R5 on LCD broken?

I have an LCD screen (http://www.mouser.com/Search/ProductDetail.aspx?R=HDM16216L-S-L30Svirtualkey68700000virtualkeyHDM16216L-S-L30S) that I attempted to mount in my project box. When I did an initial power up test, before I installed the mounting nuts & bolts, it worked just fine and displayed what it was supposed to display. I failed to notice before installing the nuts & bolts just how close some of the components were to the mounting holes, and I think I damaged / destroyed one. A picture is attached. Apologies for posting a full-res photo, but I wanted the potential damage to be seen clearly. I suspect that I caused R5 to fall out.

Now after I power on my project, the LCD only shows vertical lines on the screen. When I compared the resistor layout to a different LCD screen I have, R5 is populated with a 2.2 k resistor. In fact, R1, 2, 4, & 5 are populated with 2.2 k. On my smaller (damaged LCD screen), R1, 2, & 4 are populated with 6.8 k resistors. R3 is a 0 ohm resistor on both.

Does anyone know what function R5 has on a standard LCD module? The closest value resistor I have on hand to potentially replace it with is 4.7 k or 10 k, will one of them suffice?

Sorry your talking about the resistors on the back should of looked at your pic sure thats not part of the back light.

Two of mine are marked 222 for R5

Yes, though that is a different module to those I have seen (from eBay), the five resistors totalling 11k, define the voltage steps for the LCD multiplexing, fed from the contrast control.

In case it is not obvious from this description, the value is critical and it must be replaced with a 2k2 ("222") resistor, - SMD if you have one, but you could solder an "ordinary" resistor (1/8 watt perhaps) to the points if you are careful.

Paul__B: Yes, though that is a different module to those I have seen (from eBay), the five resistors totalling 11k, define the voltage steps for the LCD multiplexing, fed from the contrast control.

In case it is not obvious from this description, the value is critical and it must be replaced with a 2k2 ("222") resistor, - SMD if you have one, but you could solder an "ordinary" resistor (1/8 watt perhaps) to the points if you are careful.

My display has 6.8k for the other resistors. The only 6.8ks I have on hand are 1/2 W through holes, much too large. I have no 1/8 W resistors at all. The closest SMDs I have are 0805 10k and 4.7k.

Will using the wrong value damage the display? I soldered a 4.7 k in and the display seems to work just fine, although the OFF segments are a little uneven in their darkness. That's something I can live with if that's the only effect it will have.

Jiggy-Ninja: Will using the wrong value damage the display? I soldered a 4.7 k in and the display seems to work just fine, although the OFF segments are a little uneven in their darkness. That's something I can live with if that's the only effect it will have.

No, no real harm should happen, as the segments are still driven with AC-signals. The only thing that will hurt the LCD-glass is DC.

Put the 4k7 on the board, and order the correct replacement.

// Per.

Jiggy-Ninja: My display has 6.8k for the other resistors.

So it does - went back and looked at your ridiculous photograph - it does show that. Even more odd, R3 is zero.

Jiggy-Ninja: The only 6.8ks I have on hand are 1/2 W through holes, much too large. I have no 1/8 W resistors at all. The closest SMDs I have are 0805 10k and 4.7k.

Will using the wrong value damage the display? I soldered a 4.7 k in and the display seems to work just fine, although the OFF segments are a little uneven in their darkness. That's something I can live with if that's the only effect it will have.

Yes, using the wrong value will potentially damage the display because it will introduce a DC bias to the excitation voltages. If you genuinely perceive a difference in the behaviour, that is your proof. A DC bias would - amongst other things - show up as uneven illumination of the "OFF" segments.

Zapro: No, no real harm should happen, as the segments are still driven with AC-signals.

Sorry about that. Wrong! :astonished: Sit and think about it awhile.

It is the same as in one of my UPSs. They have more than one output FET in parallel. With one burnt out, it still produces AC but ... burns out power transformers in "Plug Packs".

Zapro: Put the 4k7 on the board, and order the correct replacement.

Most certainly use the correct replacement ASAP.

Interesting and entirely valid alternative - replace all the 6k8 with 4k7 and then forget it.

Paul__B:

Jiggy-Ninja: Will using the wrong value damage the display? I soldered a 4.7 k in and the display seems to work just fine, although the OFF segments are a little uneven in their darkness. That's something I can live with if that's the only effect it will have.

Yes, using the wrong value will potentially damage the display because it will introduce a DC bias to the excitation voltages. If you genuinely perceive a difference in the behaviour, that is your proof. A DC bias would - amongst other things - show up as uneven illumination of the "OFF" segments.

I would guess, that the small imbalance in the bias (in the scope of a 3$ display), it wold not be the end of the world. If something did happen in the long run.

If one of the resistors of the stepped voltage-divider is off, i would think the others would change too, rendering the net bias even.

I have no personal experience with muxed displays, i only designed circuits with static backplanes.

// Per.

I ordered a new display from Newhaven that looks like it has the resistors in a better spot than this Hantronix one, so I should be able to mount it with more than one screw.

Thank you everyone for your help.