Drok and others sell digital 200 micro amp meters but I have been unable to find any reference to the input impedance of such devices. Obviously, low is good, high is bad--anyone had an experience with one?
Thanks.

 

Are you sure that's not 200mA, not µA? Either way, I don't have your answer. I know the 10A shunt is a short piece of wire that look like ~16 gauge copper as I recall.

 

Drok comes off the boat from China and is sold as far as I know exclusively through Amazon by an importer / distributor. Some of the stuff includes basic data with some pictures but I have yet to see a comprehensive data sheet on anything they export to the US or anywhere for that matter. Most of their stuff does work. If this is the meter you are referring to: DROK 0-200uA DC Micro Amp Current Meter Digital Ampere Tester Red LED Ampere meter considering it can be had for under $9.00 USD delivered Amazon Prime you really can't expect a high end DC current meter but again they do work. Also with this and similar meters consider the product description"

Product description
Features:
Dimensions: 79 * 43 * 25 mm
Installation size: 75 x 39.5 mm
Measure rangeC 0-200uA
Operating voltage: DC +5 V +/- 5%
Operating Current: <100mA
Display: 4 digit 0.5 "LED
Display color:Red
Measurement rate: ≥ 2 times / S
Over-range display: the first shows "1" or "-1", the latter three are not displayed
Operating temperature: -10 to +50°c
Operating Humidity: 10 ~ 80% (non-condensing)
Working pressure: 80 to 106kPa

Note:
please use independent power supply
Multiple tester needs isolation powered
The length of leads does not exceed 30 cm, the diameter more than 0.5 mm

That final note is of importance! Keep that in mind.

Ron

 

Obviously, low is good, high is bad

What's your idea of high and low?

My Simpson 467 has a shunt resistance of 1000 ohms on the 200uA scale.

Some DMM state shunt resistance in terms of burden voltage. My Fluke 8010A says burden voltage is 0.3V max for the 200uA scale. This works out to 1500 ohms max.

I rarely use a meter to measure current; instead preferring to measure the voltage drop across a low resistance resistor.

 

Drok and others sell digital 200 micro amp meters but I have been unable to find any reference to the input impedance of such devices. Obviously, low is good, high is bad--anyone had an experience with one?
Thanks.

It's not so simple as low = good, high = bad. It's a tradeoff.

Most current meters are designed so that the voltage drop across them is about the same on each scale. This means that the resistance goes up as the range goes down. For 200 uA, expect a pretty high resistance. It's a tradeoff. As the resistance goes down, the voltage goes down and the signal-to-noise ratio goes down. Higher resistance, better measurement, up until the point that the resistance starts affecting the measurement.

So there's a sweet spot and what that sweet spot is depends on the application -- how accurate/precise do you need the measurement against how much perturbation can you accept in the system while making the measurement.

 

WHnovice,

Do you want to be able to measure things with minimal losses or higher accuracy? If you can make up for the voltage drop in your circuit or just do not care, then go with a higher resistance one. If you need less losses and do not need a ton of accuracy, go with a lower resistance one. Obviously chose one that is rated for your current requirements.

V=I*R
I = V/R

So your voltage drop is proportional to the flowing current. You have a set current and resistance. Less resistance means less power losses and less of a voltage drop. More resistance means the opposite.

So I am actually designing my own bench power supply that needs a current limiting and constant current feature. I am using a 100mV 50A shunt. It is actually coincidentally also made by DROK. They seem to make decent quality products, from my experience. That means 100mV represents 50 amps. 80mV 40A, etc. It is a .1V/50A = 2 milliohm shunt. If I were to use a 1V shunt, 1V*~40A = 40W wasted = (probably) smoking mess. While yes, it will mean slightly less current with a higher resistance, it is not usually a significant drop. But when you cannot handle those power losses, like for a short-circuit protection board, use a smaller resistance.

So why use a higher resistance shunt? Well, let's say you want to measure 0-10A with a 10mA accuracy. With a 100mV shunt, you need to measure differences of .1 mV. But with a 10 mV shunt, it is differences of 10 uV (VERY small). There can be noise and other interferences that will induce voltages, causing inaccurate readings. I can go on and on listing interferences, but here are a few: radio freqencies, parasitic indutances and capacities, EMI from wires, you get the point. There are ways to account for and minimize these, but they will make your life a lot harder. Plus components aren't perfect, etc. etc. And now let's say you need to use an op amp to get 0-5V for a microcontroller. That means a gain of 50 for the 100mV one, but 500 for the 10mV one. That means, again, more errors.

Based on this info, you can decide what is best for you.

Live Wire

 

Well, after a long time googling I found a meter 200 mV meter which was listed as having a 1 k resistance.

Thanks for all your help.

 

Well, after a long time googling I found a meter 200 mV meter which was listed as having a 1 k resistance.

Thanks for all your help.

Forgive my asking but why would you want a meter for measuring voltage with a 1.0 K Ohm input impedance? We started with a current meter with a range of 0 to 200 uA which is a 0.2 mA meter and really a pretty common current meter. Now based on the above we went to a 200 mV meter which while also common I can't understand a 1,000 Ohm input impedance as that would tend to load the source as a volt meter. I assume you found a 200 mA meter with a 1,000 Ohm input impedance and not a 200 uA or 200 mV meter? Even the latter makes no sense as the voltage drop across the input impedance (200 mA * 1,000 Ohms)?

Ron

 

Digital meters generally require 200mv for full scale readout. So a 200uA readout would require a 1000 ohm shunt,correct?
SG

 

Digital meters generally require 200mv for full scale readout. So a 200uA readout would require a 1000 ohm shunt,correct?
SG

Yes, and this is what it comes down to as WBahn points out. In the above case the down side is I have a burden voltage drop across my current measuring plane of 200 mV or I loose .2 Volt that I can't get back. Enter the trade off. What if I am measuring the current in a 3.3 Volt circuit? Can I afford a 0.200 Volt drop on my current measuring plane? So I have to compromise somewhere to find that sweet spot.

Ron

It's not so simple as low = good, high = bad. It's a trade off.

Most current meters are designed so that the voltage drop across them is about the same on each scale. This means that the resistance goes up as the range goes down. For 200 uA, expect a pretty high resistance. It's a tradeoff. As the resistance goes down, the voltage goes down and the signal-to-noise ratio goes down. Higher resistance, better measurement, up until the point that the resistance starts affecting the measurement.

So there's a sweet spot and what that sweet spot is depends on the application -- how accurate/precise do you need the measurement against how much perturbation can you accept in the system while making the measurement.

 

OOPS! I had been googling so long that I mistyped the meter. It is a 200 micro amp meter. And I might be able to live with the 1 k ohm resistance but I was hoping for a lower resistance. I gather, from more reading today, that it is really a 200 mV meter with a 1 k internal shunt.
I thank everyone who replied and ask forgiveness for the careless typo.