What the easiest way to incr voltage of DC/DC converter?

[John in CR]

I have some DC/DC converters designed to convert 36v-60v nominal to 12v for lights and accessories. I've run them up to 74v hot off the charger, but now that I'm going to 24s they could see right up to 100V, I don't want to blow them for no reason. What's the easiest way to make them see only their designed 60v pack voltage? btw, I don't mind wasting a few watts in the process.

John

 

[rhitee05]

The simple way is a resistor, sized appropriately based on your current draw. That could waste a fair amount of power if you're drawing significant power from your converter. If you're drawing 1A at the 12V output (12W), that would be about 250 mA at 60V input allowing for some converter losses. A resistor to drop that from 100V would dissipate 10W. I guess that's not terrible, but now you're drawing 25W from your pack to output 12W, so just under 50% overall efficiency.

You could also rig something using a 3-terminal voltage regulator like an LM317 or a zener diode/resistor combo. Those would both have similar power dissipation to the resistor-only option, just a little better stability of the output voltage for varying current. Which is better depends on whether you have a relatively constant load on the 12V side or a highly variable one.

Also, if applicable, make sure you consider the scenario when you don't have any load on the 12V output. If you only use a resistor to drop the voltage and no current is flowing, the input to the converter will rise to near battery voltage and you might pop something. A zener/resistor combo would fix this but drain some power even when you're not using 12V devices, so a cutoff switch might be in order.

Hope that helps. I can give more specific suggestions if you like, provided some more info about your application and priorities.

 

[dnmun]

just use fechter's idea of adapting an AC adapter for a laptop. sometimes you can find them in the junk piles and just find one with the output voltage you can work with(but you can modify this too). solder your 88V input to terminals of the capacitor on the input rectifier bridge on the AC adapter, then see if it will light up.

if it doesn't light with your voltage, you will have to track down the resistor on the Vin input to the pwm IC current regulator for the input oscillator circuit. rick talked about this somewhere too, i think on jeremie's thread for the power supply mod. you find the resistor on the trace from the rectified AC input that goes to the pwm chip, reduce that resistor value, and see if it lights, keep lowering the resistance until it lights up.

anyway, i would expect it to work if you had 88V. the problem could be when the battery is drawn down to 53V at the end. that would be the place to set your Vin resistance. all imho.

 

[Almasi]

I agree with 'Dnmun' , Some power laptop ac/dc power supply will work down to 24 Volt...some even less.

My Dell laptop power supply work from 20 Volt and up...

Much easier than modifying your dc to dc converter!


Robin