PSU "watchdog" with led indication

[goldvasser]

Hi there !

I have a few psu here and I would like to track they deviations with a "watchdog" circuit displaying the voltage with  3 leds marked " high, normal & low" with a settable hysteresis between the three states...
the voltage is on "normal" state between 4,9 to 5,1 volts.
Is there a simple way to achieve this ? My need is monitoring the " acceptable voltage " of the circuits plugged on psu's without a traditionnal voltage indication (certainly more understandable for non-technicians)

Thanks for your suggestions 

Cheers,
Bruno.

 

[sahib]

You need a simple voltage comparator. Three of them will give you three options. You can do that by either using an opamp or a dedicated voltage comparator. The principal is the same though. One input is tied to a reference voltage using a voltage divider. A trimpot will be easier to set. The voltage to be detected is applied to the other input. The comparator drives an LED. Depending on which input is used the LED either turns on or off. Below link has good tutorials. If you google as voltage detectors, voltage comparators etc you'll get tons and tons of circuits.

http://home.cogeco.ca/~rpaisley4/Comparators.html

 

[goldvasser]

Thanks a lot, Sahib !
I'll dig !!!
Bests
B.

 

[briomusic]

But where do you get your reference voltage from?
Another PSU?
And who is watching over that?

 

[owel]

You can use a voltage reference chip (some 2 terminals, some 3 terminals - check out Maxim).  Or a zener diode if you want to go simple.

 

[JohnRoberts]

If you have a number of PS to monitor, you can power the monitor circuit from a diode OR of all the outputs, so as long as one PS is up, the monitor circuit will get power. If all are dead that will be pretty obvious. 

I made a very simple crude threshold detector from a differential long tail pair of bipolar transistors (PNP) and a 3 lead bi-color LED (common cathode lead). With one transistor base connected to the PS output to be monitored and the other base to the reference voltage, when both are at the same voltage, both LED colors get 1/2 the current so LED lights up both red/green  (whatever color that makes). When the PS is either over voltage or under voltage by a few tens of mV, the LED is either red or green.   

Nowadays it's almost cheaper/easier to do this with a LM339 comparator than discrete parts.

JR

 

[nashkato]

that is similar to what i need/want to do right now.
i´m going to heat the preamp tubes of a Marshall clone with regulated and slowstarting 12,6VDC.
i want a LED to be lit when full 12,6 DC is reached.
this way i don´t have a reference voltage as long as i don´t rectify the 12VAC a second time ?
or is there a simpler solution to that ? (using a simple transistor ?)

 

[PRR]

> or is there a simpler solution to that ?

Yeah. Rectify and passive-filter.

Tube heaters do not need to be regulated or slow-started. You are just adding more parts to fail, then spending time "monitoring" the less-reliable design.

Anyway: It's a gitar amp. Either it works, or they yell at you. The users generally will NOT know what to do about a monitor light.

 

[coriolis]

It's a gitar amp. Either it works, or they yell at you

The ultimate in psu monitoring!  ;D

 

[CJ]

there will always be a spare amp in the house,
so use what you got until it blows up.
then borrow the house bass amp.
which will blow up.

 

[nashkato]

the only reason why i wanted to have such a "watchdog" is cause i´m using an old chassis with its faceplate which has an additional hole on the front i don´t need .
so what to do with that ? put in a "sophisticated" led. not for monitoring purposes but to give myself a little challenge than a simple tubeamp.
i agree it´s kind of overkill and that additional parts are more likely to fail .
and , yeah , the user will have to think " oh the led went from red to green i can turn on the standby " , though the Powertubes are most likely not warmed up yet .

Yeah. Rectify and passive-filter.

you made a compelling argument , i will do that , and just use a second led to cover up the hole in the faceplate.

thanks

 

[keefaz]

You can always use a resistor in serie with the led, or maybe better a zener diode, I think the led would light gradually though.