Sometimes what you say has been contradictive to me... You said the regulators "DON'T" run away, but then you said not to run them in parallel unless I make sure they are balancing the load.. So that to me seems like a contradiction.
It's not.
They don't run away, but they don't share current either.
As I've said, one will get hotter than the rest and start to go into shutdown eventually. It is not recommended to operate the devices regularly in shutdown mode.
I also am not sure you are fully reading my posts... I don't know how many times I can express.. *I* did not build this power supply. It was built by an company which happens to be an Electrical Engineering company that also has a Slot Car division..
Yeah, but you're completely re-engineering it for more current and with a higher input voltage.
The fact that you don't know what compromises and limitations the original design has doesn't mean it has none.
You are basically saying that even UNMODIFIED, the power supply was not built right and shouldn't have been built that way... Well you'll have to take that up with the manufacture...
All I am relying on is your information.
The apparent fact that the existing regulator was simply glued to the rear panel is appallingly bad design. Would I suggest you repeat this?
Was the original power supply variable? This on its own makes the design far more demanding because the amount of heat dissipated at low output voltages and high output currents increases rapidly over what you get at higher output voltages.
All I want to do is modify it to work better for my needs. Is it going to be perfect? No, not unless I completely start from scratch. I don't need or want to do that at this point, just make it work BETTER for my needs...
However you have pretty much ignored everything I've suggested.
At this point, I have identified that 1 of the regulators is hotter then the other 2, which means I need to fix that so they are equally sharing the load..
Great, you've shown that what I predicted would happen would happen.
I can't replace the 3 regulators with 1, It's not possible for the amount of current I have. The best I could do is replace the 3 regulators with 2, which may or may not help with the load sharing? I am going to order a lower voltage transformer, which from the advice given, will remove some of the heat issue.
I've covered this in detail in various earlier posts.
But I'm trying to figure out if I should order a 10v 8A transformer, or a 7.5v 15A Transformer. Either of them will have enough voltage and enough amperage. But will the extra Amperage of the 15A transformer cause more heat? or does it matter as long as I'm not actually drawing that much amperage?
I'm sorry. I've tried to answer but all that's happened is that I've been criticised for doing calculations.
I presume the power supply needs to be able to tolerate a shorted output. I assume you want reasonably good voltage regulation (i.e. you don't want it to sag significantly under load). I assume you want it to be reliable. I'm NOT assuming you want a super accurate bench power supply (although having said that, you could probably purchase a bench power supply capable of the sort of power you require for under $100). And given all that, I assume you need to keep it affordable.
Honestly, I've read and appreciate every bit of advice you have given. I have no doubt you are telling me the BEST and PROPER way to do things, but what I know is what I already have works, I just want to change a couple of things to make it a little better, with out rebuilding or redesigning the entire power supply...
You appear to be going beyond what is easily possible for a linear design. You WILL be thermally limited. Power supplies with these power ratings using linear regulators employ various techniques including switching input voltages and forced air cooling to achieve it. But all you want to use is passive cooling.
A switchmode regulator will solve many of the thermal problems by virtue of the fact that it is simply far more efficient. But it is more complex.
I recently made a 6V 5A regulator running from a 12V lead acid battery which used a very tiny heatsink. In providing 30W of energy, it wasted only a handful of watts, compared to a linear supply that would have wasted over 30W.
With the 6.6v 6A transformer they used a 33000uF capacitor, So I *think* a 56000uF capacitor would be good for the 10v 8A transformer I am looking at ordering. But that is a guess and probably wrong?
That's a pretty large capacitor, and it will have a side effect of drawing HUGE currents from the transformer near the peaks of the AC waveform, so you may find that the transformer, rectifiers and even the capacitor may get quite warm.
Incidentally a switchmode regulator allows you to have a higher voltage input that is filtered far less well, whilst not suffering from the problem that this excess voltage is converted directly into heat.
But in terms of value, it's probably OK. You'll get about 12V under load and up to about 5.5A. At 5.5A the ripple would be just under 1V, and assuming 2.5V overhead for the regulator, you could reliably adjust the voltage from 1.25V to 8.5V. The maximum power dissipation would be in the region of 70W (assuming short circuit output and current limiting not too much higher than 6A. With an output of 5A at 4V, the power dissipation would be around 40W. Given that the TO220 device has a thermal resistance between the junction and case of 4 degC/W, the maximum dissipation of the device (assuming ambient temp of 25C and die temperature of 115C would be well under 20W per device. The TO-3 devices have a lower thermal resistance so they could have a larger dissipation. Also, increasing the temperature you allow the junctions to reach will increase this. The operating junction temp maxes out at 125C, so there's only so far you can go.
If you assume the load is not continuous (and you really haven't said much at all about this) then you can raise the dissipation limit a little because you assume it has time to cool down. However this is somewhat risky as you really need to limit the duration of these high loads such that the device doesn't overheat. It doesn't matter how much time it has too cool down again if exceeds the max temp when under load.
I don't completely understand the schematics and what not, so while you may look at them and it makes perfect since, a lot of it might as well be in another language to me. So I apologize if my questions or project are upsetting you,
I am trying to keep it as simple as possible (for example I showed you a fairly complex design from the datasheet and then suggested something a lot simpler). If you don't understand what the schematics mean it's far better to ask than to remain silent and have me assume you're just ignoring me.
What you need to do is to fully understand and specify the nature of the load so we can recommend the best way to drive that load.
