
Guest
|
| Posted:
Wed Mar 16, 2005 2:31 am Post subject:
Re: Resistance of ammeter caused voltage drop |
|
|
On 14 Mar 2005 21:25:43 -0800, ahkit1021@yahoo.com (Dummy) wrote:
| Quote: | 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.
|
---
so transmit, with the ammeter in the circuit, looks like this:
1.8A--->
Power supply (Vin=7.5V)-------ammeter--------- (Vout=6.14V) radio
Which makes the total (ammeter + cable) resistance look like:
Vin - Vout 1.36
Rt = ------------ = ------ ~ 0.756 ohms
I 1.8
With the ammeter out of the circuit, the cable resistance looks like:
Vin - Vout 0.5
Rc = ------------ = ------ ~ 0.270 ohms
I 1.85
which makes the ammeter resistance look like:
Ra = Rt - Rc = 0.756 - 0.270 = 0.476 ohms
I have a Fluke 8060A which measures 0.468 ohms on the 2 amp scale, so
0.476 ohms for Ra doesn't seem far-fetched if your ammeter is similar,
but 0.270 ohms seems awfully high for the cable, what with 100 feet of
#20 being about an ohm. Just to make sure that's not a problem I'd
check out all the wiring interfaces and make sure you don't have any
high-resistance connections anywhere.
Bottom line though, after you get everything cleaned up and sorted
out, if you know that what you need to make your measurements is 7.5V
at the radio, put your ammeter in the line if that's what you need to
do, then monitor the voltage at the radio's power input, then key the
transmitter and adjust the power supply until the voltage at the
radio's power input is 7.5V. If it goes up to 8V when you stop
transmitting, so what? You can always drop it back to 7.5 if you need
to work on the receiver, although I doubt that you'll need to, since
I'm sure the receiver's not running unregulated.
--
John Fields |
|
| Back to top |
|
|
Mark
Guest
|
| Posted:
Wed Mar 16, 2005 4:26 am Post subject:
Re: Resistance of ammeter caused voltage drop |
|
|
| oh no...the thread police |
|
| Back to top |
|
|
Larry Brasfield
Guest
|
| Posted:
Wed Mar 16, 2005 6:10 am Post subject:
Re: Resistance of ammeter caused voltage drop |
|
|
"Fred Bloggs" <nospam@nospam.com> wrote in message
news:423796E7.3090905@nospam.com...
| Quote: | Larry Brasfield wrote:
Measure the current with ammeter in at the ordinary
supply voltage. Call this Iao. Measure the voltage
at the transmitter with same lashup. Call this Vao.
Measure the drop across the ammeter, Vad.
Reduce the supply voltage by an amount similar
to what the ammeter drops, leaving the ammeter
in place. Measure current and voltage, to be
called Iar and Var respectively.
Calculate Rt = (Vao - Var) / (Iao - Iar)
This is the slope of the voltage versus current
characteristic for the transmitter.
Calculate Ina = Iao + Rt * Vad
This is an approximation of the current the
transmitter draws when you have no ammeter
to reduce the supply voltage it sees.
Yeah? So the F___ what? And since when is resistance x voltage equal a current?- and you don't even have the sign right.
|
(Finally, a positive contribution.)
That line should have read, of course, thusly:
Calculate Ina = Iao + Vad / Rt
And, contrary to what the esteemed Mr. Boggs
proclaims, the sign is correct. As defined, both
Vad and Rt are positive quantities (at least if
the transmitter draws more power at higher
voltages, which is already in evidence.) Since
Ina (pronounce as: I 'n'o 'a'mmeter) represents
the current predicted when no ammeter is present,
and that is already known to be higher, adding the
positive ratio Vad/Rt to the current measure with
the ammeter present is correct for getting such a
result. I would hope that this is now obvious
even to the most critical observer.
| Quote: | Here's the deal you worthless, pretentious son-of-a-bitch-with-VD, you are a worthless p.o.s.- we are wise to your dumb ass-
|
I perhaps should engage in some name-calling on
account of the above correction, but, as a low ranking
member of the excrement class, I am not up to it.
I already said 'No', Fred. Do you think repetition
is going to be effective? (It would appear so.)
| Quote: | You could also put a lower shunt resistor across
your ammeter and calibrate the combination.
Or buy/modify a p.s. with external sense compensation,
|
Spending money was an obvious option which
I mentioned in several of its many forms. The
OP's questions led me to believe he might be
interested in using the equipment he had. We
have seen no evidence to the contrary.
| Quote: | damned worthless idiot.
|
Fred, I appreciate your opinion. Honestly.
I tried to tell you that earlier, but I suspect
my meaning escaped your notice.
--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me. |
|
| Back to top |
|
|
Fred Bloggs
Guest
|
| Posted:
Wed Mar 16, 2005 6:10 am Post subject:
Re: Resistance of ammeter caused voltage drop |
|
|
Larry Brasfield wrote:
| Quote: |
Measure the current with ammeter in at the ordinary
supply voltage. Call this Iao. Measure the voltage
at the transmitter with same lashup. Call this Vao.
Measure the drop across the ammeter, Vad.
Reduce the supply voltage by an amount similar
to what the ammeter drops, leaving the ammeter
in place. Measure current and voltage, to be
called Iar and Var respectively.
Calculate Rt = (Vao - Var) / (Iao - Iar)
This is the slope of the voltage versus current
characteristic for the transmitter.
Calculate Ina = Iao + Rt * Vad
This is an approximation of the current the
transmitter draws when you have no ammeter
to reduce the supply voltage it sees.
|
Yeah? So the F___ what? And since when is resistance x voltage equal a
current?- and you don't even have the sign right. Here's the deal you
worthless, pretentious son-of-a-bitch-with-VD, you are a worthless
p.o.s.- we are wise to your dumb ass- go away.
| Quote: | You could also put a lower shunt resistor across
your ammeter and calibrate the combination.
|
Or buy/modify a p.s. with external sense compensation, damned worthless
idiot. |
|
| Back to top |
|
|
Robert Baer
Guest
|
| Posted:
Wed Mar 16, 2005 3:16 pm Post subject:
Re: Resistance of ammeter caused voltage drop |
|
|
Jamie wrote:
| Quote: |
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.
if you want to experiment a bit, you could use an OP-Amp inputs with a
shunt ( very low value shunt). the Op-Amp can be an Amp for a simple
meter.
of course there is more that you need to do, this is just an idea for
you to ponder with.
|
Again, pay attention and use kelvin connections. |
|
| Back to top |
|
|
Robert Baer
Guest
|
| Posted:
Wed Mar 16, 2005 3:17 pm Post subject:
Re: Resistance of ammeter caused voltage drop |
|
|
Ken Taylor wrote:
| Quote: |
"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
|
I think i said that, and added a cautionary note to use kelvin
connections. |
|
| Back to top |
|
|
|
|
|
|
FAQ
Memberlist
Register
Profile
Log in to check your private messages
Log in