# 230vac / 5vdc power supply design

I am making my own basic bench power supply mainly for testing logic gate circuits, relays, flip flops etc, that I can move around to different locations.

The circuit of the entire schematic will be on a PCB inside a plastic enclosure. It is a 230Vac to 5Vdc regulated power supply. Below is my circuit design:

For the 5Vdc regulator I am using the L78S05CV voltage regulator IC.

1. Do I need to provide any other additional short circuit/overcurrent protection? According to the datasheet short circuit protection is built in to the IC (see below for sheet)

2. Any other issues/improvements with this circuit please feel free to say?

• That's rather a lot of work to get something that does the same job as a USB phone charger - 5 V, 2 A. Had you considered that? Jan 5 '19 at 18:55
• Buy a cell phone fast charger - more safe, compact and can actually reach 2 A. If you insist on making your own, at least make an adjustable voltage PSU - see the datasheet. Jan 5 '19 at 19:03
• C1 is going to have to be a lot larger value. Jan 5 '19 at 19:06
• Before you go further, calculate the power your regulator chip will dissipate, and check the thermal resistance specs in the datasheet. Now start shopping for a heat sink that can bring down the thermal resistance by a factor of 5 or 10. Jan 5 '19 at 19:12
• @Unimportant Yeah, that's a mistake in the schematic. Should be more like (1amp * 50Hz) / 1v ripple = 20,000uF cap. This is a pretty big cap. Jan 5 '19 at 19:31

1. Literally any diode can respond to a 50Hz wave. But try a full bridge rectifier that can handle >1.5A, eg from here.

2. You could, for protecting the circuits under test. But that demands other components and schematic. See the datasheet of the regulator and do a quick search on short-circuit protection, to see what fits you.

3. Some suggestions:

• Bigger caps at the input and output of the regulator. Use electrolytic and MKT/MKP in parallel in order for the PSU to be able to respond both at low and high frequencies.

• At DC side, use a fuse >1A.

• Search for a heatsink that is suitable. As you have already noticed, you have quite a lot losses. Input voltage of the regulator is $$\V_{IN} = \sqrt{2} \cdot12= 17V\$$. Maybe find a transformer with less output voltage?

Have you considered buy a phone charger for now and design a SMPS? you will gain much more knowledge and experience.

• Great answer, thank you. Yeah, I will get something temporary. Jan 5 '19 at 20:42
• You forgot the voltage drops in the bridge rectifiier. The rectified peak voltage would be 17 - 2 * 0.7 = 15.6V
– Pzy
Jan 5 '19 at 21:39
• @Pzy That's right! my apologies. But still 15-5 = 10 V are still quite many volts. A transformer with $7 \tilde{V_{SEC}} = 9.87 V_{SEC,peak} \Rightarrow 9.87-1.4 = 8.5V$ would be a must more suitable choice. Jan 7 '19 at 0:12

Learning is one thing but building this circuit based upon something that would have made sense forty years ago just does not come across as smart. You will pay more just for the heatsink that will be needed to cool the 7805 in this design than you would for a typical 5V USB charging type device.

As a matter of fact the current trend of cycling through new smart phones every couple of years means that there are a plethora of smart phone chargers laying around everywhere. For example a quick trip to a donation outlet store such as Goodwill will typically turn out a shelf full of chargers that can be had for a buck or two!

These chargers are also based upon switching technology so can provide you a whole 5V 2A output with very little heat in the converter.

• I think you are correct, I won't waste the money building this circuit. I forgot about the power loss in the 7805 IC. Jan 5 '19 at 20:34
• I agree that today it's better to get some cheap tools that others have designed and learn whatever you are motivated to learn with the use of those good tools. The times are different and sophisticated devices are nearly free, as you point out. However. That doesn't change the fact that as far as learning experiences go, starting by taking on a project that involves complex control theory, for example, isn't a good first step, either. So get the good tools, then go back and struggling with simple projects remains a good approach. Even if those simple projects can't compete anymore.
– jonk
Jan 5 '19 at 20:35
• Even beyond the simple 5V SMPS adapter, there are 'universal' power supplies like this: amazon.com/Powseed-Universal-Adapter-Household-Electronics/dp/… These are very flexible for use from 5-15V, with one important caveat """Always change the voltage with the unit turned off""". The use of break before make switches means that the output may rise between switch positions. Jan 5 '19 at 21:56