So i did not have a good Benchtop load (i used a mosfet with a pot and a battery for years to make a constant resistance load but thats not as easy to use) so i went online and got the Epic Re:Load a few months ago, out of the box it had major problems ... for one it won't REALLY work reliably under 4.5v, the standard pot it uses was hard to reach and was both not stable enough and not accurate enough, it does not display voltage or current set, its current capacity is not enough, it had a header that let you put your own voltage in to it BUT it would shove the voltage your measuring out of it if its over the voltage your putting in, and the binding posts it came with would get loose, and the feedback output is 100mv per amp and not a more usable 1v per amp
dont get me wrong i would buy this again any day of the week, it was very well designed ( modified off dave jones design) and it simply works! ... kudos to whoever made it ... but for me i had to change it
my first plan was to do this, put it in a metal box with external binding posts and a fan with a fine and coarse pot, a 10x opamp, 30v voltmeter and a 9v battery
after looking on ebay i saw allot of 0-33v volt meters, i thought to myself "why 33v? that seams arbitrary" looking at the specs i saw that they worked down to 4v meaning they probably had a 3.3v local regulator ... this is where it hit me that it uses a divide by 10 voltage divider to get the 33v fullscale output and if i removed the resistors and jumped the one it would give me a 10x multiplier for 0-3.3v signals
now for whatever reason i had it in my mind that the epic re-load used a 3.0v local regulator and not a 3.3v one so i looked on ebay and got a cheep 0-30v meter for only $2 shipped
http://www.ebay.com/itm/301119964626?var=600220973263&ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649 and i got this, it was delivered in 2 weeks to Maryland USA (fairly fast for china) and worked very well and was very well calibrated (off 1LSD with a 5v ref it gave me 5.01) it had (apparent) TTL serial i thought i could calibrate it with ... so i cracked it open
so it uses an ST STM8S003F3 micro (datasheet
http://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/DM00024550.pdf ) that actully is quite impressave .. internal 16mhz RC oscillator with external XTAL capability, 3-5v working range, 5 channel 10bit ADC, UART, SPI, I2C exc ... BUT i tried communicating over the UART as marked on silk and over all bauds i got nothing (verified on scope) with my adafruit 3.3v FTDI dongle ... there normally high signals (both of them) so i have no idea whats going on there ... but no bother the epic Re:Load had a built in output trimmer so im not too worried
it used an interesting combo of a 78.5k and a 8k resistor (they were not marked, just measured) for a division ratio of 9.813 ... guessing whatever they had and software calibrated
i removed both resistors (mistake ... read on) and shorted out one I ALSO put a wire after the local 3v regulator to power it from the LR on the Epic Re:Load that i thought was a 3.0v one (no idea why) as shown here
i than soldered the LED display back on (it was covering the circutry) than soldered it back to the blue wire to the output of the epic reload's local regulator to than relize ... THAT THE EPIC RE:LOAD USED A 3.3V REGULATOR ... as logic would dictate it would ... SOOO i threw out those plans and chose to use a 9v for an external power source snipping the wire
This worked wonderfully and after calibration i got a good readout of amps
NEXT i need to take care of the pot ... a fine and coarse pot is good but for work i got a set of 10 10 turn, 10k pots direct form manufacturer for $1.20 a peace, i tested these pots, ripped them apart and they are legit, it uses the standard resistance wire + wiper, took 25lbs of rotational force to break the shaft, 5% tolerance (tested one full turn at a time, never got out of 3.2% of expected value), 0.3%/*C .... i tested from 0-40*c and it stayed well within that .. there nice pots ... so i pulled out one from my parts bin and used that
you can buy the exact same pot (i would assume) on amazon here
http://www.amazon.com/gp/product/B00E1IL498/ref=oh_details_o05_s00_i00?ie=UTF8&psc=1NOW I KNOW I PLACED IT IN A STUPID PLACE (see below) ... THIS IS ONLY TEMPORARY i only needed to use it for 5V supplies under 2A for a limited time so it worked fine for that ... but for the future i need to case it better (see below)
for selecting what to use for power i installed a simple switch and soldered it in place where the pot mounts went, the center lead went to the input of the regulator and the left side went to the input binding post and the other side went to the voltage input so i can select the 2 (at the time i was using a bench top power supply so i only used an external source when i had to)
input protection was achieved by bodging in a jellybean diode so it does not shoot the measuring voltage back into the source voltage ... i later bypassed this (for simplicity) and used a diode on the power source
The Binding post had a few problems ... they were cheap, they were too long, they got loose very very easily i solved all but the cheapness by simply trimming them down with a dremel, rubbing some plumbing flux on them and turning my soldering station all the way up to 870*f (480 some *c ... headmath) and using a chisel tip (AND REMOVING THE PLASTIC TOPS so the threads dont melt) i soldered them in directly
this is what i got
(yes i peel labels off my beers ... im that guy XP)
PROBLEMS: - Well obviously putting my pot there is stupid ... but worked for now
- Most obvious tho is probably the voltmeter (er should i call it a current meter now?) is just flapping its dick in the breeze there ...
- the binding posts are still crap
- There is no cooling and it gets hot FAST
- The volt meter im guessing has no filtering in the input and is noisey
- There is no voltage feedback so testing voltage drop and voltage drop over wires is more difficult
- its not in a case
- no real power switch
- The parallel input impedance is extremely high contributing to charge build up when theres low values displayed and contributing to noise ... I SHOULD HAVE LEFT THE SECOND 75K RESISTOR IN PLACE
FUTURE FIXES: - I have a set of nice binding posts in the mail for another project (but i got a ton of them) that i will use
- I will put a filter on the input of the meter
- I will install a power switch
- I will use a case (see below) so i can properly mount everything
- I will install a fan
- Battery/External Source capability with switch or auto-detect
- I got a nice large voltmeter with the same purchase i will use, its only 0-30v but should be usable for most uses .. will open it up and install input protection (zener or opamp) ... onc nice feature is its real easy to calibrate ... does not come with instructions but there are "up" and "down" pins ... after fucking around with some jumpers i learned that they probably change the fullscale range ... the "up" pin shorted to ground lowers the value displayed, down does the opposite ... its far larger so i can tell them apart quickly ... you can get it here http://www.ebay.com/itm/301119964626?var=600220973265&ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2648
i decided to not spend any more money and recycle this case i used for a push-pull transformer driver i made when i was 16 (years ago) (pictured below next to case ... it was nice for something bodged together that long ago with pnp-npn totempole outputs for both inverting and non inverting running on a 555 with 2 pots and a switch for frequency range)
so thats coming in the next few days
Quickly i tested the current draw of the whole thing with my
uCurrent Gold (mods on it coming next) and it draws 22.55ma with the pot on 0 (lowers to 21.9 with pot on full 10k)
Now i must say again NOTHING AGAINST THE EPIC RE:LOAD it is GREAT for its price ... there are ebay ripoffs but the use of a truly un-killable HITFET, great manufacturing, fast shipping, did as advertized, made in the UK, and im REALLY looking forward to the re:load pro (tho i cant afford to jump on the kickstarter right now ... i hope it goes for production) ... would buy the epic reload again any day of the week
you simply can make it much more usable for low cost of parts and some bodging
total i spent just $5 on the 2 meters but if you were doing it with no parts you will need a box, 2 voltmeters, the 10turn pot, some switches, a knob, a 9v battery clip, a fan, some misc parts ... under $20 i believe
i will post here when i finish completely
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