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Old 21st Mar 2023, 12:54 pm   #1
PaulR
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Default DC-DC converter for car radio

A friend has several classic cars which are positive earth. Radios with FM which have switchable grounding bring a premium price as they are relatively rare whereas old negative earth sets are cheap.

There is a commercially available isolator specifically for the purpose but this is also quite expensive and I was thinking about buying a suitable module and mounting it in a box. There are lots of DC-DC modules about but with a bewildering variety of inputs, outputs and current ratings.

Has anyone done this and if so which one would be suitable.

Thanks
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Old 21st Mar 2023, 1:00 pm   #2
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Default Re: DC-DC converter for car radio

In the 'sixties there was a Pye unit intended for powering some of the early transistorised two-way radios that provided a floating 12V output from either a +ve or -ve earthed input.

It was an inverter using a pair of OC28 transistors.

My issue with building your own would be in regards of RFI; a while back I did a conversion of a R209 receiver to replace the failed vibrator/transformer with a cheap 12V-to-high-voltage inverter. It took a bit of effort to make it sufficiently RF-quiet.
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Old 21st Mar 2023, 3:43 pm   #3
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Default Re: DC-DC converter for car radio

Yes RFI might be a problem. I was wondering about lining the case with foil and grounding that to the car chassis. The ready made one must do something like that but I was hoping to make something for rather less than the £70 odd they are charging for it
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Old 21st Mar 2023, 4:22 pm   #4
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Default Re: DC-DC converter for car radio

My friend used to run his ex-military gear from 12V using the simplest inverter he lashed together with a 12V mains transformer driven by a couple of MOSFETs, You could do the same for an isolated 12V output by finding a small tfmr with two separate 12 or 15V secondaries. Running at 100-200Hz would increase the power rating of the tfmr. I'm sure others will advise before I can locate my notes on this
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Old 21st Mar 2023, 4:26 pm   #5
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Default Re: DC-DC converter for car radio

Without wishing to take the thread into forbidden territory, might it not be easier to change the vehicle's polarity? Positive earth vehicles generally used dynamos and changing the polarity is relatively easy.
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Old 21st Mar 2023, 4:33 pm   #6
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Default Re: DC-DC converter for car radio

As above.
It's easy to reflash the Dynamo. Firstly reverse the battery, remove the field wire from the dynamo and take a lead from the positive battery connection and stroke it across the field connection on the dynamo a few times, it'll spark but don't worry that's normal, reconnect the field connection then the system will be negative earth.
Gauges should work ok but if it's got an ammeter you'd need to reverse the connectons.
I'd not consider using a DC to DC converter due to hash.
Any further info PM me so the thread doesn't break the rules.
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Old 21st Mar 2023, 4:42 pm   #7
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Default Re: DC-DC converter for car radio

P.S. no hash at 100Hz
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Old 21st Mar 2023, 5:01 pm   #8
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Default Re: DC-DC converter for car radio

Quote:
Originally Posted by Julesomega View Post
P.S. no hash at 100Hz
The problem is the harmonics; these were less back in the day when Germanium transistors with a low Ft and slow switching were used, but modern switchers/inverters using modern transistors/MOSFETs whose Ft may go into tens/hundreds of MHz and driven with a square-wave can be potent sources of harmonics!
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Old 21st Mar 2023, 5:53 pm   #9
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Default Re: DC-DC converter for car radio

Quote:
Originally Posted by G6Tanuki View Post
Quote:
Originally Posted by Julesomega View Post
P.S. no hash at 100Hz
The problem is the harmonics; these were less back in the day when Germanium transistors with a low Ft and slow switching were used, but modern switchers/inverters using modern transistors/MOSFETs whose Ft may go into tens/hundreds of MHz and driven with a square-wave can be potent sources of harmonics!
Agree 100%.
I've a few friends that use modern types and they find too many problems when listening to Medium and long Wave
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Old 21st Mar 2023, 5:57 pm   #10
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Default Re: DC-DC converter for car radio

Quote:
Originally Posted by murphyv310 View Post
As above.
It's easy to reflash the Dynamo. Firstly reverse the battery, remove the field wire from the dynamo and take a lead from the positive battery connection and stroke it across the field connection on the dynamo a few times, it'll spark but don't worry that's normal, reconnect the field connection then the system will be negative earth.
Gauges should work ok but if it's got an ammeter you'd need to reverse the connectons.
You have to do the same on the starter and other motors (probably only the fan of the heating system on most cars).
I don't think the polarity of the (non electronic) ignition system is important.
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Old 21st Mar 2023, 6:00 pm   #11
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Default Re: DC-DC converter for car radio

Every starter motor I've ever encountered has had field windings so polarity doesn't matter.

Automotive discussions aren't allowed in the forum so let's get back on topic please.
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Old 21st Mar 2023, 6:21 pm   #12
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Default Re: DC-DC converter for car radio

There are reasons why he would prefer not to change the polarity, hence the query about a DC-DC module. There is less and less on AM these days so he he wants to use FM exclusively, but FM on car radios was coming in as +v earth was going out so changeable earth FM sets command quite a premium. I have fitted a "Aux" connection to a couple of his AM only sets so he can use an external source but it would be good to be able to receive FM.
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Old 22nd Mar 2023, 10:39 am   #13
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Default Re: DC-DC converter for car radio

I can remember some older transistorised AM only car radios dating from the 1960's could be run on both positive earth and negative earth by means of moving some wire links or a simple switch system. I am also sure that some radios of this era used a 4 diode bridge rectifier on the 12v supply input enabling the radio to run on both positive earth and negative earth.
But this is going back over 40 years so it might be my memory playing tricks.
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Old 22nd Mar 2023, 12:23 pm   #14
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Default Re: DC-DC converter for car radio

Yes it is true that some can be changed but this is fairly rare with FM sets and they bring a premium price compared with negative earth only ones
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Old 26th Mar 2023, 6:01 pm   #15
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Default Re: DC-DC converter for car radio

Would this do the job? https://www.amazon.co.uk/HOMELYLIFE-...1zcF9hdGY&th=1
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Old 26th Mar 2023, 7:23 pm   #16
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Default Re: DC-DC converter for car radio

Quote:
Originally Posted by PaulR View Post
Possibly.
The output needs to be completely isolated from the input. If not it's no good.
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Old 26th Mar 2023, 8:52 pm   #17
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Default Re: DC-DC converter for car radio

Hi PaulR

A high percentage of voltage converters use a buck or boost architecture with a common input and output ground. This allows use of a simple choke, rather than a more expensive transformer.

You need something specifically billed as a polarity changer or a fully isolated converter - if it doesn't specifically state this then it probably isn't.

Sorry if this doesn't sound encouraging

By consolation, a very long time ago I had a 6 volt car. I made a 6 to 12 volt converter to run a Philips AM/FM car radio, quite a rarity at that time to have FM. Even with elaborate screening and filtering, the hash was enough to knock out any AM reception, but FM was fantastic !
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Old 26th Mar 2023, 9:12 pm   #18
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Default Re: DC-DC converter for car radio

Thank you both, I did suspect that it wouldn't be isolated
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Old 27th Mar 2023, 12:52 am   #19
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Default Re: DC-DC converter for car radio

here's an isolated one. At that price is must be worth a try regarding HF hash? It's a reputable make.

https://uk.farnell.com/mean-well/sd-...-1a/dp/2816173
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Old 27th Mar 2023, 9:39 am   #20
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Default Re: DC-DC converter for car radio

Thanks Kevin. It seems worth a try.
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