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D.C.-D.C. Step-Up Converters Discussed
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 Posted: Fri Feb 20th, 2015 12:47 pm
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Huw Griffiths
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I think you're probably right there.

Although step up converters / "Joule thief" circuits certainly have their uses, I've always seen them as niche - partly because of likely efficiency issues - partly because of getting rid of the heat generated, thanks to the inevitable losses.

As for working temperatures for "chips" - well, any circuits really - I think it's probably best to take a line of:

Cool is cool - hot is most definitely bad news!


OK - I'm sure that some people are probably cringing at that last comment - but I suspect that many people here are agreed on the general sentiment behind it.

Many of us have also seen what happens when electronic components get too hot - or are otherwise wrecked through poor circuit design / construction or abuse.

The "magic smoke" might be an optional extra - unfortunately, the cost of replacement parts and time spent fitting them come as standard.


Personally, I might be happy with my "Joule thief" LED torch - but then I'm only talking tens of milliamps (and correspondingly low power / heat levels) - anyway, I'm sure the metal body on this thing might help with heat dissipation.

I find this torch very useful - I might also experiment with similar circuits for (very) low current applications - but I know these circuits have limitations. For this reason, i wouldn't even contemplate using them to drive anything "meaty" - each to their own, like but, as I've always been known for being risk averse, I think I'll play safe here.


Regards,

Huw.

Last edited on Fri Feb 20th, 2015 02:20 pm by Huw Griffiths

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 Posted: Fri Feb 20th, 2015 07:39 pm
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Herb Kephart
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OK--heres a question for those of you with an electrical background.

Assume radio control, a single cell and an ADJUSTABLE step up board. 

Also assume that the loco runs at the desired speed at 9 volts

Would the watts (and in this case heat) be any different if the adjustable were set to 9 volts--or---if it were set to 12 volts and the speed limited to 9 volts by the radio throttle?

I can think of arguments --some of which are obviously  bogus--both ways.

What say you?

Herb



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 Posted: Fri Feb 20th, 2015 09:31 pm
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Tramcar Trev
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Herb Kephart wrote:
OK--heres a question for those of you with an electrical background.

Assume radio control, a single cell and an ADJUSTABLE step up board. 

Also assume that the loco runs at the desired speed at 9 volts

Would the watts (and in this case heat) be any different if the adjustable were set to 9 volts--or---if it were set to 12 volts and the speed limited to 9 volts by the radio throttle?

I can think of arguments --some of which are obviously  bogus--both ways.

What say you?

Herb

Ok.... If the speed control is PWM (which it would probably be) then the motor is getting the full voltage, its just switched on and off with varying frequency (5 -10Khz or higher, when you are looking at an ESC which will give you PWM control this is the thing you should be looking at, the higher the frequency the smoother the control -so I'm told) to control the speed. This is why PWM is so good as the motor can deliver full torque at low speeds. This then opens up another can of worms cuz some electric motors do not enjoy PWM control. So that changes things.... Say half throttle = half speed the motor is drawing full voltage and full current but only half the time.
Ok, so I think what you are asking is how much waste heat is there going to be and how hot is hot and is this going to melt my plastic body? I dont know; the calculations are way beyond me but I would assume if you had one of these gizmos in your circuit and were converting (as I want to do) 9vdc to 12 VDC there is going to be a lot less heat( generated by inefficiency and the process of conversion) than if you were converting 9vdc to 18vdc infact if its linear say 50% less. Converting 9vdc to 9vdc I would assume would only use the % of "inefficiency" in the converter and convert that to heat.
This may be totally spurious but I'm not going to be offended if I'm wrong, I'm a tram driver not an electronics nerd...

Strewth the way I answered that I should have been in politics....

Last edited on Fri Feb 20th, 2015 09:33 pm by Tramcar Trev



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 Posted: Tue Feb 24th, 2015 04:39 pm
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bobquincy
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I am not much of a tram driver but I *am* an electronics nerd! ;) Trev was kind of on the right track, here is some data:

Looking at the curves given for some Pololu (and other) step-up regulators we see a similar power loss for different Vin/Vout, dependent mainly on output wattage. With Vin=3.3 and Vout =5 or Vout=12 the efficiency is similar and the power loss (heat) is also similar. It would not matter significantly if we ran our 9 v motor from 9 V at 100% duty cycle PWM or at 12 V with 75% duty cycle PWM. The average power in and the average power out are about the same, as is the average power lost.

At low (10 mA) and high (>1 A) outputs the efficiency curves are not linear but in the range that we are likely to use (100 mA to 1 A) the efficiency curves are close to a straight line.
For more curves see: http://cds.linear.com/docs/en/datasheet/35392fc.pdf

The average power lost due to inefficiency is what generates heat. The device runs at a high frequency and the temperature will not change that quickly, it takes at least a few seconds. Most of the ICs are rated to operate up to 85 C. Some devices have a thermal shutdown at about 150 C die temperature, the surface of the device will be lower. It may not be enough to melt plastic but it is probably best not to have the IC in contact with a plastic surface that could be seen, just in case. Most devices with a thermal shutdown will restart when the temperature drops by about 20 C.

If they are consistently overloaded the tipoff is repeated cycles of run-stop-run, as I had when using a converter that was just a bit too small.

boB

Last edited on Tue Feb 24th, 2015 04:40 pm by bobquincy



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 Posted: Wed Feb 25th, 2015 05:22 am
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Tramcar Trev
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Ok, glad someone is on top of this... Another solution now presents itself, a 6 cell LiPo battery which gives 22VDC is at hand. Do I a) split off 2 cells and create a 14V battery or do I b) drop the voltage via a MC78T12 12V 3A (or similar) Voltage Regulator?



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 Posted: Wed Feb 25th, 2015 08:13 am
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Si.
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Hi Trev.

Forget DC-DC converters...

...and voltage-regulators.

Only use the cells you need.

14 Volts sounds great !

No heat, no heatsinks, no thermal-shutdown, no reliability issues, no problems choosing the correct DC-DC converter for 'power' apps.

Only add extras if you ABSOLUTELY HAVE TO.

You don't have to.

Cheers.

Si.



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 Posted: Wed Feb 25th, 2015 02:47 pm
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mwiz64
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That's great advice if you can balance charge the battery. If not, you're playing with fire, literally. 3 cells wired in series (3S in the lipo world)is 12.6v at full charge.



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 Posted: Wed Feb 25th, 2015 06:23 pm
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fallen
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One thing worth thinking about is how fast you want your locos to run.

Many RTR locos will do a scale 150mph on 12v applied directly to the motor (remember that the 12v applied to the track does not all make it to the loco, there are voltage drops along the wires, along the rails, and at the points where the wheels contact the rail and the pickups).

Many modern locos will travel at realistic speeds on only half the nominal voltage applied direct to the motor, sometimes they will need even less.

I find many of my 009 locos (basically n gauge, nominal 9v) will work quite satisfactorily on a single LiPo supply of 3.7v.

I suggest trying the locos out with a voltmeter on the track to see what voltage they need for a reasonable speed, it will give you an idea of what is really needed. To be more accurate, you can attach a pair of long flexible wires to the motor and try driving those from the controller with a voltmeter across them.

Frank

Last edited on Wed Feb 25th, 2015 06:24 pm by fallen

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 Posted: Wed Feb 25th, 2015 10:01 pm
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Tramcar Trev
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mwiz64 wrote:
That's great advice if you can balance charge the battery. If not, you're playing with fire, literally. 3 cells wired in series (3S in the lipo world)is 12.6v at full charge.Each cell has a protection board across the terminals. Shuts the cell down over 5a and when the voltage drops to around 3v. This battery comes with its own charger and I suspect that the pcb,s on each cell somehow. control the charge



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 Posted: Thu Feb 26th, 2015 12:18 am
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bobquincy
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Tramcar Trev wrote: Ok, glad someone is on top of this... Another solution now presents itself, a 6 cell LiPo battery which gives 22VDC is at hand. Do I a) split off 2 cells and create a 14V battery or do I b) drop the voltage via a MC78T12 12V 3A (or similar) Voltage Regulator?I agree with Si, if you can use just batteries there is no point in adding extra devices. 

In any case we don't want linear regulators (like 7812 types) for significant current loads.  Linear regulators shed the excess voltage mainly as heat, going from 22 V to 14 V at 1 amp gives off about 8 watts!  It also gives about the same battery run time as if we used 14 V in the first place.

Switching regulators (like Pololu and others) are much more efficient then linear regulators although the linear types are ok for low current (like powering some LEDs).



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