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Modern Radio in Classic Positive Ground Vehicle

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But involves more modifications to the 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 the rev counter,

What other modifications are there being he says his car has already been converted to a negative ground system? The few old style clocks I have seen in vehicles have one wire feeding them power and a common return going to some part of the instrument panel frame or existing wiring harness Assembly so to put the little DC - DC polarity inverter power pack in that clock line is not going to be much of a modification and is certainly far less that reconverting the whole vehicle back to positive ground just because the clock doesn't work.

If he is really a purist about keeping things looking stock then he can just change the battery cable colors around so the red one goes to the frame and black one goes to the starter system. At first glance very few super nosy purists would even catch that the battery poles do not actually match with the cable colors! :p
 
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Good points. You might need to use the voltage difference between the +12v and +24v to provide the feedback into the Vfb pin 3. You could use a differencing opamp where the +24v is added and the +12v is subtracted. Yes they should work, and the single phase supply is the easiest.
 
For the third time, a switchmode supply - a transformer in it would be handy.

Something like this, with the transformer wound accordingly, and one pair of ourput transistors should be enough.

**broken link removed**


I'm sorry Nigel, I wasn't being argumentative, but having seen the circuit you suggested I now realise the theory is similar to one of the options I mentioned earlier, i.e. convert to ac, transformer, then rectify. My knowledge is possibly not much better than basic and I wasn't familiar with the name "switchmode supply" - but now I have a slightly better idea of what you mean I agree this is possibly the best option...Ok, I'm going to go totally out of charachter and make a decision here!!!...this is the one! - maybe not this exact circuit but something similar, preferably one that I can buy all the components off the shelf instead of having to wind transformers or inductors as I don't feel confident in my ability to get them right. This may be simple basic stuff to most people on this forum but for me it will be the most ambitious project I have attempted - certainly more than I was hoping to solder a couple of wires together and shut my eyes as I switch the supply on! But hey! Nothing ventured...etc.

With regards to the ouptut transformer I guess I only need a 1:1 winding with a centre tap on the primary - or would it be a 2:1 winding? i.e. does each half of the primary winding produce 12v at the secondary? And then would it be better to add a few extra turns on the secondary to allow for losses?

The description of the circuit mentions "Relatively small filter capacitors can be used" - if I'm reading it correctly it shows 4 (I would probably only need 2 with a singe winding secondary) 3300µf/50v caps to filter the output - is that correct? I would not describe these as relatively small.
 
Good points. You might need to use the voltage difference between the +12v and +24v to provide the feedback into the Vfb pin 3. You could use a differencing opamp where the +24v is added and the +12v is subtracted. Yes they should work, and the single phase supply is the easiest.

Oooh, I'd forgotten about this one! - by comparison to what we are looking at now it looks so simple I'm tempted to just try it anyway.

My lack of knowledge is showing me up again! I understand what you mean by using an opamp but can you suggest a suitable unit and show me how to connect it into that circuit?

Also, I still need to know about the 2 x HAT2165's and the 6 x 22µf caps - are these connected in parallel? Am I right that 6 x 22µf caps in parallel are 1 x 132µf cap (or nearest equivalent)? Why 6 x 22µf?
 
The 2xHAT2165's means 2, HAT2165's in parallel. I think because they are surface mount package they are sharing the heat. You could choose a suitable single device. The 6 x 22uf are 6, 22uf caps in parallel to share the ripple current. You want a low ESR(equivalent series resistance) caps.
Sorry I don't have a drawing package handy, but you need a single supply op amp, a CA3140 should be fine. Connect Vsupply of the opamp to +12v, and V-ve to gnd. Connect R1(10k) to +24v(output) and to R2(4k7), other side of R2 to gnd. Connect the junction of R1/R2 to the + input of the opamp. Connect R3(10k) to +12v and to R4(4k7) and the other side of R4 to the output of the opamp. Connect the junction of R3/R4 to the -ve input of the opamp. This is the differencing opamp. You need a resistive divider on the output of the opamp R5/R6 whose junction you should connect to pin3(Vfb).The ratio will set the output voltage. The voltage at the output of the opamp should be (Vout - V12)/2. So the feedback ratio should be 1/4 of what is shown in the diagram. Roughly replace the 229k resistor with a 68k. But check my figures, because this is off the top of my head without a calculator, and it's not a good noggin. Use at your own risk.
 
Thanks Moffy, just done a crash course in opamps today! I now think I undestand most of what you have explained but I'm still a little confused about the voltage divider on the output R5/R6 that you mentioned. Looking at the diagram (if I'm reading it right) resistors 229k and 12.4k make up this divider, so what you are saying is to replace the 229k with a 68k resistor. Is this correct? If my calculations are correct the inverting and non-inverting inputs of the opamp will be roughly 8v and the output roughly 6v, so the 229k resistor needs to be roughly 1/4 of that. (Instead of 24v it now sees 6v - is this right?)

Could you also have a look at the resistor values in the diagram - most are not the usual values - how critical are they? - could I use the nearest available?

And how about the 0.002Ω? Hardly seems necessary and where could I get this?
 
You are right about the opamp and the replacement of the 229k with the 68k. This will give, I think, a bit above the 12v needed(Vout - V12). You can probably use nearest equivalent for the other values except the 0.002, it is important as it sets the maximum current. Vital for safety. You can use some higher values in parallel i.e. 5x0.01 if you can find them. Another way is to use a suitable length of PCB track, there are guides on the net as to how to. Just make sure that the track can handle the current without excessive temperature rise. Perhaps some others might have some suggestions, as this is quite a common nuisance problem.
 
Ok, I didn't think all 10A would be flowing that way

I'm having trouble locating various parts - there are 8 x 22uf caps, 6 on the output and 2 on the input that you advised to look for low ESR's, so would this be a suitable option. I don't know what value resistance qualifies for low ESR - these are 1Ω, the lowest I could specify in the search. They are also electrolytics - the circuit doesn't ask for electolytics but that is what I would expect for a smoothing cap.

Can't find a LTC1871-7. I've sourced a LTC1871 but I don't know if they are interchangeable - does anyone know?

Also unable to find a HAT2165 but this has been suggested. Or, as the circuit specifies 2 in parallel can anyone suggest a single one that will do the job?
 
The caps are 22uf/50v ceramic X7R Digikey: 399-5797-2-ND. The LTC1871 is the part you want just be careful to order the right package. Nice FET but you need to up the voltage to at least 60v, preferably 100v, keep the 5v turn on voltage. This is because MOSFETS are very intolerant to overvoltage and small spikes kill them.
 
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