Converter or regulator from 12V Car Battery to 5.5V 3-5A, with low emi

[Economos]

Description of desired converter/regulator:
1. Low EMI to avoid interference with radio, police two-way comm, cell, etc....
2. Dual conversion required:

a. One conversion circuit (circuit 1) connected to the positive and negative battery terminals but active only when triggered
b. one conversion circuit that gets power when the ingnition circuit is on or accessory on. This circuit gets 12V from the ignition circuit and is used to trigger circuit 1.
c. The triggering is handled in the powered device - triggering is not required in the converter/regulator design- simply a low emi voltage conversion.
d. The powered device measures power at 5.5V at .5A but I want to have more than enough amperage available - if 3-5 A sounds like much more than necessary - I have no problem scaling amperage back.
e. 5V instead of 5.5V to the powered device should not be a problem.​

 

[Inquisitive]

Description of desired converter/regulator:
1. Low EMI to avoid interference with radio, police two-way comm, cell, etc....
2. Dual conversion required:

a. One conversion circuit (circuit 1) connected to the positive and negative battery terminals but active only when triggered
b. one conversion circuit that gets power when the ingnition circuit is on or accessory on. This circuit gets 12V from the ignition circuit and is used to trigger circuit 1.
c. The triggering is handled in the powered device - triggering is not required in the converter/regulator design- simply a low emi voltage conversion.
d. The powered device measures power at 5.5V at .5A but I want to have more than enough amperage available - if 3-5 A sounds like much more than necessary - I have no problem scaling amperage back.
e. 5V instead of 5.5V to the powered device should not be a problem.​

Is there a question here?

 

[MikeMl]

You want to buy, or make?

Here is a recent forum thread about why you should not consider Switch mode converters for this application.

I recommend a linear regulator, with an enable-input that comes from the ACCY contact on the key-switch. Write back if you need some help with this.

 

[bountyhunter]

You could use the old design taking a 7805 type reg and using a PNP power transistor as a pass element to get more current. You have given such confused specs I have no idea what the design is supposed to do.

How much current does the 5V output actually need to put out?

Does it need current limit protection?

Size/weight limits?

What does it power?

Is the 5V line inside another device or open to the outside?

 

[Economos]

Thanks everyone. As you can probably tell I am new to electronic circuits.
I attempted to give a description of the converter/regulator device I needed.
The device I described will be used to power an in car security camera and programable rear view mirror with 5X3 inch lcd display. Cheifly it is used for law enforcement to record various patrol events. It is a sophisticated system that records streaming GPS coordinates during the audio/video recording of patrol events and supports wireless audio recording outside the patrol car during stops. It also has the capabilty of stealthy reording of video and audio inside the vehicle (for suspects left in the car when an office steps away.) A very cool system - Digital Ally DVM 500 plus. Recording can be automatically triggered by siren, acceleration, g-force, turn signals, speed, manual activation and light bar or it can continuosly record while the ignition is on or in accessory mode.

I have 20 of these systems that I am planning to repurpose, but when they were removed from the patrol cars the power regulating modules (box with molex connections- converters/regulators) and the cabling was left installed in the cars and not included with what I received. I can purchase the power box and cabling for about $450 each or find another way to power and cable the units. I have determined the various cabes and connectors I need and now would like to complete the set-up items by coming up with a solution to power the systems.

The mirrors themselves contain the processing boards, programming and storage. I plan to strip down the 12 cables that include the recording triggers (siren, lights, acceleration, etc.) and just provide 5.5 volts of power with at least .5 amps of current to two circuits.

In order to do this I need only three cables - two are positive leads and one is ground/negative. One positive lead is attached to the ignition switch- when the ignition is turned on or turned to accessory mode - this lead triggers the mirror system to power up and be ready to record. The second positive lead is attached to the battery and provides power to the mirror when the ignition lead turns the system on - it also provides up to a 30 minute delay in power down (runs from the battery) once the ignition switch is turned off (they delay is controlled by the processors in the mirror.)

I am thinking I will either have a power module box with two circuits and in and out molex connectors or I will construct two postive leads each with an inline fuse and converter. If i go for a box - I would like it to be a 3X5 inches or 4X6 inches approximately. I envision the 12 volt coming in to the box - one ground, one direct to the positive on the battery and one positive from the ignition switch - these come in as 12 volt - and 3 cables coming out - one ground - one from the ignition circuit and one from the battery direct circuit - at 5.5 volts and a minimum of .5 amps. These each go to a different connection on the mirror/ processing unit - the mirror does the rest.

Yes- this is a question- what would you guys recommend for my power conversion/regulation, do you think I should put it in-line on each positive lead or build a box, how can I cut down on EMI and does any one have a plan / parts for such a device.

Hopefully I have been a bit more descriptive- please let me know what other details yo may nned in order to assist me.

Thanks,

Kirk

 

[bountyhunter]

I have 20 of these systems that I am planning to repurpose

I don't understand. Does "repurpose" mean they will be installed in civilian cars or will the hardware be used in other ways (like home surveillance)?

Will the units be powered from a car's electrical system?

If so, the input voltage is more like 14V than 12V. Bottom line, if a linear power converter is used to make 5V, that will mean a voltage drop of about 9V across the linear regulator which will yield about 5W of power dissipation for a 0.5A unit under power. That will require a heatsink with rating of about 15 deg C/Watt or less which is not too big and doable in your 4" x 6" limit of size..

BTW: what standby quiescent current is allowed for the thing while it is "idling" waiting to come on?

Based on what I know, I would build a 5V regulator from discrete components using the standard linear reg topology: op-amp, reference, power transistor. You can add current limiting really cheap.

There are probably linear regs in monolithic IC form that can probably work as well. You would need one with "low power" shutdown feature.

 

[MikeMl]

I would just use these. Download and read the data sheet. Adjustable, up to 1A if on an adequate heatsink.

 

[Economos]

Cables for battery and ignition to converter are awg 16 and are individual conductors. The cables coming off the converter are awg 20 multi-conductor cables with molex connectors that eventually attach to a molex recepticle at the back of the mirror system. I wanted the power module to be as small and light weight as possible- it's most probable location for installation would be under the dashboard or in the glove box.

 

[Economos]

These will be for police or non-police car event monitoring and recording (police, commercial fleet or personal.) With a 64GB memory card they can store about 53 hours of vga audio/video. So you can continuous record whenever the ignition is on or take it off continuous mode and press a button to record (it also offers pre-event recording - meaning that it has captured and will save up to 30 seconds of what happened before you press the record button along with the file.) These are very cool systems.

Thanks Bountyhunter and MikeMi- Bounty- I will have to look up "standby quiescent current" but I think I understand what you are asking. When the ignition circuit (I call circuit 2) has no power - the circuit attached to the +/- of the 12V battery (circuit 1) has a draw of zero A. Once the igintion is in the on or accessory mode - the total draw of circuits 1 and 2 is .5A. The trigger circuit is in the mirror- not in the power supply. Both circuits 1 and 2 have conductors that when active go to the mirror with about 5.5 V.

Sorry I am piecemealing the description of this system and the device I want to build to power it. I will try to put a simple schematic together and post it later today.

Thanks for you patience.

Kirk

 

[bountyhunter]

I will have to look up "standby quiescent current" but I think I understand what you are asking. When the ignition circuit (I call circuit 2) has no power - the circuit attached to the +/- of the 12V battery (circuit 1) has a draw of zero A.

No, my point was that a simple way to do the 5V reg design would be to build the regulator with an op amp and power transistor and leave that connected to the power line. Then use the "switched 12V line" to feed the bandgap reference that tells the reg to come up. The unit would draw a few milliamps sitting with no output and then come up when the keyed 12V line turns on and the reference comes up. So the reg would draw a few mA sitting in standby. Most auto electronics allow a little standby current draw.

 

[bountyhunter]

I would just use these. Download and read the data sheet. Adjustable, up to 1A if on an adequate heatsink.

LM317 does not have the turn ON/OFF feature he said he needed.