Transistor Problems - Page 1
Forum section: electronics / beginners

author: nomanland
author_rank: Newbie
timestamp: January 30, 2013, 06:29:56 am
content: I have just started making my nixie clock and am having problems with my bf469 transistors. I have one for each digit in the lamp and have them working through a shift register. They work fine running them with and led but when i hook up a nixie and the high voltage power supply the lamp stays on even with no signal to the transistor. Its probably something stupid i'm doing. I have double tested and triple tested them and am stumped.Any help would be greatly appreciated.

author: Psi
author_rank: Super Contributor
timestamp: January 30, 2013, 06:38:49 am
content: We need to see a circuit diagram.-Depending on the shift register your using the transistors base maybe floating high at times.-Your transistors maybe connected in such a way that's inverting the logic output from the shift registers.-You might need some pullup/downsIt's really hard to say without a diagram.

author: nomanland
author_rank: Newbie
timestamp: January 30, 2013, 06:53:17 am
content: the transistor is not hooked up to the full circuit when using it with the lamp, I just hook up the collector and emitter to the negative side of the lamp, no connection to the base and it lights up.

author: Psi
author_rank: Super Contributor
timestamp: January 30, 2013, 07:14:49 am
content: Quote from: nomanland on January 30, 2013, 06:53:17 amThe base of a transistor has to be connected to something, otherwise it will pickup random noise and may turn-on by itself.

author: nomanland
author_rank: Newbie
timestamp: January 30, 2013, 07:25:00 am
content: Cheers, ill set it all up and see how it goes.

author: nomanland
author_rank: Newbie
timestamp: January 30, 2013, 07:47:23 am
content: I set it up and tried them in circuit but had the same problem.http://members.shaw.ca/greg_powell/downloads/SputnixD.pdfHere is the schematic, im using a 74hc585 shift reg instead and the bf469. I am also using an arduino setup instead of the 8515.

author: Psi
author_rank: Super Contributor
timestamp: January 30, 2013, 10:46:41 am
content: Check that you're driving the shift register correctly.The 74HC595 has a storage register and output enable pins which need to be used correctly.

author: nomanland
author_rank: Newbie
timestamp: January 30, 2013, 11:31:49 am
content: I have just been using an example program that cycles each output on and off. I tested it on some leds which works fine. but when connected up all elements in lamp light up.I really have no clue what im doing wrong.

author: SeanB
author_rank: Super Contributor
timestamp: January 30, 2013, 03:45:45 pm
content: Try changing the 33k base resistors to 10k, you might have leakage current being enough to turn the transistor on. As well drop the supply voltage to around 150-180V.

author: CarlG
author_rank: Regular Contributor
timestamp: January 31, 2013, 07:37:03 pm
content: Have you inverted the output enable to the HC595? It's active high on the 14094 but active low on HC595.

author: AlphZeta
author_rank: Regular Contributor
timestamp: February 01, 2013, 04:35:58 pm
content: This sounds like an issue with 595's output is placed in high impedance mode. The circuit looks fine and should work.Here's my suggestion, could you measure the base voltage and the Vce of the transistor for a segment that you have switched on and compare those with the values of the transistor for a segment that you didn't turn on (even though it appears to be on). And I suspect that you will see that the Vce for the supposedly-off segments are higher than the turned-on segments because transistors weren't fully turned on.The reason this issue surfaced when using nixies is that they require very low drive current so they will be able to glow even when transistors are not fully on.

author: megajocke
author_rank: Regular Contributor
timestamp: February 05, 2013, 11:31:12 am
content: Are collector and emitter connected the right way round? Transistors work backwards, but with reduced breakdown voltage and current gain.

author: mark-r
author_rank: Contributor
timestamp: February 05, 2013, 04:48:21 pm
content: A pull down resistor from each base to ground might help, somethink like 10K maybe.Any reason for using BF469 transistors? They have Ic and Ft way in excess of what you need and if you're using TO126 versions they'll take a lot of room.