7812, 7912, bridge rectifier operating temps - Page 1
Forum section: electronics / general

author: ballen
author_rank: Regular Contributor
timestamp: December 20, 2020, 02:13:34 pm
content: Dear EEVbloggers,To start off: I'm new here. I hope this question is in the right forum, if not, please tell me where to move it.I've got a 1990s era precision clinometer (0.1 arcsecond resolution, 0.2 arcsecond precision) which has both AC line power and built-in NiCd rechargables. I'm replacing each of the two NiCd batteries (9v, 1100mAh) with a pair of 18650 Li-ion batteries and battery management/balance controller board.The original charging circuit for the 9V NiCds was lousy: a 39 ohm 2W resistor in series with the +-12V regulated supply and a 0.7V protection diode drop. The intent was that when the NiCd voltage was ~7V, this would provide about 100mA of charge current, reducing to a 25mA trickle when the battery had gotten up to 10V.I've replaced the 39 ohm resistor with a constant-current source made from an LM1117-adj (low drop-out LM317) plus reverse protection diode. I've tested that, and it works well. Right now I have the current source set to provide around 400mA, which charges the Li-ion batteries in ~5 hours.My only concern is the following. The designers intended their +-12V supply to provide ~150mA to power the meter plus the charger. Right now, I am drawing close to 450mA when the batteries are charging. I want to make sure that I don't do any long-term damage.The +-12V supply is simple: a center tapped ring transformer, full-wave rectifier, then a 7812 and a 7912 regulator, TO-220 package, each on a finned heat-sink, about 20mm square and 20mm high. The transformer is rated for 30 or 40VA, so no issue there. The supply circuit is fused before the regulators and after, 1A slow-blow, again suggesting that 450mA is not too much.As far as I can tell, the only critical components that I need to worry about are the bridge rectifier and the 7812/7912. DC coming in is about +-15V, so the regulators need to dissipate about 1.4W each. I put a thermocouple on the 7812 (which is providing slightly more current than the 7912) and the tab's steady-state temp in charge mode is about 70C, which would put the chip around 80C. The bridge is slightly too hot to keep my finger on it, so probably running around 75C. (It's one of those little round packages and there is air space around it, so I can probably stick on a round finned heatsink and drop that a bit.)My question for the forum is this: am I OK running the bridge and 7812/7912 at these temps? Or will it cut their lifetimes enough that I should either drop my charging current (say to 250mA) or try to fit larger heat sinks to the regulators? I know that these are rated up to around 125C junction temperature, but I'm sure it's bad practice to come any where near to that. How far away should I stay?By the way, the actual metering part of the instrument runs on +-5V internally. The +-12V supply and the +-9V (nominal) battery are only there to power LDO +-5V regulators downstream. Also, the sensitive/precision/low-offset/balanced/low-drift stuff is on the other end of the board, well away from the heat of the power supply circuitry. So the instrument function is remarkably unaffected by these mods. It's high-end gear (Taylor-Hobson Talyvel 4) and it's my impression that the designers left a lot of headroom.Cheers,Bruce

author: Simon
author_rank: Unknown
timestamp: December 20, 2020, 08:15:12 pm
content: https://www.eevblog.com/forum/projects/lm7812-lm7912-diode-bridge-operating-temperature/msg3378886/#msg3378886Please don't double post, we get complaints.....