# putting a zener in parallel with a 7805

#### mikewax

Joined Apr 11, 2016
151
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
14,831
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.

Joined Mar 10, 2018
4,057
Put 5v zener in series with input......

Regards, Dana.

#### ElectricSpidey

Joined Dec 2, 2017
1,391
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.

#### mikewax

Joined Apr 11, 2016
151
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

#### mikewax

Joined Apr 11, 2016
151
Put 5v zener in series with input......
Regards, Dana.
DOH! thanx of course that's the solution.

#### mikewax

Joined Apr 11, 2016
151
That won’t work.
you're right it won't, i'm not thinking straight. i just gotta cut the lead on the DPAK package and put the zener across it.
thanx that's it

#### ElectricSpidey

Joined Dec 2, 2017
1,391
And remember the zener goes in bass ackwards...

#### Analog Ground

Joined Apr 24, 2019
446
and remember to get a zener with a high enough power rating.

Joined Mar 10, 2018
4,057
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.

#### mikewax

Joined Apr 11, 2016
151
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.
yeah that's what i'll do. soon as i get the battery pack.
thanx