48V LED Voltage Indicator - seriously under reading

Thread Starter

cork_ie

Joined Oct 8, 2011
428
Below is the schematic of a 48V LED Voltage indicator. It seems to work but is only displaying one red led at 50.1V.
It should display at least three green led's at this voltage, i.e. fully charged.
While I understand the basic cascade for the led control, I am unsure how the BC557 and the 2N2905A, connected back to back, work. It doesn't make sense to me why the 2N2905 has a heatsink attached. I am also unsure if any of the two diodes are Zeners , but I suspect not .They are tiny and I am unable to read them. Both are glass with a black stripe at the cathode.
All components seem to be at their correct values and I have thoroughly cleaned the PCB .
I would like to be able to repair it to read correctly. I also wonder is it possible to insert a trim pot where the 33K resistor is located .to allow adjustment in service. Thanking you all in anticipation.



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AlbertHall

Joined Jun 4, 2014
8,999
I think the diode nearest to the battery on the diagram is a zener.
But what is the squiggle in series with that diode?

The two transistors form a constant current supply for the LEDs (about 27mA).

The voltage at which it works is largely set by that zener diode.
 

Thread Starter

cork_ie

Joined Oct 8, 2011
428
Thanks for your reply.
I will look up some constant current circuits to get familiar with the concept. Hopefully that will be of help.
The squiggle in series appears to be a kind of fusible link etched on the PCB.
 

Alec_t

Joined Sep 17, 2013
10,700
Here's a sim of the circuit, which indicates the input voltage needs to vary over a range of about 8V to get all LEDs to operate in turn. The start of the range depends on the Zener voltage (and/or the value of R2). I've tabulated some ranges for Zener values of 30V, 33V and 36V as examples.
7LEDVoltmeter.PNG
 

Thread Starter

cork_ie

Joined Oct 8, 2011
428
Here's a sim of the circuit, which indicates the input voltage needs to vary over a range of about 8V to get all LEDs to operate in turn. The start of the range depends on the Zener voltage (and/or the value of R2). I've tabulated some ranges for Zener values of 30V, 33V and 36V as examples.
View attachment 163594
Thank you very very much Alec for going to so much trouble. I really appreciate it. I know how long it took me to reverse engineer the board and draw the schematic and guess that you have put a lot of effort into this to help me. I will try and change out the zener if I can locate one easily, failing that I will try and reduce the value of R2 gradually to see if I can get it to read correctly.

Thanks also to Albert Hall for setting me on the right direction. I have been reading up earlier this afternoon and now know how to make a constant current source using two transistors. The entire schematic makes perfect sense to me now.
 
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