Laser Power Supply - Page 1
Forum section: electronics / beginners

author: mechelec
author_rank: Contributor
timestamp: April 26, 2011, 05:16:25 am
content: 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

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 26, 2011, 05:20:23 am
content: that is a very strange power supply. i believe its have been modded.

author: scrat
author_rank: Frequent Contributor
timestamp: April 26, 2011, 11:34:49 am
content: 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.

author: tecman
author_rank: Frequent Contributor
timestamp: April 26, 2011, 05:56:31 pm
content: 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

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 27, 2011, 12:13:39 am
content: and they should be in the black box in the first place.

author: Psi
author_rank: Super Contributor
timestamp: April 27, 2011, 07:36:20 am
content: Probably added after the product was released when they started getting returns from dead units.

author: tecman
author_rank: Frequent Contributor
timestamp: April 27, 2011, 01:50:09 pm
content: Also possibly they could be fusible resistors. Limit inrush and open like a fuse in an overcurrent condition.paul

author: scrat
author_rank: Frequent Contributor
timestamp: April 27, 2011, 02:18:25 pm
content: 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.

author: mechelec
author_rank: Contributor
timestamp: April 28, 2011, 10:40:58 am
content: 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

author: tecman
author_rank: Frequent Contributor
timestamp: April 28, 2011, 02:59:15 pm
content: Quote from: scrat on April 27, 2011, 02:18:25 pmUsually you can not tell by looking. For example:http://www.tokenonline.net/resistor/fusible-resistor.htmThey are in more things than you might think.paul

author: scrat
author_rank: Frequent Contributor
timestamp: April 28, 2011, 04:01:50 pm
content: Quote from: tecman on April 28, 2011, 02:59:15 pmInteresting... Somewhere it appears like fusible havea fifth white stripe, while herehttp://www.epanorama.net/documents/markings/resistor_colorcodes.html] 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...

author: mechelec
author_rank: Contributor
timestamp: May 01, 2011, 04:04:49 am
content: QuoteNo, 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.pdfI 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.