|Elliott Sound Products||Project 95|
Low Power Negative Car Power Supply
Rod Elliott (ESP)
I have had a lot of queries about getting a low power negative supply in cars, to power Linkwitz transform circuits and the like. There is a switchmode converter, but I must admit that it is overkill for what most people need.
Another project describes how to generate an "artificial earth (ground)", but this seems to have created more confusion than it's worth. In addition, some readers have had a great deal of trouble - especially with the transform circuit, because it has so much bass boost. In some cases, this can lead to instability - low frequency oscillation, commonly called "motorboating" because of the sound it makes.
The instability is caused by the supply varying. As the circuit powers up, it will pass some signal to the subwoofer amp, which will draw current. The current causes the voltage to drop, and this is coupled back into the transform circuit, which amplifies the small change, and causes the power amp to draw more current. This continues until a oscillation condition is established, and the sub will just make "ga-bloomp, ga-bloomp, ga-bloomp" noises (or something similar :-) but nothing will work properly.
So, in answer to this, a simple little project will provide you with positive and negative supplies, allowing a true chassis ground connection, and will be suitable for preamps, equalisers, etc., provided the current is below about 20mA or so.
The circuit is very simple, and is easily made on Veroboard or similar. Construction is not critical, and the schematic is shown in Figure 1 below.
Rectifier diodes should be ultrafast (UF4004 or similar), or you can use 1N4148 signal diodes. Losses will be slightly higher if you use signal diodes, or lower if you wanted to go to the trouble of using Schottky diodes - the latter are not warranted in such a simple circuit (IMO). The zener diode is to protect the circuit against transient overvoltage, and is optional (but recommended).
Using only a standard NE555 timer and a few other parts, this circuit should be up an running in about an hour. The 555 is configured as a minimum parts count astable (i.e. no stable states) multivibrator, and runs at around 17kHz with the values shown. The zener diode (D3) should be a 16V/ 1W type. Resistors are 1/2W carbon film, and small caps may be polyester or mylar (63V types are quite Ok in this circuit).
Use a standard (bipolar) NE555 timer - not a CMOS type. CMOS timers do not have the switching abilities of the bipolar types, and output voltage will be lower.
The circuit itself is a simple voltage doubler. You may well ask why the output is not -24V if the circuit is a doubler. Look at the circuit, and you will see that the output of the timer is AC coupled with C4, so it is actually only 6V RMS with a 12V supply. For an input of 13.8V (standard car voltage), the doubler action gives an output of -12.5V with no load. There is a small loss due to the diode losses, and additional losses in the 555's transistor switched output.
Since there are currently no plans for a PCB, make the circuit on Veroboard or similar prototype board. All currents are low (no load current is only 10mA, rising to 30mA with a 20mA output current), so special techniques are not needed. The final filter shown (R2 and C6) is needed to remove swirching noise from the output.
Make sure that you don't locate the circuit or its supply leads near pre or power amplifier input leads. The switching frequency is high enough to be inaudible (unless your hearing is very good), but could damage tweeters if it were to be amplified. If by some miracle it turns out that you can hear the switching, reduce R1 to 18k or 15k - this will increase the frequency, but will probably reduce efficiency very slightly.
Before connecting to a car battery supply, test the circuit first, using a 9V battery. If there is a problem, the IC may still be damaged, but at least it will not blow up in your face, or cause the smoke to escape from the suply leads (light duty leads are quite sufficient, as current drain is very low). From 9V, you should see at least -7V output with no load.
Note: When this circuit is used, the positive and negative supplies will not be equal. This is quite ok with the vast majority of opamp based designs, but there may be some designs that will not work if the supplies are not balanced (these will be very rare, but it has to be mentioned).
If you wanted to do so, low power 9V regulators may be used on the +ve and -ve supplies, but this will not provide any real advantages to most opamp circuits.
|Copyright Notice.This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is Copyright © 2002. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro- mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference while constructing the project. Commercial use is prohibited without express written authorisation from Rod Elliott.|