- May 11, 2008
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I am helping a friend of mine with his bench supply. To read out the voltages and currents, he bought lcd panel meters of model PM435. He downloaded a manual how to connect them and did as stated in the (correct) manual but for some reason, the diode bridge of the supply burned out and the panel meters produced a plume of smoke. He asked me to look at it and help him out with the schematics. As it turns out, he did nothing wrong , and the manual was sort of correct. But As i discovered today, there was an assembly error made by the manufacturer and solved it very creatively. Unfortunately they supplied the default cables with it. And that is why it went wrong when connected as expected.
http://weihuameter.en.made-in-china...Ur/China-PM435-3-1-2-Digital-Panel-Meter.html
What happened was the following : The led for the backlight is placed with the wrong polarity on the pcb. Thus anode on the negative terminal and the cathode on the positive terminal. It does have a series resistor of 560 Ohm so it can be connected safely to a +9V supply (Which is also the supply voltage for the meter).
The manufacturer solved this creatively by placing the socket A2 upside down on the pcb. They did this so that the supplied cables with connectors and polarity tabs would all have the same designation : The red wire is the positive wire and the black wire is the negative wire.
But in doing so they forgot one thing : The positive terminal of the socket A2 is directly connected to the V+ pin of socket A1(lcd supply voltage connection) on the pcb it self by a copper trace. And the V+ pin of socket A1 supplies the circuit of the lcd panelmeter.
But by mounting the socket A2 upside down, the black wire of the supplied cable is now connected to the positive V+ supply of the lcd panel meter.
And the red wire is connected to the anode of the backlight led.
So when using the supplied cables for the lcd meter and the backlight (red on red , black on black for both cables, red to +9V and black to ground) and connecting them to the powersupply , the powersupply for the panel meter and the backlight is shorted. If the powersupply is strong enough, a copper trace on the lcd panelmeter pcb will be burned and damaged, resulting in a open connection. The lcd meter no longer works. And this what happened when my friend powered the circuit.
The copper trace was burned out, leaving a open connection.
I solved that by soldering a small wire, bypassing the burned copper trace.
Connecting the + terminal of connector A1 to the positive terminal of (not placed) capacitor C9 on the pcb. Restoring the electrical connection.
When you have a PM435 meter with this issue, do the following :
Connect the powersupply of 9V to the lcd panel meter, thus red wire is +9V and the black wire is Ground and insert the cable with connector into socket A1.
. Do not connect the black wire of the led backlight on connector A2. Only connect the red wire of the led backlight (socket A2) to the +9V.. This way, the blue backlight led will lit up and the circuit will power up and the lcd panelmeter will show a value. The cathode of the led is already connected to the Ground connection of socket A1 of the LCD panel meter circuit through a series resistor of 560 Ohm.
EDIT:
Another alternative is to connect the red wire and black wire together from one of the supplied cables and then connect this connector to socket A2 for the led backlight. This way, the +9V on socket A2 is connected to the anode of the backlight led directly through the cable.
Connector A3 is the input of the lcd meter. Default this input accepts voltages up to +/-200mV.
Please note, these lcd panel meters need a floating supply with respect to the measured voltage connected to connector A3. This means that the ground and the positive voltage on connector A1 and A2 need there own separate 9V supply. If you do not fulfill this requirement, the meter will just show -1 all the time.
A happy (repaired) lcd panel meter :
The manufacturer als placed the lcd cover backwards. So i reversed it and now i have a shiny black cover.
Sorry for the weird pictures with a huge canvas, i forgot to resizes the canvas at first and uploaded the pictures to photobucket. As soon as i noticed, i had forgotten the canvas resize, i deleted them all. Modified the pictures and then uploaded them again to photobucket. But for some reason, when i delete something and then upload a picture with the same name, photobucket uses the old pictures instead of the new. Maybe it will solve itself later on.
http://weihuameter.en.made-in-china...Ur/China-PM435-3-1-2-Digital-Panel-Meter.html
What happened was the following : The led for the backlight is placed with the wrong polarity on the pcb. Thus anode on the negative terminal and the cathode on the positive terminal. It does have a series resistor of 560 Ohm so it can be connected safely to a +9V supply (Which is also the supply voltage for the meter).
The manufacturer solved this creatively by placing the socket A2 upside down on the pcb. They did this so that the supplied cables with connectors and polarity tabs would all have the same designation : The red wire is the positive wire and the black wire is the negative wire.
But in doing so they forgot one thing : The positive terminal of the socket A2 is directly connected to the V+ pin of socket A1(lcd supply voltage connection) on the pcb it self by a copper trace. And the V+ pin of socket A1 supplies the circuit of the lcd panelmeter.
But by mounting the socket A2 upside down, the black wire of the supplied cable is now connected to the positive V+ supply of the lcd panel meter.
And the red wire is connected to the anode of the backlight led.
So when using the supplied cables for the lcd meter and the backlight (red on red , black on black for both cables, red to +9V and black to ground) and connecting them to the powersupply , the powersupply for the panel meter and the backlight is shorted. If the powersupply is strong enough, a copper trace on the lcd panelmeter pcb will be burned and damaged, resulting in a open connection. The lcd meter no longer works. And this what happened when my friend powered the circuit.
The copper trace was burned out, leaving a open connection.
I solved that by soldering a small wire, bypassing the burned copper trace.
Connecting the + terminal of connector A1 to the positive terminal of (not placed) capacitor C9 on the pcb. Restoring the electrical connection.
When you have a PM435 meter with this issue, do the following :
Connect the powersupply of 9V to the lcd panel meter, thus red wire is +9V and the black wire is Ground and insert the cable with connector into socket A1.
. Do not connect the black wire of the led backlight on connector A2. Only connect the red wire of the led backlight (socket A2) to the +9V.. This way, the blue backlight led will lit up and the circuit will power up and the lcd panelmeter will show a value. The cathode of the led is already connected to the Ground connection of socket A1 of the LCD panel meter circuit through a series resistor of 560 Ohm.
EDIT:
Another alternative is to connect the red wire and black wire together from one of the supplied cables and then connect this connector to socket A2 for the led backlight. This way, the +9V on socket A2 is connected to the anode of the backlight led directly through the cable.
Connector A3 is the input of the lcd meter. Default this input accepts voltages up to +/-200mV.
Please note, these lcd panel meters need a floating supply with respect to the measured voltage connected to connector A3. This means that the ground and the positive voltage on connector A1 and A2 need there own separate 9V supply. If you do not fulfill this requirement, the meter will just show -1 all the time.
A happy (repaired) lcd panel meter :
The manufacturer als placed the lcd cover backwards. So i reversed it and now i have a shiny black cover.
Sorry for the weird pictures with a huge canvas, i forgot to resizes the canvas at first and uploaded the pictures to photobucket. As soon as i noticed, i had forgotten the canvas resize, i deleted them all. Modified the pictures and then uploaded them again to photobucket. But for some reason, when i delete something and then upload a picture with the same name, photobucket uses the old pictures instead of the new. Maybe it will solve itself later on.
Last edited:
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