Multi-voltage step down

[Poley]

Hey all!

I have a problem I hope you can help with, if at all possible.

Our research group has built a rig for an experiment next week which contains a number of components (stepper motors, light gates etc), all requiring different voltages. As it stands, it needs a 12V feed (with 1-2 amp draw, depending on how many motors are running), and a 5V, 3V and 1.2V with minimal current draw. I'd ideally like to power everything off one power supply adaptor, although I realise due to the massive differences in current draw, it might be simpler to have the 12V drivers on an isolated power supply, this is no problem.

Is there a straightforward means of creating seperate 5V, 3V and 1.2V feeds from the one 5V adaptor?

I looked into simple resistor based voltage dividers, but I fear that the components would interfere with one another as they are switched on and off.

If you could point me in the right direction, that'd be fantastic. Thanks in advance!

 

[GreenGiant]

either current divider resistors like you mentioned (which as long as there is enough supplied power should not change voltages anywhere, but this may mean finding a 3-4 amp power supply)
or zener diodes, they drop a specific voltage (sold at different voltage ratings) and can pass multiple currents so having a setup of these should do you fine, even setting up a bunch in series with branches off the middle for different voltages should work as long as you don't exceed the current requirments

 

[duke37]

Wikipedia gives details of computer power supplies. These will give 12V, 5V and 3V.
The 1.2V could be almost supplied with a LM317T 1.25V regulator from the 5V supply.

 

[CocaCola]

Since you mention that the lower voltage rails are low current, and assuming those lower volt rails don't need to be perfectly regulated you could cheat and use a 12V main supply, use a 7805 to establish a 5V rail and use some series diode off the 5V rail to cheat on the 3V and 1.2V rails...

It all really depends on how tight you want those lower voltage rails...

 

[(*steve*)]

The important detail is that since the 12 volt rail is powering motors, you may have to decouple your lower voltage rails from it in order that the motor noise (spikes on the 12V rail) do not affect things.

Whilst voltage regulators will help with this, some form of additional filtering -- essentially a low pass filter -- may be required to reduce it to acceptably low values.

 

[Poley]

Thanks for all your responses, they're very helpful!

I need a fairly quick solution, as the experiment is next week, and so running a separate 12V and 5V power supply is no issue.

The lower voltage rails don't have to be exact, bar the 1.2V as it's powering what have proven in the past to be ridiculously temperamental IR LEDs.

For the time being, although I can order parts (the zener diode idea I didn't think of, thank you!) I have as many resistors as one could need. For a simple solution, is the resistor based voltage divider the way to go?

Thanks again!

 

[(*steve*)]

Is a resistive divider ok... maybe. but only for *very* low currents.

1.2V supply for LEDs? Sounds like you're trying to run LEDs from a voltage source. If you are, that could easily explain why you then say they are:

ridiculously temperamental

Perhaps a quick explanation of what the rails are for and the currents you expect to be drawn from them...?

 

[CocaCola]

The lower voltage rails don't have to be exact, bar the 1.2V as it's powering what have proven in the past to be ridiculously temperamental IR LEDs.

As Steve hinted, LEDs are current driven not voltage driven... With a properly calculated resistor to limit current you should be able to run that LED from any of the rails...

What is the 3V and 5v rail for?

If it's logic chips or mircos, look at the datasheet for voltage ranges that the chips will work with, you might be able to run them off a single voltage source to simplify design...

 

[BobK]

You need to read the LED tutorial:

https://www.electronicspoint.com/got-question-driving-leds-t256849.html

Bob

 

[Poley]

I don't know why that didn't occur to me, thank you!

The 5V is for a few ICs and some phototransistors, the 3V is powering a couple of low power lasers and the 1.2 is the LEDs, but of course I could just protect the lasers/LEDs with resistors. The ICs and phototransistors can run at much higher voltages, but the signal returned from them is dependent on the input voltage, and it needs to be 5V.

Ask a daft question, get a great answer haha. Thanks again all.

 

[(*steve*)]

but of course I could just protect the lasers/LEDs with resistors

Not *could*, but *must*.

Running a LED from a constant voltage is asking for all sorts of trouble.

 

[CocaCola]

of course I could just protect the lasers/LEDs with resistors.

That would be the proper and correct way to do it, in this instance...

So you take your 12V supply, use a 7805 to drop down to 5V to run the rest of the circuit... A few resistors to protect the LEDs and lasers and possibly some filtering to keep noise on the 12V rail from messing with stuff on the 5V rail...