Hello TEA fellows,
I want to take the opportunity to show you an example
of a little side aspect of TEA: hopelessly over-engineered DIY test equipement.
For another little project of mine I was in need of a resistor decade.
There are a lot of them outsinde, most of them nice and dandy but
for some reason I had the idea I must build one of my own.
![Face Palm :palm:](https://www.eevblog.com/forum/Smileys/default/xfacepalm.gif.pagespeed.ic.EBDwh1hCfo.png)
The specifications aren't that hard:
5 Ranges from 0.1Ohm steps to 1000Ohm steps.
Tolerance: 1%
max power: 0.5W
I looked into my parts collection and found a lot of relays (omron G6K 5V)
and some nice 7-segmet red LED displays from HP.
With some BCD coded switches, some Li-Ion batteries and a nice case I thought,
this could be an easy one.
Well, yeah...
![Grin ;D](https://www.eevblog.com/forum/Smileys/default/xgrin.gif.pagespeed.ic.QVVz6XIT20.png)
This is an image of the finished box:
![](https://i.imgur.com/mcwdjua.jpg)
Box opened:
![](https://i.imgur.com/Dp7cHPS.jpg)
First, I ordered a whole bunch of MELF resistors.
Then I measured the contact resistance of every single relais and took
the best five ones with the lowest contact resistance.
Soldered the relais on a single sided experimental board and added some
ULN 2003L.
Top view:
![](https://i.imgur.com/uDw8auT.jpg)
and the bottom view (lots of fumbling was involved
![Tweak :-/O](https://www.eevblog.com/forum/Smileys/default/smiliey_cal.gif.pagespeed.ce.yYMcAvxBJk.gif)
):
![](https://i.imgur.com/XjkVP8f.jpg)
Then I drilled some holes for the switches in the cover plate and
made a cutout for the 7-segment display. Soldered all on experimental
boards as well.
Top view of the modules:
![](https://i.imgur.com/KbNSx8a.jpg)
Backside of the display module:
![](https://i.imgur.com/DnA1Guf.jpg)
This is the middle part, which contains the Li-Ion batteries (Ansmann, 3600mAh),
the two charger modules (TP 4056 with protection), the LDO (LM 1117, 5V) and the on/off switch.
Top view:
![](https://i.imgur.com/WqzPWzZ.jpg)
Bottom view:
![](https://i.imgur.com/JWSvuGW.jpg)
Detail Li-Ion charger module:
![](https://i.imgur.com/GLssjKp.jpg)
Those TP 4056's are getting really hot, while they charge the Li-Ion batteries
with 1A (at the beginning, descends after a while).
![](https://i.imgur.com/HwxZKZD.jpg)
The batteries on the other side stay cool:
![](https://i.imgur.com/7NOnX6e.jpg)
It started with 2A and this is after 1.5h charging:
![](https://i.imgur.com/hhcQnqD.jpg)
One battery full, the other still ongoing:
![](https://i.imgur.com/t4tx9WV.jpg)
For the on/off switch I'm using a bistable relais (omron G6AK-274 5V)
and some reed sensors. Works really nice and no ugly switch visible.
![Cheesy :D](https://www.eevblog.com/forum/Smileys/default/xcheesy.gif.pagespeed.ic.KPzuH4J-TG.png)
![](https://i.imgur.com/P240Ek9.jpg)
The are the output binding posts from Hirschmann, gold-plated:
![](https://i.imgur.com/EwnRH1U.jpg)
But how does it perform?
I did some measurements with my DMM7510 and 34401A and I am very
pleased with the result:
This is the starting resistance without being nulled out:
![](https://i.imgur.com/I4psJkF.png)
nulled out and 100mOhm step:
![](https://i.imgur.com/qiqX389.jpg)
600mOhm step:
![](https://i.imgur.com/oWwyU6h.jpg)
At 1650Ohm:
![](https://i.imgur.com/oyAQ3ZX.jpg)
At 9999.9Ohm:
![](https://i.imgur.com/1P2OK5F.jpg)
In comparison with the 34401A:
![](https://i.imgur.com/pyQWJDG.jpg)
And here a nice one at 30Ohm:
![](https://i.imgur.com/b3tZoTS.jpg)
Here are the steps from all five ranges, measured with the DMM 7510, 10PLC:
100mOhm steps:
![](https://i.imgur.com/E9oTX3L.png)
1Ohm steps:
![](https://i.imgur.com/UwbUfNB.png)
10Ohm steps:
![](https://i.imgur.com/ZaSjMYO.png)
100Ohm steps:
![](https://i.imgur.com/S52zpug.png)
1000Ohm steps:
![](https://i.imgur.com/g0qZmTy.png)
Hope you enjoyed my little project.
![Popcorn :popcorn:](https://www.eevblog.com/forum/Smileys/default/EatingPopcorn.jpg.pagespeed.ce.yJ6TmC5fSa.jpg)
Cheers,
BU508A
Edit: wrong link to the picture of the 10Ohm steps corrected.