How hot would *you* run a 1 Watt Zener?

[cvanc]

See attached photos.  The 2 hot components side-by-side are 3.3V 1W Zeners in series.  They're over 300°F (~155°C).

I know most 1W Zeners are rated up to 200°C, but these are by far the hottest things in the DUT and I'm a bit concerned.  And this measurement is with the cover off sitting on my bench - it'll only get hotter when I button it up.  They're in the upper right corner on the schematic.

Replace with 5W units, maybe?  What would you do?  Thanks.

[maginnovision]

I probably wouldn't want them running so hot next to some aluminum electrolytic caps. Is there anything you can do to minimize heating in the first place? Higher wattage won't change much if the same current at the same voltage is crossing them.

[bob91343]

My rule is to run at one-fourth rating.  A 1 Watt diode, then, runs at 250 mW.  That's usually the level in the specification so you can count on the voltage being correct.

Any deviation from that requires refiguring based on its impedance.  And of course, depending on the ambient temperature and heat flow, one should take into account the temperature coefficient of voltage as well.

Read the spec sheet to see the standard test conditions; I try to duplicate them as closely as the application warrants.

[amyk]

That circuit is strange and not in a good way.

If I'm reading it correctly, it's a bunch of linear regulators that turn 55V into 26V, then 18V? It looks like the designer just wanted to drop 6.6V, to reduce the heat on Q41? How hot does Q41 get? If it's more below its maximum rating than the diodes, I'd try to bypass one, then both of them, and measure the temperature of the transistors.

Regardless, that's a pretty steep drop for a linear regulator...

[NiHaoMike]

Measure the actual current through the zeners, then add a small incandescent bulb in parallel to divert a large amount of the current.

[jmelson]

See attached photos.  The 2 hot components side-by-side are 3.3V 1W Zeners in series.  They're over 300°F (~155°C).

YIKES!  Are you sure the device that is powered is not drawing too much current?  If not, then what idiot designed this stuff?  Sure couldn't get that past a safety lab.

I'm not sure going to a 5 W Zener is really going to help, you still have the same amount of heat being created.

[jdragoset]

My question is why they did not use the 6.6V drop for the negative pass transistor but felt it was needed for the positive pass transistor?
Other than the two series ZD's, the positive and negative regulators mirror each other.

[T3sl4co1l]

They sure went crazy with the "Q capacitors" and filtering... (C41-C44, C47/C48, C413/C414).

No current limiting, hFE (suicide biasing) doesn't count.  Except for 12VTRIG oddly enough (but with terrible regulation, a stack of Vbe's below a resistor divider).

Yes, replacing the diodes with bigger ones, or attaching heatsinks, would do.  Mind that diodes mainly dissipate heat through their leads, and if there isn't much copper pour on the PCB they'll get awfully hot, even at rated current.

The diodes (and series collector resistors R45/R46) aren't necessary if the transistors are rated for the SOA (including adequate heatsinking, mind).  TIP31C (and complement) may be better in that regard.  Since the TO-126's shown aren't rated for much.

A much better solution overall is likely possible, but I'd have to see the rest of the circuit.  In a repair setting, this isn't a very useful exercise though.

Tim

[golden_labels]

Mind the derating curve. 200°C is the temperature at which the curve touches 0W, not safe operating temperature at full power! See datasheets, but 1W zeners in the DO-41 package can typically dissipate 1W only below 50°C–100°C.

[soldar]

You can replace the zener with a transistor and place the zener between base and collector. This way most of the heat will be produced by the transistor which can have an adequate heatsink.

[SeanB]

Replace with 5W devices, and heatsink them. Otherwise use a power transistor in TO220 package, and arrange it as an amplified zener diode, connecting a 6V2 1W zener from collector to base, with a 100R series resistor with the zener diode, and another 470R resistor from base to emitter. that transistor will need a small heatsink ,but being a lot bigger it will run cooler.

Only reason I see this using the zeners is because the power pass transistors suffered punch through with some devices with startup at high mains voltages, so they put in some measures to both drop the input voltage ( the zener diodes) with R45 there as a rough current limit and fusible link for shorted pass devices.