This one may be a little more interesting.
I'm trying to figure out the simplest way to dim some instrument gauges on a patrol boat.
The boat has 2 engines (inboard) and is not using any CAN for monitoring. Everything is analog.
There is a Blue Sea System dimmer (7504) that dims using 128Hz PWM. Some of the gauges have incandescent backlighting and work fine using PWM.
The 7504 dimming is controlled by a (on) off (on) spdt switch.
There are 3 VDO gauges per motor that can not dim using PWM. This has be confirmed by the factory as well as bench testing. Dimming of these can only be accomplished by varying DC voltage on the illumination circuit in the wire harness.
I can't add any switches to the control panel of the boat and any solution needs to be IP67. The dimming switch currently installed could be replaced with a dpdt switch so 2 independent devices/circuits could be controlled.
I have built a 1st order LPF that didn't work and a 2nd order would require very large LC values.
This boat is in the eastern part of the world and I don't want to make it complicated i/e using large circuits with opamps etc....
If a good dimming solution can't be found, the last resort is to connect the gauge illumination to a blackout switch already in the boat.

Does anyone have any ideas?
I appreciate any input.

 

How much current (per gauge), and what voltage, does the VDO gauge illumination need at full brightness? What voltage at the required dim level?

 

For low values of current an RC LP filter with a sufficiently large capacitor should work.

For higher currents a inductor-capacitor filter could be used.
To minimize inductor size, a higher frequency PWM signal would be needed.

How did you determine the PWM frequency of the 7504 device?

 

Why does a PWM Signal not work on the VDO Gauges ?

An "On/Off" Switch is just an extremely low Frequency PWM-Signal,
so, at what Frequency does the Illumination-Input STOP working ?

Do your Gauges even have a separate Illumination-Input-Wire ?

Please post the PDF File for the particular Model of VDO-Gauges that You are working with.
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How much current (per gauge), and what voltage, does the VDO gauge illumination need at full brightness? What voltage at the required dim level?

That would be my question. Consider long before we ever knew what PWM was. Automotive dash board illumination was controlled by a simple variable resistor controlling incandescent lamps, plenty of them.

Ron

 

How much current (per gauge), and what voltage, does the VDO gauge illumination need at full brightness? What voltage at the required dim level?

I haven't metered it but the manual says <200mA nominal... at 24VDC I would expect it to be <100mA
These are 24V gauges so dimming from 0 to 24Vdc

For low values of current an RC LP filter with a sufficiently large capacitor should work.

For higher currents a inductor-capacitor filter could be used.
To minimize inductor size, a higher frequency PWM signal would be needed.

How did you determine the PWM frequency of the 7504 device?

I did considered low vs high LC values. The PWM was measured with a scope. 7.8ms period was measured.

Why does a PWM Signal not work on the VDO Gauges ?

An "On/Off" Switch is just an extremely low Frequency PWM-Signal,
so, at what Frequency does the Illumination-Input STOP working ?

Do your Gauges even have a separate Illumination-Input-Wire ?

Please post the PDF File for the particular Model of VDO-Gauges that You are working with.
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The illumination LED is built onto the gauge main board. Maybe PWM would work at a much higher f but that is not an option.
There is a separate illumination wire.
I can't vary the f but at a certain duty cycle threshold, the illumination begins to flash and then goes out.

SingleViu
These are the 52mm gauges.

That would be my question. Consider long before we ever knew what PWM was. Automotive dash board illumination was controlled by a simple variable resistor controlling incandescent lamps, plenty of them.

Ron

I have even searched for a simple variable resistor that could be controlled with a (on) off (on) dpdt switch...

 

I haven't metered it but the manual says <200mA nominal... at 24VDC I would expect it to be <100mA
These are 24V gauges so dimming from 0 to 24Vdc



I did considered low vs high LC values. The PWM was measured with a scope. 7.8ms period was measured.


The illumination LED is built onto the gauge main board. Maybe PWM would work at a much higher f but that is not an option.
There is a separate illumination wire.
I can't vary the f but at a certain duty cycle threshold, the illumination begins to flash and then goes out.

SingleViu
These are the 52mm gauges.



I have even searched for a simple variable resistor that could be controlled with a (on) off (on) dpdt switch...

You will need some idea of the lamps used on the gauges and the current draw total. Play around with "dashboard illumination potentiometer old cars" gets me some hits. If I recall correctly these potentiometers were basic low resistance wire wound pots with an ON/Off switch built in. Low resistances like 10 Ohms and high wattage like 10 watts or greater. This would be as simple as it gets.

Ron

 

I have even searched for a simple variable resistor that could be controlled with a (on) off (on) dpdt switch...

If PWM doesn't work, and you can't find a suitable variable power resistor, then you might consider using a pot controlling an NPN power transistor as an emitter follower output to vary the DC voltage.
It's not efficient, as the transistor power dissipated will be the total current at the set voltage times the voltage difference between the 24V supply and the set voltage, which may require a heat-sink, but it will give a smooth, variable DC voltage.

 

You will need some idea of the lamps used on the gauges and the current draw total. Play around with "dashboard illumination potentiometer old cars" gets me some hits. If I recall correctly these potentiometers were basic low resistance wire wound pots with an ON/Off switch built in. Low resistances like 10 Ohms and high wattage like 10 watts or greater. This would be as simple as it gets.

Ron

Thanks for the reply.. unfortunately, I can't and any controls to the panel. Also, this is a harsh marine environment and the solution needs to be IP67 rated.

Total current of all lamps should be <600mA total.

 

If PWM doesn't work, and you can't find a suitable variable power resistor, then you might consider using a pot controlling an NPN power transistor as an emitter follower output to vary the DC voltage.
It's not efficient, as the transistor power dissipated will be the total current at the set voltage times the voltage difference between the 24V supply and the set voltage, which may require a heat-sink, but it will give a smooth, variable DC voltage.

If we could figure out how to use a (on) off (on) switch... that is the only way I can control any dimming solution for these VDO gauges.

 

Maybe PWM would work at a much higher f but that is not an option.

Can't you just add a simple '555-based circuit to chop up the existing 128Hz at, say, ~10kHz, making it easier to low-pass filter with small components? If you only have an on/off switch available to control the dimming then that could be used to select either full brightness or a pre-selected dim level.
level

 

The dimming switch currently installed could be replaced with a dpdt switch so 2 independent devices/circuits could be controlled.

Maybe I haven't understood the question correctly, but if the low voltage side of the 3 VDO bulbs are connected together and that is wired through a resistor to ground then the resistor would drop some voltage and if the switch is switched it would short the resistor so all the voltage is seen across the bulbs? The value of the resistor both in terms of ohms and power consumption would need to be suitably selected. To control the voltage more exactly (to avoid the changed brightness if one or more bulbs is daed) maybe an LM338. Not sure why IP67 is so important, but could be in the same enclosure as the switch?

 

I can't and any controls to the panel.

Okay.
If a fixed dimming level is okay then how about changing to a DPDT switch, with one pole controlling the PWM and the other pole connecting the LEDs either directly to the 24V or to the 24V through a power resistor.
You would likely have to experimentally determine the value and power dissipated in the resistor for the desired dimming, to select the correct power rating and resistance value.

 

I have built a 1st order LPF that didn't work and a 2nd order would require very large LC values.

Use a 2nd order RCRC filter.

 

Maybe I haven't understood the question correctly, but if the low voltage side of the 3 VDO bulbs are connected together and that is wired through a resistor to ground then the resistor would drop some voltage and if the switch is switched it would short the resistor so all the voltage is seen across the bulbs? The value of the resistor both in terms of ohms and power consumption would need to be suitably selected. To control the voltage more exactly (to avoid the changed brightness if one or more bulbs is daed) maybe an LM338. Not sure why IP67 is so important, but could be in the same enclosure as the switch?

Currently, a spdt (momentary) switch controls the PWM dimmer (7504) for the gauges that will dim.
I don't think a momentary switch could work the way it needs to.

 

Can't you just add a simple '555-based circuit to chop up the existing 128Hz at, say, ~10kHz, making it easier to low-pass filter with small components? If you only have an on/off switch available to control the dimming then that could be used to select either full brightness or a pre-selected dim level.
level

That could be a solution. I'm trying to keep it as simple as possible so far as circuitry goes
There are only 2 switches that are usable.
1. a momentary off momentary spdt which could be changed to a dpdt
2. a blackout switch that controls a solenoid which in turn cuts all navigation lights as well as some other lights.

 

Use a 2nd order RCRC filter.

RCRC could work.
I am blocked when clicking on that link.

 

Okay.
If a fixed dimming level is okay then how about changing to a DPDT switch, with one pole controlling the PWM and the other pole connecting the LEDs either directly to the 24V or to the 24V through a power resistor.
You would likely have to experimentally determine the value and power dissipated in the resistor for the desired dimming, to select the correct power rating and resistance value.

The switch is momentary.

 

RCRC could work.
I am blocked when clicking on that link.

Work out the first RC for the cutoff frequency. Then use 10xR and C/10 for the second RC. That gives a critically damped (Q≈0.5) second order filter.
Use the same values of R and C for both sections and you get Q=0.33
The combined response is 6dB down at the cutoff frequency.

 

Work out the first RC for the cutoff frequency. Then use 10xR and C/10 for the second RC. That gives a critically damped (Q≈0.5) second order filter.
Use the same values of R and C for both sections and you get Q=0.33
The combined response is 6dB down at the cutoff frequency.

I think I'm going to revisit this and give it another shot.