Hello all. I'm repairing a circuit board from a Crate PA6FX mixer amp, and it has some scorched zener diodes with a very clear part number 1N5353B. This comes up as a 5 watt, 16v diode. There is an additional number below this on the diode, 1061; I haven't found that to match any part numbers or mean anything relevant to the part's rating.
When they arrived, the new diodes look similar; black body with silver markings, but the body of the new diodes are at least 30% smaller than the diodes that are in place. I looked under the part number listings again to see if I had perhaps picked the wrong wattage rating, but none of the diodes under that part number have a different wattage rating or different physical size. Has diode composition improved that much in the past 10-15 years or so? Am I missing something?
Thank you in advance for any insight you can offer.
The additional 4 suffixes marked on the part would be for Date/Lot Code information. Also, this part is offered in several through hole packages. The part number does not call out a specific package case size. The case sizes I see offered for this part are 5w. So as long as the dimension will work for the application I see no reason why you can't use another case size.
The relatively small 1N5353B zener diode achieves its specified 5 watt rating by heatsinking through the leads. The datasheet specifies 3/8" leads to an infinite heatsink. This cannot be easily achieved in practice and so the practical rating is much less than 5 watts.
I've used two "tricks" to cool off these types of zener diodes. One is to use the entire lead length as a heatsink. To shorten the linear length each lead is formed into a spiral around a drill bit. The other is to cut a copper penny in half and solder it to the lead right at the diode body.
I once saw a design in which the engineer ran 5 watt diodes at 5 watts. Being mounted on the underside of the PCB they would melt the solder and fall out. The engineer's fix was to slather the PCB with epoxy to hold the diodes in! That is not a good design.
Thank you Wayne, and Dave!
I had not thought of the heatsink aspect of it, but I did solder in the new parts with as much extra lead legth as I could for the space. I surmised there would at least be more air flow and heat dissipation if I moved them away from the surface of the board; the original parts were scorching the board. I liked the suggestion about coiling the leads, never thought of that either!
That's a good point so let's work the numbers. The datasheet specs are at DC and the inductance effect is of course at AC, but there there's enough data to ball park it.
The Microsemi datasheet shows that at 75 mA a 16 volt 1N5453 has a dynamic impedance of 2.5 ohms. Using a total length of 2" and the rule-of-thumb of 20 nH/inch the zener has an inductance of 40 nH. We'll ignore the zener capacitance for now. At what frequency is the inductve reactance equal to the dynamic impedance? That's 40 nH and 2.5 ohms and the frequency is 10 MHz. So, unless the zener is being used without a shunt capacitor above 10 MHz the lead inductance is not an issue.
As to the zener capacitance, to have a reactance of 2.5 ohms at 10 MHz it would have to be 6 nF. I recall measuring a 1N5353 many years ago - I forget what breakdown voltage it was - and the capacitance was less than 1 nF. So, I think we can ignore the capacitance for this exercise.
Thank you for the excellent explanation, I believe I understand what you were saying. Since the circuit I'm dealing with is a simple inverter delivering +/- 16v, it's never going to see any frequencies that could make the inductance an issue.
MJ, this paper on Zener diodes tells the whole story. Figure 1.18 shows voltage vs. capacitance.