Power Supply design

[viks_raj]

Hi guys

I am desiging power supply for my project.

Input voltage will be around +12V from CAR battery.
So it will be not exactly +12V.

I need four output out of it.
+/-12V , +5V and +3.3V.

I was thinking to use thinking to use COSEL DC-DC converter
**broken link removed**

After that I can use simple ICs like 7805,LM2937 .

Do you think this approach is OK.
I need to provide this power to following boards.
Ezdsp2812(3.3V), one LCD(+5V) and interfacing board which had 10 Relays and some Optocouplers (+12/-12V).

And comments would be very helpful.
Thanks,

 

[Nigel Goodwin]

What are your power requirements?, the +/-12V requirement also looks rather strange?.

 

[viks_raj]

thanks for your reply Nigel, I appreciate it.

+12/-12V is for Opamp which is generating +9 to -9 V signal.

150mA for LCD, 500mA for DSP board and I am not sure about interfacing board which I am desiging, it will have 12-15 solid state relays with some Optocouplers.

Total current requirements should be within 1.5A - 2A.

 

[Optikon]

you know that the 12V car supply voltage can be 80V at times while running right? Just be sure to use adequate overvoltage protection.

 

[viks_raj]

you know that the 12V car supply voltage can be 80V at times while running right? Just be sure to use adequate overvoltage protection.

Thanks for reply. I will take care of it.

This kind of advise I am looking for, thanks again.

 

[RDL2004]

you know that the 12V car supply voltage can be 80V at times while running right? Just be sure to use adequate overvoltage protection.

Okay, you are pulling his leg - right?

 

[dieplz]

you know that the 12V car supply voltage can be 80V at times while running right? Just be sure to use adequate overvoltage protection.

Umm.... no?
Sure the voltage across the battery terminals increase when the alternator is pumping to power your car electronics and charge the battery, but 80V is a pretty high approximation. It's probably more around 14-16V I would say. Take a look at a voltage gauge on a car when its running, its typically 14-14.5V, so I would say 16V would even be a pretty high end for a pretty typical car.

 

[audioguru]

When the battery cable becomes loose on the battery then the voltage spikes from the alternator will be 60V. Maybe the voltage also spikes to +60V when you turn off the rear window defogger.

I use the LM2931 low-dropout regulator. It features +60V load dump protection and -50V reverse transient protection for use in cars.

 

[mneary]

If you're directly across the battery then you are assured of less than 16-20V. But you're probably using the ACCessory or IGNition circuit. It's not always connected to the battery. Various inductive loads might not be well behaved when their battery connection is suddenly lost, and 80V is not uncommon.

 

[Diver300]

Car battery voltages are very unreliable.

80 V for 1/2 second can occour if the battery is being charged, then it gets disconnected. The alternator continues to output loads of volts until the field current in the alternator decays.

The problem is that a battery connection can break at any time.

On the low side, some modern vehicles have current limiters instead of fuses. This is great in that a the vehicle recovers from a breif short circuit. The trouble is that the current taken by filament lamps turning on looks like a short circuit and the voltage after the limiter dips badly.

Recent example. Ford Transit, hazard lights. The voltage feeding them drops to 4V each time they light.

Good luck. You'll need it.

 

[picbits]

I've seen examples of up to 200 volts during load dumps.

I build stuff for cars and my input protection circuitry costs almost as much as the rest of the designs.

 

[Nigel Goodwin]

I've seen examples of up to 200 volts during load dumps.

I build stuff for cars and my input protection circuitry costs almost as much as the rest of the designs.

I think you all need to put it in to perspective! - yes a car is a fairly nasty electrical environment - BUT, fairly minimal precautions are almost certainly all that's required. Car audio systems don't have any complicated protection systems, and I've never seen (or heard) of one being damaged by the electrical connection.

 

[picbits]

Aye but overdesigning the power supply input for stuff used on cars in a commercial environment makes sound business sense.

The latest input filtering and protection circuit I've got on the drawing board will cost me an extra £5 per unit (unit retail price is £50) but will make the unit virtually bombproof meaning that the return rate under warranty should be close to zero. It should also mean that the units will operate without problem for many years for the customer which is as important to me as making sure they last until the warranty period.

The cost of a customer sending a product back , repairing or replacing it, sending it back out would cost me a lot more than £5 a unit in both postage and time.

For a hobbyist though an LM2940 (or even a 7805) would normally suffice as if it goes wrong you just grab another regulator/PIC from the junk box and swap the bits over.

 

[picbits]

P.s. I've been at a garage watching the mechanic trying to start a Citroen Diesel.

His 180 amp booster charger wasn't quite up to the job of getting it going so he whacked it up to the 24 volt range.

The car started (eventually) but we noticed smoke coming out from the dashboard and it was in fact the radio that was on fire. After ripping it out of the dashboard and throwing it across the yard he proceeded to drive the car off for its MOT.

Oh and we may be veering off topic slightly as the OP said it was to run off a 12v car battery rather than in a car itself.

 

[Hero999]

What about the bulbs, ECU and all the other electronics?

Did the speakers make a loud sound? They were also probably also knackered.

 

[picbits]

I didn't hang around much longer after watching that - I took my car to a different mechanic lmao.

 

[viks_raj]

Actually I will be using Car battery just as power source.

I am not using it with Car or Car electronics.

My main concern is to convert +12 V to +/-12V.

I was looking for Cosel DC-DC converter but I have never used it.
Anyone want to suggest something for it ??

 

[Nigel Goodwin]

Like I said originally, it all depends on the power requirements, in particular that required from the -12V line. You don't explain exactly what you're trying to do, and if you actually do need +/-9V swing on the opamps?.

 

[viks_raj]

I am sorry for not explaning my problem properly.

This is my project.

I have 1 DSP board F2812, 1 LCD, 1 Signal conditioning and I/O board.

Opamp in I/O board generate output in range -9/+9 which goes into AVR to control generator voltage. Generally it never goes into that range but it MUST suppport output between -9 to +9.

Right now I am using different power supply for each , but I need to design one power supply for all of three (+12,-12,+5,+3.3)
and input to this supply will be +12V battery.

 

[Nigel Goodwin]

In that case measure the current drawn on the -12V rail, make sure you try it under all possible circumstances. Pretty silly spending a lot of time, money and effort generating an amp or two at -12V, only to find you require 10mA or so.