Dangerous Prototypes Programmable dummy load January 26, 2012, 12:44:19 pm Here is what we came up with for a programmable dummy load.*PIC18F2550*MCP4725 I2C 12bit DAC*USB and UART*Fuse*2x16 HG44780 LCD..*30V and 3A(probably more)This is our initial test version, If you have any suggestions for future versions, plz leave them here. Last Edit: January 01, 1970, 01:00:00 am by Guest
Re: Dangerous Prototypes Programmable dummy load Reply #1 – January 26, 2012, 01:22:31 pm Interesting work, few questions- why not use poly-switch instead of glass fuse?- why not use few 2W resistors instead of one to220 resistor? few 1% 2W are way more precise and way cheaper then precise low value to220 one (yes they take bit more board space but...)- why not measure input voltage too? Without input voltage monitoring it's not nearly as useful.. almost all load's have input voltage monitoring feature (both ones from ITead for e.g.), for any case where I use electronic load one without voltage measurement is unusable..- why lm358? Input offset is up to 7 mV !! Since most of the use with this devices is in the low current range that might be an issueSome suggestions - programmable or selectable gain op-amps - microCurrent schematic from Dave (eevblog) for low end of the measurement scale Last Edit: January 01, 1970, 01:00:00 am by Guest
Re: Dangerous Prototypes Programmable dummy load Reply #2 – January 26, 2012, 03:29:22 pm [attachment=0]here is one i have been working on its super simple and not meant to exceed 2.5mA but could be adjusted for higher currents. Last Edit: January 01, 1970, 01:00:00 am by Guest
Re: Dangerous Prototypes Programmable dummy load Reply #3 – January 26, 2012, 03:37:17 pm If you already have R1, why you need R2-R5 ? Last Edit: January 01, 1970, 01:00:00 am by Guest
Re: Dangerous Prototypes Programmable dummy load Reply #4 – January 26, 2012, 04:01:55 pm [quote author="arhi"]If you already have R1, why you need R2-R5 ?[/quote]r1 is meant for the current sense, r2-5 are the load resistors they are meant to set the full scale current as the npn darlington is regulating the voltage between the emitter and ground, r2-5 should be 1k for a max load of 2.5mA, @ 100 ohm it will max at 25mAR1 should be 0.1 ohm for 1v/mAthe ts100 current sense amp will not work below ~1.5v that is why it is on the high side. Last Edit: January 01, 1970, 01:00:00 am by Guest
Re: Dangerous Prototypes Programmable dummy load Reply #5 – January 26, 2012, 05:28:44 pm just add a resistor between the darlington and the op amp output, and you can drive the darlington to ground (remove the 4 resistors, they are superflus)...The op amp will change its output level depending on the difference between the ts current monitor and pot...this lever will increse or decrease on one end of the resistor, the other end of the resistor will always be at around 1-1.2V( the (the be voltage of the darlington) Last Edit: January 01, 1970, 01:00:00 am by Guest
Re: Dangerous Prototypes Programmable dummy load Reply #6 – January 26, 2012, 05:34:30 pm my 2c...I'd like it to have, other then the already available prog load, input voltage monitoring, and another current and voltage monitor. something to be attached at the input of a PS, to calculate efficiency...also a handy thing would be to add a test pin on the DAC output..this would allow you to drive the load with a preselected pulse signal, that could also be used as a trigger for the scope...showing you how the V-out changes when the load changes from 20mA to 500mA...in a pulse... Last Edit: January 01, 1970, 01:00:00 am by Guest
Re: Dangerous Prototypes Programmable dummy load Reply #7 – January 26, 2012, 05:42:22 pm i have yet to bread board this/ at the low currents i can prob get away with a 2n3904.i don't want to use the current sense resistor as the load resistor for stability reasons.using a high value resistor on the emitter of the transistor will limit the max current across the transistor.i'm going to test this today/ ill get back to you with results. Last Edit: January 01, 1970, 01:00:00 am by Guest
Re: Dangerous Prototypes Programmable dummy load Reply #8 – January 26, 2012, 06:35:16 pm there is no load resistor on your schematic...It only serves the purpose to limit the NPN's base current from the op amp...it has the same function if you place it between the base and opamp as it doeas now...only you can use any cheap resistor and have no load on it...on other Prog load designs that resistor is at the same time used as a feedback for the op-amp buffer, so the op-amp keeps the level on the resistor at the same level of the other input..Since you are using a different feedback routine...you don't need that resistor at all....the only load bearing device on your schematic should be the transistor...The current sense resistor will have the exact same stability whether or not you remove those bottom "load"resistor..there will be absolutely no change to the way it acts, or functions... Last Edit: January 01, 1970, 01:00:00 am by Guest
Re: Dangerous Prototypes Programmable dummy load Reply #9 – January 26, 2012, 07:04:30 pm [quote author="arakis"]The current sense resistor will have the exact same stability whether or not you remove those bottom "load"resistor..there will be absolutely no change to the way it acts, or functions...[/quote]i will experiment with this, i am curious as to the diffrences Last Edit: January 01, 1970, 01:00:00 am by Guest
Re: Dangerous Prototypes Programmable dummy load Reply #10 – January 26, 2012, 08:02:34 pm just be sure that if you remove the 4 bottom resistor, you replace them with one between the op amp and transistor...otherwise you could fry the transistor... Last Edit: January 01, 1970, 01:00:00 am by Guest
Re: Dangerous Prototypes Programmable dummy load Reply #11 – January 26, 2012, 10:20:59 pm i have done some quick testing, i'm using a lm324 instead of an 321the resistor on the emitter serves 2 purposes 1. to set the max current that the transistor can disparate, considering that i don't want massive currents and only to max at either 250 or 25mA. and could be changed to a single resistor since the resistor won't heat up that much at low currents.2. most importantly it gives a more linear adjustment of the set current.i did add the resistor on the base, just siple oversight.i changed the v+ of the op amp to vbatt as the max output of the opamp is (V+ - 1.5) and was limiting the range.and changed to a 0.1uf cap on the output of the current sense amp to help stability.the values could still use some tweeking[attachment=0] Last Edit: January 01, 1970, 01:00:00 am by Guest
Re: Dangerous Prototypes Programmable dummy load Reply #12 – January 26, 2012, 10:30:07 pm use rail 2 rail op-ampQuote to set the max current that the transistor can disparateYou do that with base resistor. Max current trough collector-emmiter is base-emmiter current * hFE, and you know hFE for your transistor, base-emmiter current you set with the resistor (max voltage that op can output - 0.7V drop on the transistor) / base resistor ....Quotemost importantly it gives a more linear adjustment of the set current.how come?Note that you are using BJT here and not FET so the current trough transistor is controlled by current trough base, not like with fet with voltage on "base" Last Edit: January 01, 1970, 01:00:00 am by Guest
Re: Dangerous Prototypes Programmable dummy load Reply #13 – January 26, 2012, 11:17:42 pm I have a few thoughts:You've placed single headers underneath the LCD footprint, I assume these are mainly for testing and debugging, but why not place then out from underneath the LCD. Might save you some trouble down the line.The LM358 might not work well with lower voltage input, eg testing a 3v supply with 1Aa rail to rail would be more expensive, but would probably help immensely. Microchip have a bunch of really cheap 5v ones.Not sure why that source trace has to be so large coming back from the FET. All the current should be going through the resistor. Unless it's just to reduce resistance... Last Edit: January 01, 1970, 01:00:00 am by Guest
Re: Dangerous Prototypes Programmable dummy load Reply #14 – January 26, 2012, 11:43:02 pm figured it out, to much delay in feedback loop, and it turned into an oscillator.update:the resistor just made it harder to notice oscillation, it was basically acting like a switching regulator.i need a cc source capable of regulating in the 1uA range for some testing. i can do away with the current sense amp and go conventional, plus it will cost less. Last Edit: January 01, 1970, 01:00:00 am by Guest
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