Fixing a laboratory DSC

[vrasp]

Re: Fixing a laboratory DSC

Here is where you can find a cheaper fuse series

https://www.amazon.com/Littelfuse-BL.../dp/B07DSPZPX7

Thanks I ll have a look.

[sam_sam_sam]

Re: Fixing a laboratory DSC

Here is where you can find a cheaper fuse series

https://www.amazon.com/Littelfuse-BL.../dp/B07DSPZPX7

[vrasp]

Re: Fixing a laboratory DSC

Thanks guys. It s much clearer. I think I ll be using that bulb method much more often from now on.

[sam_sam_sam]

Re: Fixing a laboratory DSC

One note KTK-R-1 is a faster blowing than a KTK-1 fuse

There are cheaper fuses if you do not need 600 volt voltage rating

I once troubleshooted a machine that was blowing fuses on the controls circuit used the incandescent light
( 100 watt ) bulb instead of the the fuse to find the short in this machine if your light bulb is very bright and you only have a one to two amp fuse in this circuit then you definitely have an issue with the this circuit

[eccerr0r]

Re: Fixing a laboratory DSC

Well the example did not indicate what the device under test (DUT) is represented by - it's just generic behavior of the circuit. The DUT could have been either the parallel or single bulb in series. So yes if you hook up two in parallel and treat the singleton one as the DUT you can see that the DUT will need to pass more current and is indicated by such as it'll burn brighter.

That being said you don't want to allow 6A to go through continuously which is what will happen if you hook up a lot of bulbs in parallel. You only want only one or a few to start off to find "obvious" or critical shorts and then increase them as your confidence that the short has been solved.

[vrasp]

Re: Fixing a laboratory DSC

I had to go back to reading on electricity basics but I think I understand the use of lamp bulbs as a current limiter.

The part I have a hard time understanding is the number of bulbs to use. If I want to test the device that takes in 6A at 230v, wouldn't the one bulb at 1A be super bright? Then I wouldn't be able to tell if it's bright due to a short or if It's normal.

Same with the other device that runs at 2A 230v. Wouldn't just one bulb be super bright even if there is no short?

[stj]

Re: Fixing a laboratory DSC

lamps in parallel will allow more current.
lamps in series are pointless unless they are the wrong voltage.
i mentioned 2 in series because i thought you were going to do it with the 240v side and being canadian your lamps are 120v

and dont give it 6a or a fault could cause a fire.
if you disconnect the load from the power supplies then 1a should be more than enough to get them started.
which is unlikely because we are searching for a fault!

[vrasp]

Re: Fixing a laboratory DSC

It s more clear but there is still something I dont get.

Will bulbs in parallel limit more current then in series?

120v with 100w bulbs is about 0.83 amps. Should I put 7 of these in series to test the 6A Midec?

[eccerr0r]

Re: Fixing a laboratory DSC

Think of it this way: one bulb straight connected to 120V... will not burn out
Two bulbs connected parallel straight to 120V... will not burn out, and more the same... but more current is being drawn with each bulb added.

Now connect two bulbs in series. Note that each bulb will be dimmer than before. Measure the current flowing through the two bulbs in series. It's necessarily lower than before! Double up on one of the bulbs. Note that the parallel bulbs are now dimmer than the single bulb...which will now be brighter! Now short out the parallel bulbs... The parallel bulbs will go out but you won't trip a fuse, and the single bulb will be full brightness.

Don't know if that blurb above makes sense but that's how we're taking advantage of series bulbs to limit the amount of current going through a circuit.

---

Hmm... that thing... now you might have to open it and look inside...

[vrasp]

Re: Fixing a laboratory DSC

My mistake. Yes, the fuse is before the transformer so it s at 120v. I have more pictures of the setup but it s quite messy. Lots of cables everywhere. The Mitec is on the left.

So if I have let s say, hypothetically, five 120w bulbs in parallel instead of the fuse, and there is a short to ground somewhere. This is what I understand: The current drawn will try to increase until something breaks and the circuit is open. Let s say each bulb only allow 1 amp. Will the rest be dissipated as heat? Or will the bulb break?

[stj]

Re: Fixing a laboratory DSC

the limit is the lamps - so in that instance, 100w
at 240v that would be about 400mA

your just trying to stat the power supply's and not run the whole unit at this time.

[eccerr0r]

Re: Fixing a laboratory DSC

F**K those KTK fuses, that's those stupid fast blow midget fuses they use in multimeters and we had that other rant thread somewhere on BCN.

The reason why light bulbs "limit" the current is that it's connected in series with the load. Now unless your device somehow has negative resistance (I'd love for someone to give me a resistor with negative resistance... things that are piecewise negative don't count) the amount of current going through the bulbs can only max out if your device was completely shorted out.

Now you probably don't need a whole bunch of bulbs in parallel - yet. Unless you're sure the short is in the cooling/heating circuitry. However since this is a KTK fast blow fuse, it might be troubling and more pictures of how things are connected and what it's driving would be interesting. Normally you do not use KTK fuses on SMPS's because of inrush!

As a rule of thumb, since 120V at 1A is 120W, a 100W bulb is a bit under 1 amp. You can make a rough guess at how many bulbs you need in parallel.

So where is the fuse actually, is it in the 230V side or 120V side, I thought it was 120V wall plug - switch - fuse - step-up transformer 230V - that MITEC PSU that needs 230V.

[vrasp]

Re: Fixing a laboratory DSC

no, your using it to limit the current - not allow it all
a couple of 100w 120v lamps in series will do for now

How do I know how much current I m limiting? Wouldn't I want enough bulbs that the current doesn't go over 5A so I dont damage anything? Also the voltage on the line is 230v.

Thank you!

[stj]

Re: Fixing a laboratory DSC

no, your using it to limit the current - not allow it all
a couple of 100w 120v lamps in series will do for now

[vrasp]

Re: Fixing a laboratory DSC

series lamp-bulb time.

btw, can you show us this amazing $50 fuse??

The fuse is a KTK-5 Bussmann. 5 amps 600VAC. It s actually $50 canadian.

Can I really use the lamp bulb method in this case? The line is 230v. If I understand correctly, I would need a 230v*5A=1150w bulb? That sounds like a pretty powerful lamp bulb. Maybe as expensive if not more than the fuses. Correct me if I m wrong.

[stj]

Re: Fixing a laboratory DSC

series lamp-bulb time.

btw, can you show us this amazing $50 fuse??

[eccerr0r]

Re: Fixing a laboratory DSC

just a hunch because they went out of business it seems (website is gone?!) and switchmode PSUs tend to have their own set of failure mechanisms that aren't shared with linear PSUs.

But need to look at that PSU and confirm, it's only a speculation at this point.

[vrasp]

Re: Fixing a laboratory DSC

Of course! Haha

How do you know Midec is unreliable? Or are you saying this because of the switching power supply?

[eccerr0r]

Re: Fixing a laboratory DSC

BadCaps.Net

[vrasp]

Re: Fixing a laboratory DSC

Interesting. I ll take a closer look at that Midec. It does have a fan on the side which I suppose is next to a heatsink inside.

By the way what's BCN?