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I have a VFD with a control board I am trying to fix. It requires an external variable resistor to be connected between the +10V terminal, analog reference terminal and ground. I found that on connecting the VR between the +10V terminal and ground, the voltage at the +10V terminal drops from 10V to around 8.78V. However, a similar board did not behave in this manner. What could possibly be the cause of this?

Many thanks in advance.

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    \$\begingroup\$ To answer the question, we'd need a schematic of the control board. If you have one, please add it to the question. If you don't, the question is off-topic because it's about using a device rather than designing one. \$\endgroup\$ – The Photon Jun 7 at 18:55
  • \$\begingroup\$ Unfortunately, I don't have a schematic of the control board. However, I would like to know generally why Vcc would drop on connection a voltage divider between it and ground. \$\endgroup\$ – Topa Jun 7 at 19:14
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    \$\begingroup\$ Then all we can say is "either it's broken, or it's not designed to supply as much current as your voltage divider is drawing". \$\endgroup\$ – The Photon Jun 7 at 19:55
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I can't say what the cause is without more information, but since you ask "What could possibly be the cause of this?" I'll bite...

The cause is that more current is being drawn than the supply can give, so the voltage is drooping. You say that a "similar" board doesn't have the problem--do you mean a board with an identical schematic and layout? If so, the problem is less likely to be with the design, and more likely to be a flaw in the physical board or components.

Start by checking the actual resistance across the resistor. Also, look for things like soldering problems that could cause a partial short between VDD and GND.

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