A New Approach to a Bench Top Ammeter

RE: A New Approach to a Bench Top Ammeter

jw0752 — Mon, 16 Oct 2017 04:02:45 GMT

As mentioned in my previous post 12 days ago I decided to build a one channel 30 volt Power supply and incorporate the now dual range Ammeter as an auxiliary test option. I will not go through all the process for building the new piece of test equipment as you have endured enough of my power supplies. Here is the finished product. In the two pictures below I am running an efficiency comparison between a standard linear 7805 voltage regulator and a Recom R785.0-1.0. Here is the 7805 in action:

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From the display we can see that the input to the 7805 is 8.5 volts at 350 mA for a total power input of 2.89 Watts. The auxiliary ammeter is reading the current on the 5 volt output into a 14 ohm load. This was perhaps a bad choice as it is also displaying 350 mA which may be confusing. However 350 mA at 5 volts is only 1.74 Watts and if we divide this by the input wattage we get about 60% efficiency. The extra 40%, of course if being shed as heat in the 7805.

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Now we have the R785.0-1.0 DC to DC converter in place of the 7805. Since we still have 5 volts output into the 14 ohm load we still have a 350 mA current displayed on the output. Notice however that the input current has dropped to 210 mA. At 8.5 volts this translates to 1.79 Watts and if we divide this into the output wattage of 1.75 watts the result is 98% efficiency. Here is a slightly better picture of the new unit:

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John

RE: A New Approach to a Bench Top Ammeter

jw0752 — Thu, 05 Oct 2017 03:22:42 GMT

After having a few days to think about it I realized that I was approaching the idea of expanding the ammeter to a dual range version all wrong. Sometimes I get a wrong idea in my head and it takes a while to see past it. In this case I was envisioning having to switch different shunts into the circuit in order to have a dual range device. Yesterday the light in my head finally flickers and brightened a bit so I realized that the shunt should be a fixed 2 stage voltage divider and that I only need switch the input of the meters ADC from one point on the shunt to another while also switching the display decimal point one place. Here is a schematic for what I decided to do:

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Now in theory the resistors are the values that are printed on them and everything turns out pretty but this seldom happens, at least in my world. In this case the theory only got me to the point where one scale was off from the other by about 10 percent. If I had a reading on 2 amps on the 2 amp scale the 20 amp scale would read 1.8 amps. After an hour of adding various resistors to the Trim resistor I finally had both scales on the same page. Here a re a couple pictures of the display reading 318 mA

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2 Amp Scale

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20 Amp Scale

Here is a picture showing the messy shunt with the extra 2 trim resistors across the 0.1 ohm shunt.

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Note that the input to the ADC starts with a series 1 M resistor. At least by switching the ADC and not shunts the resistance of the switch contacts are irrelevant.

This is a different unit from the one that I built above in the black case. I am still trying to decide if I am going to make that one into a dual range as I did not plan for a range switch and there isn't a really good place to put one. Perhaps I will have to wait for the lights in the old head to flicker on again.

Last picture shows the whole raw ammeter and the switch. I am currently planning on building a one channel 30 volt bench supply with an auxiliary ammeter and this one will get used in that build.

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John

RE: A New Approach to a Bench Top Ammeter

Jan Cumps — Sat, 30 Sep 2017 10:38:54 GMT

my time (hint: not worth much at this stage of life)

Your time is priceless and you spent it well [View:/resized-image/__size/16x16/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-c5e0796a-5c46-4010-9ddc-ec81c4ef46ea/contentimage_5F00_3.png:16:16]

RE: A New Approach to a Bench Top Ammeter

balearicdynamics — Sat, 30 Sep 2017 07:32:20 GMT

John it's really impressive. You have the ability to generate something professional (not only professional looking) from cheap stuff. It's a great quality [View:/resized-image/__size/16x16/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-c5e0796a-5c46-4010-9ddc-ec81c4ef46ea/contentimage_5F00_1.png:16:16]

Enrico

RE: A New Approach to a Bench Top Ammeter

DAB — Fri, 29 Sep 2017 19:10:21 GMT

Excellent project John.

Well done.

DAB

RE: A New Approach to a Bench Top Ammeter

shabaz — Fri, 29 Sep 2017 09:17:53 GMT

Hi John,

That's a nice looking device! Impressive job with the assembly too, with the steel cut-outs looking so clean!

RE: A New Approach to a Bench Top Ammeter

mcb1 — Fri, 29 Sep 2017 08:00:14 GMT

Another quality product for use on any general workbench.

Well done.

Mark