SAFE VOLTAGE

SUZTEK · 23rd December 2010, 07:09 AM

Using 2x Mj15004 and 15003 as output transistors for a power amp.I have a DC power supply of +/- 77volts. Will it be safe to operate these transistors at these voltages?

richie00boy · 23rd December 2010, 08:27 AM

They are rated at 140V, so no that voltage is too high really. You might get away with it but I would not try.

SUZTEK · 23rd December 2010, 08:35 AM

thanks richie, will take your advise and use the correct transformer with an output of 48 0 48 v.Should then be within the safe operating specs.

AndrewT · 23rd December 2010, 10:15 AM

you said your PSU was +-77Vdc.
Under what operating conditions do you measure +-77Vdc?

Are there any normal or even exceptional operating conditions where the measured PSU voltage is not = +-77Vdc.

A 230:40+40Vac transformer gives ~+-59Vdc when fed with 240Vac and loaded with the bias current of two ClassAB amplifiers.
If mains voltage rises or falls then PSU voltage varies.
If one amp is off then the voltage rises on the other amp.

For worst case conditions I think 48+48Vac is too high for 140Vce0 devices.
41Vac gives a worst case in the UK of about +-63Vdc

You might get away with 44+44Vac, but you should check for worst case before committing to purchasing.

SUZTEK · 23rd December 2010, 11:00 AM

The supply mains voltage in South Africa is 220v. If i take the Vceo of 140 v less 10% of 140 divided by 2 gives a safe operating voltage of 63 volts. The transformer output of 48 v at 220v gives a dc output of 67 volts. Now as you rightly pointed out, this is an unloaded measurement. If the amps are connected the supply should drop. But to be on the safe side i will take your advice and use a 46/0/46 which should give an output of be64 vols unloaded.
thanks
Willy

AndrewT · 23rd December 2010, 12:13 PM

What is the range of supply voltage from your 220Vac supplier?

mt490 · 23rd December 2010, 12:20 PM

Yeah, AndrewT's point must not be discounted. I live in the land of -6% +10% (216-253V, 230V nominal), the benefit is that I'm actually told that the power should meet that spec - not always the case for every country.

You can push the limits for personal use but just don't be surprised if the occasional smoke release occurs. Perhaps some sort of over-voltage shutdown should be included.

djk · 23rd December 2010, 11:12 PM

"use a 46/0/46 which should give an output of be64 vols unloaded."

Only if the transformer has 100% regulation.

23rd December 2010, 07:09 AM	#1
SUZTEK diyAudio Member Join Date: Sep 2005	SAFE VOLTAGE Using 2x Mj15004 and 15003 as output transistors for a power amp.I have a DC power supply of +/- 77volts. Will it be safe to operate these transistors at these voltages?

23rd December 2010, 08:27 AM	#2
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	They are rated at 140V, so no that voltage is too high really. You might get away with it but I would not try. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

23rd December 2010, 10:15 AM	#4
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	you said your PSU was +-77Vdc. Under what operating conditions do you measure +-77Vdc? Are there any normal or even exceptional operating conditions where the measured PSU voltage is not = +-77Vdc. A 230:40+40Vac transformer gives ~+-59Vdc when fed with 240Vac and loaded with the bias current of two ClassAB amplifiers. If mains voltage rises or falls then PSU voltage varies. If one amp is off then the voltage rises on the other amp. For worst case conditions I think 48+48Vac is too high for 140Vce0 devices. 41Vac gives a worst case in the UK of about +-63Vdc You might get away with 44+44Vac, but you should check for worst case before committing to purchasing. __________________ regards Andrew T.

23rd December 2010, 12:13 PM	#6
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	What is the range of supply voltage from your 220Vac supplier? __________________ regards Andrew T.

23rd December 2010, 11:12 PM	#8
djk diyAudio Member Join Date: Feb 2001 Location: USA	"use a 46/0/46 which should give an output of be64 vols unloaded." Only if the transformer has 100% regulation. __________________ Candidates for the Darwin Award should not read this author.


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23rd December 2010, 08:35 AM	#3
SUZTEK diyAudio Member Join Date: Sep 2005	thanks richie, will take your advise and use the correct transformer with an output of 48 0 48 v.Should then be within the safe operating specs.

23rd December 2010, 11:00 AM	#5
SUZTEK diyAudio Member Join Date: Sep 2005	The supply mains voltage in South Africa is 220v. If i take the Vceo of 140 v less 10% of 140 divided by 2 gives a safe operating voltage of 63 volts. The transformer output of 48 v at 220v gives a dc output of 67 volts. Now as you rightly pointed out, this is an unloaded measurement. If the amps are connected the supply should drop. But to be on the safe side i will take your advice and use a 46/0/46 which should give an output of be64 vols unloaded. thanks Willy

23rd December 2010, 12:20 PM	#7
mt490 diyAudio Member Join Date: Jun 2010	Yeah, AndrewT's point must not be discounted. I live in the land of -6% +10% (216-253V, 230V nominal), the benefit is that I'm actually told that the power should meet that spec - not always the case for every country. You can push the limits for personal use but just don't be surprised if the occasional smoke release occurs. Perhaps some sort of over-voltage shutdown should be included.

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