DC to AC conversion... ?

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scm6079
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DC to AC conversion... ?

Post by scm6079 » Fri Mar 28, 2003 3:58 pm

I am working on an automotive project, and find myself needing a 9V-AC supply line to power an existing component. I know I can regulate the 12V DC auto line to 9V DC with a power regulator, but I wasn't sure the best way to turn it to AC. <p>I was thinking about using a pair of 555 timers (or a 556) to turn on and off a pair of transistors -- one powering the circuit with a reverse connection compared to the other. The thought would be I would end up with a 60-HZ square wave, probably 'good enough' to power the device (an equalizer) designed to run off wall power through a 9V AC box. <p>Commercial power inverters seem like a waste of power, and don't seem to have good duty cycles. I'd like to leave this running continuously. <p>Any one out there have any circuit advice?! A better way to do this? Will my approach even work? <p>I scoured the 'net and couldn't seem to find anyone with a similar circuit. :( <p>-Scott

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Externet
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Re: DC to AC conversion... ?

Post by Externet » Fri Mar 28, 2003 7:22 pm

Hi Scott.<p>Chances are your device in need of 9VAC is meant to be powered by a domestic juice wall adapter; and it has rectification and regulation internally to get somewhere around 5V for the circuitry operation.
I would open it, make a diagram of the power input stage, study it, and probably you will find the unit can work well if you feed 12V DC instead, as the bridge rectifier would steer the polarity.<p>Or,
It is very probable the power is first rectified, then filtered and regulated to a given value.
All you have to do is install an additional power connector, attach very few components to achieve the same given value from a 12V DC source and you can feed 12V DC trough it.<p>Good luck,
Miguel
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Dimbulb
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Re: DC to AC conversion... ?

Post by Dimbulb » Fri Mar 28, 2003 8:28 pm

A 555 does 60Hz square or RC does sine.
A two stage transistor to power transistor takes the signal to 9VAC If you don't do the math then at least use rule of thumb to limit the current to the transistors.<p>http://ourworld.compuserve.com/homepage ... #phase.gif<p>The RC network in an op-amp low frequency sine wave generator fixed at 60 Hz uses NPO timing capacitor and 1% resistors. A single supply op-amp is a derivative. If you need a sine.
Notice that the discription says sine looks very clean on the scope.<p>[ April 01, 2003: Message edited by: 1206DX ]</p>

scm6079
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Re: DC to AC conversion... ?

Post by scm6079 » Mon Mar 31, 2003 3:57 pm

Wow -- sincere thanks for the great responses. I'll 'crack the case' on the EQ and see what it has. I hadn't considered they may go through the effort to make a 9V AC wall adapter, and bridge it to DC internally... Hmm.. <p>If it turns out to not be flattened to DC, and I need to build the DC-to-AC converter, what do you think is the best way? I didn't understand the comment on the 'RC does sine'. My understanding was the 555 acted as a timer, or with RC circuit to calculate period, a square wave -- not sine. As a cheaper circuit to build, I came up with this:<p>:D

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Edd
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Re: DC to AC conversion... ?

Post by Edd » Mon Mar 31, 2003 4:31 pm

SCOTT:
Go for what Sr Miguel suggested as hopefully you might find a LM 7805,7806,or even a 7809 inside the eq unit as a regulator for its power supply. The manufacturer was wanting DC on the units supply in the end, anyway. Thus the ~12VDC from the car batt should not be excessive for the Regulator to compensate for. For that type of device one should only expect lower scale milliamperes of consumption. IF it’s a cheapie unit it could have even used a zener or discrete bipolar pass/zener combo.
At very low level audio inputs there might be a slight possibility of alternator whine if inadequately filtered, however,I’m suspecting you to be up in the ~500mv range.<p>
73's de Edd
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Re: DC to AC conversion... ?

Post by Chris Smith » Mon Mar 31, 2003 10:43 pm

All stereo equipment eventually some where on the board, is DC driving the circuits.

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Re: DC to AC conversion... ?

Post by chessman » Tue Apr 01, 2003 8:29 am

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by Scott Moore:

If it turns out to not be flattened to DC, and I need to build the DC-to-AC converter, what do you think is the best way? I didn't understand the comment on the 'RC does sine'. My understanding was the 555 acted as a timer, or with RC circuit to calculate period, a square wave -- not sine.
<hr></blockquote><p>To get the sine wave that you are after, run the square wave through a large cap ~1000uF or so and it'll filter out to a sine wave.

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Re: DC to AC conversion... ?

Post by scm6079 » Sun Apr 06, 2003 8:21 pm

Okay-- I pulled the EQ apart... and was rather surprised by what I found. They have a diode built bridge rectifier -- and TWO voltage regulators -- a +12V and a -12V! Aack. After tracing their power circuit, it is clear that the chips inside use both the +12v and -12v lines. <p>Soo... The idea of feeding in a simple 12V is out. :( Now, the question is, is it better to take the 14.4V from the car and feed it into the first regulator, and build/buy a DC/DC converter to feed the -12V line (anyone know where to buy an ISR like this??) or just make the 9V AC adapter from the 12V DC line of the car? I was thinking since they are regulating the supply anyways, perhaps I could just create a cheap 12V square wave with some transistors... or a 555 and some transistors and feed this in. <p>I also noticed that there is a shared signal and power ground. That means there would be an audible hum if I use a high frequency (in audible range) 555 style voltage inverter. <p>I appreciate all the replies... Hopefully I can return the favor soon.<p>-Scott<p>PS - For those wondering why the effort -- the EQ is a high fidelity 100 band pro-audio unit, not your typical car EQ. Bach never sounded so good...<p>[ April 06, 2003: Message edited by: Scott Moore ]</p>

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Re: DC to AC conversion... ?

Post by Dimbulb » Sun Apr 06, 2003 9:50 pm

I think now that you have decided to splice into the internals. You could get the data sheets on the LM7812 and LM7912 or whatever they are to find pin labeling. (in/out/gnd)<p>next plan a way to connect the two power wires
usually (red and black)from each of the two series regulators through the rear panel. This may be easier at the diode rectifier. Somewhere inside the case you should add a 0.1 uF disk capacitor across these leads for decoupling. A fuse (red/black) assembly can usually be found at radiojack could be added also.

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Re: DC to AC conversion... ?

Post by rshayes » Sun Apr 06, 2003 9:55 pm

I would advise against directly connecting the automotive battery line directly to your equipment. Some kind of DC/DC converter is probably necessary. While you are building a DC/DC converter, you might as well provide both +12V and -12V outputs.<p>The reason for this is that vehicle power is not "clean" in any sense of the word. I am unfamiliar with the commmercial standards, but the military standards for both aircraft and ground vehicles allow short transients of several hundred volts and longer surges of nearly a hundred volts on a 28 volt line. I would not expect commercial standards to be much better.<p>Usually it is easiest ot suppress thes transients and surges in the DC/DC converter. The transients are often caused by switching inductive loads, such as motors and solenoids, off. The surges result from sudden decreases in load where the voltage regulstor takes some time to readjust. If the equipment isn't designed for them, these conditions will probably destroy the equipment.<p>An isolated DC/DC converter is desirable to avoid coupling noise on the ground of the vehicle supply into the equipment.

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Re: DC to AC conversion... ?

Post by gadgeteer » Mon Apr 07, 2003 3:06 am

You can probably buy a surplus brick that will change 12v into &#177 12v; OR, you'll probably find that the -12v uses only a couple of milliamps, making it adaptable to a 7660 charge-pumped-inverter. Simple IC, 8-pins, couople of capacitors, diode; puts out I think 10mA of negative voltage...<p>www.mpja.com always has lotsa bricks; you could even use the car's 12v, and invert the 12 into a single -12 if need be...<p>[ April 07, 2003: Message edited by: gadgeteer ]</p>

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Edd
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Re: DC to AC conversion... ?

Post by Edd » Mon Apr 07, 2003 7:31 pm

Mot Score:
Well, it was either dyslexia …or the devil made me do it :D <p>Seeing that it is using both +- 12 v regulated, looks like it’s a higher ended design using differential amps instead of some Norton based amps or a derived differential ground reference.
Thus we should expect an almost equal demand upon the neg supply as well, and not merely a miniscule drain as is sometimes used in circuity that is utilizing it as a bias or reference fuinction.
I might offer an easy solution if you can provide some mo’ info’…specifically, open up the outputs of the regulator(s) one at a time and insert a milliameter in series and then power up the unit on AC power with a 500mv AF signal in and see what current demands are needed for both the supplies. Fill me in on the specs and I’ll check my stock, as I believe /know that I have some OEM DC/DC converters that put out separate +15 and _15 VDC…all potted and super shielded in about a 2 inch square X ~3/8 to 1/2 in height of footprint. I need to check specs myself… They were used in the front end of some hi dollar –hi tek blood analysis lab equipment.
Hit me with the current drain specs and if within parameters of this unit I’ll donate one to your cause.
Thusly getting you into a mobile mode speedier and taking in the lilting etudes of Bach…esp
Jesu:
http://www.bachfaq.org/odetojoy.html <p>73's de Edd
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MrAl
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Re: DC to AC conversion... ?

Post by MrAl » Tue Apr 08, 2003 1:11 am

Hello there,<p>Starting to sound like building a 9v ac adapter
for the car would be the best thing to do, saving
you time trying to figure out the best config
for a dual dc-dc converter.<p>Two points to keep in mind:
1. Whatever you build up, you will probably need
complete isolation from the 12v automobile supply.
This probably means you need a transformer on
the output.
2. Since your unit has plus and minus 12 volt
regulators inside, if you use a square wave output
(easier to build then a sine wave converter)
you will need at least 13 volt peaks (or more)
in order to properly drive the regulators after
the 'ac' is rectified.<p>An 'ac' converter isnt hard to build if you can
get by with square waves. All you need is a push
pull circuit with transformer that has a center
tapped primary. The center tap goes to +12 while
the other ends of the primary windings go to
the collectors of the transistors.
The transistors are turned on and off one at a time,
usually with some 'dead time' between which gives
the previous transistor time to turn off before
the other one turns on.
There are chips made to drive transistors like this,
that even have the 'dead time' worked into the
chip so you dont have to build your own delay
networks or whatever.
Mosfets work even better, and can be bought
fairly cheaply even with high current ratings.<p>Good luck with your circuits,
Al
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Re: DC to AC conversion... ?

Post by scm6079 » Sat Apr 12, 2003 9:36 am

I think that building the 9vac adapter would be the 'smarter' choice, since the transformer should isolate the circuit, and shouldn't create the high frequency 'whine' that comes with most dc-dc converters. <p>That said, in from the car battery is from 12v-15v depending on the alternator state. Knowing I want 9vac that would be a square wave of amplitude +/-12.726. Knowing the internal regulation going on, I would think anywhere in the range of 12-14 should be fine (rms 8.4-9.9v).<p>This circuit:<p>Image<p>Seems right... except that it's hte wrong transformer. Now, given a small drop in power from the circuit (guess 0.6V, because of the diodes/transformers?) what transformer would work? <p>I'm thinking as close to a 1:1 transformer as I can get/build? The reason for the transformer at all would be to isolate the noise from the car side input from the output to the eq side output. <p>Thanks again for everyone's help!<p>-Scott

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Edd
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Re: DC to AC conversion... ?

Post by Edd » Wed Apr 23, 2003 2:23 pm

SCOTT:
Pulled one of the DC-DC converters and checked it out.I checked the DCoutput at three current loads and found that a unit puts out +and- 15.12 Vdc @ 25 ma loads or 14.86 Vdc @ 50 ma loads or 14.56 Vdc @ 75 ma loads. The primary power input requirement is 5Vdc @ 200 ma. The mid range is probably what your unit will be pulling since you never had a come back on the systems power consumption. Thereby, you would only need to put together a 5V regulator and filtering unit to supply the 5V to the unit from your mobile source supply of the typical 11.5-13.5.battery source.
A 7805 reg circuit or even a simple zener diode and companion power pass xstr shud suffice.
The unit is metal encased, 2 in square by 3/8 height. Hit me at the attbi addee if U want a freebie .<p>73's de Edd
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