Lab PSU modding — generating negative voltage for LCD - Page 1
Forum section: electronics / projects

author: tooki
author_rank: Super Contributor
timestamp: November 05, 2016, 02:21:12 pm
content: Hey everyone,So one of the projects I've been working on for a while is modding a cheap chinese PSU (it's an unbranded Mastech HY3005 or clone) to give it a more precise voltage/current readout, output cutoff switch, etc.I found an eBay module that does the trick (shows V and A down to 1mV/1mA resolution on a 1602 LCD), and it basically works great. But I want a larger LCD, which works fine except for one little snag: the jumbo LCDs need negative voltage on their contrast pins! (In the range of 0.3-0.7V, depending on the color.)I breadboarded up a little 555 timer based charge pump to generate negative voltage, and it works fine, with the LCD drawing a whopping 0.4mA of current on the contrast pin. So I've got a solution if need be. But I also took a closer look at the PSU's own circuit, and found some interesting things. One of them is that the power connector to the original panel meters actually has an unpopulated -5V pin. Tracing out all the traces and unpopulated components shows it to be a simple zener-regulated rail pulled from the main PSU circuit. This connector, however, does not have its ground referenced to the main PSU circuit's ground. The ground and +5V on the panel meter power connector are supplied by a 7805 regulator run off an independent transformer winding (i.e. completely floating relative to the main PSU circuit), so the -5V on this connector necessarily requires that it get its ground elsewhere. Careful examination of the main PSU circuit shows that circuit ground is actually the V+ of the output.Meanwhile, the LCD panel meter module from eBay uses +5V for power, and uses a common ground for power as well as for the voltage and current to be measured, such that V- of the output is grounded.Obviously I can't have both V+ and V- of the outputs be at circuit ground, so I've got to find another solution. (Right?)I noticed that the transformer winding for the panel meter supply has an unused tap. It appears to produce about 9V AC between taps 1 and 2, and 3V between 2 and 3. (The 5V supply is generated off of the 9V between taps 1 and 2, through a bridge rectifier to the 7805, with tap 3 left totally unconnected.) Is there a way to use that extra 3V AC tap to generate the negative 1V DC I need? Every circuit I found for making dual-rail supplies from a center-tap transformer is based on the premise of symmetrical taps, using a diode bridge from the outer taps, using the center as ground. But would that work if asymmetrical, i.e. if I either a) left the 5V rail running off of taps 1 and 2, and added a whole separate bridge between 2 and 3, leaving a minuscule amount of voltage to work with, or b) changed the bridge to taps 1 and 3, using 2 as a ground?Code:—————— 9V (1)SSSS—————— 0V (2)SSSS—————— 3V (3)

author: Kleinstein
author_rank: Super Contributor
timestamp: November 05, 2016, 04:37:21 pm
content: It should be possible to use a kind of charge pump driven by the 50/60Hz AC of the transformer.You could also use the extra tab to get a negative supply. Just one diode from the extra tab to a filter cap for the negative supply. This should give something like -4 V, half wave rectified so with 50/60 Hz ripple. So still need a -1 V stabilization circuit. At only 0.5 mA this should be OK from the transformer to have a little DC current.For showing the current, it might be Ok to have the neg. signal-input connected to the panel meter ground and positive output. For the voltage reading, one might need an OP in inverted configuration to get a positive output.

author: tooki
author_rank: Super Contributor
timestamp: March 31, 2017, 10:50:08 am
content: So I came to the conclusion that trying to use the extra winding is a fool's errand, and ordered some ICL7660/7660S chips off fleabay. Unsurprisingly, wiring one up externally, and again using a pot between V- and V+ works just fine.And then I took a closer look at the LCD module itself. It's got component footprints that for all the world look exactly like the ICL7660 circuit, including the location of Vin and Vout, the capacitors, etc. And the Vout of that footprint is in fact connected to the contrast pin of the LCD.After overthinking it for a bit, I found a datasheet that indicated that even small LCD modules may need the negative voltage to run off 3.3V (whereas big ones may need it even for 5V, like mine.) A quick confirmation showed that 3.3V LCDs just have the ICL7660 populated, and that you just have to wire the contrast pot differently. (Namely, between contrast and ground only, without the V+ connection.) Turns out itdoesn'tneed to be a voltage input on the pin, as it's actually part of a big resistor divider network that's part of the LCD driver.I populated the ICL7660 and the 10uF caps, and voila, it works!! At least in the case of my jumbo LCD modules, the contrast pot needs to be a small value to be of any use: when set to good contrast, the contrast pot is only at around 10 ohms.