Newbie question regarding fuse's and resistor ratings

neil_kaye · 2nd July 2006, 01:10 AM

Hi,
I have several questions regarding resitor, LED and Fuse selection thaty i was hoping someing might help me with:
1. I have a transformer that is rated 120VA with 22VAC secondaries. The rectified voltage I calculated will be 31.1 Volts (120VAC * 1.414). For use with a LM3875 opamp
I would like to use a red LED with forward voltage range of 2.2Vdc 20-30ma
http://www.partsexpress.com/pe/pshow...70-015&scqty=6
I calculated that the resistor i would need is (33.1-2.2)/0.02= 1.4K. So i chose a 1.5kOHM 1w resistor http://www.partsexpress.com/pe/pshow...3-1.5K&scqty=1
Is this the correct resistor to be using? This set up is for a chipamp. Would it be better to use a 1.5 ohm 12w Mills type of resistor for this applciation:
http://www.partsexpress.com/pe/pshow...number=005-1.5
How do i connect the LED to a dual secondary transformer?

2. I would like to us a power jack with 10a built in fuse:
http://www.partsexpress.com/pe/pshow...90-443&scqty=2
The fuse i selected is a 4a 3AG slow blow fuse. Would this work? The concern i have is that the IEC says that i need to use a fuse rated at 250v. However i can't find a 250v fuse in slow blow so can i use a 32v rate slow blow fuse with this IEC?

Thanks for your help

poobah · 2nd July 2006, 01:25 AM

The first resistor you mention is correct.

I don't know what the second resistor is about.

What is the LED for? If it is for an "ON" indicator, you simply connect the LED/resistor across one of the large caps following the rectifier.

HERE is one of several fuses that fit the bill. The voltage rating of a fuse is known as the interrupting rating. The rating of the fuse should be greater than the voltage the fuse would see if it were open (in this case 120V).

neil_kaye · 2nd July 2006, 02:04 AM

Hi poobah,

Thanks for the response. I still don't quite understand the fuse situation. Is the fuse that i origianlly chose incorrect?
Is there a slow blow fuse that i can use with the IEC i chose? All the slow blow fuses on the parts express website seem have an interupting rating of 35V?

Thanks for your help.. .this is all new to me

poobah · 2nd July 2006, 02:09 AM

Yeah dude... there is a fuse. Click where "Here" is highlighted in blue in the last post.

AndrewT · 2nd July 2006, 08:39 AM

Hi,
thanks for that Cooper/Bussman link.

that's the first time I've noticed the Voltage drop across the fuse at rated current.
65mV to 75mV for all the fuses I have used!

If only all manufacturers were as informative.

neil_kaye · 2nd July 2006, 03:14 PM

All right, but i still am not quite sure how to choose the amp rating of the fuse. If the torroid has an maximum draw of 2.7 amps do i choose a fuse with a rting slightly higher such as product GDC-3.15A?

Cheers and thanks again

AndrewT · 2nd July 2006, 05:02 PM

Hi,
I would put in the smallest fuse that allows the amp to work over a wide range of operating conditions.
Theoretically a 120VA on 120/110Vac will run on a T1A fuse but I doubt it will start on a fuse that low.

It may run on T1.6A or T3.1A or the two in between.

I would not recommend T4A.

The closer the fuse rating to the actual operating current the more likely it will blow when the amp/operator misbehaves.
Similarly the lower the fuse rating the less heat/energy generated when it does go wrong.

cliffforrest · 2nd July 2006, 08:29 PM

With regard to the LED.

That 20-30mA is more likely to be the top end of the working range and will be incredibly bright!

Experiments are in order, but I would not normally run an indicator LED at more than 5mA.

On your 30v(ish) rail that would need 5K6, and a lot less power to waste and worry about!

LEDs vary enormously in efficiency and brightness for a given current from type to type.

2nd July 2006, 01:10 AM	#1
neil_kaye diyAudio Member Join Date: Dec 2005 Location: San Francisco	Newbie question regarding fuse's and resistor ratings Hi, I have several questions regarding resitor, LED and Fuse selection thaty i was hoping someing might help me with: 1. I have a transformer that is rated 120VA with 22VAC secondaries. The rectified voltage I calculated will be 31.1 Volts (120VAC * 1.414). For use with a LM3875 opamp I would like to use a red LED with forward voltage range of 2.2Vdc 20-30ma http://www.partsexpress.com/pe/pshow...70-015&scqty=6 I calculated that the resistor i would need is (33.1-2.2)/0.02= 1.4K. So i chose a 1.5kOHM 1w resistor http://www.partsexpress.com/pe/pshow...3-1.5K&scqty=1 Is this the correct resistor to be using? This set up is for a chipamp. Would it be better to use a 1.5 ohm 12w Mills type of resistor for this applciation: http://www.partsexpress.com/pe/pshow...number=005-1.5 How do i connect the LED to a dual secondary transformer? 2. I would like to us a power jack with 10a built in fuse: http://www.partsexpress.com/pe/pshow...90-443&scqty=2 The fuse i selected is a 4a 3AG slow blow fuse. Would this work? The concern i have is that the IEC says that i need to use a fuse rated at 250v. However i can't find a 250v fuse in slow blow so can i use a 32v rate slow blow fuse with this IEC? Thanks for your help

2nd July 2006, 08:39 AM	#5
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, thanks for that Cooper/Bussman link. that's the first time I've noticed the Voltage drop across the fuse at rated current. 65mV to 75mV for all the fuses I have used! If only all manufacturers were as informative. __________________ regards Andrew T.

2nd July 2006, 05:02 PM	#7
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, I would put in the smallest fuse that allows the amp to work over a wide range of operating conditions. Theoretically a 120VA on 120/110Vac will run on a T1A fuse but I doubt it will start on a fuse that low. It may run on T1.6A or T3.1A or the two in between. I would not recommend T4A. The closer the fuse rating to the actual operating current the more likely it will blow when the amp/operator misbehaves. Similarly the lower the fuse rating the less heat/energy generated when it does go wrong. __________________ regards Andrew T.

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2nd July 2006, 01:25 AM	#2
poobah diyAudio Member Join Date: Nov 2005	The first resistor you mention is correct. I don't know what the second resistor is about. What is the LED for? If it is for an "ON" indicator, you simply connect the LED/resistor across one of the large caps following the rectifier. HERE is one of several fuses that fit the bill. The voltage rating of a fuse is known as the interrupting rating. The rating of the fuse should be greater than the voltage the fuse would see if it were open (in this case 120V).

2nd July 2006, 02:04 AM	#3
neil_kaye diyAudio Member Join Date: Dec 2005 Location: San Francisco	Hi poobah, Thanks for the response. I still don't quite understand the fuse situation. Is the fuse that i origianlly chose incorrect? Is there a slow blow fuse that i can use with the IEC i chose? All the slow blow fuses on the parts express website seem have an interupting rating of 35V? Thanks for your help.. .this is all new to me

2nd July 2006, 02:09 AM	#4
poobah diyAudio Member Join Date: Nov 2005	Yeah dude... there is a fuse. Click where "Here" is highlighted in blue in the last post.

2nd July 2006, 03:14 PM	#6
neil_kaye diyAudio Member Join Date: Dec 2005 Location: San Francisco	All right, but i still am not quite sure how to choose the amp rating of the fuse. If the torroid has an maximum draw of 2.7 amps do i choose a fuse with a rting slightly higher such as product GDC-3.15A? Cheers and thanks again

2nd July 2006, 08:29 PM	#8
cliffforrest diyAudio Member Join Date: Sep 2004 Location: UK	With regard to the LED. That 20-30mA is more likely to be the top end of the working range and will be incredibly bright! Experiments are in order, but I would not normally run an indicator LED at more than 5mA. On your 30v(ish) rail that would need 5K6, and a lot less power to waste and worry about! LEDs vary enormously in efficiency and brightness for a given current from type to type.

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