Regulator ByPass

[3v0]

I would like to be able to use the same input for regulated or unregulated power.

Is it OK to put a jumper across the onboard regulator's in and out pins when you need to bypass the regulator (use regulated power) ? Schematic attached.

 

[ecerfoglio]

I would like to be able to use the same input for regulated or unregulated power.

Is it OK to put a jumper across the onboard regulator's in and out pins when you need to bypass the regulator (use regulated power) ? Schematic attached.

It should work if, with the jumper "ON" you don't feed the circuit with more than the regulator´s nominal voltage.

But it seems to be dangerous, or as they say "an accident waiting to happen" If you plug in your unregulated supply with the jumper "ON" you may damage both the regulator and the circuit that expects no more than the regulated voltage

If you want to use a single input you may leave the regulator allways in circuit and feed a little more voltage. Say you need 3V (as your name implies ), if the regulator you use needs at least 2 V hearoom you may feed it with a 5V regulated supply or with an unregulated one (say 6 V).

 

[Hero999]

Use a low dropout regulator, that way when you connect it to your regulated power source it might drop out but you'll only loose <300mV if you choose your regulator carefully enough.

 

[3v0]

Thanks to both of you for the good advice. I will use a low drop out regulator.

 

[Hero999]

What voltage do you need at what current?

What's the minimum acceptable voltage?

I might be able to suggest a suitable regulator for you.

 

[3v0]

What voltage do you need at what current?

What's the minimum acceptable voltage?

I might be able to suggest a suitable regulator for you.

Current is low in that it just need to run an 18 pin PIC and and one or two LEDs. 100mA should be fine.

I was thinkiing of using LM2931AZ50R for 5V use but am thinking that I may want to try some boards with a lower voltage. Microchip claims the 16F88 will run down to 2V. The one I found at Mouser that would fit the bill is the TI **broken link removed**.

Background:
Current I am working on a simple robot controller for use by my students. Some of the robots use the Radio Shack 1.5-3V motors so running the controller on 2 or 2.5V would make sense.

The TPS79920YZUT is a bit spendy but I can live with it.
Are these good selections ?

Thanks.

 

[eng1]

Microchip claims the 16F88 will run down to 2V.

That's true for the LF version and up to 4 MHz. The standard version wants 4 V min. over the entire frequency range.

 

[3v0]

That's true for the LF version and up to 4 MHz. The standard version wants 4 V min. over the entire frequency range.

Thanks. We talked about that in another post but I had forgotten. Checked the datasheet for the non LF version but did not see it.

Maybe the thing to do for the 1.5-3V motors is to use a 9V (or some lighter battery) for the processor and 2 AA cells to drive the motor. Four AA cells would be too much for these little robots.

I think I will get a 16LF and a 2V regulator to play with.

 

[eng1]

It looks like you could power the whole circuit from 2x1.5 V batteries?
The PIC16F628A can runn off 3 V, but your choice of the 16F88 makes me think that you need A/D too? I would suggest a PIC16F690 that can run off 2 V @ 10 MHz, is loaded with periphereals and is cheap too. Just a note: only the high nibble of PORTB is available (just in case you have to port old code...).

If you want to use an LDO regulator, you might consider the LM1086 (TO220, 1.5 A, but cheap and common) with 3 or 4 1.5V batteries [other options might be better].

 

[3v0]

A lot to think about.

This is for use by my students building robots. By the end of the year I hope to have them laying out their own boards. For now I just need a platform(s) for them to start on.

I choose the 88 because it was a newer chip with a 8Mhz internal osc. For low voltage use it looks like the PIC16F690 is the way to go.