One for any electronics experts

[Mick J]

You will find the plastic coating comes away easy enough, more of a problem is the soldering, you need to be quite quick and not cook, short or damage the strip, bit of shrink over after looks good.  Clip on connectors are available for led strip. 

[Demonperformer]

Right. No "shorted" connection, but can attach red led  across them  (with suitable resistor). This will be much easier for me that breaking into the circuit outside.

Thanks.

[Demonperformer]

43 minutes ago, Mick J said:

You will find the plastic coating comes away easy enough, more of a problem is the soldering, you need to be quite quick and not cook, short or damage the strip, bit of shrink over after looks good.  Clip on connectors are available for led strip. 

Thanks for the tip. Think I will try the soldering approach first. Got a very fine tip so hopefully won't have too much trouble (relative term!). Becoming ever-more pleased that I have an extra couple of sections to practice on. 

[Gina]

Good luck ?

[Demonperformer]

Trying to use the information provided previously to determine the best value for the resistor to go with the red LED. Now my power supply is 12v, as this is what is required by the uv strip. The LED I am considering has the following specs:
* Forward voltage = 2.25v
* Reverse voltage = 5v
* Power dissipation = 120mW
* DC forward current = 25mA
* Peak forward current = 120mA

So, if my understanding of the formulae provided on p1 of this thread is correct, I need to drop the voltage from 12v to 2.25v, which is 9.75v. I then need to divide this by the current, for which I have two values: 25mA or 120mA. If I use 25mA I get 9.75/0.025 = 390 ohms and if I use 120mA I get 9.75/0.12 = 81.25 ohms. Both of these are considerably smaller than the 2200 ohms (or more) of recent recommendations.

Now if a higher resistence just means that the LED shines less brightly, that is not a problem for me. The red LED serves no purpose other than to confirm that power is flowing through the circuit, so as long as it is visible I'm OK with it. However, 2200 is between 5.6 & 27 times my calculated figures, which strikes me as a bit excessive.

I have no doubt that the problem is with my understanding/calculations, not the advice I have been given, so can anyone explain where I am going wrong?

Thanks.

[John78]

How about considering RS number 228-5607, its a whole 20p, but, its 12v.

https://uk.rs-online.com/web/p/visible-leds/2285607/

 

[Demonperformer]

Well, 24p including VAT, but I'm not going to throw the rattle out of the pram over that.

So would that mean that I would not need to include a resistor at all - just attach it directly to the end of the uv strip?

[Demonperformer]

Ah, reading a bit further down (why didn't I do that in the first place?) I see that it does.

Nice one,John. Thanks.

[Stub Mandrel]

25 minutes ago, Demonperformer said:

Now if a higher resistence just means that the LED shines less brightly, that is not a problem for me. The red LED serves no purpose other than to confirm that power is flowing through the circuit, so as long as it is visible I'm OK with it. However, 2200 is between 5.6 & 27 times my calculated figures, which strikes me as a bit excessive.

The higher figure is an instantaneous peak value - the most you can apply for a fraction of a second without blowing it.

The smaller value is for maximum brightness. We're assuming you just want to see if it's on or off and want to maximise it's lifetime.

Also, bear in mind that the eye perceives brightness logarithmically so a 5.6x reduction will not be hugely noticeable.

 

[Demonperformer]

Thanks, Neil, that makes sense.

I'm thinking now the 12v one suggested by John will be better as I will save myself the cost of a resistor, the space it would occupy and one solder connection.

[John78]

4 hours ago, Demonperformer said:

Thanks, Neil, that makes sense.

I'm thinking now the 12v one suggested by John will be better as I will save myself the cost of a resistor, the space it would occupy and one solder connection.

Correct the 12v version includes the right resistor embedded in the package, they're especially useful on boards where space is constrained.

[Demonperformer]

Perspex discs have arrived and have to say I am unimpressed with the frosted one. Massive crack across one surface which shows up nicely as a black mark in this photo taken with daylight through curtains:

My initial thought was to go ballistic, but, as this is not a particularly good photo (needed a third hand - one to hold the disc, one to hold the phone, one to push the focus/capture buttons) I had the inspiration of using my current ELP to light it on the table, and got this:

the crack has disappeared.

Now I don't know whether this is going to be any use for a flat field or not - first photo would suggest 'not', second photo would suggest 'why not?'. If I am going to shout at the manufacturer, I really ought to get on with it, but if it's going to be all right, life's too short. Unfortunately, I am nowhere near sorting out the problem of the "container" (I should have got one disc bigger than the other to make life simpler), so I'm not going to be able to set it up and test it before I should complain if I am going to.

So what do people think? Will this work or will I need a replacement?

Thanks.

[The Admiral]

31 minutes ago, Demonperformer said:

So what do people think? Will this work or will I need a replacement?

Can't be sure of course, but since the flat light source will be far from in focus when on the scope, unlike in the photo where it is in focus, I'd have thought it would be OK.

But that's not the point really. The fact is you've been sold a faulty item, and who knows, the crack might propagate. If you email them with the first picture, they might just send you another. You don't know until you ask.

I had a disk cut by SimplyPlastics, and was happy with the service.

Ian

[Demonperformer]

Thanks, I'll do that.

Invite them to comment before I post my review to Trustpilot ...