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Electronic dummy load questions

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exodia:
Hello there

I'm into building an electronic dummy loader. I want to describe my idea and thoughts and tell me what you think about.

I'm planning on making a power loader circuit to test the durability of Photo voltaic batteries according to different loads.

So far i've made a network of 7*25watt 10 Ohm resistors in parallel. Thats 175Watt max and 0,7Ohm Rtotal in theory but i plan to reduce it to 150 max or maybe less for safety reasons

Also i'm thinking of using a max4173 IC for sensing current draw from resistor network. Any other alternatives?

My main question is if i can use PWM with 2-3 logic level gate mosfets to get whatever wattage i want instead of 25-50-75-100-125...* and if yes what kind of mosfets and what precautions for interferences from PWM should i take account for? The voltage will be about 12-14 and the amps about 10 so i think an IRLZ44 will do the trick?

The circuit will have the battery then the charge controller which will have only the battery installed and not the PV panel and should make output voltage about 12 , then my circuit and finally the resistor network.

one last thing i want to know if i can use separate voltage for the mcu and the various ic's instead of using voltage from the battery. My concern is that if i use separate voltage for mcu i will have two different grounds one from battery to resistor network and one from the supply of mcu. I'm wondering if i tie the grounds together because they are on different voltage and maybe the 12 volt rail due to pwming it will be noisy will interfere with my MCU

enough questions for 1st post

Thanks in advance guys

Simon:
why not have a look at daves recent blog about a dummy load he made ?, I think you have a bit to learn before going head long into it.

exodia:
Yea i saw Dave's approach

i want to use MCU cause i'll do some other stuff with it like logging, also i already have bought that 7 power resistors and i want to use them. Any feedback on my approach?

TechGuy:

--- Quote from: exodia on August 13, 2010, 01:44:31 pm ---Yea i saw Dave's approach

i want to use MCU cause i'll do some other stuff with it like logging, also i already have bought that 7 power resistors and i want to use them. Any feedback on my approach?

--- End quote ---

Well using a PWM to control the load might cause some issues with measurements, since your load won't be constant. I believe that most common adjustable dummy loads use a bank power BJT transistors operating in linear mode to adjust the power load. Really all you need is a set of power BJTs attached to a heatsink and a potentimeter to adjust the base current to the transistors to control the load. You can use your power resistors in series with the power BJT's avoid adjusting the base current into saturation. The Power resistors would limit the max current the device would handle. Just make sure that the power BJT's you buy have a high operating voltage incase you wish to re-use it to test power supplies with higher output voltages.

toli:

--- Quote ---So far i've made a network of 7*25watt 10 Ohm resistors in parallel. Thats 175Watt max and 0,7Ohm Rtotal in theory but i plan to reduce it to 150 max or maybe less for safety reasons
--- End quote ---

The way I see it, 7 resistors of 10ohm in parallel are 1.43Ohm :)

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