Low Cost Programmable Decade Resistor for Prototyping Convenience

[gerrysweeney]

All,

Just a quick note for those of you that purchased these items while I have had them on back order.  I finally have the tracking number and they are hold up in Heathrow, they have cleared customs so should be delivered to my door on Monday (tomorrow).  Assuming there are no manufacturing issues (i am pretty sure they will be absolutely fine), I will get the shipping rolling during the course of the week.

Over the last four weeks a significant backlog of pre-orders have built up so me and the misses will be kept pretty busy over the next few days.  Thank you everyone for supporting this project by buying one or more of the units. Also, an extra special thank you for those of you that have ordered knowing I was waiting for stock and have waited so patiently over the last four/five weeks.

I have also designed a five decade capacitor board using the same form-factor and design approach which I have had made at the same time, I get those tomorrow soon. Quite a few people asked about this so I am taking a punt on the fact that people will find them useful and have funded their manufacture myself rather than doing a crowd source exercise - I will do a video soon for this and probably start another forum thread here on the EEVBlog forum.

Thanks,

Gerry

[Fsck]

Got mine from the backorder batch. They're neat.
However, one of my header pins was brutally bent over. Easily fixed with some physical therapy.
Minor quibble: some of the fiberglass from the PCB keeps flaking off the edges.
little bit of flux here and there, again, minor.

most annoying (still not that bad) is the modification to the 9 pins is extremely uneven. Some of them are pretty mangled, but doesn't really affect the product, just its appearance.

personal note: I'm not sure I like the terminal block thing. I'd be more for a banana jack so it'd be completely tool less.

I'll probably add some little feet to mine

[ddavidebor]

i've received the two boards. alredy changed the screw connector with a couple of banana ones, soldered the 2.4mm couple of pin for convenient use with breadboard.

[Zbig]

Sorry for the off-topic post but there's one thing which puzzles me since some time regarding resistance decades, be it this one (great idea, BTW!) or pretty much any other commercially available small size decade/substitution box I've seen. Is there any practical advantage of having all the resistors rated for the same maximum power dissipation? While I have no trouble understanding the need of having small valued ones rated for as high a power as it's practically achievable, I'm not sure I understand the need for using the same power rating, e.g. 0.5W for high values like anything beyond 100k or so as well. What I mean is, one would need to apply over 200 Volts across the 100k resistor to get 0.5W of power out of it and that wouldn't be the wisest thing to do with a densely SMD-populated, jumper-bridged PCB, right? Let's not even go into the megaohm area. Is it purely to keep the solution "elegant", the resistors consistent in regards to their specifications and not to overcomplicate things?

[gerrysweeney]

Got mine from the backorder batch. They're neat.
However, one of my header pins was brutally bent over. Easily fixed with some physical therapy.
Minor quibble: some of the fiberglass from the PCB keeps flaking off the edges.
little bit of flux here and there, again, minor.

most annoying (still not that bad) is the modification to the 9 pins is extremely uneven. Some of them are pretty mangled, but doesn't really affect the product, just its appearance.

personal note: I'm not sure I like the terminal block thing. I'd be more for a banana jack so it'd be completely tool less.

I'll probably add some little feet to mine

Thanks for the feedback, comments noted.

Gerry

[gerrysweeney]

i've received the two boards. alredy changed the screw connector with a couple of banana ones, soldered the 2.4mm couple of pin for convenient use with breadboard.

Glad you received them ok.

Gerry

[gerrysweeney]

Sorry for the off-topic post but there's one thing which puzzles me since some time regarding resistance decades, be it this one (great idea, BTW!) or pretty much any other commercially available small size decade/substitution box I've seen. Is there any practical advantage of having all the resistors rated for the same maximum power dissipation? While I have no trouble understanding the need of having small valued ones rated for as high a power as it's practically achievable, I'm not sure I understand the need for using the same power rating, e.g. 0.5W for high values like anything beyond 100k or so as well. What I mean is, one would need to apply over 200 Volts across the 100k resistor to get 0.5W of power out of it and that wouldn't be the wisest thing to do with a densely SMD-populated, jumper-bridged PCB, right? Let's not even go into the megaohm area. Is it purely to keep the solution "elegant", the resistors consistent in regards to their specifications and not to overcomplicate things?

Its mostly elegance.

Gerry

[Lightages]

Finally mine arrived after the usual 1 month of slow mail in Chile, and another 1 month because the mail workers went on strike. I like these things and I am sure they will get good use in testing and prototyping.

[gerrysweeney]

Finally mine arrived after the usual 1 month of slow mail in Chile, and another 1 month because the mail workers went on strike. I like these things and I am sure they will get good use in testing and prototyping.

Glad they made it, thanks for posting.

Gerry

[Alex]

Gerry,

In case this has not been covered, the 10R decade on my board was shorted by a thin solder bridge under the pin header. Once located, it was easy to repair by reheating the solder joints. I assume this can be traced back to the assembly process. Hopefully this will help anyone with the same issue.

I otherwise share everyone else's comments 

Alex.

[gerrysweeney]

Gerry,

In case this has not been covered, the 10R decade on my board was shorted by a thin solder bridge under the pin header. Once located, it was easy to repair by reheating the solder joints. I assume this can be traced back to the assembly process. Hopefully this will help anyone with the same issue.

I otherwise share everyone else's comments 

Alex.

Hi Alex,

Sorry about that. Regrettably the quality of the assembly was less than optimal - I am not going to name the assembly house at this time, I have been trying to work it out with them. Quality aside they are reasonable to deal with. I will know on the next project if they actually do what they say they will.  Anyways, if you want me to send you a replacement I will gladly do so.  Sorry you had to spend time to find a problem, my main goal was to provide a simple and usable (out of the box) tool for my fellow EE'ers, I missed the boat in this case obviously.

Gerry

[Alex]

Hi Alex,

Sorry about that. Regrettably the quality of the assembly was less than optimal

Gerry

Your support is honest and thank you for your offer. Hopefully the feedback will help you iron out any issues with the assembly house, which was my intention.

After all, some people would pay more for equipment in need of repair.


Alex.

[zapta]

Just wonder, can you use BCD scheme (1, 2, 4, to cut the resistor count by half? The thumb switches for example do come with bcd scheme.

[Mike Warren]

Just wonder, can you use BCD scheme (1, 2, 4, to cut the resistor count by half? The thumb switches for example do come with bcd scheme.

All the thumbwheel switches I've seen have a common terminal. For resistors you would need the switches across each resistor.

You would also have to think in reverse; open the switch to select the value.

The other problem would be that uncommon values would be needed.

Code: [Select]


   +--/--+--/--+--/--+--/--+
   |     |     |     |     |
---+--1R-+--2R-+--4R-+--8R-+---




[Bryan]

Could one not use 7 rotary switches (1 pole 10 position) for each decade instead of the jumpers and mount in a nice project box?.

[ddavidebor]

but it would cost 75$ and not 15$ and it would be much bigger

[Bryan]

Yes, I know. I already have the decade resistor. Wanted to modify it so it is easier to use. Looking at the schematic I can see no reason why not.

[sync]

Why modifying it? Resistors are cheap. Build a complete new one.
btw: use make-before-break switches.

[dragonflight]

This is a great idea, but it could be made better (IMHO) if you added a 10th resistor to each decade.
Why?
As it stands you are potentially limited to resistance values +/- 1%. As an extreme example if you want 100.000K?, but your 100K is 1.01K and your 99.999K is 98.999 (though it is likely to be higher), the best you can do is 100K +1% or 100K -1%.

With a 10th resistor you are guaranteed of covering all the resistance values over the 7 decades.

So now that we can cover all the values with a little setup work (accurately measure the resistance of the first 4 decades) we can set the resistance to .1% (100?-999.9K?)!

To accurately set a resistance to .1% (and by extension .02% or .03% with constant room temperature and a lot more calculation ...), the user would have to measure and build a table of offsets for each of the first 4 decades (32 entries) and add these offsets to the desired value, before setting the jumpers

an example
to set 43.34K +/- .1%

look up 40K and the value is -.31K
look up 3K   and the value is -.023K
so you want 43.34 - .31 -.02 = 43.01

if the the error sum is negative then you redo the calculation and "borrow" 1 from the first 2 digits
so to set 43.31 look up 42K + 1.31K

40 K is -.31K
2K is +.020K

so you want 42. + 1.31 -.31 +.02 = 42. + 1.02
so you set 0(100K), 4(10K), 2(1K), 10(100), 2(10), 0(1)

This may seem difficult (it ain't easy), but the complexity is a small price to pay for a .1% accurate resistance box again IMHO at this price.

[Flump]

Does anyone know what kind of resistors Gerry's programmable resistor uses as I released the magic smoke
in two of them! and a lot of smoke it was too lol.

I only need 2 (100R) but would buy 10 or so, so I have some spares
can you buy them in small amounts like that, and preferably in the UK ?

(How I did it)
I made a 4:1 Balun for a HF antenna and I needed a 200R carbon resistor and was looking around for one
and spotted the programmable resistor, so I set it to 200R and hooked it up the the Balun.
For about 10 seconds the VSWR looked great, totally flat from 7Mhz upwards then the magic smoke released,
I guess it didn't like 10watts of RF going into it.

That will teach me for messing around with things at near 1am in the morning.

[Fsck]

they're just plain old 1210 smd resistors.

[gerrysweeney]

Does anyone know what kind of resistors Gerry's programmable resistor uses as I released the magic smoke
in two of them! and a lot of smoke it was too lol.

I only need 2 (100R) but would buy 10 or so, so I have some spares
can you buy them in small amounts like that, and preferably in the UK ?

(How I did it)
I made a 4:1 Balun for a HF antenna and I needed a 200R carbon resistor and was looking around for one
and spotted the programmable resistor, so I set it to 200R and hooked it up the the Balun.
For about 10 seconds the VSWR looked great, totally flat from 7Mhz upwards then the magic smoke released,
I guess it didn't like 10watts of RF going into it.

That will teach me for messing around with things at near 1am in the morning.

Hi Mark,

They are just 2010 SMT resistors - not rated at 10W (for very long at least).  You should be able to get them from e-bay or any electronics component distributor but if you are buying 10 of them you are likely to pay a big premium for shipping/handling, ordinarily they cost less than a penny each but you need to be buying 5000 on a reel.  If you get really stuck I can probably find a few spares for you

Gerry

[Flump]

Thanks for the info Fsck & Gerry
I found some on ebay I think

http://www.ebay.co.uk/itm/271226754188?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT
RESISTOR, SURGE, 1210, 1%, 100R
Resistance:100ohm
Resistance Tolerance:± 1%
Power Rating:500mW
Voltage Rating:200V
Temperature Coefficient:¬± 100ppm/¬_C
Resistor Element Material:Thick Film
Resistor Case Style:1210
Resistor Mounting:SMD
External Depth:2.5mm
External Length / Height:0.6mm
External Width:3.2mm
No. of Pins:2
Operating Temperature Range:-55¬_C to +155¬_C
Overload Voltage:400V
Package / Case:1210
Reel Quantity:5000
Series:ERJP
Termination Type:SMD

are they the right ones ?
if so i will get them as you never know I might do it again 

[gerrysweeney]

Hi Mark,

Yes those will do...£6 for 25 of em, told you they would be expensive....

Gerry

[gerrysweeney]

Mark,

The offer is still there, if you want to buy a stamp and an envelope and post me a pre-stamped empty envelope I will stick a few in the post for you - will save you a few quid

Gerry