Help designing a power supply

I need to create a power supply that will accept 12V DC and output 5V DC All devices to be through hole.
I have a 811-2195-5-ND which is a 3.3V supply and so I’d need the same thing in 5V if I use it. However, it’s pretty pricey as I need to create 10 of these.

I have a commercial board that uses a JQ6500 board and provides the wanted specs. I wonder if I can duplicate it for my purposes. It uses a bridge rectifier to protect against reverse connections on the 12V side. It is marked as MDD FJ Is that a current part available from DigiKey? I did not find it by using the search bar.

It has a 30V 100 MF capacitor just after the bridge rectifier. Black with a grey stripe. What is a Digikey part number for this cap?

Next, it uses a LT7806CV for a regulator. I found L7805CV that appears to be the same. Is it?

That’s it for the commercial board. Is there a good reason to use other components for my application?

Hello bart - our Applications Engineers are not in the office today, but will be back tomorrow and will be able to offer you suggestions on your design. Here are some options for you to look at for DC-DC converter 5V output. I wasn’t able to identify the part that is marked with MDD FJ. We don’t have any 30V 100uf capacitors in stock, but we do have 35V capacitors in that value. I wasn’t able to locate a part number LT7806CV, so I’m wondering if that is an old, obsolete part number. We do have stock on L7806CV.

7806 is a 6V output regulator, 7805 is 5V output.

How much current do you need to supply to your load?
What is the minimum input voltage you need to allow?
What is the maximum input voltage you need allow?
What is the ambient operating temperature range you need?
Will it be exposed to free air or enclosed in a housing? (if housing what about ventilation)

These are all important considerations when using a linear power supply since all the excess voltage will be converted to heat.

That’s just how linear regulators work and why they are banned for many uses this century (extreme inefficiency compared to switching regulators).

Examples: for a 5V supply, 10V input = 50% power converted to heat, 15V input 66% of power converted to heat

Thanks for your reply.

Sorry about the typo. It’s a LT7805. I need 5 V
I need 1 amp max.
I’m expecting 12 V DC as input. So, between 10 and 15 should be sufficient. I also want to put a bridge diode in the system to guard against reverse input connections. So it should be able to accept 10 V DC.
Temp would be room temp.
Free air.

What would it take to create a switching supply?
And, can you suggest a suitable bridge diode device?

Also, about the capacitor? I don’t seem to be able to figure out a suitable part.


The very popular 78xx series regulators were introduced in the early 1970s by either National Semiconductor (LM78xx), or Fairchild (UA78xx), can’t remember who came first. They accept up to 35V in and can deliver up to 1A output at the regulated voltage designated by the xx in the part number. The companion 79xx series is the same but generates a negative output voltage, e.g.LM7905 is -5V.

A very good, and my favorite, bridge rectifier is the classic “DF” series.

Since the input is DC, not AC, you can use any small 100uF @ 25V or higher electrolytic for the input filter (for AC input you’d need 1000uF or more).ürth-elektronik/860010573007/5726978

I also like to add 0.1uF ceramic capacitors at both the input and output to keep the high frequency noise down.

The big catch is that you need to dissipate nearly 10W of heat (15V-5V*1A). So you’ll need a big heatsink to keep the regulator from releasing the magic smoke, this one should do the trick:

Test that the heatsink is sufficient by holding a finger against the 7805 body while it powers up and runs. If it gets too hot to keep touching you’ll need a bigger heatsink. Effectively you want to keep the body temperature of the 7805 below 70°C so you can be pretty sure the actual junction temperature is below 100­C.

Generally I limit my use of 78xx series regulators to deliver less than 0.4A and keep the input output differential between 2 and 4 Volts so that I can use a much smaller heatsink. Fun fact, if you look at the wall warts for a lot of early network gear you’ll find they used 7V or 7.5V DC wall warts to keep the power dissipation down. They had to do this because back last century switching power supplies cost 10 to 100 times as much as a linear supply with a suitable heat sink. Currently almost everything that draws 0.5A or more uses a switching power supply.

Designing a good switching power supply is a very difficult job compared to a linear supply. This is why I don’t hesitate to buy a great switching power supply design as a module for $5.00US when I need 0.5A or more for a dozen so devices.


What would it take to create a switching supply?

You may want to check out these 5V Switching Linear Regulator Replacements as they are much more efficient than Linear Voltage Regulators and they’re a lot easier to use than rolling your own switching power supply.


If your load would never exceed 1A, then here are some additional switching regulators which will reduce your cost compared to the higher current options previously offered.

Make note that most, if not all of these will require external capacitors for proper operation. If you find ones you are interested in, then carefully read the associated datasheets. They will usually suggest recommended capacitors for proper operation.

An additional point regarding using a linear regulator is that, unlike a switching regulator, you’ll need a heatsink for higher current applications, you have to factor in the additional cost of the heatsink, the fasteners, the thermal interface materials (either grease of TIM), and the labor required to attach the heatsink. The total cost can be significantly higher than just the cost of the regulator itself.

1 Like

I have chosen the 1470-VR20S05-ND. As I understand the data sheet, it shows I can use just two capacitors and should be good to go. Do I understand this correctly?

I saw this not too long ago… Hello @bart .

I wanted to show off some ideas I found outside of DigiKey, i.e. if that would not be too upsetting. Outside of DigiKey’s Reference Design Library, I was thinking some other ideas may help too. So, while on your journey: What is a Switch Mode Power Supply | Jameco Electronics .

Anyway, any knowledge should be referenced to their site at that link.


P.S. It is basic but full of info. The data on DigiKey’s Reference Design Lib, , may help even more.


However I notice the data sheet does imply it will fail FCC Class B conducted emissions testing. So to sell it in a product in the USA would require more filtering, like they show, and verification testing.

As long as this is a personal hobby project located within the borders of the USA it’s OK to use it without the extra filters. If you are NOT in the USA you will need to make sure there is a hobbyist exemption for emissions that gives a one time get out of jail free card for emissions. (In the USA second offense can carry steep fines)

The conducted emissions may also add noise to other signals in the environment. So things like audio and video systems may present undesirable output.

For myself I try to choose models like the Murata/Oki products that don’t require extra filtering to meet legal emission requirements.

1 Like

Thanks for your reply.
I guess I better add the extra caps. I am pretty cost conscious.

I think you missed the point, if you are cost conscious do NOT add the three extra caps plus one inductor to make that supply FCC Part B compliant.

For lowest cost use the “Typical Application” circuit without the inductor and with only two caps instead of five.

If you really want FCC class B compliance I would recommend one of the other module choices as most do not require extra extremal parts for emissions compliance and would therefore be less expensive. (inductors are usually relatively expensive)