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Your Layout and Models => Electronics => Topic started by: Bealman on October 03, 2019, 06:29:37 AM

Title: Resistance Substitution Box: A Review
Post by: Bealman on October 03, 2019, 06:29:37 AM
There has been some interest lately on the SMD diodes thread about these cheap resistance substitution boxes manufactured in China. A post by ntp3 first alerted me to these (thanks, mate!  :thumbsup:), so I decided to buy one, as I thought it would be a useful bit of kit.

Mine arrived on Monday, and here, for what they're worth, are my thoughts.

First impression was that it was bigger than I thought it would be. Then I realised that this one was a different version to the one ntp3 has. Mine has an extra knob, covering the x0.1-0.9 Ohm range, hence the larger case.

(https://www.ngaugeforum.co.uk/SMFN/gallery/82/255-031019052845-82287874.jpeg)

Second impression was cheap and nasty! The plastic is poor quality - in fact, I almost thought it was second-hand! The binding posts are particularly cheap and nasty - there is no hole to pass a wire through before tightening the screw, so basically the wire will have to be wound around the post at least once before tightening in order to get a secure connection.

There are no holes on the top of the post where a banana plug could be inserted, another way of obtaining a secure connection.

The knobs are big and easy to read, and have a positive action and a satisfying click when rotated.

The instructions, of course are completely in Chinese, but Ohm's Law is the same in China I guess, so that is no big deal.

(https://www.ngaugeforum.co.uk/SMFN/gallery/82/255-031019052720-822861539.jpeg)

On a more positive note, my multimeter revealed that the accuracy of the resistors contained therein did not seem to be too bad:

All knobs set to zero gave 0.4 Ohms
1 Ohm gave 1.5
10 Ohm returned 11.4
100 Ohm returned 101.3
1 KOhm (1000 Ohms) returned 0.999K
9999.9 Ohm returned 9.94K.

I then decided to take a look inside.... big mistake!

(https://www.ngaugeforum.co.uk/SMFN/gallery/82/255-031019052847-822892230.jpeg)

Like others, I was dismayed to find that the selector switches were not wafer selector switches at all, but a jury-rigged affair of tiny SMD resistors soldered to extremely thin copper PCB tracks. When I say thin, I'm referring to the thickness of the copper tracks. I can envisage them wearing through pretty quickly if the unit has frequent use.

The rotating part of the switch is a piece of metal held in place by a spring clip, as the photo shows. Ntp3 has already noted in the diode thread that these "switches" are smothered in copious amounts of grease, and he discovered a piece of copper thread which shorted out a switch and destroyed a diode he was testing! @Malc (https://www.ngaugeforum.co.uk/SMFN/index.php?action=profile;u=497) also blew a diode by rotating a switch during testing, and recommends removing power before rotating the switches.

To add insult to injury, I couldn't get the case back together properly! Somehow I managed to lose one of the four screws, and the others had worn the plastic as I removed them, and wouldn't tighten up again!!

Judging by the teacher and blackboard on the box, along with the circuit diagrams in the instructions, I'd say this unit is meant to be used in schools to teach Physics:

(https://www.ngaugeforum.co.uk/SMFN/gallery/82/255-031019052848-822911100.jpeg)

All I can say in that regard is, that if it was connected up to the 0-15V high current power supplies I had under every bench in my lab when I was a high school Physics teacher, those SMD resistors and probably the case as well, would be TOAST!!

Anyway, wot ya pays for is wot ya get, I suppose, so I'm not really losing any sleep over the purchase. Used with care, and as Malc suggests, removing power before switching ranges, should make it a useful bit of test gear.

Comments from other users most welcome.  :beers:
Title: Re: Resistance Substitution Box: A Review
Post by: Malc on October 03, 2019, 09:32:39 AM
I was trying to find the value of resistance to use with some SMD Street lights. I wired one up to a 12v supply through the resistance box. Starting at 2k I wound the resistance up to 5K. I decided 5K was too dim, so wound it back to 4K and the led stopped working. All I can think is that somehow the box shorted out between ranges and put the full 12v across the diode. I checked all the connections, and the lamp on its own. The connections were OK, but the diode didnít conduct. Fortunately I have another 19 to play with.
Title: Re: Resistance Substitution Box: A Review
Post by: ntpntpntp on October 03, 2019, 11:54:09 AM
Yeah, as per my post last night in the SMD LED thread, the internals of the box aren't as well engineered as I'd expected, but then again the box was cheap! 

The connection posts are disappointing, I may drill them out to take a mini banana plug and also put a hole through the sides to take thin wire, or better still just replace with decent binding posts when I find a cheap pair.

Thinking about it, I may indeed have blown the LED I was playing with in the same manner as @Malc (https://www.ngaugeforum.co.uk/SMFN/index.php?action=profile;u=497) described above - changing ranges while power was being applied.  The wiper connections are "make before break" which is not ideal. The piece of copper wire/track I found inside may not have been the actual cause, but I am obviously going to go over the innards carefully and clean up the tracks.

This box will do the job for what I need now that I know its limitations.  I wonder how much a properly engineered version would cost?

Title: Re: Resistance Substitution Box: A Review
Post by: Bealman on October 03, 2019, 12:05:10 PM
Thanks for the input!  :thumbsup:


Actually, you've just put an idea into my head I'd  never even thought of until your post!

Instead of all the drilling, etc you suggest, just pull the binding terminals out all together, and replace them with decent ones from the local electronics store!

 :beers:
Title: Re: Resistance Substitution Box: A Review
Post by: DCCDave on October 03, 2019, 12:06:05 PM
This looks better built but they dare not publish a price!

https://www.fishersci.com/shop/products/eisco-6-decade-resistance-box/s96527 (https://www.fishersci.com/shop/products/eisco-6-decade-resistance-box/s96527)

Cheers
Dave
Title: Re: Resistance Substitution Box: A Review
Post by: Bealman on October 03, 2019, 12:09:56 PM
Thanks!

Wonder what it looks like inside?  ;)

Didn't like the price... A$94.

And the PSU below it.... A$439? They jest.
Title: Re: Resistance Substitution Box: A Review
Post by: Malc on October 03, 2019, 12:50:00 PM
Yeah, as per my post last night in the SMD LED thread, the internals of the box aren't as well engineered as I'd expected, but then again the box was cheap! 

The connection posts are disappointing, I may drill them out to take a mini banana plug and also put a hole through the sides to take thin wire, or better still just replace with decent binding posts when I find a cheap pair.

Thinking about it, I may indeed have blown the LED I was playing with in the same manner as @Malc (https://www.ngaugeforum.co.uk/SMFN/index.php?action=profile;u=497) described above - changing ranges while power was being applied.  The wiper connections are "make before break" which is not ideal. The piece of copper wire/track I found inside may not have been the actual cause, but I am obviously going to go over the innards carefully and clean up the tracks.

This box will do the job for what I need now that I know its limitations.  I wonder how much a properly engineered version would cost?


R.S. Do some semi pro ones for £2-300 and some better ones for £1100+
Title: Re: Resistance Substitution Box: A Review
Post by: njee20 on October 03, 2019, 01:09:40 PM
This looks better built but they dare not publish a price!

https://www.fishersci.com/shop/products/eisco-6-decade-resistance-box/s96527 (https://www.fishersci.com/shop/products/eisco-6-decade-resistance-box/s96527)

Am I missing something? Says $94 on the RH side.

6 decades of resistance should keep you going for a while though.  :confused1:
Title: Re: Resistance Substitution Box: A Review
Post by: DCCDave on October 03, 2019, 02:49:41 PM
Am I missing something? Says $94 on the RH side.

6 decades of resistance should keep you going for a while though.  :confused1:

No it's me missing the price, it was hiding behind the sign up box.

Must get some new eyes!

Cheers
Dave
Title: Re: Resistance Substitution Box: A Review
Post by: njee20 on October 03, 2019, 02:50:36 PM
Probably be cheaper than their PSUs!
Title: Re: Resistance Substitution Box: A Review
Post by: RailGooner on October 03, 2019, 02:59:57 PM
~£80 at Farnell (https://cpc.farnell.com/unbranded/model-901/model-901-resistance-box/dp/IN00081?ost=IN00081&ddkey=https%3Aen-CPC%2FCPC_United_Kingdom%2Fsearch). Or 7 decades (https://cpc.farnell.com/tenma/72-7270/resistance-decade-box/dp/IN04175?ost=IN04175&ddkey=https%3Aen-CPC%2FCPC_United_Kingdom%2Fsearch) for ~£60.
Title: Re: Resistance Substitution Box: A Review
Post by: Nigel Cliffe on October 03, 2019, 03:52:18 PM
Seems an expensive and effective way to blow up LEDs.....

This is what I've done, worked for years and years:
1 - bought a multi-packet of resistors for under £20, covering a huge range of sizes.  Mine came from Rapid, but that's because I used to live in cycling range of their trade counter, there are lots of other sources.   
2 - use a simple electronics "breadboard" to connect components - resistors, LEDs, etc. when trying out circuits.
3 - check each resistor before use with a digital multimeter.  I trust the makers, but don't trust my ability to either read resistor codes, or always put any spares back into the correctly labelled packet.  If one assumes a mistake may be made its safer.
4 - always start with a higher resistance than you might think you need.

I do have a posh lab power supply, but only because I was given it.   Before then, any old 12v PSU brick would be used.
Title: Re: Resistance Substitution Box: A Review
Post by: Steven B on October 03, 2019, 04:26:18 PM
Three potentiometers (1k, 5k and 10k) work fine for me when combined with a multimeter. They don't take up much space either.

Steven B
Title: Re: Resistance Substitution Box: A Review
Post by: Bealman on January 14, 2020, 07:42:25 AM
Thanks for the input!  :thumbsup:


Actually, you've just put an idea into my head I'd  never even thought of until your post!

Instead of all the drilling, etc you suggest, just pull the binding terminals out all together, and replace them with decent ones from the local electronics store!

 :beers:

Finally got round to doing that today (well, attempting that today).

Turns out nowt's easy. Bloody holes need drilling out as the new binding posts have a wider diameter than the existing hole.

No big deal really, but since the new kitchen came along, my workbench is the one and only table (note table cloth and circular table edge, not good).

I gave up on it and put it back together!

(https://www.ngaugeforum.co.uk/SMFN/gallery/86/255-140120073441-86088253.jpeg)

Title: Re: Resistance Substitution Box: A Review
Post by: Bealman on January 22, 2020, 06:28:28 AM
Me being me, and looking for an easy way out, I saw this at the local Jaycar store for ten bucks. I used me reward points, so I've got yet another bit of Chinese product in me toolbox.

Unfortunately, the spade connectors are a bit on the big side for the terminals on the Chinese resistor box!

There's no free lunch  ;)

(https://www.ngaugeforum.co.uk/SMFN/gallery/86/255-220120062150-864681011.jpeg)
Title: Re: Resistance Substitution Box: A Review
Post by: keerout on January 22, 2020, 05:46:37 PM
 TANSTAAFL !  :no:

Gerard  :wave:
Title: Re: Resistance Substitution Box: A Review
Post by: Railwaygun on January 22, 2020, 08:12:42 PM
There ain't no such thing as a free lunch

Title: Re: Resistance Substitution Box: A Review
Post by: ntpntpntp on January 23, 2020, 08:55:44 PM
** A WARNING ***   

Just been playing with my resistance box and some 12V strip LEDS, working out what additional resistance to add to each strip to tone them down to the effect I'm after for my loco sheds.

The design of box I have shorts out to 0 when moving between 4 and 5 on the dial!  I witnessed it on the x1000, x100 and x10 dials, and pretty sure the x1 does it too although changing by 5 ohms is difficult to see the difference - I didn't bother to get my multimeter out.

This confirmed what I thought might be the case as I managed to blow some other LED mast lights a few weeks ago.

What a nuisance, still at least I know to avoid that in future.
Title: Re: Resistance Substitution Box: A Review
Post by: Malc on January 25, 2020, 05:09:05 PM
Yes, I too found this out the hard way. Fortunately I only blew one street light.
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