Modern Radio in Classic Positive Ground Vehicle

Hi Guys, I wonder if anyone can help.

The plan is to fit a modern radio in an old positive earth car. I know one option is to convert to negative ground - actually that has already been done but firstly the time clock does not work and I'm stuck with it as it is part of ther rev counter, and secondly I would like to keep the car as original as possible so I plan to convert it back to positive earth.

Regarding the radio the most obvious choice may be to isolate the car radio from the car body but I'm not entirely happy with that as it is too vulnerable to the occasional short and possible fire. I know fuses can protect things but I'm still not happy with that idea! The other option I gleaned from the internet may be to use a voltage doubler/multiplier, connect the input positive to the vehicle ground, and take the output between the vehicle ground, (+12v) and the output positive (+24v). In theory I guess this would work but I don't know what type of circuit to use. Also the output would have to be fairly consistent between 12 and 14 volts.

I've come across many dc voltage doubler circuits but most of them use 555 timers or similar devices that I don't think will handle the current requirements. I also suspect the problem with some circuits would be that if the 24v output is well regulated, the voltage as described above would drop as the vehicle charging system starts to work. Unless anyone can tell me otherwise it seems that the only option may be to use an inverter to convert the dc to ac, a transformer to boost the voltage, and then a rectifier to convert back to dc with some kind of voltage regulation. In this setup would it be possible to earth the positive input and the output negative together? I think that the use of a transformer will isolate the two so there would be no problem doing this. Can anyone confirm?

Or does anyone have any other ideas?

The present radio in the car (as someone has previously converted it to negative earth to fit this - as I said before I plan to convert it back) has an input fuse rated at 10 amps so this circuit must be able to deliver that as a minimum. This won't be the actual radio going in (this one doesn't work - another project for another day!) but I guess it will be something similar.

Over to you guys!...

Hi,

I've worked with 240v high current rectifier circuits (10 killowatts and higher) with diodes that mount right to the metal heat sinks. The diodes were made in two versions, one with anode stud and the other with cathode stud, just for this very reason so that they can be mounted directly to the heat sink without any insulating washer of any kind. That puts 340v peak DC voltage right on each (two or more heat sinks) heat sink metal. The whole project is often housed in a large metal cabinet.

Why bring this up? Because even at those high voltages, there are ways to electrically isolate the heat sink metal safely from the project chassis, and we're talking a lot more voltage than 12 volts DC here.

One way this is accomplished is to use fiber glass board material. The heat sinks are bolted to the fiber boards using one set of bolts, the fiber boards are then bolted to the chassis using another set of bolts, possibly with metal standoffs so the heads of the first set of bolt screws cant touch the chassis.
This kind of setup lasts much longer than the life of the equipment and even freight and overseas shipping. Earthquake proof? Maybe with thick enough fiber board and decent size bolts.

So there's one tried (over years and years) and proven method. Mount the radio to insulated board material, then mount that to the car chassis. If it's mounted inside the dash that will protect it from falling or loose metal debris.

As you suggested either a voltage doubler or a 12v to 12v isolated power supply. Found a link but it could be expensive.
http://www.mgpower.com/?module=product&class=10&mode=0

Thanks moffy, I can't find a price on the web site but it looks huge! More to the point though I was hoping for a suitable circuit that I can build for myself - it's more fun that way!

I guess by what you are suggesting that the transformer will isolate input and output so it will be ok to connect the positive of the input to the negative of the output. Will this have any effect on the output?

Also, thank you MrAl, yes I could mount it in fiber board and it could be ok, but I'm still not happy about the idea - there is also the problem of mounting the aerial that has to be mounted on the body.

Please find the attached link. https://www.electro-tech-online.com/custompdfs/2011/11/dn371f.pdf
Gives you a practical circuit. But it's surprising how much time and cost can be involved in a 1 off. Sometimes it's cheaper and more reliable to buy. But I agree it's more fun to build.

Why not convert the car to negative ground? I suspect it doesn't have a computer or anything similar. The clock problem can be more easily fixed than the radio problem. Besides, isn't putting a modern radio in the car inconsistent with keeping it as original as possible? Is it a 6V or 12V system originally?

John

jpanhalt said:

Why not convert the car to negative ground? I suspect it doesn't have a computer or anything similar.

John

Click to expand...

The car is already converted to negative ground but the time clock cannot work like this and I can't change that as it's part of the rev counter - I found this out when I took it apart to find out why it wasn't working, so I plan to convert it back to positive earth - also to keep it as it was when new

moffy said:

Please find the attached link. https://www.electro-tech-online.com/custompdfs/2011/11/dn371f-1.pdf
Gives you a practical circuit. But it's surprising how much time and cost can be involved in a 1 off. Sometimes it's cheaper and more reliable to buy. But I agree it's more fun to build.

Click to expand...

Will either of the suggested circuits be ok? If the only difference is the voltage ripple the single phase with a 200mv ripple should be ok. It would be interesting to know how this circuit works.

As I will be using the voltage across the input and output, what would happen when the dynamo starts charging and the input voltage rises? If the 24v output is regulated then the voltage between the +12v input to the +24v output that I will be using will drop - or will it? Can that be regulated?

Looking at the diagram it shows HAT2165 x2 and 22µf x6. Am I reading this right? Do we connect 6 capacitors in parallel and 2 mosfets in parallel?

You don't mention the current needed for the replacement negative ground radio? Anyway, and I may be wrong on this, what would happen if you used a DC to DC converter similar to one of these and used for example a Model ZUS151212 which gives you an input voltage of 9 to 18 VDC totally isolated from the output. Then take the negative output and connect it to the chassis which we know id 12 VDC. Then take the positive output and connect it to the radio. The radio is a 12 volt negative ground but in reality all the radio should care about is that the power in be 12 volts above its own common or ground. I am just not sure how it would work out. Maybe I just had too long of a workday to think about this? The speaker output(s) would need isolated I think?

Ron

And don't forget about the antenna.

Reloadron said:

You don't mention the current needed for the replacement negative ground radio? Anyway, and I may be wrong on this, what would happen if you used a DC to DC converter similar to one of these and used for example a Model ZUS151212 which gives you an input voltage of 9 to 18 VDC totally isolated from the output. Then take the negative output and connect it to the chassis which we know id 12 VDC. Then take the positive output and connect it to the radio. The radio is a 12 volt negative ground but in reality all the radio should care about is that the power in be 12 volts above its own common or ground. I am just not sure how it would work out. Maybe I just had too long of a workday to think about this? The speaker output(s) would need isolated I think?

Ron

Click to expand...

Thanks Ron, yes this is one of the options we were discussing earlier - however the unit you suggested only delivers 1.3A/15W - I did mention at the start that the current radio has the power fuse rated at 10A so I guess it may need something at least able to deliver this power. I am looking at moffy's suggestion at the moment (this one) as I would prefer to build my own but if you look at my post from late last night I still have a couple of queries about it that I am hoping someone will answer shortly.

KeepItSimpleStupid said:

And don't forget about the antenna.

Click to expand...

No I haven't! I did mention it further up the thread and this is one of the problems to deal with if we isolate the radio from the chassis. If we use a voltage doubler or isolated dc/dc supply this won't be a problem.

The antenna shouldn't be a problem as it can be DC isolated from, and AC coupled to, the radio by one or two capacitors.

alec_t said:

The antenna shouldn't be a problem as it can be DC isolated from, and AC coupled to, the radio by one or two capacitors.

Click to expand...

I don't know how to do this. I don't plan to go down this route but could you please show me how this would be done on a diagram? - for educational purpses. Is it a capacitor in series?

alexb1406 said:

I don't know how to do this. I don't plan to go down this route but could you please show me how this would be done on a diagram? - for educational purpses. Is it a capacitor in series?

Click to expand...

Yes, simply a capacitor in series - there may even be one already.

But really what you need is a switch-mode power supply, to take in +12V and output -12V instead.

Nigel Goodwin said:

But really what you need is a switch-mode power supply, to take in +12V and output -12V instead.

Click to expand...

If by a switch mode power supply you mean output -12v instead of +12v then no, that is not what I need.

Ground is at +12v (positive earth) and the supply to the car electrics is at 0v, (or 12v below the ground voltage). What I need is a +24v supply (from a 12v dc input) to power the radio which will be grounded at +12v, so the radio will effectively see a negative earth 12v supply. The positive of the input of the supply/voltage multiplier will be grounded as will the negative of the output, so whatever circuit I end up using has to be able to be connected this way.

The other option, and I am beginning to favour this one at the moment, is for a 12v dc to 12v ac inverter, with a transformer to isolate the input and output, followed by a rectifier. Would someone please have a look at this and tell me if this would, with a suitable transformer, deliver 10A to the radio. Also, there is a warning not to run motors off this circuit. What would have to be done to enable me to operate either a tape or CD player - both have motors.

Given that the clock takes very little power why not use the low power DC - DC converter to drive that in the positive ground way but keep the rest of the car at the common negative ground setup?

As far as keeping the electrical system original positive ground there is no real advantage to it if anything modern ever needs to be used in it such as plugging into the cigarette lighter as a power port to charge something.

alexb1406 said:

If by a switch mode power supply you mean output -12v instead of +12v then no, that is not what I need.

Click to expand...

Sorry - wrong way round -12V IN and +12V OUT

The other option, and I am beginning to favour this one at the moment, is for a 12v dc to 12v ac inverter, with a transformer to isolate the input and output, followed by a rectifier. Would someone please have a look at this and tell me if this would, with a suitable transformer, deliver 10A to the radio. Also, there is a warning not to run motors off this circuit. What would have to be done to enable me to operate either a tape or CD player - both have motors.

Click to expand...

That's circuits absolutely crap anyway - as I said you just need a switchmode PSU, an isolation transfomer wouldn't be a bad idea for that though, and was what I had in mind.

The motors restriction on the naff inverter circuit is for attempting to power large mains powered motors, not little low voltage ones.

tcmtech said:

Given that the clock takes very little power why not use the low power DC - DC converter to drive that in the positive ground way but keep the rest of the car at the common negative ground setup?

Click to expand...

Nice idea

But involves more modifications to the car.

Nigel Goodwin said:

That's circuits absolutely crap anyway - as I said you just need a switchmode PSU, an isolation transfomer wouldn't be a bad idea for that though, and was what I had in mind.

Click to expand...

Any good circuit ideas?

tcmtech said:

Given that the clock takes very little power why not use the low power DC - DC converter to drive that in the positive ground way but keep the rest of the car at the common negative ground setup?

As far as keeping the electrical system original positive ground there is no real advantage to it if anything modern ever needs to be used in it such as plugging into the cigarette lighter as a power port to charge something.

Click to expand...

Yeah! I never thought of that! But I'd still prefer to keep the car original - I've made a little adaptor for plugging into the cigarette lighter using a bridge rectifier so that it will always provide the correct polarity regardless of the ground system on the car.