Improvements and fine tuning
As mentioned in my last post on the dummy load, though I am very pleased with the results, there is always room for some fine tuning and improvement. Also, some parts needed to be bought. First off, here’s a revised version of the schematic:
I cleaned up some wiring and substituted a few parts. Most notably, I swapped the opamp from the LM324 quad opamp to the LM358 which is basically the same thing in a dual package. I didn’t see the logic in wasting two opamps and the part cost a whole fifty cents retail at the brick and mortar. I also changed the 10kΩ 10-turn pot for a 100kΩ one, simply because I wasn’t paying attention when buying pots yesterday. This is not a big deal and the circuit was easily redesigned for this change by swapping the 5.6kΩ with 56kΩ. In testing, I changed that resistor again to 47kΩ to be assured of hitting the 3A desired maximum.
Other than that, I simply swapped the 7805 5V regulator for it’s low power cousin the 78L05 since really only a few scant milliamps are in play and again, why waste a higher power part? The function is identical. Package is also a TO-92 which is much smaller than the TO-220. Bit of a trap for young players: the 7805, when looking at from the front has a pinout IN-GND-OUT. The 78L05 is the REVERSE (OUT-GND-IN). So put the part in backwards. Why on earth do they do that?
In quick testing, I found the operation pretty much identical from last time. The LM358 and 78L05 are direct substitutions from their predecessors so I wasn’t expecting any difference. I find in switching the battery charger from 12V to 6V I lose a few mA and I’m unsure exactly as to why. It could be my circuit and associated parts are too loosey goosey to properly regulate over a variety of voltage sources, or it could be that I’m hitting the limit of the power supply to the opamps again which is probable and likely.
A few more tests need to be run. Most importantly is determination of the proper power supply for the opamps to get a nice stable regulation across a wide variety of voltage sources. I will be checking the gate voltage for issues and trying out higher voltage supplies to see if I can give it a lot of wiggle room to regulate well and stable. I have not, thus far met any show stoppers or things that are really bugging me so completion is near. I will need to do more thermal tests as well.
In terms of the final construction, I have small proto boards which will work great for this. The circuit is small and compact and easy to deal with. I estimate a twenty minute soldering job for the lot.
I will be adding a power switch, power LED, binding posts, and a DC barrel connector. I did not add a diode to the power input as polarity protection since I didn’t want to drop 0.8V to the opamps. If I use a higher supply with enough headroom, I may add it later. It may add a problem later if the 78L05 is not drawing enough quiescent current to turn the diode on, but I highly doubt it. Also the opamp will be draining a trickle so it should be okay if I go that route.
Most notably missing is a panel meter for the thing. This is because I’m actually having a problem sourcing one that is suitable. Home Hardware (née Supremetronic) didn’t have a single LCD or LED 7-seg voltmeter in stock! If you can believe it. The analog meters they had on hand were likewise of unsuitable specification. Creatron was no help either. All they had was one voltmeter that basically was only good for measuring it’s own supply and could not measure below 4.5V making it bloody useless.
The douche at Creatron even said that this was “usual” which proves to me he has no frickin clue what he is talking about. Panel voltmeters are “usually” 200mV full scale and configurable with jumpers and input divider resistors for other ranges. Pathetic. The brick and mortars seriously failed me this time. I will have to go to A1 or Active to get one. Hopefully they have something acceptable. Either that or I will order one online. I love visiting A1 anyway :).
A brief rant
I love Arduino, and Arduino culture. I do. I think it’s marvellous how they’ve brought micros to the masses and made them accessible. I too use them and love them. I do, however, hate what they are doing to my hobby. It seems the brick and mortars are putting all their focus on Arduino toys and completely neglecting basic analog electronics (let alone the huge range of more advanced analog and digital electronics that are not of the Arduino ecosystem). Home hardware has been out of stock of 10Ω 1% 2W resistors FOR OVER A YEAR. Not having acceptable panel meters is also pretty damn basic. C’mon guys, if you want to call yourself a shop for the electronics hobbyist, stock appropriate parts and stop licking the butt of Arduino fanboys.
A person who hooks an LED to an Arduino and dumps in some code copied from the internet cannot call themselves an electronics hobbyist. That’s not arrogance, it is a truth.