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Topic: Diode voltage drop (Read 6172 times) previous topic - next topic

Paul__B

#30
Jul 27, 2015, 07:58 am Last Edit: Jul 27, 2015, 07:58 am by Paul__B
Somebody is now going to prairie dog and say that is a bad idea!

raschemmel

Obviously paralleling regulated p.s. requires that BOTH P.S. be calibrated for the SAME output voltage. Other than that, there is no limit to how many p.s. you may parallel. It is best if they are identicsl make & models but that isn't mandatory.
There may be some residual current differences but nothing worth worrying about. If you have any doubts about it yiu can try it yourself. No P.S. knows tge ithers exidt .It adjusts it's output until it sees the setpoint. It is not capable of outputting a negstive voltage so all it can do is turn maintain the setpoint by toggling the output device.
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septillion

@Mark, you need to do some reading. The module he is using is a switch mode step down module. It's not linear nor a zener diode. And even a switch mode step down gets hot with this drop and current. Doing it linear would be a disaster....


And you, two (non fixed output) steps downs in parallel is a bad idea or at least not working that great... If the set voltage of one is set a little bit higher then the other the first one will do all the work and the second almost non.

But if you modify the modules so you use one potentiometer to adjust both it can work. Just desolder the pot on one and find out which pin goes off to the feedback (hint, it's pin4) and connect the feedback pins of both modules.
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raschemmel

#33
Jul 27, 2015, 04:24 pm Last Edit: Jul 27, 2015, 06:14 pm by raschemmel
Quote
If the set voltage of one is set a little bit higher then the other the first one will do all the work and the second almost non. 
That's right. It's only true if there is a large discrepancy in the setpoints because BOTH  P.S. will SHARE the load if both see a difference between output voltage and setpoint. Using only one pot is a smart approach though

If you question the fact that paralleled P.S. with similar setpoints will share the load,
insert DMMs in current mode in series with each P.S. and you will see that BOTH have the same current until you get to the voltage range that corresponds with the difference in setpoints. If one has a setpoint of 11.80 V and the other a setpoint of 12.25 V then the former will basically turn off it's output at 11.8V and the later will be the only P.S. delivering current in the range between 12.25 V and 11.80 V. Below 11.8V, both supplies will be at work and above 12.25, only the one.
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Domino60

Quote
Those LM2596 modules are cheap enough that you can put two of them in parallel. to eliminate your overheating problem.
It's working really great as it is, i just added a 8 x 8cm fun on the side and it's cooling down really well/fast.

Why would i need 2 of them to drop down the voltage if they work really good as it is.



As you see they can handle the voltage and the current I need, If i will need to charge more than 3A then that will be a problem. But right now I don't charge more than 2A it's getting hot but as i said the fun do great job, cooling it down from 80C` to room temperature.
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raschemmel

Your call.

Quote
I got as well a 12v 800mA power supply which i charge my battery drill but when i tested the output of that  power supply i got over 19 volts so i can't understand why the hack do that.
That's the "No Load" voltage.

The battery drill represents an 8.75 ohm load.

(19V -12V)/0.800 A = 8.75 ohms
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MarkT

#36
Jul 28, 2015, 01:36 am Last Edit: Jul 28, 2015, 01:37 am by MarkT
@Mark, you need to do some reading. The module he is using is a switch mode step down module. It's not linear nor a zener diode. And even a switch mode step down gets hot with this drop and current. Doing it linear would be a disaster....
The OP asked for a way to drop many volts, I pointed out a common circuit for this
function, so what's the issue?  You can drop 7V from any voltage source, it doesn't matter
what it is internally.  I said it will dissipate a lot of power.

As for reading does The Art of Electronics editions 1, 2 and 3 count?
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polymorph

Your call.

That's the "No Load" voltage.

The battery drill represents an 8.75 ohm load.

(19V -12V)/0.800 A = 8.75 ohms

No... that would be the internal resistance of the power supply.

The drill charging resistance, if it were using 800mA while at 12V, would be 12/0.8 = 15 ohms

Of course, all this is an extreme oversimplification...
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raschemmel

Quote
No... that would be the internal resistance of the power supply.

The drill charging resistance, if it were using 800mA while at 12V, would be 12/0.8 = 15 ohms
 
Yes of course you are correct. Actually that's what I meant but I misstated it. Thanks for the correction.

Quote
Of course, all this is an extreme oversimplification... 
Yes, maybe so, but that's probably appropriate for this thread.
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polymorph

Steve Greenfield AE7HD
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septillion

It's working really great as it is, i just added a 8 x 8cm fun on the side and it's cooling down really well/fast.
It's funny how you keep saying fun instead of fan :D

Why would i need 2 of them to drop down the voltage if they work really good as it is.
As you see they can handle the voltage and the current I need,
Quote
No it does not really. The charger can easily overload the module. That's the whole point ;)

It's like, better save then sorry. If you build it into a set and you use it for some time you tend to forget stuff like that. And then you think, ow, a 6S, I can charge that and damage your setup in the process. A second module is cheap and makes it more rugged.

But right now I don't charge more than 2A
Like told before, only the charge current says nothing about the current though the step down. 2A is the current to the pack. With 3 LiPo's and an efficiency of 80% (not uncommon) you are already up to 2,9A for the step down!


@Mark, you just said "Its an emitter-follower driven from a zener...", not pointing to common circuit in my opinion but "it's" referring to the step down. If you meant another circuit it's a good idea to say so / post it. So it's just a matter of misunderstanding ;)
As for reading does The Art of Electronics editions 1, 2 and 3 count?
It certainly counts ;) But because of your "it's" reference I meant the topic itself because I thought you missed the point of the module being a switch mode step down.
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Domino60

I use fan for fun :D

The step down converter works, I cool it down with a fan who cares if it burns I'm gonna buy other,
It's working so no problem, probably gonna buy a better one in the future a more wattage one for bigger power supp. and Amps.
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