Confused by circuits - need some help please!

[timtim86]

Hi everyone,

I have a problem that I need to sort and I'd be really grateful if you could help me out.

At work we have a 15" LCD screen built into a stand that shows a slide show of pictures from an SD card. It is powered by a 12V battery (they have to be mobile) and recently it broke.

Now the powerpack (the battery and a few other bits that I'll come to) is housed in a box and this is what has died. I'll try and explain the circuit as best I can.

It goes - 12V battery (Yuasa NPC24-12), wired to a box with a turnable knob on top (protected by a 5A fuse), wired to a 2A fuse, then to an output to the monitor.

I have no idea what the box with the turnable knob on top is - would anyone be able to tell me? It seems like a variable resistor or something, would that be right? Would I be utterly stupid to think that it acts to lower the current between the battery and the screen? Basically it is this box that is broken - I've looked at it and it's fried on the inside, one of the connectors on the PCB board is melted. I need to buy a new one of these...

Thank you! Tim

[timtim86]

Possibly it's a potentiometer?

By the way, there are two inputs (+ve and -ve) into the circuit with the variable knob from the battery, and two outputs (+ve and -ve).

[peterb]

Sounds more like the cap of the fuseholder. Picure would help. I can't see why what is essentially is probably a fixed voltage regulator would need a potentiometer.

If a connector has melted, it has been carrying too much current which indicates that there is a failed component on the cct board. Although in that instance it should have blown one of the fuses.

Does the monitor have a low voltage powersupply (most of the small ones do) in which case the box will be a fairly simple regulator (can it charge the battery too) or does it have a mains type input (in which case the box will be a voltage invertor)

[timtim86]

This is probably ridiculous, but i've never actually known how to upload a photo to these forums...any idea? I have them on my desktop.

[timtim86]

Nevermind my last one!

The black thing at the bottom of the blue box is the fuse holder.

[timtim86]

Sounds more like the cap of the fuseholder. Picure would help. I can't see why what is essentially is probably a fixed voltage regulator would need a potentiometer.

If a connector has melted, it has been carrying too much current which indicates that there is a failed component on the cct board. Although in that instance it should have blown one of the fuses.

Does the monitor have a low voltage powersupply (most of the small ones do) in which case the box will be a fairly simple regulator (can it charge the battery too) or does it have a mains type input (in which case the box will be a voltage invertor)

I know one of the fuses should have blown before the circuit board - I bet someone has put the wrong fue in at some point.

The battery is actually attached to a charger of its own, indepent of the blue box. You only run the monitor off the battery.

I have no idea if the monitor has a low voltage power supply!

[peterb]

Check the power plug on the monitor - if you can get to it - but it will probably be low voltage, te cct board is probably just a voltage regulator.

(Think youi needto post photos on some other site that accepts them - then link to it)

[timtim86]

Check the power plug on the monitor - if you can get to it - but it will probably be low voltage, te cct board is probably just a voltage regulator.

(Think youi needto post photos on some other site that accepts them - then link to it)

The photos are up there ^^^

Would it be a voltage regulator even if the fuses go from 5A into the blue box, then 2A from the blue box to the screen? Cos it seems like the amps are being reduced?!

[peterb]

Wherever you posted the phots is blocked by the system firewall...

Fuses protect the circuit they are connected to. The 5 amp fuse protects the battery if the cct board fails. The 2 amp protects the cct board if the monitor fails. The voltage regulator won't be 100%efficient, so there will be more current drawn from the battery than will be supplied to the monitor (the difference gets turned into heat by the regulator.

5amp and 2 amp fuses, in the place you describe, are not unreasonable values for this scenario.

[aidanjt]

Yes, that's the purpose of a votlage regulation circuit, to manage the level of current coming in, and going out.

[peterb]

[Duplicate - deleted]

[peterb]

Yes, that's the purpose of a votlage regulation circuit, to manage the level of current coming in, and going out.

Yes - indirectly. You have a 12 volt battery (as a lead acid cell, cpable of delivering several hundred amps under short cct conditions.

Say the monitor has an input voltage of 5 volts. Apply 12 volts to the monitor, and some components will break down (in an electronics sense), too much current will flow, destroying the semicionductor junctions that have broken down, heat, and misery all round.

The voltage regulator will reduce the 12 volts (DC) to 5 Volts - so the monitor operates correctly. The fuse is there to protect the voltage regulator if the monitor fails for some other reason (or the power cable shorts) . Monitor draws too much current - fuse blows. However a well designed voltage regulator should also have a current monitor in the circuit too - which will shut off the power if too much current is drawn. The fuse is then a backup. (And see my post http://forums.hexus.net/hexus-hardwa...nary-tale.html - which describes what happened with a poorly designed cheap (computer) power supply)

[timtim86]

So there's been another twist in the tale - I went out and bought exactly the same voltage regulator again (it was from Maplin) and wired it all up in the same fashion. Now the 5A fuse between the battery and the voltage regulator is blowing everytime, do you have any idea why this would be? It's really bugging me! The fuse is sort of wired in parallel to the battery. Help!

Thanks, Tim

[peterb]

Ok - I can see the pics now (and that is a variable resistor - which I guess is to control the output voltage - or perhaps the overcurrent threshold) - what is the Maplin part number?

[timtim86]

The part number is RN41U on the maplin website. Thanks Peter, hopefully we'll get to the bottom of this one!

[timtim86]

I did notice actually that the max constant current for this part is 3A. I used a multimeter on the battery and it said that it was putting out 5.5A. But the whole screen/battery package did absolutely work when we first received it...very confusing.