# 12V to 24V dc (Voltage Doubler)

Discussion in 'Electronic Design' started by Sean J, Apr 5, 2004.

1. ### Sean JGuest

If any one can help, I've run into another problem, Roland sent me this
circuit (it works fine), but it runs the battery flat quite quickly

http://www.aaroncake.net/circuits/6-12conv.htm

" I have a 12V dc car battery and I need to run some truck electronic
equipment, approx. 24V / 25mA "

I wonder if anyone can help me with a schematic of a Voltage doubler from
12V dc to 24V dc. I have seen one in a Electronic mag a long time ago. It
used 2 transistors which oscillated, and 1N4007, a cap, etc. I only need
25mA at 24V dc at the output. Please if anyone can help, I would greatly
appreciate it, thanks.

Sean

Search for a DC to DC converter. www.jameco.com has several. Ex: Part#
155694CF is: 9-18V input, 24VDC @ 625ma output for \$35. These off the shelf
units also sport nice features such as filering, I/O isolation, continuos
short circuit protection, etc...

3. ### Walter HarleyGuest

That seems good advice in general. However, the OP is concerned about
battery drain, and a converter capable of 625mA is probably going to be
rather inefficient at only 25mA (what the OP requested).

Jameco does not seem to have many 24VDC output converters. But since the
output is isolated from the input, another option would seem to be to use a
12V converter and put the output in series with the input, like this:

.--------------------------------.
| |
| .-----------------------. |
| | | |
12V in --o---|in+ +out|----|----- 24V out
| 12V isolated | |
| DC-DC converter | |
GND-----o---|in- -out|----'
| | |
| '-----------------------'
|
'--------------------------------------- GND

Jameco has an 84mA 12V DC-DC converter, part 216830CA, for only \$10. The
only downsides I can think of to doing it this way are that the output
voltage is only as regulated as the input is, and that any noise generated
on the input side by the converter will also show up on the output. But
those problems would be true of the circuit he envisions, as well.

I'm not sure I agree that the 625ma converter would be inefficiant at the
25ma load, but I can see your concern. Jameco #161701CF is a dual output
+12V -12V unit. The datasheet shows separate output lines, I don't see a
reason that they could not be put in series! It's rated for 125ma (each
output) 3W - for only \$21.

5. ### SpajkyGuest

http://w1.859.telia.com/~u85920178/power/vconv_01.htm

-- Regards, SPAJKY Â®
& visit my site @ http://www.spajky.vze.com
"Tualatin OC-ed / BX-Slot1 / inaudible setup!"
E-mail AntiSpam: remove ##

6. ### Bill BowdenGuest

You might do it with a 555 timer and couple caps and diodes
but you won't get the full 24 volts. Look at the circuit
"Generating -5 volts from 9 volt battery" at:

http://ourworld.compuserve.com/homepages/Bill_Bowden/page10.htm#5volts.gif

Leave out the zener diode and it will produce about -10.5 from
a 12 volt battery so there will be about 22.5 volts from the negative
output to the + side of the battery.

I don't know it it will deliver 25mA or not, it's rated at 12mA.
Maybe try larger capacitors.

-Bill

7. ### Walter HarleyGuest

The basic issue is that quiescent current starts becoming an issue;
higher-powered converters tend to draw higher quiescent current.
That would probably work, too. Do you know of a good reason to spend the
extra \$11 over the solution I suggested?

-w

8. ### MNQGuest

Why not use intersils ICL7660sipa. this ic will convert your 12 volts
to -12 volts. You should be able to use this as a 24v supply but current
supply may be limited to about 10ma you will have to check the data sheet.
It also seems to be efficient. Checkout the data sheet.

Naveed

9. ### R.LeggGuest

What is the battery current drain?

Is Q3 getting warm? This is supposed to be a darlington. If you're
using something else, it may be running linearly.

There are some things you can do to cut down on consumption, or to
make a non-darlington work correctly.

1) Increase R1 through R4 by a factor of 2. If Q3 is really a
darlington, R5 and R6 can be increased similarly.

2) Decrease C1 and C2 by a factor of 10 to .01uF.

3) Keep the inductor larger than 470uH. 1mH is a good ballpark figure
at this low power level.

4) Disconnect the collector of Q4 from the base of Q3 and reconnect it
to the base of Q1.

These all tend to keep Q3 from operating linearly during a switching
interval, at a reasonably high operating frequency, for the power
level expected.

Turn the circuit on and off at the 12V battery, not at the 24V
terminal.

To reduce overshoot at turn-on, due to mvb early operation, stick a
feedforward network across the zener string. 10K and .01 should do it.

Stick a snubber across D2 - 2N2/220R if it interferes with the radio.
If you can get a faster part for this position, please use it.

This is a pretty flakey circuit, but can be made to function and uses
no parts that are hard to find or to substitute.

RL

As you stated in your first email:
"The only downsides I can think of to doing it this way are that the output
voltage is only as regulated as the input is..."

The 161701CF has an input range of 9-18 VDC giving better regulation and
being dual output provides complete isolation!

11. ### Walter HarleyGuest

Ah, but neither of those seems to be important to the OP (judging by the
circuit he was looking for). So I still think mine is the best solution to
the problem as posed

12. ### N. ThorntonGuest

If you lack tronic knowledge, you can set a relay self oscillating by
wiring it so the coil is on, but when the relay closes the coil is cut
off.

Now, direct your 12v via 2 contacts to 2 different large capacitors,
put the caps in series... 24v.

Very crude but works.

Regards, NT

13. ### Roger GtGuest

X-No-Archive: yes
"N. Thornton" wrote
: "Sean J" wrote
: > If any one can help, I've run into another problem, Roland
sent me this
: > circuit (it works fine), but it runs the battery flat quite
quickly
: > http://www.aaroncake.net/circuits/6-12conv.htm
: > " I have a 12V dc car battery and I need to run some truck
electronic
: > equipment, approx. 24V / 25mA "
: >
: > I wonder if anyone can help me with a schematic of a Voltage
doubler from
: > 12V dc to 24V dc. I have seen one in a Electronic mag a long
time ago. It
: > used 2 transistors which oscillated, and 1N4007, a cap, etc. I
only need
: > 25mA at 24V dc at the output. Please if anyone can help, I
would greatly
: > appreciate it, thanks.
: >
: > Sean
:
: If you lack tronic knowledge, you can set a relay self
oscillating by
: wiring it so the coil is on, but when the relay closes the coil
is cut
: off.
: Now, direct your 12v via 2 contacts to 2 different large
capacitors,
: put the caps in series... 24v.
: Very crude but works.
: Regards, NT

Okay, this is a "Vibrator converter" Relays are not really rated
for this application. IT WILL WORK, but switching Capacitors is a
pretty rigorous requirement and the relay will likely fail after a
few months. Put it on a socket and use one with a 5 Ampere rating
for this application. (You can draw about 500ma max with a 10
times rating on the contacts.) Caps should be at least 470mFd to
1KmFd.

Also it is Noisy. Vibrators in Car radios were loud enough you
could tell if the radio was on from the vibration of the dash
board!

14. ### Walter HarleyGuest

That would hardly meet the OP's stated goal of improved efficiency

15. ### N. ThorntonGuest

After rereading the q I guess youre right. A 1w coil relay running 50%
duty cycle = 0.5w, at 12v thats about 40mA.

Regards, NT

16. ### Rich GriseGuest

For 25 mA, why not just 12V worth of lithium batteries?

Cheers!
Rich

One simple solution would be to use a single chip switched cap voltage
converter made by linear (www.linear.com) using an LTC1144 (just type
that in to the search) There is a voltage doubler circuit in the
datasheet. You would need a couple of caps and some diodes.

Hope it hepls