| Electronics > Repair |
| 18V/3A HY1503 PSU - weird CC/CV behavior while charging battery cells |
| (1/15) > >> |
| iMo:
Hi, I've been coping with following crazy issue - I modded an older HY1503 classic far east PSU I got as a gift - I set it to 18V (was 15) and added a several ohm shunt in series with the existing 0.3ohm, with a switch to select 200mA or 3A CC range. When shorting the leads or messing with potentiometers under a set load the current limit seems to work :) . Now the issue: When I've been charging say 16V (an example) cell with set 1A (and set 16V without the load) the current is perfectly stable and then it starts to increase slowly (like ~5 minutes of rise) when the voltage approaches the set 16V. Like from 1A to 1.25A, and not stopping even it crosses the set 16V. When I move the voltage pot a liiitle bit down the current jumps to zero. I've spent some time to observe the behavior but cannot find the clue. My current understanding is when the battery voltage is approaching the set voltage the current shall start to decrease to zero (with let say plus minus some 10-30mV of a hysteresis, my guess). :palm: PS: I do not think it is a thermal stuff because it runs for 2 hours stable current 1A into the battery, and the last minutes around the set voltage the current starts to rise.. PPS: for example below charging 3S 18650 from 11.5V, PSU set to 12.62V and 0.99A. |
| iMo:
After spending a whole day with the issue, while measuring every node on the pcb around the critical point of the rising current, I found out the touching the pin 5 of the 741 (CV control) with the voltmeter's probe caused an immediate jump of the current to zero (when in the region of rising current). So I removed the C10 (a small 100p ceramic cap) and the stuff started to behave normally, it steadily drops the current into the battery to zero at exact set voltage. The C10 there is wired from the 741's output pin 6 to pin 5 (one input of the offset nulling). That is an interesting wiring, perhaps they put C10 there in order to speed up the CV transition?? Perhaps experts here know more?? PS: Or, perhaps they used to use a different opamp in past with an "overcompensation" at the pin5 (like the LT1028 has got)?? |
| floobydust:
I find these PSU circuits are copied and some designs go back to the 1970's. Like the ubiquitous 0-30V 2mA-3A kit from china. The ICs get updated, changes made by people who don't really understand the circuits. I think they make bad changes, get a stability problem and then fart around trying to stop it oscillating in production. Mastech had the same cap C10 in their HY3020, and on the voltage (error-amp) op-amp at pin 5. Pin 5 is +ve feedback I thought on a 741 but not for a 709. It must be an archaeological artifact. |
| iMo:
Yep, all those PSUs share common concept and schematics inclusive the same values.. The 741s there have the 100pF capacitor at pin5 which is offset zeroing input in 741 (it is internally freq compensated), not the freq compensation like in 748 or others. In LT1028 it is overcompensation pin for an external capacitor. The 748 has pins 1 and 8 for the freq compensation as well as the LM301. The LM709 has got pin 1 and 8 for "input freq compensation" and the pin 5 for "output freq compensation", however.. The concept of those PSUs is not bad, imho, I've put the schematics into LTspice and it oscillates in sim with the 741 and 324 opamp models I have handy. The real stuff does not oscillate under normal situation, but the 100pF feedback needs some investigation as it showed problems with slow input signal. In the simulation I even tried the 741 made of transistors I found in Examples, there is the pin 5 and pin 1 for offset setting, but it worked the same. I will try at least with a simple 741 sim to see how the 100pF impacts the 741 speed.. |
| xavier60:
Try a higher value for R21. |
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