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Old 19th February 2009, 09:25 AM   #221
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by AndrewT
Check the voltage across R2.
Check the voltage across the Zener. Check the voltage Vds across the FET.
These should add up to the supply voltage. I suspect that either the Zener is back to front and/or the FET is damaged.

Quote:
Originally posted by Tripmaster
R2 has the same voltage as output and the J309 has a reading of less than a volt.

Quote:
Originally posted by Tripmaster
Im not getting an output from the Zener
now total the voltage drops across all three devices.
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Old 19th February 2009, 09:56 AM   #222
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Quote:
Originally posted by AndrewT







now total the voltage drops across all three devices.

Hi Andrew

I will do this tonight.

The voltage decreases the longer I leave the power supply powered. Is this going to lead to an inaccurate figure or should I wait a while between each component?

Does Vds stand for ‘voltage drain to source’?

Thanks

Richard
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Old 19th February 2009, 10:12 AM   #223
AndrewT is offline AndrewT  Scotland
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yes,
Vds = Voltage Drain to Source.

Is the Zener the right way round?

This is the negative rail!!
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Old 19th February 2009, 11:19 AM   #224
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Quote:
Originally posted by AndrewT
yes,
Vds = Voltage Drain to Source.

Is the Zener the right way round?

This is the negative rail!!

It did cross my mind but the Zener symbol in the schematic faces in the same direction as the v+ side. Is this an error on the schematic?

It’s the first time I have built a circuit by following a schematic…and boy does it show!
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Old 19th February 2009, 01:38 PM   #225
AndrewT is offline AndrewT  Scotland
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Hi,
the Black Band is towards the more positive voltage.

If you have +ve supply at the top, Zero volts from the supply in the middle and -ve supply at the bottom, then both Zeners "point" in the same direction - towards the more positive rail.
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Old 19th February 2009, 01:45 PM   #226
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Quote:
Originally posted by AndrewT
Hi,
the Black Band is towards the more positive voltage.

If you have +ve supply at the top, Zero volts from the supply in the middle and -ve supply at the bottom, then both Zeners "point" in the same direction - towards the more positive rail.
Thats ok then.
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Old 19th February 2009, 07:48 PM   #227
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Here are the voltage readings


Output voltage=21.5v

J309
Drain is connected to Zener =00.7v
Source connected to 1k resistor=00.7v
Gate connected to R2=1.6v

Zener output=00.7v

Voltage across R1=20.6v
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Old 19th February 2009, 10:46 PM   #228
AndrewT is offline AndrewT  Scotland
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21.6V across the resistor explains why it's burning.

0.7V across the Zener indicates it's back to front, i.e. forward biased, check it's direction again.
J309 may now be damaged but fix the Zener & resistor first.

Buy yourself a plug board. A couple of £ from Maplin and many others. Ideal while your learning and continues to be useful long afterwards.
Save that expensive Veroboard for proved circuits.
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Old 20th February 2009, 08:09 AM   #229
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Quote:
Originally posted by AndrewT
21.6V across the resistor explains why it's burning.

0.7V across the Zener indicates it's back to front, i.e. forward biased, check it's direction again.
J309 may now be damaged but fix the Zener & resistor first.

Buy yourself a plug board. A couple of £ from Maplin and many others. Ideal while your learning and continues to be useful long afterwards.
Save that expensive Veroboard for proved circuits.

Morning Andrew

Thanks for checking those figures

I'm not trying to challenge your advice, but when I measured the voltage across the Zener, before I cut the tracks and added a couple of links as per Audio1st diagram, I took a reading of 36v from this side of the Zener. Should the anode or cathode be connected to the drain pin of the J309?

Click the image to open in full size.

Zener Data Sheet

http://www.datasheetcatalog.org/data.../105630_DS.pdf

I have some spare parts to swap out the faulty components and will have a go at this tonight
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Old 20th February 2009, 08:57 AM   #230
AndrewT is offline AndrewT  Scotland
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Hi,
what power of Zener is that?
A 400mW glass packaged Zener will do the job better and cheaper. Rapid sell them for 2p each.
A 24V 400mW Zener requires at least 0.8mA (5%) to operate properly and better to have at least 1.7mA (10%) to operate well.

A 1.3W device asks for too much current to get around the knee in the I vs V curve.
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