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Topic: Dummy load (Read 8 times) previous topic - next topic

Techone

@retrolefty

Nice site. Good work and nice idea using mineral oil for cooling and a recycle paint can. Very "McGyver".  XD

@Nick Gammon

I did the simulation using Circuit Wizard. http://www.new-wave-concepts.com/

During the simulation, the pot was set around 65 %, the current going through the MOSFET and the resistor was 2.8 A. A saturated value. I increace the R value, the current max was reduced, but the pot at low setting ( under 10 % ) the current at R was under 100 mA. Decreasing the R, will increase the I across the R. The low setting of the pot will be increase ( start around 250 mA ). Also, it depend the MOSFET type ( resistance across the MOSFET ) , the voltage at the gate ( to optain saturation value )  and the power resistor ( controlling the max current ).

I will figure out a better way to design and make the variable dummy load. It a matter of tweeking the circuit so I can get a low current through the R ( at low pot setting ) and set a max value ( at max pot setting ), so I can test my PSU I built / test.

I may use the Arduino to monitor the current going through the R. Right now, a current meter wil do the job.

Nick Gammon


I used 20 1,000 1% resistors ordered on E-bay for just a couple of bucks.


I initially read that "I used 20,000 resistors". Wow, I thought, you have a lot more patience than me!

Interesting article, thanks.

BTW, when measuring the AC component of the dummy load it seems quite high (a few hundred millivolts) when the MOSFET kicks in, at least at the "edge" point. Would you recommend a capacitor somewhere to smooth that out? If so, what one and where?

retrolefty



I used 20 1,000 1% resistors ordered on E-bay for just a couple of bucks.


I initially read that "I used 20,000 resistors". Wow, I thought, you have a lot more patience than me!

Interesting article, thanks.

BTW, when measuring the AC component of the dummy load it seems quite high (a few hundred millivolts) when the MOSFET kicks in, at least at the "edge" point. Would you recommend a capacitor somewhere to smooth that out? If so, what one and where?


Hum, well the mosfet rather high gate capacitance coupled with the limited output impedance (current drive) of the op-amp could very well make that circuit somewhat unstable during initial transition. Not sure if adding more capacitance would help or make it worst.

Lefty

Techone

@Nick Gammon

I just finish a simulation using Circuit Wizard. I re-do the circuit of your "Dummy Load" circuit, And the circuit can be control be Arduino using a D/A for the 5 V section ( Vin ) and the current monitor, the voltage at the Power resistor can be scale down ( to 5 V ) and going into an analog pin.

The analog circuit follow this formua.

Vout = 9/5 * Vin + 3

First Op-Amp section :  Vout-a = - ( Vin * 1.8 )

Second Op-Amp section :  Vout = -( Vout-a + -3 V )

Therefore :  Vout = - ( - ( Vin * 1.8 ) + - 3 )

Here the schematic.

draythomp

I've been messing around with lead acid battery chargers and working on a load to test capacity.  I have a heck of a lot of trouble with batteries here in the desert and I want to get it under control.  So, I stumbled across this thread in passing. 

Nick, that is a cool idea.  I've had several instances where a wall wart didn't come close to supplying it's rated power and it really annoyed me.  In one instance it was a USB cable that had enough resistance to limit a 5V 2A wall wart to around 200ma; that was a pain to find.  This little device would solve that troubleshooting problem for me.  It could also help isolate the bad NiCd batteries that turn up in solar accent lights bordering my walkway.  They seem to die too often; this could help me figure that one out too.

Lefty, I have a couple of questions on your RF load.  You have folks adjust the voltage up by .4 to allow for the diode, doesn't it have a .7V forward drop?  Also, you show two diodes in the picture and only talk about one in the description.  I realize that .3V or even 1V with two diodes isn't statistically significant in the scheme of things, but it might confuse someone (besides me).  I also wonder about the voltage reading since you only use a half wave to charge the capacitor and RMS calculations usually use the entire wave form, but then again this would only be a small error at HF and could probably be ignored.  The other question is the value of the capacitor.  At 3MHz it's impedance should be (if my math isn't too rusty) less than an ohm rising to around 5ohms at 50Mhz.  What am I missing?
Trying to keep my house under control http://www.desert-home.com/

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