Dimming the lamp

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Doc ID 5656 Rev 10

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Dimming the lamp

The lamp is dimmed by changing the working frequency. The minimum working frequency

is:

Equation 18

To change the working frequency, one has to change the current that flows from pin 4. Using

the circuitry shown in Figure 15, the current that flows from pin 4 also depends on the

Equation 19

So the working frequency will be:

Equation 20

For example, we can calculate a maximum frequency of ~53 kHz (V4 = 2 V and assuming

Vdiode3 = 0.5 V).

The dimming level is set by changing the value of the P1 potentiometer, which causes the

amplifier’s positive reference to change from 20-30 mV to 110-120 mV. If R16 is lowered, the

maximum working frequency is increased and the dimming level lowered as well (the higher

the frequency, the lower the current in the lamp’s arc).

If one tries to dim the lamp towards a low power range (<20% of the electrical arc power) a

common effect is the presence of stationary waves along the lamp tube and/or some

flickering effects.

A common trick to get rid of these disturbances is to add a small continuous current flow

inside the lamp (a few mA). The easiest way to do this is to add a resistor in parallel to the

half battery capacitor (C17 in Figure 13 and Figure 18). For instance, one can add 50 k

Ω

(I = 200 V/50 k

Ω = 4 mA), which is already very effective. Of course, the resistor has to be

able to sustain the power dissipation caused by the current flow, so it is common to use

many resistors connected in parallel.

Remember that this is only a tip. It helps but is not enough, and should be used together

with the right frequency settings that must be chosen according to the type of lamp.

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