Simple 12v overvoltage circuit.. - Page 1
Forum section: electronics / beginners

author: almamater
author_rank: Regular Contributor
timestamp: August 17, 2021, 11:03:02 am
content: Hello everyone.I made a simple circuit with (some 555, Leds and Buzzers), ~80mA -100mA máx. 12v.For the power supply, i want to use a simple transformer (Transformer 220V-12V ~300\500mA), but I thought it would be better to add an additional protection, if the transformer breaks down and more than 12v shows up..So, if i put something like this:12v 3W Zener diode.Will it protect the circuit?

author: iMo
author_rank: Super Contributor
timestamp: August 17, 2021, 02:39:55 pm
content: It will - provided the source voltage (the "input voltage" before the fuse) will be 2 to 3V lower than the zener voltage.Otherwise the zener gets conductive and a large current will flow through the zener to ground.The zener does not work as a "precise voltage switch" - the so called "zener current knee" is rather wide.So, for say a 12V "input voltage" you would need something like 15V zener (or a better choice would be a 15V TVS aka "transil" diode).PS: the "input voltage" has to be a DC voltage - thus after the transformer (AC output) you would need a diode rectifier with capacitor.Likehttps://electronicsclub.info/powersupplies.htm

author: Zero999
author_rank: Super Contributor
timestamp: August 17, 2021, 07:34:51 pm
content: The circuit will blow up, if it's connected to a 12V transformer, bridge rectifier and capacitor. Transformers are specified in RMS output voltage, whenunloaded. The voltage on the DC side of the rectifier will be the peak voltage, minus a small voltage drop in the diodes. The peak voltage is √2 the RMS voltage of a sine wave and the unloaded voltage can easily be 20%, over the specified voltage, for a small transformer, giving 1.2*1.424*12 = 20V.If the 12V circuits you're powering don't need a regulated power supply, use a 9V transformer and make sure they can still handle higher voltages, up to say 18V, as the mains voltage could also be higher, than expected. If you need a regulated supply, use a slightly higher voltage transformer, say 15V, to give plenty of headroom and the LM7812.

author: almamater
author_rank: Regular Contributor
timestamp: August 17, 2021, 07:47:20 pm
content: Thanks for the answers so far, I was actually thinking about using a 'transformer' but like this:(adapting it to my circuit\project.)

author: Zero999
author_rank: Super Contributor
timestamp: August 17, 2021, 09:44:52 pm
content: Quote from: almamater on August 17, 2021, 07:47:20 pmThat's not a transformer. It's a regulated power supply.

author: almamater
author_rank: Regular Contributor
timestamp: August 18, 2021, 02:22:40 pm
content: I apologize for my lack of detail, in any case I appreciate the explanation above which is very useful.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 18, 2021, 02:58:29 pm
content: boa Tarde. Try this classic pé de cabra,https://theorycircuit.com/crowbar-overvoltage-protection-module/Make sure your thyristor is rated well above the fuse rating.A zener as show will pass unlimited current as the voltage inreases and then melt.The transformer in your power supply is very unlikely to fail and create an over voltage. Usually they fail and there is no voltage!You might want to protect the input from overvoltage spikes and transients with an MOV across the primary eg MOV-07D391Khttps://www.electronics-project-design.com/Varistor.html

author: mariush
author_rank: Super Contributor
timestamp: August 18, 2021, 03:25:47 pm
content: I'd just use a linear regulator with low dropout voltageFor example, LP2950 / LP2951 has a dropout voltage of 0.38v at 100mA - put it on the input, configure the output to 11.5v and you're done.It's around 20-30 cents even in small quantities :https://www.digikey.com/short/th80mz3rOne more observation : don't expect a power supply (that wall wart adapter, or whatever) to output exactly as it says.For example, let's say that wall wart adapter is advertised for 12v 0.5A ... 6 watts maximum. The adapter may be configured to intentionally output more than 12v, let's say 12.4v, to account for the losses in the wires from the wall wart all the way to the end of the cable when something consumes 0.5A.So a device that consumes only 50-100mA may see 12.4v , but a device that consumes 500mA may see 12v or even less, because the wires between the adapter and the plug have resistance and that resistance causes a voltage drop.

author: TimFox
author_rank: Super Contributor
timestamp: August 18, 2021, 03:37:14 pm
content: Quote from: Zero999 on August 17, 2021, 07:34:51 pmWorse than that: transformers themselves are normally specified by rms voltage at a given (usually max rating) current, so the no-load (at zero or low current) is actually higher. This can be estimated (lacking a spec) if you know the efficiency at rated load.

author: Zero999
author_rank: Super Contributor
timestamp: August 18, 2021, 07:42:15 pm
content: Quote from: TimFox on August 18, 2021, 03:37:14 pmYou're right. I got that backwards. I should have said loaded.In addition to the above.Quote from: almamater on August 18, 2021, 02:22:40 pmIn addition to my previous comment about it being a regulated power supply. It's also switched mode, which means it might have a lot of high frequency noise on the output, which will require filtering for some circuits to work properly, especially things like microphone amplifiers, which have low level signals.