Laser Power Supply

[mechelec]

Hi,

I recently bought a secondhand power tool that has a non functioning laser.
I narrowed the problem to the power supply for the laser.
The PS has what appear to be 10 ohm resitors on the 240VAC lines.
One of them measured ~12 ohm and the other was opem circuit.
Can any one tell what the purpose of the resistors are and whether I can replace the faulty one with a normal 10 ohm resistor,

Thanks,

James

[Mechatrommer]

that is a very strange power supply. i believe its have been modded.

[scrat]

They're maybe there for inrush limiting, ie to limit the initial current that flows in when the rectifier's capacitor is discharged.
If you replace them with other similar value resistors (same nominal resistance and power), I see no reason why it shouldn't work.

[tecman]

There would NOT be a 10 ohm resistor across the line.  As Scrat mentioned, it could be a thermistor, but that should be in series with the line, to limit inrush to the filter cap.  Usually they should be somewhat higher than 10 ohms.

paul

[Mechatrommer]

and they should be in the black box in the first place.

[Psi]

Probably added after the product was released when they started getting returns from dead units.

[tecman]

Also possibly they could be fusible resistors.  Limit inrush and open like a fuse in an overcurrent condition.

paul

[scrat]

Is there anyone that can be almost sure about those resistors not being real resistors? They are marked as resistors, so why should they be something else?
IMHO, this seems like a bad patch (not even a little plastic/rubber packing) on a power supply not requiring high efficiency, so could be resistors.
Anyway, since now it doesn't work, I'd try with resistors of the same size, I doubt it would break anything.

[mechelec]

Thanks for all your responses.

I had the same thought as you scrat, anything other than a straightforward resistor would not have standard markings.

Also if they were fused it would be strange to have them on both lines.

Btw they did have a nice heat shrink cover when I opened it up originally.

Anyway I will solder a 10 ohm resistor when I get a chance and see how it goes.

James

[tecman]

Is there anyone that can be almost sure about those resistors not being real resistors? They are marked as resistors, so why should they be something else?
IMHO, this seems like a bad patch (not even a little plastic/rubber packing) on a power supply not requiring high efficiency, so could be resistors.
Anyway, since now it doesn't work, I'd try with resistors of the same size, I doubt it would break anything.

Usually you can not tell by looking.  For example:  http://www.tokenonline.net/resistor/fusible-resistor.htm

They are in more things than you might think.

paul

[scrat]

Is there anyone that can be almost sure about those resistors not being real resistors? They are marked as resistors, so why should they be something else?
IMHO, this seems like a bad patch (not even a little plastic/rubber packing) on a power supply not requiring high efficiency, so could be resistors.
Anyway, since now it doesn't work, I'd try with resistors of the same size, I doubt it would break anything.

Usually you can not tell by looking.  For example:  http://www.tokenonline.net/resistor/fusible-resistor.htm

They are in more things than you might think.

paul

Interesting... Somewhere it appears like fusible havea fifth white stripe, while here http://www.epanorama.net/documents/markings/resistor_colorcodes.html] [url]http://www.epanorama.net/documents/markings/resistor_colorcodes.html[/url ] they say all white or blue resistors are flame resistant or fusible.

Is it white the color in the posted pic? Light it quite weak there...

[mechelec]

Interesting... Somewhere it appears like fusible havea fifth white stripe, while here http://www.epanorama.net/documents/markings/resistor_colorcodes.html] they say all white or blue resistors are flame resistant or fusible.

No, the resistors are a grey colour.

However as paul mentions you can't tell by looking, and I'd hazard that the colour is arbitrary as well.

This application note gives a good overview of the use of fusible resistors and as far as I can tell the resistor bodies are grey.

http://www.welwyn-tt.co.uk/pdf/application_notes/EMC2_AN_A.pdf

I guess I will try and get hold of a fusible resistor and replace it with that.

But I still can't figure out why they would have fusible resistors on both the active and neutral side.

Surely if the idea is to limit the inrush current you would just use a 22R on the active side.