Help with Diode information for aftermarket radio

[GoodysGotaCuda]

Father-in-law purchased a aftermarket radio that meets his needs for his 1976 Warlock (D100, I know it's no ebody, but same dual resistor ballast wiring). Only complaint that I have seen in this particular radio is a complaint of the radio not booting up when the car is started quickly. They mention the key needs to be turned on, then started. I'd like to cure this problem before installing it.

From what it sounds like it is caused by the "switched 12v" source not letting the radio start up and it locks up. I figured by piggy-backing off the ballast/coil wiring I can resolve this. Since then the radio would be powered while cranking as well. (Which I know could be a concern in dying battery situations, but the small current draw doesn't concern me. And the vehicle is well maintained.)

Problem with that is full voltage would then be fed to the coil positive versus the dropped voltage it is designed for. Anyone have diode information on what I am looking for? I understand the concept, but do not know what style/size/rating/etc I am looking for.

Any advise?

Thanks- 

[quagmire]

I'm assuming you want to use a diode inline between the coil power and the switched 12v wire so the radio has constant 12v while cranking and in run and it will not see a voltage spike low during cranking.

For what I think you are trying to do, I would just use a standard diode with a wattage rating 15-20% higher than what the radio is rated for at max power.  Really you could probably get away with much lower, but better safe than sorry.  A little bigger won't hurt in that situation.

Radioshack should have several generic diodes that would work.  As long as they meet the voltage and wattage requirements you are good to go.  For this app, there are literally thousands of diodes that would fit the bill.

[GoodysGotaCuda]

Since pictures are worth 1,000 words, here is how i would like to use the diode. To keep the Run 12v from feeding the coil direct 12v via the 'start' wire that goes straight to the coil +

hopefully that makes it clear. max that circuit should draw is MAYBE 2amps (probably in the ma). Since it's just a sensing wire for the radio to turn on.

[quagmire]

Yeah, that 's what I figured.  Should work just fine.  Just beware not all radios use the ignition switched wire as a low current turn on only, some use it to power the internal amp itself while the constant 12v wire is used for KAM.  Either way, any normal diode with a wattage rating slightly greater than what you will see on that circuit will work.

[GoodysGotaCuda]

Yeah, that 's what I figured.  Should work just fine.  Just beware not all radios use the ignition switched wire as a low current turn on only, some use it to power the internal amp itself while the constant 12v wire is used for KAM.  Either way, any normal diode with a wattage rating slightly greater than what you will see on that circuit will work.

'normal' diode? Any particular name/style? And does the band (on the diode) go towards the radio or the start 12v in that diagram?

Sorry for all the questions 

[quagmire]

I would use whatever is readily available and cheap to be honest.I usually use an inline style that has leads to solder on, just like a typical resistor.  As long as it is rated for the wattage, current, and voltage you'll be fine.  You'll never have a reverse bias voltage on it without 12v on the other side so it won't ever be breaking a sweat.  By normal I mean a rectifier diode.  The band end will be closest to the radio.

[GoodysGotaCuda]

I would use whatever is readily available and cheap to be honest.  A long as it is rated for the wattage, current, and voltage you'll be fine.  You'll never have a reverse bias voltage on it without 12v on the other side so it won't ever be breaking a sweat.  By normal I mean a rectifier diode.  The band end will be closest to the radio.

I'm thinking this should do, 6a. I don't need four but they may have singles in store
http://www.radioshack.com/product/index.jsp?productId=2062591&numProdsPerPage=60&y=4&x=11&retainProdsInSession=1
 

[quagmire]

That should work perfectly.  Even if your radio draws more than that at peak, I really doubt you'll be blaring it at max volume very much while cranking.  Even if you were it would be able to take it for the few seconds before it the car starts without any issue. 

I've bought tons of semiconductors from there for all sorts of projects, most of the resistors and diodes are sold in 4 packs.  At least they're cheap and you'll have spares.

I also use Digikey and Mouser Electronics for the harder to find stuff, but for what you need it'd be a waste of money after shipping to get it from them.

[Changin Gears]

My advice would be to return the radio and buy one that doesn't have known defects.

[Changin Gears]

If you do decide to put the fix in you really need 2 diodes, one each in the start and run lines.  If you don't you will be suppling current to the whole vehicle through the radio fuse and the diode by "back fackfeeding".

[quagmire]

The diode on the starter wire will prevent backfeeding through the whole car.  The other wire normally supplies 12v anyways.

[Changin Gears]

The diode on the starter wire will prevent backfeeding through the whole car.

The diode in the start wire will keep from backfeeding during run.  During start you need the 2nd diode.

The start wire is providing the voltage.  This then goes back through the run wire (back through the radio fuse then to the rest of the vehicle) during cranking. 

[quagmire]

Yeah, thats true.  Forgot all about that.