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?
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?
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