# schematic symbol from power supply, two triangles pointing together

What does the symbol marked Z1 (two triangles pointing towards line) represent in this schematic?:

This is coming from the reference design on page 10 of the LD7550-B pulse width modulator datasheet (Jan 2005 version). You can get the datasheet here. It's not written in the datasheet anywhere I can see, and in the table of part numbers the component value is written as "N/A".

• Bidirectional TVS diode, I suppose from looking at the symbol only. Jul 17 '16 at 15:46
• Seems to fit from the context. I have another pretty naive question: in this schematic there are multiple places where two resistors are placed in series with nothing else connected, for example R1A and R1B, and also R2A and R2B. Why would they do this instead of using a different resistor? Just because of part availability, or perhaps to change the failure mode? Jul 17 '16 at 15:56
• Two resistors in series have a higher voltage withstand/rating and/or power rating. Jul 17 '16 at 15:58
• Because it is labelled with 'Z,' I think that is a back-to-back Zener diode. This is a type of TVS (transient voltage suppressor), so I am agreeing with Bence. Just providing more detail. If the part number is "N/A" I think that maybe it is not actually installed. Like they put it in the schematic just in case, but then found out they didn't need it in production. Jul 17 '16 at 16:11
• Umm... I hope this is post a transformer because the dclink negative is tied to chassis which will me more than likely the neutral of the AC. Jul 17 '16 at 16:15

'Z' is often a designator for Transzorb, as is the symbol. However any SMPS I have ever seen normally use a 20mm MOV, which has a higher transient absorption rating. Confusing the issue even more is that 'Sidacs' use the same symbol, but are very expensive, often in the $20 to$40 price range.