Tapping 5V of 43V powersource.

[Witsenburg]

Hi,

I just purchased 3 drivers and a 43V powersource. But the breakout board needs a 5V feed. I was thinking about using a 7805 to tap the 43V source, but I believe the 7805 doesn't take more then 30-ish volt input voltage. Any ideas?

Thanks,

Hugo

[gar]

070503-0644 EST USA

What is the maximum current required at 5 V?

If you use a resistive means to lower the voltage (this includes any series pass regulator means such as a standard 7805), then the power loss is (43-5)*I in Watts for current in Amps. At 100 ma, this is 3.8 W, at 1 A it is 38 W.

You are correct the normal peak rating of the 7805 is below 43 V.

If you use a switching regulator circuit you can greatly reduce the wasted power loss of a linear regulator.

Consider a small transformer and associated components to feed your 7805 regulator. There are inexpensive small wall transformer DC supplies in the 400 to 500 ma range rated at 9 V that would probably work for your 7805. You will probably need to heat sink the 7805 even with a source voltage of 9 V. Heat sinking will depend on power dissipation, ambient temperature, and thermal resistance. The load current and voltage drop from the source to load (like 9 - 5) determines the power dissipation the 7805. At 100 ma load and 4 V drop across the 7805 the dissipation is 0.4 Watts.

.

[vulcom1]

You did not mention if your power supply is DC on the output or if you need to use a rectifier. I just built a bow foam cutter power source and although it is variable output I can not see why you could not make it fixed for your purpose. I use the LM338K but I believe the LM350 would also work. You are border line for max input but it is worth a try.
John

[Al_The_Man]

Originally Posted by Witsenburg Hi, I just purchased 3 drivers and a 43V powersource.

You didn't state the nature of the power source?
If it is a linear unregulated supply, one of the most efficient is to add a 5v source winding to the transformer, this is relatively easy on Toroidal types but a bit tricky on Square Lamination type, but not impossible.
Al.

[pminmo]

A lm317AHV will do, so will putting a 10V Zener inline with a 7805.

[Witsenburg]

Thanks for all the input, it's up and running. I have a fully stabilized 43V= for the drivers, I hooked up an LM317 to half this and then put a 7805 in series to get a stable 5V=. Some elcos here and there and 2 diodes for safety and I was done. I know I probably could have gotten away with only the 317 but I feel saver having the 7805 doing the 5V.

Cheers,

Hugo

[Oldmanandhistoy]

Originally Posted by Al_The_Man You didn't state the nature of the power source? If it is a linear unregulated supply, one of the most efficient is to add a 5v source winding to the transformer, this is relatively easy on Toroidal types but a bit tricky on Square Lamination type, but not impossible. Al.

Hi Al_The_Man,

Does adding a coil for the 5Vdc have any affect on the VA rating of the secondary?
As you may or may not remember I needed 6Vdc for my breakout board and ended up using a wall wart (I think that’s what you guys call them over there). The current need of the breakout board is 1Amp. What wire gauge would be needed for 6V 1Amp?

Thanks in advance for your time and superior knowledge ,

John

[Al_The_Man]

Originally Posted by Oldmanandhistoy Hi Al_The_Man, Does adding a coil for the 5Vdc have any affect on the VA rating of the secondary?

Technically yes, but generally the current drawn by the 5v is a very small percentage of the overall rating.
For that current, I have used 18awg.
Al.

[Oldmanandhistoy]

Thank you Al.

John