Power supply for 555 timer based flip/flop - Page 1
Forum section: electronics / projects

author: Jim-0000
author_rank: Regular Contributor
timestamp: April 30, 2019, 04:25:09 am
content: I have a little circuit wired up and working using the schematic on this page:http://www.circuitbasics.com/555-timer-basics-bistable-mode/It works fine for showing the state of an associated device when powered up with a battery; powered with anything from 6 volt to 12 volts.However, I want to power it up using a suitable power supply from the mains (240volt).I've salvaged a capacitor (transformerless) power supply out of another device given to me for the purpose (not sure what it is). This puts out 12 volts.When I power my flip/flop with this, it will illuminate the main LED, but will not reset when I press the reset switch. I just reverts to the first LED.I assume that the output is not clean enough ie: has too much ripple to allow the timer to function as designed.Corrections in this regard invited please?Secondly, what is an alternative power supply that will be clean enough?I have a very small output transformer that has a primary impedance of 1k which I might try to use in a transformer, full bridge rectified, electrolytic cap smoothed power supply. I don't know if this will work or not?Is someone willing to provide some advice and tips here please?Jim.Edit: I have found a recycled USB power supply that will run this unit at around 5.2 volts. However, I would like a 6.6/6.8 volts supply in order to supply a float charge to a LiFe backup battery ( 2x 3.3 volt cells).

author: Zero999
author_rank: Super Contributor
timestamp: April 30, 2019, 07:53:57 am
content: A transformerless power supply is a bad idea. It has no isolation from the mains, so the whole circuit needs to be treated, with the same precautions as the mains i.e. adequate creepage and clearances and the pushbuttons need to be double insulated.No one knows whether the small transformer can work because you've not stated the primary and secondary voltages. Presumably 1k is the series resistance?Presumably you're talking about a LiFePO4battery? In which case it's the safest type of lithium ion battery.

author: Jim-0000
author_rank: Regular Contributor
timestamp: April 30, 2019, 11:17:40 am
content: Quote from: Zero999 on April 30, 2019, 07:53:57 amYes, I will go for a transformer type.QuoteThat is the primary resistance. Secondary voltage is unknown at this time. But, what I am aiming for is around 6.6 to 6.8 volts to "float" charge the back up LiFe battery.QuoteYes. That is why I chose that type.Thanks for the reply.Jim.

author: mikerj
author_rank: Super Contributor
timestamp: April 30, 2019, 11:50:20 am
content: Quote from: Jim-0000 on April 30, 2019, 04:25:09 amBy "output transformer" do you mean an audio coupling transformer? If so you should not connect this to the mains, the insulation is unlikely to be sufficient for this application.

author: Zero999
author_rank: Super Contributor
timestamp: April 30, 2019, 12:32:23 pm
content: Yes, I missed that: an audio output transformer is not suitable for connecting to the mains!Regarding LiFePO4batteries: use use a power supply with a higher voltage and charge it with a current limited supply.

author: Jim-0000
author_rank: Regular Contributor
timestamp: April 30, 2019, 09:52:39 pm
content: Quote from: Zero999 on April 30, 2019, 12:32:23 pmYes, that is the plan for the LiFe charging system.

author: Jim-0000
author_rank: Regular Contributor
timestamp: April 30, 2019, 09:54:33 pm
content: Quote from: mikerj on April 30, 2019, 11:50:20 amYes, I since realised that. Thanks for stating this; most important.