# Small Dedicated Ammeter For Breadboard Projects

#### RAMBO999

Joined Feb 26, 2018
222
As the title says I am looking for recommendations for a small ammeter that I can include in circuits to monitor the current in the circuit at any point. I feel the need to do a really in depth study into what's going on in BE and CE circuits of BJTs to help me better understand of how they work and what's going on when I cary the BE and CE circuit source supply voltages. A range < 1 amp would be fine because I am only using the small TO-92 package type. I would need two. One per circuit. Digital would be good. Down to uA. Anybody have anything they could recommend? There are few on Amazon and eBay but I just thought I would garner opinion.

Thanks.

#### AlbertHall

Joined Jun 4, 2014
9,281
Digital multimeters are available very cheap, depending where you are in the world, and would do the job you have in mind but would also be useful for other things.

#### RAMBO999

Joined Feb 26, 2018
222
Digital multimeters are available very cheap, depending where you are in the world, and would do the job you have in mind but would also be useful for other things.
I have three already. I just want something that I can slip into the circuit with no leads and very little internal resistance.Absolte minimal footprint.

#### dl324

Joined Mar 30, 2015
10,002
As the title says I am looking for recommendations for a small ammeter that I can include in circuits to monitor the current in the circuit at any point.
All ammeters will introduce a resistance in the circuit that could affect circuit operation.
I feel the need to do a really in depth study into what's going on in BE and CE circuits of BJTs to help me better understand of how they work and what's going on when I cary the BE and CE circuit source supply voltages.
You'll be better off measuring the voltage drop across a resistor and calculating the current.

In all of my decades of working on circuits, I can't remember when I really needed to actually measure current. Calculating the current was always sufficient.

#### BobaMosfet

Joined Jul 1, 2009
1,009
@RAMBO999 - Honestly, why not use this as an experience to help you learn? Make your own. You *CAN* do it. Making it yourself allows you to determine the way it measures current, how it displays it, and the size & form of the package. Just figure out what your goal is and what you're willing to trade in order to reduce complexity.

#### RAMBO999

Joined Feb 26, 2018
222
@RAMBO999 - Honestly, why not use this as an experience to help you learn? Make your own. You *CAN* do it. Making it yourself allows you to determine the way it measures current, how it displays it, and the size & form of the package. Just figure out what your goal is and what you're willing to trade in order to reduce complexity.
I might do that. Looking for a quick and easy solution. I just want to sit down with a few BJTs and their datasheets and play around particularly where current gain is concerned.

#### AlbertHall

Joined Jun 4, 2014
9,281

#### crutschow

Joined Mar 14, 2008
24,328
The problem with ammeters is that you need to break the circuit connection to insert in in series with where to measure. and it's series resistance can affect the circuit current..

That's why is usually easier to just measure the voltage across a resistor in the circuit and calculate the current from that.

#### RAMBO999

Joined Feb 26, 2018
222
Here is one circuit,

If shorted e/l no leds light.
If shorted e/n led 1 off.
If shorted l/n led 2 off.
If open e leds 1, 4 on.
If open l leds 2, 5 on.
If open n led 2, 3 on.

View attachment 199447
Thanks fot that.

#### RAMBO999

Joined Feb 26, 2018
222
Thanks for the input. I have decided to go analogue. A couple of old school 0 - 100 mA panel ammeters. That should suffice.

#### RAMBO999

Joined Feb 26, 2018
222
The problem with ammeters is that you need to break the circuit connection to insert in in series with where to measure. and it's series resistance can affect the circuit current..

That's why is usually easier to just measure the voltage across a resistor in the circuit and calculate the current from that.
True but it's not a problem on breadboards.

#### dl324

Joined Mar 30, 2015
10,002
True but it's not a problem on breadboards.
It's not the inconvenience of connecting a meter. It's the fact that it can affect circuit operation.

#### crutschow

Joined Mar 14, 2008
24,328
True but it's not a problem on breadboards.
It is a problem if it's a soldered vectorboard type breadboard.

#### Audioguru again

Joined Oct 21, 2019
872
The 4 digits current meter you earmarked cannot measure voltage below 0.1V and cannot measure current below 1mA. Reviews said it begins to make an accurate current measurement above 5mA or 10mA. It has no detailed spec's that say what is its its shunt resistance for you to calculate the changes it makes to the circuit it is measuring.

#### RAMBO999

Joined Feb 26, 2018
222
The 4 digits current meter you earmarked cannot measure voltage below 0.1V and cannot measure current below 1mA. Reviews said it begins to make an accurate current measurement above 5mA or 10mA. It has no detailed spec's that say what is its its shunt resistance for you to calculate the changes it makes to the circuit it is measuring.
I read those reviews myself and reached the conclusion that resolution might be a problem. I don't know wheter the analogue ones I have ordered will be any better of course. Shall just have to see. I will compare them with my multis when they arrive.

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#### KeepItSimpleStupid

Joined Mar 4, 2014
3,889
process meters can measure 0-20 mA without breaking the circuit. They are pricey.

You can easily build an I-V converter or zero resistance ammeter and power it with two 9V batteries. they are simple to build and will only drop a mV or so.

20 mA is super simple. +-100 mA is easy too. Just put an LT1010 buffer (use the datasheet) in the feedback loop of a decent OP amp.

Low current ranging is easy with mercury wetted relays. The meters I built had a 40 pA offset current. I only had 4 bi-polar ranges 100, 10, 1 and 0.1 mA full scale at +-10V out. You can add a clipping indicator or over-range indicator and later add auto ranging and zeroing when you decide to get into microprocessors.

A 4-terminal I-V converter is much harder to build.

Your general problem might be resolution.

The OP amp selection is primarily based on low Vos and low Ib.

Without doing fancy stuff, low current may not wet a switch. A switch switching 10 uA probbaly won;t reliably work.

Look at schematics for the Keithley/tektronix 480 and 485

What a shame: Search for "Keithley 485 Autoranging Picoammeter for parts or repair" on the bay. A parts/repair unit without buttons. A common easy to fix problem is only a few ranges work.

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#### RAMBO999

Joined Feb 26, 2018
222
process meters can measure 0-20 mA without breaking the circuit. They are pricey.

You can easily build an I-V converter or zero resistance ammeter and power it with two 9V batteries. they are simple to build and will only drop a mV or so.

20 mA is super simple. +-100 mA is easy too. Just put an LT1010 buffer (use the datasheet) in the feedback loop of a decent OP amp.

Low current ranging is easy with mercury wetted relays. The meters I built had a 40 pA offset current. I only had 4 bi-polar ranges 100, 10, 1 and 0.1 mA full scale at +-10V out. You can add a clipping indicator or over-range indicator and later add auto ranging and zeroing when you decide to get into microprocessors.

A 4-terminal I-V converter is much harder to build.

Your general problem might be resolution.

The OP amp selection is primarily based on low Vos and low Ib.

Without doing fancy stuff, low current may not wet a switch. A switch switching 10 uA probbaly won;t reliably work.

Look at schematics for the Keithley/tektronix 480 and 485

What a shame: Search for "Keithley 485 Autoranging Picoammeter for parts or repair" on the bay. A parts/repair unit without buttons. A common easy to fix problem is only a few ranges work.
A bit out of my price range those Keithley/tektronix 480 and 485s. All I want to do is try a few of the "Test conditions" detailed on the transistor datasheets and see what I get. having sais that there is a note on some of them that the tests are conducted using "Pulsed: Pulse duration = 300 ms, duty cycle £ 2 %" signal. Whether I can match that remains to be seen.

#### KeepItSimpleStupid

Joined Mar 4, 2014
3,889
The $30.00 bid was a good price. I got a few broken ones for under$100.00. Like I said, they are easy to fix. Some have IEEE-488 and some battery powered, so I have enough parts to make a battery powered IEEE-488 iterfaceable meter with handles. I haven't had the time to put it together.

Anyway, go here: https://www.eevblog.com/forum/repair/keithley-480-picoammeter-repair/

and it will take you to here: http://w.ko4bb.com/manuals/108.16.178.184/Keithley_Model_480_Picoammeter_Complete.pdf#page=42

The meat is at the top left of this page. You have a power supply and DVM IC's.

You can cobble something together easily.

#### peter.zechmeister

Joined Aug 21, 2019
1
For something between do-it-yourself and an expensive instrument, look at the ADM00805 Microchip eval kit (\$60).
It comes with PC USB interface with 4 channels of V and I measurement with default Rsense of 4mohm (25A scale), which gives you down to 1mA resolution. You can easily change Rsense to be higher to go to finer resolution.