L4973V5.1 Power supply giving too much output voltage

[dicky96]

Hi guys
This power supply is from an FM Radio transmitter

It takes an input of 28V and should output some lower voltage. When I first looked at it I found that there is a zener diode D3 directly across the V out to ground and it was short circuit.

I removed the Zener, there are no markings to determine it's correct value, and I don't know what Wattage it should be either, plus it broke in half. I took a photo comparing it to a 1N914 signal diode and a smaller zener which I have (which I think are 0.5W but I am not sure) and also a resistor so maybe someone can work out the wattage from the physical size?

Anyway the zener is directly across the output so it obviously acts like a crow bar if there is an over voltage condition

When I power up the supply (with the load pcb disconnected) I get 12.5V output - but this could be too high which is would explain the zener went short.

The power supply uses a L4973V5.1
https://pdf1.alldatasheet.com/datash...L4973V5.1.html

This is a fixed 5.1V buck regulator, but can be used to generate higher voltages using a resistor divider from pin 12 L4973V5.1 to V Out and Ground

My circuit has the resistor network on pin 12 and most closely resembles the example in Fig 4 of the datasheet. Plus the zener.

I have resistors R4 and R5 (see pics attached). These look like R4 7.5K 1% from Vout to pin 12 and R5 5.11K 1% from ground to pin 12. The datasheet shows some example values but doesn't seem to contain the formula to calculate the output voltage from the resistor values

In circuit there is a capacitor charging/discharging but on my DMM, once it settles, I read pin 12 to ground 4.86K and pin 12 to Vout7.06K


So my questions are:
Is the PSU giving the correct voltage output?
Is it too high, hence the short circuit zener?
What is the correct rating for the zener?

[stj]

Re: L4973V5.1 Power supply giving too much output voltage

that diode has markings.

from the size it could be 1.3w

[stj]

Re: L4973V5.1 Power supply giving too much output voltage

datasheet has the info,

the reference pin expects 5v1
so you need to calculate what ratio that resistive divider is.
then for 5v1 out, you can calculate what has to be going in.

[dicky96]

Re: L4973V5.1 Power supply giving too much output voltage

Hi stj good thinking - pin 12 has 5.08V

I also found the schematics for the transmitter now, its not an FM radio transmitter as I assumed it was

[dicky96]

Re: L4973V5.1 Power supply giving too much output voltage

Bahh it's a different version to mine- it as 15V main supply not 28V. But I will try contact OMB and see what they say

[dicky96]

Re: L4973V5.1 Power supply giving too much output voltage

OK stj so back to your observation that pin 12 should be 5.1V, which it is near as damn it, then if my maths is right the V out would be 12.6V and I am reading 12.5V (see pics)

So I powered it up and it works (or at least it comes on and the LCD display is working)

it doesn't explain why the zener went short but I guess a 13V 1.3W would be a good substitute?

[stj]

Re: L4973V5.1 Power supply giving too much output voltage

13v may be too low - it starts to switch below the rating.
maybe thats why it failed.
check the zener datasheets

[dicky96]

Re: L4973V5.1 Power supply giving too much output voltage

Ok. I ordered an assortment pack of Zeners that include 12V 13V 14V 15V so I could put a higher voltage rated one. The board that is powered by this PSU has some LM393, TL084 and 4000 series CMOS chips plus a big square one in a socket that looks like a programmable device, maybe an FPGA or something.

I don't know the original value of course, I can only guess.