PDM running 24V

[Martin]

Ive read that temperatures should be reduced when running 24V, why is this?


Surely if you disconnect an Inductive load like a FAN at 24V and the PDM clamps at -17V on the 20A outputs the inductive load energy will be easily surpassed?


Any advice or comments?

[figgie]

because at a given voltage, if the amperage is the same, the power will be higher in the 24v system v the 12v system.

Ohm's law applies here

[Martin]

This is regarding my first question right? You mean at the same resistance and current the power in the 24v system will be higher?

Temperature is not in Ohms Law

Heat generated is proportional to the current squared times the resistance.

For the PDM the on resistance of the outputs are 10Mohm and 6Mohm. With the load staying the same, the on resistance will increase with a increase in junction temperature and this will cause the risk of thermal runaway, but:

Lets say,
PDM32 12v at 120A reaches x degrees in y seconds
PDM32 24v at 120A reaches x degrees is z seconds

Lets also say the time that it takes to reach x degrees is different, still the max operating temp limit of the PDM must be the same regardless of what voltage you run through it(or not?)..........so my question is......Why does Motec want it to run cooler when supplying a higher voltage?

[figgie]

yes it will martin, and yes, Ohms law does apply here as the temprature is directly proportional to the power (wattage) as that power has to be disipated somehow and somewhere.

Power = volts^2/ohm

so if the resistance does not change, but voltage increases, that means POWER increases. Has no choice.

12v^2/1000 ohms = .1444 watts
24v^2/1000 ohms = .576 watts or an increase of .4316 watts.

That power has to be disipated somewhere


Why would MoTeC want to run cooler with higher voltage? Simple, more over head as at 24v you have decreased the overhead as now it is easier to pull more amperage which means more power draw.

IE let say that you pull 100amps from the 12v system v 100A of the 24v system.

That is 1200 watts total from the 12v system v 2400 watts of the 24v system for a net increase in 1200 watts, which if you were to run the PDM at the SAME temps, the 24v would reach it's temp limit threshold before it even gets to 60 amps @ 24 volts. Hence run it cooler.

[Martin]

I understand that P=VI=V^2/R=I^2R . I also understand that V=IR also no problem there. And you have convinced me that running double the wattage through the PDM will heat it up more.


Lets get back to my original post.

I want to run PDM at 24V I do not intend to run it with 12V devices coupled to it. so the current should be half that on the 24V system.


So, why cant I run it in the same temperature environment? Why should the max operating temp be less?

[figgie]

so the current should be half that on the 24V system.


So, why cant I run it in the same temperature environment? Why should the max operating temp be less?

Well you can, just not at the max amperage (at least, I myself don't see why you can not). Now be aware that the current limit might also be because of the autosport pins dimension which will also have to carry that extra power.......

With that said, It should be simple enough to add two 90mm or 120mm computer fans to push/pull the air over the PDM (if talking about the PDM16/PDM32). If you are really are creative, you can adapt a heatsink from the "ancient" Pentium II "Hershey bars" for additional heat dissipation which should be able to be sourced from Ebay, or your local used computer store.

http://en.wikipedia.org/wiki/Pentium_II

[Martin]

Great, now we are getting somewhere with the temp thing and 24v


So the main problem with running 24V is not the temperature as a result of more power but rather keeping the temp within spec, which will be more of a job if power figures are higher.

Any comments on the inductive load energy on a 24V system?

[Walk]

so if you run a pdm at 24v the max temp is lower ?

also what will you be running on the pdm ? how much current will you intend to run ?

the temps should be reduced becouse the current should be lower 24V vs 12v

[DarrenR]

Hi Guys,

Good discussion, but Martin is 100% correct. Running on 24V Vs 12V, the voltage drop across the driver for a given current is the same no matter what the supply voltage is. This is the power that is dissipated. Of course the device the output is driving gets twice the amount of power!

The off resistance of the driver is so high this is not a consideration either.

The reason the ambient temp need to be lower is mainly because of the power supply for the CPU, this is a linear regulator. It drops the voltage from the battery voltage to 9.5v, then another reg drops it further to 5v for the CPU. The CPU only draws a very small amount of current (~30mA) but the regulator goes from dropping 2.5V @ 12V bat volts (~0.075W), to 14.5V at 24V bat voltage (~0.435W).
The regulator needs to dissipate that (small amount of) power to the ambient temp inside the PDM so the ambient temp outside needs to be lower so it doesn't over heat locally.

There are a few other small factors like the input pullup resistors when a input is grounded and a few other components needing to dissipate more heat at a higher bat voltage, but it is to do with not overheating these components locally and nothing to do with the output drivers.

Cheers,