putting a zener in parallel with a 7805

Thread Starter

mikewax

Joined Apr 11, 2016
118
hi guys,
i got one of those Bird-style electric scooters and i want to replace the battery pack of 29.4V with a bigger pack of 37.8V.
my problem is that the scooter's motor control unit has a uController, which runs off of a ST L78M05 regulator. i looked up the regulator and it turns out that it can regulate from an input of no more than 35v. if i give it an input of 37.8v it's bound to fail before long.
so what if i get a 30 volt zener and solder the anode of the zener to the input pin of the regulator, the cathode of the zener to a resistor, and then connect the other end of the resistor to ground?
i'm estimating the load on the 7805 regulator at no more than about 50mA.
 

Papabravo

Joined Feb 24, 2006
12,576
hi guys,
i got one of those Bird-style electric scooters and i want to replace the battery pack of 29.4V with a bigger pack of 37.8V.
my problem is that the scooter's motor control unit has a uController, which runs off of a ST L78M05 regulator. i looked up the regulator and it turns out that it can regulate from an input of no more than 35v. if i give it an input of 37.8v it's bound to fail before long.
so what if i get a 30 volt zener and solder the anode of the zener to the input pin of the regulator, the cathode of the zener to a resistor, and then connect the other end of the resistor to ground?
i'm estimating the load on the 7805 regulator at no more than about 50mA.
Compute the power dissipation as (30-5)*0.050 = 1.25 watts. You're going to need a pretty good heat sink or it will pop. Try a buck regulator.
 

ElectricSpidey

Joined Dec 2, 2017
739
hi guys,
i got one of those Bird-style electric scooters and i want to replace the battery pack of 29.4V with a bigger pack of 37.8V.
my problem is that the scooter's motor control unit has a uController, which runs off of a ST L78M05 regulator. i looked up the regulator and it turns out that it can regulate from an input of no more than 35v. if i give it an input of 37.8v it's bound to fail before long.
so what if i get a 30 volt zener and solder the anode of the zener to the input pin of the regulator, the cathode of the zener to a resistor, and then connect the other end of the resistor to ground?
i'm estimating the load on the 7805 regulator at no more than about 50mA.
That won’t work.

I would place a properly rated Zener in series with the positive input of the regulator instead.

Say 5 volt 1 watt…assuming the current requirement is as you said. You could also use a 5 watt Zener if you have any doubt.
 

Thread Starter

mikewax

Joined Apr 11, 2016
118
Compute the power dissipation as (30-5)*0.050 = 1.25 watts. You're going to need a pretty good heat sink or it will pop. Try a buck regulator.
so that's why the existing regulator is a DPAK. a buck would certainly be better but i'm just tryin to do the quick n dirty solution. ideally i would be able to unsolder the 7805 from the board and just get another higher-rated one. but that's not an option with such a small board.
thanx
 

danadak

Joined Mar 10, 2018
3,631
If you have Iz =x, Vz = y, then P = Iz x Vz.

At 50 mA, say Vz 5.1V, that only 255 milli watts. 1/2 W Zener would be fine.


Regards, Dana.
 
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