Voltage regulation in old vehicle

Archived from groups: rec.audio.tech (More info?)

In <Zub%c.135845$X12.114623@edtnps84>, on 09/07/04
at 05:16 AM, "Jack" <jacksimpson@telus.net> said:

>Hi.
>Looking for info on a simple technique of protecting a small (solid
>state) radio from supply voltage spikes.
>I have an old classic truck with 6 volt (neg ground) electrics. The
>old tube radio is now toast ..well, too expensive to fix: power
>transformer appears shot.
>I want to sub the guts of a more modern trans radio into it and that
>radio also runs on 6 volts (4 c cells). However, I am a tad leary of
>destroying the new radio by the voltage spikes that the generator will
>produce during charging: up to 8.5 volts. I need something to smooth
>out the voltage. I was thinking of using an LM317 to regulate the
>radio but I suspect it won't work properly as the supply will vary
>from 5.8-8.5 volts. So, I was also intrigued with the idea of using a
>zener but not sure how to go about that.
>The radio appears to draw a max of 100 ma @ 6 vdc.

>Any (simple) ideas are appreciated.

There could easily be some short duration spikes that are much higher
than the nominal charging voltage.

The zener diode is a decent approach, but designing with them can be
tricky.

Keep the zener's power handling capacity in mind.

Start with a series resistor between the car's electrical system and
your radio. Pick the resistor such that it will not burn itself or
something nearby if the full 8.5 volts is supplied from the car and the
radio or the zener shorts. This value must be low enough that the 100mA
drawn by the radio will not cause the voltage to fall below what the
radio needs.

Now pick a zener diode of about 6 Volts. Slightly higher is not a
problem because your "C" cells could be as high as 1.55 Volts. (also
keep in mind that there could be a 5,10, or 20% tolerance on the
zener's voltage) Now check to see that the zener will be happy. The
worst case for the zener will occur when the radio is off and the
charging system runs at 8.5 volts. Calculate the current in the
resistor, which will drop 8.5 minus the zener voltage. Once you know
this current, multiply it times the zener voltage to see if you are
within it's power dissipation and current limits.

The first time you go through these calculations, something won't be
happy and you'll have to fiddle with the resistor value or the zener's
power rating and repeat the calculations.

Note that this crude approach will not result in a regulated supply for
the radio, but it will protect the radio from the charging system.

The above minimal approach will result in a power source that is not
very "stiff". The voltage seen by the radio will vary with the power
demands of the music. (calculate the voltage drop across the series
resistor for various currents from about 25mA to the full 100mA) Some
radio designs tolerate this, some don't. You can fuss with lower values
for the series resisor. This will result in a "stiffer" supply, but the
cost of the zener will rise. Adding a capacitor in parallel with the
zener will increase overall stiffness and add some protection from high
voltage spikes from the charging system and absorb high frequency
"trash" that might ride on the 6V power line. Make sure the zener is
mounted to allow proper cooling.

After you fuss with this arrangement for a while, you'll realize why
high power or low power energy sensitive systems don't use it. But it
is simple.

-----------------------------------------------------------
spam: uce@ftc.gov
wordgame:123(abc):<14 9 20 5 2 9 18 4 at 22 15 9 3 5 14 5 20 dot 3 15
13> (Barry Mann)
[sorry about the puzzle, spammers are ruining my mailbox]
-----------------------------------------------------------

Archived from groups: rec.audio.tech (More info?)

>Thanks, Barry. As I digest your info, I had also thought about using a
>fixed value LM7805 VR. However, the concern of the voltage drop used by
>that device bothers me. I'm wondering if it would even work when the
>generating system is at the 6vdc resting voltage ..being such a low
>threshold, would that even operate the 7805? I surmise 8.5 volts would
>though.

I believe your surmise is correct, in both cases. The LM7805 states
that the regulator has a 2.0-volt forward voltage drop at 1 ampere...
it won't regulate properly at below about 7.0 volts of input.

From the internal schematic it looks to me as if the dropout is
probably two diode drops (about 1.4 volts) plus the saturation voltage
of the pass transistor.

There are three-terminal regulators which have lower dropout
voltage... down to 1.5 or below, I believe... but even those may not
work properly in an application with so little "headroom".

One alternative approach is as other folks have suggested - use a big,
hefty zener diode shunted across the radio, and feed the radio (and
zener) through a current-limiting resistor. It's likely to be tricky
to pick a resistor whose value is low enough that it doesn't starve
the radio for current when the auto system voltage is low, and still
limits the zener current adequately when the system voltage is high.
Adding a big, hefty filter capacitor (say, 10,000 uF 12 WVDC) across
the zener would probably be an excellent idea.

Another approach is to build a custom very-low-dropout voltage
regulation circuit. This could probably be done using a PNP (or
P-MOSFET) pass transistor, with the base/gate being driven from the
output of an operational amplifier which takes its supply from the
(unregulated) auto voltage rails (perhaps filtered somewhat) and which
compares a divided-down version of the output voltage with a reference
voltage. You'd have to play around with the gain, and frequency
response of the op amp's feedback loop in order to ensure proper
stability of the circuit.

>And while on the subject how about this alternative. If I wanted to just
>stick in a (stand alone) 12 volt am/fm radio, how could I boost the 6 vdc to
>12 vdc (just for the radio and nothing else powered in the truck)?

For this, you would need a "boost-mode" switching voltage converter (a
"DC/DC converter" ), and possibly an additional noise-filtering or
post-regulation stage. These are not inexpensive (especially as you'd
probably need several amps of current out) but may be available
surplus.

--
Dave Platt <dplatt@radagast.org> AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Archived from groups: rec.audio.tech (More info?)

Nah, Bruce. I don't think 4.9 volts will operate the radio. As an
experiment I tried using a LM317 VR and as you suggested, it needs a head
clearance of about 1.5 volts to work. So, as the truck's electrics run
anywhere from 5.5 (no charge) to 8.5 (charging) the regulation device would
have to _maintain_ at least 5.5 volts (and not more than about 6.5 volts) to
the radio for it to work properly ..and without damage. That's where the
suggestion of a zener might be in order I guess.
Jack



"Bruce Chang" <bechang@swspambegonebell.net> wrote in message
news:V7o%c.8808$yp2.4446@newssvr30.news.prodigy.com...
>
> "Jack" <jacksimpson@telus.net> wrote in message
> news:jxm%c.102073$A8.11739@edtnps89...
> > Thanks, Barry. As I digest your info, I had also thought about using a
> > fixed value LM7805 VR. However, the concern of the voltage drop used by
> > that device bothers me. I'm wondering if it would even work when the
> > generating system is at the 6vdc resting voltage ..being such a low
> > threshold, would that even operate the 7805? I surmise 8.5 volts would
> > though.
> >
> > And while on the subject how about this alternative. If I wanted to
just
> > stick in a (stand alone) 12 volt am/fm radio, how could I boost the 6
vdc
> to
> > 12 vdc (just for the radio and nothing else powered in the truck)?
> >
> > Jack
> >
>
> With my limited knowledge, you could use a 7806 and that would get you 6v
as
> long as you stayed above about 7.4 volts. If intuition leads me in the
> right direction a good 6V automotive battery should measure at 6.3V. At
> rest, you'd be getting somewhere around 4.9 volts.
>
> To boost the voltage to a 12V radio, you'll need to use a switching power
> supply. I would imagine they're not hard to come across.
>
> -Bruce
>
>
>

Archived from groups: rec.audio.tech (More info?)

Thanks Greg.
In using the Zener then, are you using the full amperage to trigger the
device or a % of the amperage? Please correct me if I'm wrong but from what
I've read, 25% is all that is needed to trigger the zener. So, using Mr.
Ohm and his Law, I show that it would work out to a 30 ohm, 1/2 watt
resister. Now that was based on a peak supply voltage of 8 volts ..as when
the generator is charging flat out. My ??? though would be if the battery
was discharged at say, 5.5 volts? Or somewhat charged at 5.8 volts? or
fully charged at 6.2 - 6.5 volts? How does one calculate a limiting
resistance when the supply is so variable, as in this old truck?
As I am not a trained electronics guy (just a dickering hobby) I am not
familiar with low drop out regulators. Do you have any specific part
numbers that would be appropriate here?
Jack









"gregs" <szekeresREMOVE@adelphia.net> wrote in message
news:h5WdnZoPPNicw6PcRVn-hg@adelphia.com...
> In article <Zub%c.135845$X12.114623@edtnps84>, "Jack"
<jacksimpson@telus.net> wrote:
> >Hi.
> >Looking for info on a simple technique of protecting a small (solid
state)
> >radio from supply voltage spikes.
> >I have an old classic truck with 6 volt (neg ground) electrics. The old
> >tube radio is now toast ..well, too expensive to fix: power transformer
> >appears shot.
> >I want to sub the guts of a more modern trans radio into it and that
radio
> >also runs on 6 volts (4 c cells). However, I am a tad leary of
destroying
> >the new radio by the voltage spikes that the generator will produce
during
> >charging: up to 8.5 volts. I need something to smooth out the voltage.
> >I was thinking of using an LM317 to regulate the radio but I suspect it
> >won't work properly as the supply will vary from 5.8-8.5 volts. So, I
was
> >also intrigued with the idea of using a zener but not sure how to go
about
> >that.
> >The radio appears to draw a max of 100 ma @ 6 vdc.
>
> A simple zener type would probably work. With the radio only using only
> a 100 ma. draw, a zener shunt type would probably work, rather than
> using it in conjunction with a transistor pass element. With a 20 ohm 10
watt series resistor
> feeding the 6 volt 1watt zener, and a 2200 mF parallel 16 volt
electrolytic, I believe that would
> work. I did some quick calcs. Put it in a metal box with a 1 amp series
fuse and an on off
> switch, since it will draw current even with the radio off.
>
> There are a ton of low dropout
> regulators on the market today. 100-300 millivolts is about all they
loose.
> Many are small packages, but there are some To-220's.
> A DC to DC converter is another approach, but may inject some RF noise.
> If you have some know how, search around the low dropout regulator
> lists.
>
> greg

Archived from groups: rec.audio.tech (More info?)

In article <bfv%c.76229$S55.33743@clgrps12>, "Jack" <jacksimpson@telus.net> wrote:
>Thanks Greg.
>In using the Zener then, are you using the full amperage to trigger the
>device or a % of the amperage? Please correct me if I'm wrong but from what
>I've read, 25% is all that is needed to trigger the zener. So, using Mr.
>Ohm and his Law, I show that it would work out to a 30 ohm, 1/2 watt
>resister. Now that was based on a peak supply voltage of 8 volts ..as when
>the generator is charging flat out. My ??? though would be if the battery
>was discharged at say, 5.5 volts? Or somewhat charged at 5.8 volts? or
>fully charged at 6.2 - 6.5 volts? How does one calculate a limiting
>resistance when the supply is so variable, as in this old truck?
>As I am not a trained electronics guy (just a dickering hobby) I am not
>familiar with low drop out regulators. Do you have any specific part
>numbers that would be appropriate here?
>Jack

Just as an example, scroll down to about the middle of the page,
under TO-220
http://www.micrel.com/product-info/ldos.shtml

Figuring out the zener values, just compromise power drain,
power dissipation, and regulation. Its better regulation to draw some power, but things
get hotter. Need more room and airflow.

greg


>
>
>
>
>
>
>"gregs" <szekeresREMOVE@adelphia.net> wrote in message
>news:h5WdnZoPPNicw6PcRVn-hg@adelphia.com...
>> In article <Zub%c.135845$X12.114623@edtnps84>, "Jack"
><jacksimpson@telus.net> wrote:
>> >Hi.
>> >Looking for info on a simple technique of protecting a small (solid
>state)
>> >radio from supply voltage spikes.
>> >I have an old classic truck with 6 volt (neg ground) electrics. The old
>> >tube radio is now toast ..well, too expensive to fix: power transformer
>> >appears shot.
>> >I want to sub the guts of a more modern trans radio into it and that
>radio
>> >also runs on 6 volts (4 c cells). However, I am a tad leary of
>destroying
>> >the new radio by the voltage spikes that the generator will produce
>during
>> >charging: up to 8.5 volts. I need something to smooth out the voltage.
>> >I was thinking of using an LM317 to regulate the radio but I suspect it
>> >won't work properly as the supply will vary from 5.8-8.5 volts. So, I
>was
>> >also intrigued with the idea of using a zener but not sure how to go
>about
>> >that.
>> >The radio appears to draw a max of 100 ma @ 6 vdc.
>>
>> A simple zener type would probably work. With the radio only using only
>> a 100 ma. draw, a zener shunt type would probably work, rather than
>> using it in conjunction with a transistor pass element. With a 20 ohm 10
>watt series resistor
>> feeding the 6 volt 1watt zener, and a 2200 mF parallel 16 volt
>electrolytic, I believe that would
>> work. I did some quick calcs. Put it in a metal box with a 1 amp series
>fuse and an on off
>> switch, since it will draw current even with the radio off.
>>
>> There are a ton of low dropout
>> regulators on the market today. 100-300 millivolts is about all they
>loose.
>> Many are small packages, but there are some To-220's.
>> A DC to DC converter is another approach, but may inject some RF noise.
>> If you have some know how, search around the low dropout regulator
>> lists.
>>
>> greg
>
>

Archived from groups: rec.audio.tech (More info?)

Ted Azito wrote:

> I would either convert the vehicle to 12 volts or fix the original
> tube radio. There are rpl parts readily available if you know where to
> look. A solid state vibrator is easy to build.

Re : "solid state vibrator".... isn't that a contradiction in terms ? You
mean inverter I assume ? There are some pretty fancy adult toys these days
OTOH. ;-)

Graham

Archived from groups: rec.audio.tech (More info?)

"Ted Azito" <larboard34@hotmail.com> wrote in message
news:b53da461.0409121758.75d24da5@posting.google.com...
> I would either convert the vehicle to 12 volts or fix the original
> tube radio. There are rpl parts readily available if you know where to
> look. A solid state vibrator is easy to build.

Thanks, Ted. But I was told that constructing a SS 6-volt radio vibrator
was too onerous and to just buy one from Antique Audio ..sure. By the time
it's landed here in Canada it costs close to $65! So, just how easy is it
to build, Ted?
Jack