circuit to extract 7.5v from car battery

[Dave]

I am looking for a circuit that will output 7.5V from the 12V car battery?
What is the best solutiosn for this using as little components as possible.
TIA

You have not given a very useful specificiation.

What sort of accuracy - +/- 2V, 2mV, 2uV ?

What sort of current - 100A, 1A, 1mA or whatever ?


It could be as simple as 2 resistors if the accuracy requirements are
lax, and the current small. It will need some form of complex swith mode
design if you want to take 100Aa.

 

[ehsjr]

I am looking for a circuit that will output 7.5V from the 12V car battery?
What is the best solutiosn for this using as little components as possible.
TIA

You didn't specify how much current you need, or
what the application is.

Here's a standard LM317T circuit with an added TVS diode
that might meet your needs. It will provide up to 1.5 amps,
if you mount the LM317T on a heatsink:

--------
+12 ---+---+---Vin| LM317T |Vout---+--------+--- +7.5
| | -------- | |
| | Adj [R1] 240 |
| | | | | +
[D1] [C1] .1uf +------------+ [C2] 1 uF
| | | |
| | [R2] 1.2K |
| | | |
Gnd ---+---+----------+---------------------+---Gnd


D1 is a 15 volt TVS diode like Digikey part# 1.5KE15CADICT-ND

Ed

 

[Rich Grise]

I am looking for a circuit that will output 7.5V from the 12V car battery?
What is the best solutiosn for this using as little components as possible.
TIA

http://www.radioshack.com/category.asp?catalog_name=CTLG&category_name=CTLG_009_001_002_004&Page=1

Good Luck!
Rich

 

[Orc General]

I am looking for a circuit that will output 7.5V from the 12V car battery?
What is the best solutiosn for this using as little components as possible.
TIA

 

[Tam/WB2TT]

I am looking for a circuit that will output 7.5V from the 12V car
battery?
What is the best solutiosn for this using as little components as
possible.
TIA

You didn't specify how much current you need, or
what the application is.

Here's a standard LM317T circuit with an added TVS diode
that might meet your needs. It will provide up to 1.5 amps,
if you mount the LM317T on a heatsink:

--------
+12 ---+---+---Vin| LM317T |Vout---+--------+--- +7.5
| | -------- | |
| | Adj [R1] 240 |
| | | | | +
[D1] [C1] .1uf +------------+ [C2] 1 uF
| | | |
| | [R2] 1.2K |
| | | |
Gnd ---+---+----------+---------------------+---Gnd


D1 is a 15 volt TVS diode like Digikey part# 1.5KE15CADICT-ND

Ed

The 15V TVS will blow. I would go with something like 27V, followed by an LC
filter. If the current is low, he could put a resistor ahead of the TVS.

Tam

 

[ehsjr]

I am looking for a circuit that will output 7.5V from the 12V car
battery?
What is the best solutiosn for this using as little components as
possible.
TIA

You didn't specify how much current you need, or
what the application is.

Here's a standard LM317T circuit with an added TVS diode
that might meet your needs. It will provide up to 1.5 amps,
if you mount the LM317T on a heatsink:

--------
+12 ---+---+---Vin| LM317T |Vout---+--------+--- +7.5
| | -------- | |
| | Adj [R1] 240 |
| | | | | +
[D1] [C1] .1uf +------------+ [C2] 1 uF
| | | |
| | [R2] 1.2K |
| | | |
Gnd ---+---+----------+---------------------+---Gnd


D1 is a 15 volt TVS diode like Digikey part# 1.5KE15CADICT-ND

Ed

The 15V TVS will blow.

Why? It'll handle up to 71 amp transients. Is the
steady voltage over 15 volts? Shouldn't the battery
hold it lower? I've had one of them across my 2 meter
rig for years. Fed from the battery with #12 - say
20 feet total wire - with a 30 amp relay at the
rig, energized by the accessory line.

The LC is a good idea, but is contrary to what he
wanted - "as little components as possible". I
think he might get by with the standard 317 circuit
with the added TVS and no LC to fulfill that "little"
requirement (which I take to mean parts count).

Ed

I would go with something like 27V, followed by an LC

 

[Tam/WB2TT]

Orc General wrote:

I am looking for a circuit that will output 7.5V from the 12V car
battery?
What is the best solutiosn for this using as little components as
possible.
TIA



You didn't specify how much current you need, or
what the application is.

Here's a standard LM317T circuit with an added TVS diode
that might meet your needs. It will provide up to 1.5 amps,
if you mount the LM317T on a heatsink:

--------
+12 ---+---+---Vin| LM317T |Vout---+--------+--- +7.5
| | -------- | |
| | Adj [R1] 240 |
| | | | | +
[D1] [C1] .1uf +------------+ [C2] 1 uF
| | | |
| | [R2] 1.2K |
| | | |
Gnd ---+---+----------+---------------------+---Gnd


D1 is a 15 volt TVS diode like Digikey part# 1.5KE15CADICT-ND

Ed

The 15V TVS will blow.

Why? It'll handle up to 71 amp transients. Is the
steady voltage over 15 volts? Shouldn't the battery
hold it lower? I've had one of them across my 2 meter
rig for years. Fed from the battery with #12 - say
20 feet total wire - with a 30 amp relay at the
rig, energized by the accessory line.

The minimum breakdown voltage on the 1.5KE15 is 13.5V.That is not good.
Depending on how the car is wired up, and where the battery is located, you
could have more than 15 VDC at the cigarette lighter socket. My previous car
ran about 15.2V. Original battery was still good when I sold the car; so,
this was not a problem. Battery was under the rear seat.

The 317 is good for 40V, but noise suppression is poor at high frequencies.
That's the reason for the LC filter. Might work OK without the filter, but
inductors don't cost anything if you can afford to spend 5 minutes to make
one.

Tam

 

[ehsjr]

Orc General wrote:


I am looking for a circuit that will output 7.5V from the 12V car
battery?
What is the best solutiosn for this using as little components as
possible.
TIA



You didn't specify how much current you need, or
what the application is.

Here's a standard LM317T circuit with an added TVS diode
that might meet your needs. It will provide up to 1.5 amps,
if you mount the LM317T on a heatsink:

--------
+12 ---+---+---Vin| LM317T |Vout---+--------+--- +7.5
| | -------- | |
| | Adj [R1] 240 |
| | | | | +
[D1] [C1] .1uf +------------+ [C2] 1 uF
| | | |
| | [R2] 1.2K |
| | | |
Gnd ---+---+----------+---------------------+---Gnd


D1 is a 15 volt TVS diode like Digikey part# 1.5KE15CADICT-ND

Ed


The 15V TVS will blow.

Why? It'll handle up to 71 amp transients. Is the
steady voltage over 15 volts? Shouldn't the battery
hold it lower? I've had one of them across my 2 meter
rig for years. Fed from the battery with #12 - say
20 feet total wire - with a 30 amp relay at the
rig, energized by the accessory line.

The minimum breakdown voltage on the 1.5KE15 is 13.5V.

The datasheet for the specified part does not agree. It
gives 14.3 V. See:
http://www.diodes.com/datasheets/ds21503.pdf

That is not good.

Depending on how the car is wired up, and where the battery is located, you
could have more than 15 VDC at the cigarette lighter socket. My previous car
ran about 15.2V.

Ok. That makes your point that a higher voltage TVS is
indicated.

Original battery was still good when I sold the car; so,

this was not a problem. Battery was under the rear seat.

The 317 is good for 40V, but noise suppression is poor at high frequencies.
That's the reason for the LC filter. Might work OK without the filter, but
inductors don't cost anything if you can afford to spend 5 minutes to make
one.

The reason for the LC filter is not in question. For the OP's
requested minimum parts count, it must not be included,
unless mandatory. Of course it could be argued that the
OP might be able to use a zener and a resistor and nothing
more. We're not talking best design, here, we are talking
about a minimal circuit that might meet his needs.

Ed

 

[Tam/WB2TT]

...............................

Of course it could be argued that the

OP might be able to use a zener and a resistor and nothing
more. We're not talking best design, here, we are talking
about a minimal circuit that might meet his needs.

Ed

That may not be bad at all. A zenar with an emitter follower current
booster. (unless he only wants 10 ma)