Author Topic: Electronic dummy load questions  (Read 9616 times)

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Offline exodia

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Electronic dummy load questions
« on: August 13, 2010, 08:06:40 PM »
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

Offline Simon

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Re: Electronic dummy load questions
« Reply #1 on: August 13, 2010, 10:48:51 PM »
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.

Offline exodia

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Re: Electronic dummy load questions
« Reply #2 on: August 13, 2010, 11:44:31 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?


Offline TechGuy

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Re: Electronic dummy load questions
« Reply #3 on: August 14, 2010, 04:48:42 AM »
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?

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.




Offline toli

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Re: Electronic dummy load questions
« Reply #4 on: August 14, 2010, 04:05:53 PM »
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

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

Offline exodia

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Re: Electronic dummy load questions
« Reply #5 on: August 16, 2010, 04:24:27 PM »
@toli yes you are correct, thanks for correcting my excel calculator :)

@TechGuy i'll do some more research

Edit Dave's approach at the last mins of video is to connect the mcu to a voltage follower though PWM and a smoothing filter. That way with 50%duty cycle the op amp will get say 2,5 volts or a couple of 5/0 pulses???. The mosfet will conduct fully at 2,5 volts? BTW does pwming a mosfet that is connected to bank of parallel resistors affects the Rtotal value according to pwm?
« Last Edit: August 16, 2010, 06:24:20 PM by exodia »

Offline scrat

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Re: Electronic dummy load questions
« Reply #6 on: August 16, 2010, 08:38:12 PM »
If you use a bank of parallel transistors, I'd rather place MOSFETs instead of BJTs, because of the BJT's positive thermal coefficient (the more they heat, the more they conduct, so if one is more conductive, it will burn almost surely, followed by the others).

A good dummy load for measurement should be linear, not PWM, but it will be huge if power is quite high. PWM seems to me a good solution (especially if you plan to use a converter on the real system).
So, filter inductance * PWM period becomes the real matter. Higher values will make current more constant.
The issues with low inductance or low frequency values will be on the sensing side (current will be a heavy triangular wave added to a DC, but this can be filtered out), and on the measurement side, since the triangular ripple causes added losses (it has null mean value, but not null RMS value). The total load for the battery will be more than expected (you must take into account battery internal resistance and inductance ESR)...
This can or can not be a trouble, depending on the application. If you plan to convert battery voltage with a switching converter, a good thing could be to reproduce the same conditions in terms of ripple on the load, so the measure of duration will be very accurate.

Maybe I went a little off topic, or said thing you already know...

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Offline scrat

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Re: Electronic dummy load questions
« Reply #7 on: August 16, 2010, 09:00:54 PM »
BTW theoretically PWMing a power supply and extracting from it a certain average current is the same as using a resistor which draws the same current. Pratically, the troubles are with series resistance...
One machine can do the work of fifty ordinary men. No machine can do the work of one extraordinary man. - Elbert Hubbard

Offline exodia

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Re: Electronic dummy load questions
« Reply #8 on: August 16, 2010, 10:08:22 PM »
I'm thinking of using a MOSFET driver for better compatibility and to be sure i can drive the mosfets at higher frequency than 20Khz to avoid noise. I don't think i have to use an op amp like Dave's approach. My plan is to prompt the user the wattage he wants, then i can take a reading of the current from a max4173^2 and divide it with the Rtotal in parallel which i know. That will give me the power. The i will make adjustments in PWM and fortunately i can come really close to the wattage requested. That is at least my theory, i don't know how close to implementation is that.

At home i'll try connecting a 7,5 ohm 30watt bank to 2 AA batteries and measure the current with an ohm meter (only think is will the ohm meter be able to measure current from pwming the mosfet, don't know if RMS capable meter has to do with it)

BTW i get the following quote from another forum but i don't know if it stands real, can anyone confirm?

Quote
Yes, you can definitely PWM a resistor to create an intermediate resistance.

Example: If you PWM a 20 Ohm resistor on 50% duty cycle, you can get 40 Ohms

What i have understood from PWM is that it chops the time source is fully on or fully OFF and if the frequency is high enough the load gets the average voltage. Haven't heard anything about resistance choping.My only thinking is because  of Ohm's law is that you can somehow simulate the lower voltage getting from PWM by doubling the resistor?
« Last Edit: August 16, 2010, 10:28:27 PM by exodia »

Offline TechGuy

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Re: Electronic dummy load questions
« Reply #9 on: August 17, 2010, 07:00:05 AM »
If you use a bank of parallel transistors, I'd rather place MOSFETs instead of BJTs, because of the BJT's positive thermal coefficient (the more they heat, the more they conduct, so if one is more conductive, it will burn almost surely, followed by the others).

Not that big of deal when operating in linear mode. It's fairly easy to set up a feedback loop to regulate current to offset temperature changes. This can be done using a current sensor and a comparator that compares the current sensor voltage with a pot output voltage. The current sensor could be a low ohm resistor that monitors the voltage drop across the resistor.

Mosfet are voltage controlled devices, BJT are current controlled devices. It will be easier to design an adjustable current sink using a current controlled devices than voltage controlled devices. Mosfets are usually very sensitive to minor voltage noise when operating in linear mode. Your control circuit will need to be powered using a very low noise power supply to prevent significant current fluxuations. Even a 50 mv noise at the gate voltage can cause significant impendance changes when using an MOSFET in linear mode.

The issue with PWM is that it will be difficult to smooth the current demand on the source side. A LC filter will smooth out the load side, but the source side will remain fairly choppy, because the switching transistor will transition from infinite resistance to near zero resistance. The inductor will also make it difficult to adjust the load with a fixed frequency because the inductor will have a fixed value. You can only push so much current per cycle through a fixed inductor with a fixed frequency. A low value inductor will permit a lot of current to pass, but it will also make the source input current a lot more choppy because it will have a low impedience. A high value inductor will have a higher impediance, but will limit the power you can push through.
In my opinion, A linear mode sink is really the only way to design a simple adjustable load dummy.

Choosing to control the load using a PWM or MOSFET in linear mode is just going to make the project a whole lot more complex.




Offline scrat

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Re: Electronic dummy load questions
« Reply #10 on: August 18, 2010, 06:26:26 PM »
I must admit I didn't think much in deep on what I wrote..
About MOSFETs instead of BJTs... I was thinking of dissipating all the power needed on the transistors, but now I realize it will be quite a silly approach, since it will have no advantage over the linear one (the power rating of transistors will have to be the same as for linear control). If transistors are used to drive resistors, the issue with thermal drift doesn't exist (negative feedback from emitter resistance), but this surely limits power.

@TechGuy
I don't agree on the fact that inductance limits current, since you are not forced to make your PWM "converter" to work in discontinuous mode (eg with current going to zero at each switching period). So I think current can be as high as desired, provided that the inductor used is rated for that (I'm thinking about a boost topology).

Chopping a resistor could be a good idea. To explain the sentence from the other forum, simply think at current and voltage, instead of resistance: at the battery side, voltage is fixed, while average current depends on the time the load is connected. So
equivalent (average) resistance = source voltage / average current = source voltage / (source voltage / load resistance * duty cycle) = load resistance / duty cycle.
The same is for dissipated power: why will you use current and resistance measurement to obtain power? Why don't you use voltage and current, which are both easily measurable (and cut off resistance thermal dependence)?
One machine can do the work of fifty ordinary men. No machine can do the work of one extraordinary man. - Elbert Hubbard

Offline exodia

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Re: Electronic dummy load questions
« Reply #11 on: August 18, 2010, 08:14:09 PM »
As a matter of fact i was thinking exactly why use resistor and amps to make the total power when i read about the thermal coefficience of the resistor. Guys why should i use an inductor. I'll connect the PWM output to a pair of bjt's or driver that will drive the MOSFET gate.

Offline scrat

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Re: Electronic dummy load questions
« Reply #12 on: August 18, 2010, 10:15:39 PM »
Inductor smooths current waveform, since otherwise it will be a square PWM wave.
A 0 - V PWM current wave will not dissipate on the resistance the same power as a DC current equal to its average (this is clear if you consider its rms value).
One machine can do the work of fifty ordinary men. No machine can do the work of one extraordinary man. - Elbert Hubbard

Offline TechGuy

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Re: Electronic dummy load questions
« Reply #13 on: August 19, 2010, 12:37:02 AM »
Inductor smooths current waveform, since otherwise it will be a square PWM wave.
A 0 - V PWM current wave will not dissipate on the resistance the same power as a DC current equal to its average (this is clear if you consider its rms value).

But on the output side not the input! When the PWM switching transistor shuts off, it also shuts off the current demand from the power supply. When the switching transistor switchs off, the inductor supplies the current to the dummy load, by draining the magnetic field stored in the inductor's core. Sure you have a nice smooth load output, but that isn't important You can't really use a PWM if you want a smooth constant current drain on a power supply.

Quote
I don't agree on the fact that inductance limits current, since you are not forced to make your PWM "converter" to work in discontinuous mode (eg with current going to zero at each switching period). So I think current can be as high as desired, provided that the inductor used is rated for that (I'm thinking about a boost topology).

An unsaturated inductor has impediance (ie resistance) that limits current flow. The higher the inductance the higher the impedance. The only way you can adjust the impedance of a fixed inductor is to have a DC winding so that you can apply current to hold the inductor at a point closer to saturation causing the inductor's impedance to fall. But this gets tricky. Usually it done with a second core with a DC winding in close proximity to the inductor. The magnetic field alters the inductance of the inductor. this is how magnetic amplifiers work.

You must switch off current flow to the inductor to prevent it from saturating. You can't operate an inductor in continuous DC mode because it will quickly saturate and it will lose all impediance, leaving just the DC winding resistors. In DC mode, and Inductor operates as a power resistor.


Quote
Chopping a resistor could be a good idea. To explain the sentence from the other forum, simply think at current and voltage, instead of resistance: at the battery side, voltage is fixed, while average current depends on the time the load is connected.

But not on the source (power supply side) side. When you turn off the switching transistor the current load disappears on the source side. A PWM driving into a resistor bank will create average power load. Current and voltage fluxuate with the switching transistor.

When testing a power supply you want a stable adjustable current demand from the load dummy. How smooth the output is, is irrelavent.

Try for yourself, build a simply Buck regulator using a 555 or a waveform function generator as your PWM controller. Then take a look at the waveform from the transistor high side (assuming you use either a NPN BJT or an N-Channel Mosfet). See what the output looks like. Really you need a current probe so you can see the current flow as the switching transistor cycles on or off. You will also need to apply some output load on the buck regulator, enough so that it drops the voltage across the power supplies output caps.




Offline scrat

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Re: Electronic dummy load questions
« Reply #14 on: August 19, 2010, 03:03:00 AM »
But on the output side not the input! When the PWM switching transistor shuts off, it also shuts off the current demand from the power supply. ... Sure you have a nice smooth load output, but that isn't important. ...
Of course a linear load will be better in terms of precision, dynamics and noise, but it will be very large above certain power values (over a few kW could also be impossible to make it linear)

You must switch off current flow to the inductor to prevent it from saturating. You can't operate an inductor in continuous DC mode because it will quickly saturate and it will lose all impediance, leaving just the DC winding resistors. In DC mode, and Inductor operates as a power resistor.

In switching control you should be controlling current properly (to reach the desired mean value), but even if you don't control current, there is a steady-state value in which average current will settle. Otherwise, switching converters won't have any Continuous Conduction Mode, and will only work in DCM (Discontinuous ...)

When you turn off the switching transistor the current load disappears on the source side. A PWM driving into a resistor bank will create average power load. Current and voltage fluxuate with the switching transistor.
That's true if you consider a buck topology, but for a boost it isn't. In a buck, the transistor is in series with the input, so of course current from the source is discontinuous. Otherwise, in a boost, input is in series with the inductor, so current can be maintained above zero for all the time (provided that an appropriate switching period is choosen).

However, the doubt remains if a switching load (chopped resistance) could be an adequate testing device. I'm not so experienced to say this ;)
« Last Edit: August 19, 2010, 03:49:50 AM by scrat »
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