Repair of a thermal printer - Page 1
Forum section: electronics / repair

author: shockpoint
author_rank: Contributor
timestamp: November 04, 2024, 01:31:09 am
content: Hi all,Question about selecting a fuse if the SMD fuse marking has been blown off by the heat generated when the fuse blew.I am repairing a QL 1100 NWB Thermal printer from Brother.These have a Texas Instruments 54335A voltage buck converter on board. These are notorious for failing short, where the input pin is shorted to GND. I believe there is another post on EEVBlog that shows its failure in a Bose Speaker.I have isolated the issue to the 54335A Buck converter on the board. On the Vin line, there is a fuse that links the Power Board's +25V input to the buck converter.The buck converter's function is to produce 5V, which is used directly, and indirectly as 3.3V, both of which are for powering the microprocessors on the PCB. The +25V input also goes directly to the mechanical roller and thermal heating function.On the board, the section marked for the 54335A reads: 50V, 2AThe fuse unfortunately is illegible. It is an SMD fuse and I think it is 0805 or 1206 size. What I can make of it suggests it might be a LittelFuse N or a P marking.I've gone to the datasheets and this corresponds to a rating of 2A and 3A respectively. That seems to be consistent with the marking on the PCB itself.Can someone shed light on guidelines on how to select a fuse? Anything else I need to consider? I don't want to over-rate a fuse and end up with current blowing my traces up.Thank you

author: BILLPOD
author_rank: Frequent Contributor
timestamp: November 04, 2024, 07:10:23 pm
content: Good Morning Shockpoint, I would start at the Brother Manual page:https://support.brother.com/g/b/manualtop.aspx?c=us&lang=en&prod=lpql1110nwbeusGood luck and let us know how you made out.

author: fzabkar
author_rank: Super Contributor
timestamp: November 04, 2024, 08:38:53 pm
content: These are the only photos I could find:https://www.jabsol.com/assets/Image/Product/detailsbig/D00LWG001.jpghttps://www.jabsol.com/assets/Image/Product/detailsbig/D00LWG001-1.jpghttps://www.jabsol.com/assets/Image/Product/detailsbig/D00MG2001.jpgI think I can see a fuse (F1 ?) which is identified as "50V ?A TI" . It is located between the 4-wire cable and electrolytic capacitor in the second photo.

author: shockpoint
author_rank: Contributor
timestamp: November 04, 2024, 11:54:32 pm
content: Hi fzabkar, thanks for your reply.Thank you for the photos.The fuse in question has been marked with a red circle on the photo you kindly shared.This area has the marking 50V 2A TI.This fuse has a marking that LOOKS like an N or a P or an R. I'm very inclined to think it's a P because what marking that remains looks a lot like an F (which is only 0.5amps)Do you think that my suspicion is on the right track? Or should I be rating my fuses higher than that?I think I will purchase both the R and the P for good measure.Does anyone know what size these fuses on the board are? do they look more like 1206?Thank you

author: shockpoint
author_rank: Contributor
timestamp: November 04, 2024, 11:55:45 pm
content: Quote from: BILLPOD on November 04, 2024, 07:10:23 pmThe brother manuals are not service manuals, and I am actually using the service manual for a QL 700 and QL 1050 to get an idea of the block diagram and the mechanical side of things.they do not contain pcb component numbers

author: fzabkar
author_rank: Super Contributor
timestamp: November 05, 2024, 12:49:58 am
content: What is the marking on the fuse on the other side, and how does this marking relate to the writing on the PCB? Does "TI" refer to time delay or time-lag fuse??? Texas Instruments???

author: shockpoint
author_rank: Contributor
timestamp: November 05, 2024, 11:50:33 pm
content: Hi fzabkar,I looked at the fuses on the other side of the PCB, they have a P fuse for 50V 3A.I clarified the TI, I think it actually is [50V 2A T], so it's a square bracket.What's confusing for me is that I don't know what 50V 3A stands for because the P fuse is rated for 3A; my assumption is that it blows at 3AIt would make sense if 50V 3A is a stated maximum rating for that area of the board. But if it's a running norm, then a 3A fuse would likely blow all the time.So then, I'm not clear on the 50V 2A rating; do I pick something for 2A or get a higher rated fuse?

author: fzabkar
author_rank: Super Contributor
timestamp: November 06, 2024, 01:34:46 am
content: Quote from: shockpoint on November 05, 2024, 11:50:33 pmA 3A fuse should be able to carry 3A for its entire service life.You can see the behaviour in "Average Time Current Curves":https://m.littelfuse.com/media?resourcetype=datasheets&itemid=5efd7580-3a45-4eb9-9869-f350543371fa&filename=littelfuse-fuse-483-datasheet(page 3)I expect that "T" would mean "time lag" or "time delay" or "slow blow".

author: shockpoint
author_rank: Contributor
timestamp: November 08, 2024, 02:46:38 am
content: Hi fzabkar,Thanks very much for the insight - was wondering if it's slow blow or fast blow, your comment certainly helps.OK, that clears it up! in that case sounds like a 2A fuse means it can carry 2A for its service life. Had a quick peek at the time current curve too.Will report back!