5V to 3V3

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Hello!

Can anyone tell me a good way to make 3V3 from 5V to power an MMC card, but I still need to keep the 5V to power the uC, so simply using a Zener Diode won't work AFAIK, my only idea would be to use two Diodes with ~1V drop, so that would be around the 2V-3V3 range of the card.

Any better ideas, or can you confirm that mine is O.K. to use?

thanks,
axos88

axos88

The diodes will work as long as you dont need to pass more than a few mA through them. I use this trick all the time for logic lines.

Otherwise, a low dropout linear regulator will do the trick fine, something like a LM2937-3.3...

Datasheet says it needs less than 100mA. Is that okay?

axos88

I find that three 1N4001 in series drops 5v down to almost exactly 3v; my current is a lot lower than yours so your voltage might be a little higher.

1N4001's are rated up to 1A continuous forward current, and forward voltage drop @ 100mA should be around 800mV per diode. So two of them in series should do the trick from 5v to 3.4v... Add a 1N5817 schottky to drop 0.3v more...

If the load will be reactive, or you plan to add to your circuit, you are probably better off with a regulator, since otherwise the forward voltage drop of the diodes will change depending on the load on them...

Thanks guys, because the load will change depending on the current action (read/write/standby), I think I will use the regulator.
But can you tell me how you calculated that 800mV drop @ 100mA, so I can do it myself later on?

axos88

There is no simple way to calculate the voltage. It is possible but one would need a lot of prameters first.
So its just a look in the datasheet or may be just remembering some typical values.

There's a graph with typical forward voltage drop versus forward current in the datasheet... ;)

Low-dropout regulator, LE33, etc.

The price doesn't hurt, therefore there is no reason to try to be as cheap as possible.

barnacle wrote:
I find that three 1N4001 in series drops 5v down to almost exactly 3v; my current is a lot lower than yours so your voltage might be a little higher.

Would this method (2 1n4001 in series) suffice for avr-usb circuits until I can order some proper 3v6 diodes? I can get the 1n4001 at the local Radio Shack but not the zener diodes called for in this type of circuit.

There are a lot of ics with very low drop out voltage (if you need less than 50mA)like:
TC55
MCP1701
TCS71330 (if I remember well)
LM2951

and many more.

Michael

Michael.

User of:
IAR Embedded Workbench C/C++ Compiler
Altium Designer

Barney - all I can say is 'suck it and see'. I needed *approximately* 3v at fairly low current, so I just stuck three diodes in series, and low and behold, they dropped exactly two volts between them. Using two, you won't be *far* off but I'd be surprised if you get exactly 3.6v.

Hello, I had a similar problem for a battery powered device and used a dual feed with both versions (3.3 and 5 volts) of the LD1117 LDO regulator. Certainly the diode idea is much simpler and more efficient. I suppose you cannot run the micro on 3.3?

I remember searching and searching for a DC-DC convertor with both the 3.3 and 5 volt output but, if they make such a thing I certainly couldn't find it...they had many other dual output voltage converters??Makes no sense to me.

John

Just some guy

Okay, solved it, I bought a 3V3 voltage regulator. But there is a question that made me wonder. If my AVR runs at 5V, it's signals (HI 1, and LO0) will be at 5V and 0V too. Won't that damage my chip running at 3V3? Or I should insert some 3V3 zeners to regulate the communication voltages also?

axos88

Driving the memory device with 5V is NOT a good idea. Are you using the SPI interface? If so, the data is one way in each logic line. 74LCX125 can be used to convert both ways. Power it from 5V and use the open drains pulled up to 3.3V. Normally, you see these as inverting buffer BUT if you put the signal into the output enable and set the normal logic input to low, you can do some amazing things.

Jim

Until Black Lives Matter, we do not have "All Lives Matter"!

Great idea, thanks. But it's a bit "hacky"... I'm thinking of a way to use simple BC547 NPN (because i have a lot of those at hand) transistors would work converting 3v3 to 5v, but i wonder if it would work the other way around?
What happens if i connect 5v signal to base and 3v3 to collector? Would the 5v from base go through to the emitter and (probably) damage my memory device?

axos88

I use this:
http://www.futurlec.com/Mini_Log...

(for 4.90 USD ! I don't know how they can do that!)

For breadboarding but, the 74LC245 is obsolete now...have to use Jim's recommendation. I usually do not mess with resistors or diodes in a logic circuit but, other more experienced guys do with success.

John

Just some guy