Can I connect voltage regulators in series?

[jcarter]

Dear Clever Forumers,

I am working with a stepper motor that requires 36v to work at its best, however I only have a 14v power supply.

What I want to know is if I can (and how would I) connect three 7812 voltage regulators in series to get 36v to drive my stepper with?

If I cant, is there any other way for me to generate 36v at approx 1amp from a 14v 5amp supply?

Thanks for any help!
Regards,
James

 

[birdman0_o]

You cannot connect 7812's in series.
Look into DC-DC converters, it can be done with the power supply you have to meet your requirements.

 

[ke5frf]

You cannot connect 7812's in series.
Look into DC-DC converters, it can be done with the power supply you have to meet your requirements.

Hmmm, is this a switchmode based circuit? Just curious. I've never used a DC-DC.

For the original poster, regulators are by nature not used this way. You always have a ceiling voltage at which it operates, and the output will be less than the input by a certain measure to work efficiently and properly. The common way to increase voltage is to step it up through a transformer. And with DC, as in your 14 volts, it would normally require converting it to high frequency AC with a switching circuit and stepping it up through a transformer, which can be made small in high frequency circuits.

Just remember, you give up current capacity in return for the higher voltage, which might be important if you are loading a 14V supply of predetermined power specifications.

 

[bankian]

Just resurrecting this thread with a slight twist on it...

I've been thinking about making a dual USB charger for the car. Most of the circuits I've seen use a 7805 or equivalent to drop the voltage from the car's 13 or so volts down to 5v, but this is working the regulator quite hard and so would generate quite a lot of heat, require a big heat sink, drain the battery more quickly etc etc.

I've been wondering about connecting two 5v Regulators in series across the supply. That way they could each power a USB device and the dropout voltage would be much less, therefore less heat generated.

The dropout voltage requirement on a 7805 is a bit too high to do this, but with the LM2940CT-5.0 it's only 500mv, so two of these in series ought to work with voltages of 11V and above.

Any reason why this should not work?!

 

[crutschow]

Read the answer in post #2. You can't connect two linear regulators of the same voltage in series. The first regulator will still have to drop all the voltage and dissipate all the power from 13V to 5V. Then there is no need for the second regulator.

If you want to reduce the power dissipation you will need to go to a switching regulator.

 

[MrAl]

Hi there,


The reply for the older thead:

You can use one regulator as the reference to another so that the voltages appear to add (such as 12+12=24) but there are better ways to do this such as with a higher voltage version of the LM317.

But your other problem is that you dont have the required input voltage. The input voltage would have to be higher than 36v to drive a 36v regulator. That's why other members are telling you to use a switching regulator. The type you need is usually called a Boost Switching Regulator. It can boost the voltage from lower to higher and also provide regulation with better efficiency. You should start asking about this kind of regulator next.

The reply for the newer post:
You should start your own thread for this question not use an older thread for your own post. Start a new one and you'll get more responses.

 

[Diver300]

It would be possible to design a regulator that feeds two 5 V loads from one 12 V supply, and the current taken is the maximum of the two loads.

However, you certainly can't do it with 7805 regulators. You need to allow for the two loads taking unequal currents, even if they are rated the same. Two phones will become fully charged, and so take less current, at different times.

If you had two devices on a regulator like that, the earths of the two devices would be at different voltages, so if you connected the audio output of one to the line-in of the car radio, you could get problems.

It is far easier to find a good switch-mode regulator that will reduce 12 V to 5 V more efficiently, and to run the devices in parallel from that.