RFID reader no power. - Page 1
Forum section: electronics / repair

author: macd@polo
author_rank: Newbie
timestamp: December 18, 2023, 07:22:50 am
content: Hi all i am working on Rf Id reader bit all of sudden it went dead. i am getting voltage if i measure at the input. but no LED's are coming on.after opening i discover one leg TVs diode ( 1.5KE24CA ) is burned . could that be the reason for no power in circuit. what can i used if don't have TVs diode in Rf Id reader. what type of diode can i used cause i don't have 1.5KE24CA and will 1.5KE18CA not work?

author: m k
author_rank: Super Contributor
timestamp: December 21, 2023, 06:19:42 am
content: TVS is a protection device.It's not an active part during normal operation.There is something else that is a result of something big.Left of diode seems to be a voltage converter.It might be a victim.

author: macd@polo
author_rank: Newbie
timestamp: December 21, 2023, 05:49:14 pm
content: Hi Thanx for feedback , i have attached new clear picture, which one is the voltage convertor, is it the one i have circle in black?

author: m k
author_rank: Super Contributor
timestamp: December 21, 2023, 07:56:15 pm
content: No, both white and black are surge protectors.The voltage converter is the whole section on right.How much is input voltage?Type parts 24 and 18 of TVS' are intended voltage ranges above what the protector starts protecting.Yellow standing round thing beside the blue connector is named F3, so it seems to be a fuse of some sort.S-E of 330 thing is LED10, it should lit when voltage converter is operational.A bit more right and down is TP_5V, from there you can measure that 5V is present.A bit under F3 is D1, both sides of it must have input voltage.After that things become more complicated.

author: macd@polo
author_rank: Newbie
timestamp: December 28, 2023, 10:45:43 am
content: thanks very helpfull.

author: MathWizard
author_rank: Super Contributor
timestamp: December 30, 2023, 02:19:38 am
content: F3 looks like a thermistor or re-settable fuse. The white protection thing looks like a gas discharge tube or something similar like a MOV. If the black one is a TVS, with one end on Vcc in, where does the other end go besides to F3 ?The diodes for the switching section are D1 and D2.Is there a label on the IC ? What voltages are on it ?

author: BertB
author_rank: Newbie
timestamp: December 30, 2023, 03:10:34 am
content: The item you circled in black seems to be a Gas Discharge Tube. It is a surge device where it shorts when there is excess voltage. It should not be shorted in normal operation, and also when the board is not energized. You could check the resistance across it.Also, in your first picture, it looks like the component E2 is not securely on the board. It looks a bit sideways on the board in the clearer picture. Is that E2 properly attached?

author: macd@polo
author_rank: Newbie
timestamp: January 01, 2024, 07:12:34 am
content: the other end from tvs goes to ground.

author: inse
author_rank: Super Contributor
timestamp: January 01, 2024, 08:25:33 am
content: BS removed

author: darkspr1te
author_rank: Frequent Contributor
timestamp: January 02, 2024, 08:00:01 am
content: U8 is the L4978 from ST .https://www.st.com/resource/en/datasheet/l4978.pdfThere is pinout and example schematic in the datasheet.darkspr1te