Positive Linear Voltage Regulator - Variable or Fixed?

[daimoku]

I was testing this voltage regulator and accidentally shorted the pins which blew the 3A fuse. I'm ordering some replacement components and I want to order a replacement regulator just in case.

Is this regulator variable or fixed output?

I understand it's a positive linear regulator. Before I shorted the leads, I measured 2.613V on Vout and 5.19V on Vin. Is it safe to assume this is a 2.5V or 2.6V fixed output?

As you can see in the picture, it says "CAC 1084 0727CL." Thanks a bunch!

[daimoku]

Re: Positive Linear Voltage Regulator - Variable or Fixed?

Here are the potential replacements I was looking at:

Link 1
Link 2

[Pyr0Beast]

Re: Positive Linear Voltage Regulator - Variable or Fixed?

Adjustable
LDO probably

[Pyr0Beast]

Re: Positive Linear Voltage Regulator - Variable or Fixed?

Here you have datasheets.

http://www.datasheetcatalog.com/data.../AME1084.shtml

You could use a fixed one. Just that you would need to remove those resistors and connect directly to ground.

[kaboom]

Re: Positive Linear Voltage Regulator - Variable or Fixed?

That looks like it's adjustable. If fixed, it would say '1084-xxx' where xxx was the voltage. 1084-2.5 if 2.5V non-adjustable. Also, the two resistors tell the tale...

I'd remove it and check for shorts in your 2.5V load(s). If you shorted 'VI' to 'VO' and put 5V into the 2.5V side, parts may be damaged.

I assume the fuse is in the 5V feed.

It was probably designed for 2.5 volts out, but because of tolerances of the 'adjust' resistors, ended up a little off. I'd use an LM1084-ADJ or just LM1084. A fixed reg won't have a ground on the leftmost pin if used on this board. Just use a short jumper.

-Paul

PS- ya beat me, Pyr0

[Pyr0Beast]

Re: Positive Linear Voltage Regulator - Variable or Fixed?

Hhehe )

[severach]

Re: Positive Linear Voltage Regulator - Variable or Fixed?

Measuring the voltage on the pin labeled "GND" would have told you. ground=0V=fixed, >0V=adjustable. If you don't want to power the unit back on you can measure resistance from 1084-GND to ground.

Labeling the pin "GND" sure suggests it's fixed.

But it's not. It's an adjustable regulator. R43 and R40 give it away. Look at a datasheet for any of the linear regulators: 1084, 1117, 1581, 1585, AME8807, LM117, LM217, LM317. For the adjustable variants you'll see the two resistors in that configuration. The fixed regulators ground the pin labeled "GND."

Linear regulators are heavily protected and are hard to burn out. They should handle short circuits easily without blowing the fuse. They are also easy to test. 2.6v out, it works! Low ripple? The caps work too.

Shorting 5v to 2.6v is more likely to burn out connected chips than the regulator. If a chip that uses 2.6v and 5v shorted 5v to 2.6v to ground then the fuse would burn. Low voltage chips are quite intolerant of over voltage. Don't order any parts until you are certain the 2.6v and other supplies aren't shorted to ground or each other.

A bench supply would allow you to power these circuits and measure the current without the uncertainty that the voltage will be too high or too low.

Adapting the circuit to use a fixed 2.5v regulator isn't a good idea. The voltage is probably boosted intentionally. They wouldn't have spent the money on those two resistors if a fixed part would do. D-Link boosted a lot of 1.5v supplies to 1.62v. You need to do a lot of quality testing if you change the voltage.

[daimoku]

Re: Positive Linear Voltage Regulator - Variable or Fixed?

It appears we have a unanimous vote...thanks! I went ahead and ordered the adjustable one.

Thanks for teaching me about the resistors. I saw those in the datasheet, but I didn't really understand that the resistors adjust the regulator output until you mentioned it again. I'm still learning.

Paul - you're correct. The blown fuse is in the 5V feed.

I'm not really sure how to trace the 2.5V Vo circuit from this point without a schematic. The traces aren't visible...they go to a copper hole (for lack of a better description) and that goes somewhere else, but there's no visible trace to follow.

[daimoku]

Re: Positive Linear Voltage Regulator - Variable or Fixed?

If you don't want to power the unit back on you can measure resistance from 1084-GND to ground.

I measure 120 Ohms from 1084-GND to the ground where the cable connects.

A bench supply would allow you to power these circuits and measure the current without the uncertainty that the voltage will be too high or too low.

I'll have to add that to my list of things to get.

Thanks for the advice. I went with the adjustable regulator.

[Pyr0Beast]

Re: Positive Linear Voltage Regulator - Variable or Fixed?

Maybe they got a bulk discount on adjustable regulators, who knows. I've actually rarely seen fixed regulators in cheaper networking equipment.

[daimoku]

Re: Positive Linear Voltage Regulator - Variable or Fixed?

Yeah, I bet they bought a boatload of variable regulators at a steal haha

What specs are most useful in a variable power supply in your experience? I was looking at one that goes up to 30V, 5A. Shouldn't that be sufficient for about everything? Thanks!

[severach]

Re: Positive Linear Voltage Regulator - Variable or Fixed?

Maybe they got a bulk discount on adjustable regulators, who knows. I've actually rarely seen fixed regulators in cheaper networking equipment.

Maybe, but when buying 50,000 you're already getting all the bulk discount you can get. Ordering 500,000 so you have extras for the next 9 production runs is going to waste extra savings in stock cost.

I've seen plenty of both but what I haven't seen is an adjustable linear regulator set to a voltage that is available in a fixed linear regulator. The two resistors don't just add to the BOM, they eat up slots in the pick-n-place machine and at least one of them is a precision resistor that isn't useful anywhere else. Adjustable SMPS regulators often duplicate fixed voltage SMPS regulators.

I bought a HY1505D 15V 5A linear and immediately discovered it was too small for some jobs. 5A is too small to power automotive electronics and 15v is too small to power 24v devices. Nothing wrong with buying one too small. It works for most things and once you get one, you'll see that having more than one is useful too. Someday I'll get the HY3010E Mastech. Those (defective) 1 ohm power leads will have you thinking your supply is broken until you figure out where the fault lies.

[Pyr0Beast]

Re: Positive Linear Voltage Regulator - Variable or Fixed?

>The two resistors don't just add to the BOM, they eat up slots in the pick-n-place machine and at least one of them is a precision resistor that isn't useful anywhere else.

So do two regulators c c c.. ..

[daimoku]

Re: Positive Linear Voltage Regulator - Variable or Fixed?

I bought a HY1505D 15V 5A linear and immediately discovered it was too small for some jobs. 5A is too small to power automotive electronics and 15v is too small to power 24v devices. Nothing wrong with buying one too small. It works for most things and once you get one, you'll see that having more than one is useful too. Someday I'll get the HY3010E Mastech. Those (defective) 1 ohm power leads will have you thinking your supply is broken until you figure out where the fault lies.

Thanks. I guess "everything" is too general haha I am mostly looking at computer/tv applications, but it could be handy for those other uses too.

Replaced the fuse, powered the tv on and got the same symptoms as before the fuse blew. Replaced one voltage regulator LM1117MP-ADJ and the tv is alive! Thanks again for the edumacation!