26-03-2017, 11:03 AM
(26-03-2017, 09:06 AM)unrealdave Wrote: Well I put the PS together and got 6.3V out – Perfect! These sets ran on 6V batteries so when fully charged they had about 6.8V
I connected the set up, then pulled the on switch (LT).
I saw these on ebay '5W Zener Diode Voltage Regulator 6v'. is there a reason people use voltage regulators rather than these?
The mean level of a single lead acid cell is 2.1V, hence heater voltages of 6.3V and 12.6V rather than 6V or 12V.
The reason for using voltage regulators and why a 5W zener wouldn't work in your case is because you already had your 6V PSU up and running before you 'pulled the on switch'.
The load would be connected across the zener with both fed by a series resistor. Thus the combination of supply voltage and resistor value would allow both for the load current and the current through the zener. As the load current varies, the current through the zener varies in the opposite direction to keep the voltage constant or, more likely, it is the supply voltage which varies, so the zener passes more current as the voltage rises or less when it drops.
In your case, the zener would have to pass whatever current you designed it for plus the load current when the load is absent - in your case before you 'pulled the on switch'.
Without the load, the zener will have to dissipate 6*1.177W or 7.062 watts over and above its design rating, so you would need a pretty hefty zener for that!
Rather than shunting excess current as with the zener, a series voltage regulator acts as a variable series resistor which drops the difference between the supply voltage and the voltage required by the load. Therefore, the current through the regulator and hence the power dissipated is (virtually) zero if the load is removed, increasing as the load is added. Thus the maximum dissipation is equal to the maximum supply voltage minus the load voltage multiplied by the maximum load current.
If you had, say, a maximum supply voltage of 12V, say, the maximum dissipation would be (12-6.3)*1.177W or 6.7089W.
The regulator spec will include a minimum voltage drop figure. If you can be sure that the supply voltage can never dip below the input level this translates to, but is not much higher, the dissipation will be reduced.
