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Building 12vdc power supply

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troutman

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I have a transformer 120vac with some step down legs that put out 8v and 15 v. When I install a single wave rectifier I wind up with about 15vdc. The transformer is about .250kva. Iam building this for my sons Ham radio. It is a mobile unit that he will have in the house for now.
I think I should install a capacitor across the dc terminals with possibly a resistor and diode in series but in parallell to capacitor. Diode installed reverse to polarity so as not to create a short.
What size resistor and capacitor should I use. It seems like i've seen this type of power supply before but I can't remember where.
I know I could buy a power supply but what would this site be without people like us. It's also a teaching moment for him. He is 15 and despises learning from a book but has had to learn the rules for his techninians license from pdf files. This has been good for him and he is interested in building things.
 
I have a transformer 120vac with some step down legs that put out 8v and 15 v. When I install a single wave rectifier I wind up with about 15vdc. The transformer is about .250kva. Iam building this for my sons Ham radio. It is a mobile unit that he will have in the house for now.
I think I should install a capacitor across the dc terminals with possibly a resistor and diode in series but in parallell to capacitor. Diode installed reverse to polarity so as not to create a short.
What size resistor and capacitor should I use. It seems like i've seen this type of power supply before but I can't remember where.
I know I could buy a power supply but what would this site be without people like us. It's also a teaching moment for him. He is 15 and despises learning from a book but has had to learn the rules for his techninians license from pdf files. This has been good for him and he is interested in building things.

Great project for you both. I think once you see how badly the single wave circuit makes clean DC power you will want to make a full wave circuit next.
**broken link removed**
 
This type of simple power supply is unsuitable for a radio transceiver.
You do not mention the current requirements for the radio, but from experience I expect that on receive it will be a couple of hundred milli-amps and on transmit several amps, depending on the transmitter power.

15 volts is rather high for a "12volt" radio, any fluctuations due to the mains supply could cook the radio.

You will need a lot of smoothing capacitance to eliminate the mains hum on transmit.

All in all, you need a proper stabilised power supply, not a simple capacitor filtered supply.

JimB
 
Thanks for the reply. I have seen power supplys of the type nsaspook has drawn and that is what I was thinking of. They were used in relay logic controller applications. I have not checked the output on ac volts to see what kind of leakage there may be but in more advanced supplys I have, and if we found more than .5 volt ac the supply would be replaced. I'm assuming this is the type of supply JimB is referring to.
His radio puts out 55 watts. We would fuse it all the way around. We will look for a commerciallly built supply and maybe just build the other for fun and learning experience.
He likes to build stuff out of what most people throw away. One day I came home from work and he had his garden tiller running of of a 1lb. propane bottle. I put a stop to that as he had no kind of real flame arrestor or good regulator. He said he got it off the internet. I told him that doesn't make it safe!!!!!
 
Something doesn't seem right

What size filter capacitor did you use?

Did you use one at all?

The transformer is 15V.

If you'd use a filter capacitor, I'd expect the voltage across the rectifier to be around 20V with no load connected, 15V sounds too low.

I was going to suggest the LM7812 but then I realised it won't be able to give 10A or so which your radio probably needs.

It's possible to add another transistor to the LM7812 so it can output mor current.

**broken link removed**

You'll probably need several transistors for 10A.
 
Ok troutman is my dad I am the on that needs to have a power supply or a battery to power this thing its a Yaesu 1900r the manual says that in needs 12 to 13 volts at 10 amps on high (55 watts) I want to build one because its going to be cheaper than buying one and me and my dad will have the satiation of building it from old elevator parts

**broken link removed** :)
 
You need a "regulated" supply. I would use a well documented, well debugged circuit because if it screws up, you will likely blow up your rig. Look at back issues of QST or the Handbook; there have been good designs there.

Wa7ark
 
You need a "regulated" supply. I would use a well documented, well debugged circuit because if it screws up, you will likely blow up your rig. Look at back issues of QST or the Handbook; there have been good designs there.

Wa7ark

Do you mean QST mag? and I dont have a Handbook.
 
Even years old ARRL Handbooks will have power supply circuits. Your library might have one.
 
I am going to deviate a little from the suggestions to date. Receive power is nothing so we won't worry about it. The high power transmit power is as you stated about 11 amps. Nice rig by the way, enjoy! Anyway that rig is designed for mobile operation in a vehicle. If you look at the actual power requirements the nominal working voltage is 13.8 volts, pretty typical automotive system. Additionally that 13.8 volts is plus or minus 15% so anywhere between 11.73 and 15.87 volts should be fine. Mobile rigs in automotive applications are designed for a varying input voltage. I don't see a need for a well regulated source. I would take that more than adequate .250 KVA transformer and hang a full wave bridge on it (with a heat sink) and apply some good filtering.

We ran a buddy of mine's Yaesu (WB8DOK) but I forget the model number off an ATX form factor PSU on the 12 volt rail for years. He needed 20 Amps of 12 volt power too.

Anyway, since it is a rig designed for mobile use when running as a base station I wouldn't overly worry about regulation. I would filter well and make sure I fuse it. If you have a scope at your disposal I would check for ripple under a full load.

Just My Take & 73's
Ron
 
√2 C Er F =I for the filter cap.

1.414 Capacitance 1/2 volt of ripple 120Hz = 10 amps

118,000 microfarads at 25 volts.
 
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