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Aux/Add-on Variable P. S. for Existing 12vdc Power Supply

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TGC357

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I would like to build an add-on project box to use with an existing fixed 12vdc power supply (20A, 13.8vdc, Regulated) that I already own and is intended for use with a ham transmitter.

What I have in mind is a bench tester / battery eliminator that would allow me to produce 1.2vdc to 12vdc , at least. ... Since what I have is fairly substantial, I'd like to built it to 5A. potential, although most of its use will be at 1.5A or less.

I've Googled & found some information & some confusion to me ... (I'm just a hobbyist & it's been ~ 40 years since I built a few Heathkits :rolleyes:).

The LM338 looks good to me, but then I read a mention of the LM338K and the LM338T ... So o o o, your experience can save me a lot of trouble and frustration and head me in the right direction. Thanks for your patience, Tom.
 

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Look at the **broken link removed**. It shows the package options. You will be dissipating a lot of power, so the regulator must be bolted to a large heatsink. Buy whichever is easier for you to mount. Don't forget an mounting insulator kit.
 
Look at the **broken link removed**. It shows the package options. You will be dissipating a lot of power, so the regulator must be bolted to a large heatsink. Buy whichever is easier for you to mount. Don't forget an mounting insulator kit.
Thank you very much for your reply, Mike!

They recommend an input bypass capacitor (0.1uF disc or 1uF solid tantalum) and then say, "If the bypass capacitor is used, it is sometimes necessary to include protection diodes to prevent the capacitor from discharging through internal low current paths and damaging the device."

Then they say, "For output capacitors of 100uF or less at output of 15V or less, there is no need to use diodes."

Would you recommend the diodes for my application? ... Also, what rating on the 5K Pot would you suggest? ... Would you suggest the Steel TO-3 case over the Plastic TO-220?
 

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Use a 10uF electrolytic output bypass, shunted with a 10 to 100nF ceramic RF bypass, and no diodes.

Choice of package is up to you. TO220 is easier to mount. Tab must be isolated.

Current through the pot is only 10mA, so rating not important. If pot wiper hits a dirty spot on the pot element, the voltage will spike high. I like to shunt the pot with a zener which will limit the highest possible output voltage to say 15V (14V Zener, anode to ground).
 
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Use a 10uF electrolytic output bypass, shunted with a 10 to 100nF ceramic RF bypass, and no diodes.

Choice of package is up to you. TO220 is easier to mount. Tab must be isolated.

Current through the pot is only 10mA, so rating not important. If pot wiper hits a dirty spot on the pot element, the voltage will spike high. I like to shunt the pot with a zener which will limit the highest possible output voltage to say 15V (14V Zener, anode to ground).
Great! ... You've definitely got me headed in the right direction! ... Thanks again!

Input bypass cap .. **broken link removed** .. **broken link removed** .. **broken link removed** .. Zener .. **broken link removed** .. Heat Sink

If you wouldn't mind, could you check these? ... Wait until I get the bill for all your help, right? :D

.
 
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The voltage rating on the 10uF is too high, the heatsink is too small.
 
Would you recommend the diodes for my application?
I would add the diodes since this will be a bench supply. The diodes will protect the regulator if you applied power to the output when the AC switch was off. Some people like to charge batteries with their bench supplies or put supplies in parallel, etc.
Would you suggest the Steel TO-3 case over the Plastic TO-220?
While the TO220 is easier to mount, since you want to dissipate lots of heat the TO-3 is the best package IMHO.
 
The voltage rating on the 10uF is too high, the heatsink is too small.

OK ... went from 450v down to 50v (they don't have any smaller) & found a larger heat sink that's 43mm x 45mm x 10mm ... What do you think?

I would add the diodes since this will be a bench supply. The diodes will protect the regulator if you applied power to the output when the AC switch was off. Some people like to charge batteries with their bench supplies or put supplies in parallel, etc.

While the TO220 is easier to mount, since you want to dissipate lots of heat the TO-3 is the best package IMHO.

re: Diodes - OK ... thanks for the suggestion ... would that change the values of the capacitors?

OK ... good point ... the TO-3 looks like it has more surface area & maybe use even a larger heatsink?

Yes, they do mention a couple times in the Data Sheet that there is going to be heat to dissipate. I saw a nice little FAN unit - should I go with that?

.
 
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...
OK ... good point ... the TO-3 looks like it has more surface area & maybe use even a larger heatsink?

Yes, they do mention a couple times in the Data Sheet that there is going to be heat to dissipate. I saw a nice little FAN unit - should I go with that?

.

See if you can find a make-on-rise 50 to 70degC thermo-switch. Put that in series with the fan. That way you dont have to listen to the fan when you dont need it. I put one of those on the fan on the back of my ICOM 2M radio which is mounted in a tight space where it doesn't get sufficient convection.
 
See if you can find a make-on-rise 50 to 70degC thermo-switch. Put that in series with the fan. That way you dont have to listen to the fan when you dont need it. I put one of those on the fan on the back of my ICOM 2M radio which is mounted in a tight space where it doesn't get sufficient convection.

Great idea! ... Pepi USA ... Available HERE @ 3/99 cents! ... Does that look like yours?

I located it as a "normally open thermal switch" :)
 
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