why are these transistors getting so hot?! - Page 1
Forum section: electronics / beginners

author: kasumyku
author_rank: Regular Contributor
timestamp: April 17, 2013, 07:16:41 pm
content: I am trying to figure out if this is normal or one of the components failed.Some people say it's normal some say it's not, I would would really appreciate a second opinion.Those two parts of the schematics are from a JVC RX-8010 receiver.I was wondering if these 6 power transistors suppose to be so damn hot that I can't even touch them or its just normal.I have used a infrared thermometer and on the heat sink it reads somewhere around 75 C degrees when the volume is just 1/2 way up.Comparing these power transistor with the output transistors, the output ones are just barely reading 25 C degrees.thanks!!!

author: megahz
author_rank: Regular Contributor
timestamp: April 17, 2013, 07:21:32 pm
content: the ones I used get pretty hot but its wired correctly, if its the kind of transister that comes with a heatsync on the back tho

author: kasumyku
author_rank: Regular Contributor
timestamp: April 17, 2013, 07:25:09 pm
content: they are attached to heat sinks...

author: madires
author_rank: Super Contributor
timestamp: April 17, 2013, 07:58:53 pm
content: Each of the 6 transistors is a part of a zener voltage regulator (called power zener or zener with follower).

author: Unknown
author_rank: Unknown
timestamp: April 17, 2013, 08:03:49 pm
content: Your top image is showing power supplies series regulators for the whole receiver system, thus they would have much more demand for current being drawn from the receiver overall.The bottom image, is similarbut they are for individual dedicated power supply stages,and would draw less current on average but, their temperature can rise intermittently, as more demand is required from these regulators.

author: kasumyku
author_rank: Regular Contributor
timestamp: April 17, 2013, 08:12:24 pm
content: Quote from: madires on April 17, 2013, 07:58:53 pmSo it suppose to get that hot?

author: madires
author_rank: Super Contributor
timestamp: April 17, 2013, 08:19:45 pm
content: Quote from: kasumyku on April 17, 2013, 08:12:24 pmThe transistors have to dissipate the voltage difference between input and output multiplied by the current drawn. It's like you would use a 7812.

author: IanB
author_rank: Super Contributor
timestamp: April 17, 2013, 08:25:32 pm
content: Quote from: kasumyku on April 17, 2013, 08:12:24 pmSometimes transistors get hotter with the case open than with the case closed. This may happen if there are cooling fans and with the case open the airflow is not properly directed over the heat sinks.

author: Unknown
author_rank: Unknown
timestamp: April 17, 2013, 08:33:12 pm
content: QuoteYes, it's normal...

author: edpalmer42
author_rank: Super Contributor
timestamp: April 18, 2013, 01:31:52 am
content: 1. Measure the output voltage from each regulator. Is it correct?2. Measure the voltage drop across the collector resistors. Calculate the power dissipated by those resistors. Is it within the wattage rating of the resistors?If the answers to both questions are 'yes' then the circuits are likely working as designed. Looking at the entire schematic, it looks like they should be dissipating only a few watts since the input power is coming off of main capacitors that are rated for 35V. One other thing to do is make sure that the screws or clips that are holding the transistors to the heatsink are snug.By the way, I wouldn't necessarily trust your infrared thermometer. They can give incorrect results depending on the type of surface you're pointing them at. Also, the transistors are rated for a junction temperature of 150C so a surface temperature of 75C isn't out of line. People often think that components shouldn't be more than 'warm' to the touch. Wrong!Ed

author: kasumyku
author_rank: Regular Contributor
timestamp: April 18, 2013, 02:42:11 pm
content: Quote from: edpalmer42 on April 18, 2013, 01:31:52 amThank you very much Ed!!!