Device planning: Question about powering indicator lamps - Page 1
Forum section: electronics / beginners

author: RacerR
author_rank: Contributor
timestamp: March 12, 2022, 03:39:28 pm
content: Hello, I'm currently planning to build a small benchtop device. I'm working through some of the basics - physical layout, external connections, and status lighting.My current question is around how best to power the indicator lamps. This device will be directly connected to mains power and I'm planning on using separate switches for the primary and secondary sides of the power transformer. I've put together a drawing showing two simplified versions of the power supply circuit:- Figure 1 shows what I think is a typical Power/Standby switch layout using a single pole single throw switch (1P1T) and an indicator lamp on both sides of the transformer.- Figure 2 shows an alternate layout where the both the primary and secondary indicator lamps are powered from the primary side.- Figure 3 shows another layout powering both lamps from the primary side, using a relay in this case.Are there any rules, best practices, or other guidelines I can consider here?Drawing updated to include Figure 3, using a relay.Thanks, any input provided is appreciated.-Rob

author: Ian.M
author_rank: Super Contributor
timestamp: March 12, 2022, 04:24:51 pm
content: Fig 2 is unsafe unless the switch has reinforced insulation between its poles. Many switches only have functional insulation internally.The current drawn by a LED indicator for low voltage circuits or a Neon indicator for mains voltage AC circuits is totally negligible compared to typical line frequency transformer secondary current ratings. It is only likely to be an issue if using ultra-miniature sub-1VA transformers.

author: Decapitator
author_rank: Contributor
timestamp: March 17, 2022, 09:54:23 am
content: If you wish to use identical 120v indicators for whatever reason, you could use a triac type optocoupler such as an MOC3010 or similar as a lamp driver. The LED side of the coupler only needs about 10mA DC which could be derived from a current limiting resistor, a diode, and a small capacitor.

author: Berni
author_rank: Super Contributor
timestamp: March 17, 2022, 10:49:51 am
content: An indicator LED should be drawing so little power that it does not matter unless you have a really really tiny transformer.It is nice to have indicator lights directly powered from the relevant power supply since then the indicator not only indicating that the switch is turned on but also indicates that the transformer is working and there is indeed power present on the output.If the issue is getting identical looking indicator lamps then you can just buy 2 identical neon indicator lamps, then take apart one of them, snip out the neon bulb and replace it with a LED. As long as the indicator front glass is diffused enough it should look pretty close to a neon bulb. You might also want to use a different color lamp, in that case you don't have to worry about matching the LEDs light color with the neon lamps color.

author: gbaddeley
author_rank: Regular Contributor
timestamp: March 17, 2022, 10:54:04 am
content: Unless you can guarantee that the incoming active & neutral can never be swapped (due to bad mains wiring or bad power cable), the first switch should be double pole, to switch both active & neutral lines. The transformer primary should also be fused (eg slow blow, at say 2x maximum load current). Fuse the secondary as well for more protection.

author: RacerR
author_rank: Contributor
timestamp: March 17, 2022, 03:32:18 pm
content: Quote from: gbaddeley on March 17, 2022, 10:54:04 amAgreed, drawing updated.Quote from: gbaddeley on March 17, 2022, 10:54:04 amAlso agreed - I've left fusing out of drawing to keep focus on the power switch and indicator lamp planning.Thanks!-Rob

author: RacerR
author_rank: Contributor
timestamp: March 20, 2022, 07:14:26 pm
content: I've updated the drawing to show a 3rd layout (Figure 3) using a relay as the switch on the secondary side.Are there any concerns with running the relay coil in series with a neon indicator lamp?Thanks.-Rob

author: Ian.M
author_rank: Super Contributor
timestamp: March 20, 2022, 07:24:00 pm
content: Yes, The Neon runs at approx. 1mA and the relay coil will need far more current so in series wont work.

author: RacerR
author_rank: Contributor
timestamp: March 20, 2022, 08:51:36 pm
content: Quote from: Ian.M on March 20, 2022, 07:24:00 pmThanks, that makes sense. Drawing updated showing the indicator lamp parallel to the coil.-Rob