# A question regarding Step Down Transformers

#### B Bobby

Joined Oct 6, 2018
8
I need to order a custom step down transformer to output 16v 12amp
My area has power fluctuation which ranges between 210v to 245v, I am confused what should be the target voltage for input 230v or 240v or something else?

#### Hymie

Joined Mar 30, 2018
877

#### drc_567

Joined Dec 29, 2008
912
... Your target power level is approximately 200 VI ... volt-amps. However, you should order a 500 VI rated transformer ... so that you avoid heating inefficiency. The secondary voltage should be somewhat greater than the 16 volt target voltage. A 24 volt secondary has a peak value of 1.41* 24= 34 volts, which is a little high. A 12 volt secondary transformer has a peak sine voltage of about 17 volts, which may work, depending on the target requirements. The secondary of the transformer will produce 34 volts or 17 volts after it goes through a diode bridge rectifier and electrolytic filter capacitors are placed at the input of a voltage regulator of your choosing. The target voltage of 16 volts is a little unusual, so you may want to get another opinion as far as voltage regulation goes.

#### B Bobby

Joined Oct 6, 2018
8
... Your target power level is approximately 200 VI ... volt-amps. However, you should order a 500 VI rated transformer ... so that you avoid heating inefficiency. The secondary voltage should be somewhat greater than the 16 volt target voltage. A 24 volt secondary has a peak value of 1.41* 24= 34 volts, which is a little high. A 12 volt secondary transformer has a peak sine voltage of about 17 volts, which may work, depending on the target requirements. The secondary of the transformer will produce 34 volts or 17 volts after it goes through a diode bridge rectifier and electrolytic filter capacitors are placed at the input of a voltage regulator of your choosing. The target voltage of 16 volts is a little unusual, so you may want to get another opinion as far as voltage regulation goes.

Thanks for the detailed answer I was looking to build a manual charger for Flooded Lead Acid (FLA) Battery , I need upto 14.8v to 15v to equalize them therefore I was in perception that a 16v Transformer will work, Of course combined with bridge rectifier +Capacitor + Constant current Constant Voltage module
(I will buy off the shelf modules and just connect them together this is where my skills end)

#### Hymie

Joined Mar 30, 2018
877
If the thread starter is after 12Vdc at 16A, there are plenty of switch mode power supplies that will do the job.

#### B Bobby

Joined Oct 6, 2018
8
If the thread starter is after 12Vdc at 16A, there are plenty of switch mode power supplies that will do the job.
But my google searches suggest that a transformer based power supply is recommended for charging large FLA batteries that is why I was interested in transformer based solution. Can you please give your opinion about it?

#### Hymie

Joined Mar 30, 2018
877
No, a 16V transformer will give a peak of 22V after rectification – too much to charge a 12V battery; unless the Constant current Constant Voltage module can handle this.

#### drc_567

Joined Dec 29, 2008
912
... Charging lead acid batteries can be problematic sometimes. Leaving a constant current charging on the battery can lead to internal damage of some sort. ... If you want to be sure that charging is done correctly, leading to longer battery life, consider getting one of these devices ... used one for several years ... no problems.:

... don't know if they can be used on multiple batteries in parallel ... inquire to the manufacturer.

#### B Bobby

Joined Oct 6, 2018
8
... Charging lead acid batteries can be problematic sometimes. Leaving a constant charging on the battery can lead to internal damage of some sort. ... If you want to be sure that charging is done correctly, leading to longer battery life, consider getting one of these devices ... used one for several years ... no problems.:

... don't know if they can be used on multiple batteries in parallel ... inquire to the manufacturer.
I have one of these smart chargers but they can't be used to revive dead batteries

"Why smart and automatic battery chargers SUCK. manual charger may be better"

#### Hymie

Joined Mar 30, 2018
877
I have one of these smart chargers but they can't be used to revive dead batteries

"Why smart and automatic battery chargers SUCK. manual charger may be better"
In my experience, once a vehicle battery has reached the end of its service life – it cannot be resurrected, Lazarus like, by applying some voltage/current.

#### MrChips

Joined Oct 2, 2009
22,069
I have one of these smart chargers but they can't be used to revive dead batteries.
More often than not, a dead battery is just that, dead, and cannot be revived.

#### B Bobby

Joined Oct 6, 2018
8
More often than not, a dead battery is just that, dead, and cannot be revived.
Sometimes smart chargers simply refuse to charge a battery which somehow went below a certain threshold, one such incident can clearly be seen in the youtube video I attached above , this is where a manual charger can help to save such battery

#### nsaspook

Joined Aug 27, 2009
7,724
Sometimes smart chargers simply refuse to charge a battery which somehow went below a certain threshold, one such incident can clearly be seen in the youtube video I attached above , this is where a manual charger can help to save such battery
Can you abuse the battery chemistry with a manual charges to the point where some parts of the plates might reactivate due to hard sulfate flaking off due a few hot spots excessive heat? Yes, you can but it's crippled for life, with a hair trigger for death, not saved.

#### Hymie

Joined Mar 30, 2018
877
One of my earliest electronic projects was a PSU rated up to 30Vdc, 4A. Before its demise, I occasionally used it as a car battery charger (normally set on constant current).

But once a battery was dead, it was dead no matter what voltage/current was applied.

#### drc_567

Joined Dec 29, 2008
912
For a secondary transformer voltage of 12 VAC, giving a rectified, filtered voltage of 17 volts DC,.. A voltage regulated, zener diode circuit, producing 15 volts at 10 amps, is shown here:
https://www.electronics-tutorials.ws/diode/diode_7.html
.

A 15 volt zener regulating diode resulting in 15 volts regulated output, is available here:
https://www.digikey.com/product-detail/en/microsemi-corporation/1N3314A/1N3314A-ND/4377747

... The resistor, R, used in the circuit is calculated to be 0.20 ohms, and should handle at least 20 watts... due to the 10 Amp current specification

... A 0.2 ohm resistor @ 30 watts is listed here:
https://www.digikey.com/product-det...onics-inc/MP930-0.20-1/MP930-0.20F-ND/1284411

... You will have to install the zener diode in a metal heat sink, making sure that the negative end connects to the + voltage ... that's the way zener diodes work. Wear your full coverage safety glasses in case one of the batteries is a little touchy ... accidents can happen. ... suggest not leaving a battery connected to this circuit unattended. ... hydrogen gas can build up leading to an explosion. ... an experience that should be avoided.

#### MrChips

Joined Oct 2, 2009
22,069
Wear your full coverage safety glasses in case one of the batteries is a little touchy ... accidents can happen. ... suggest not leaving a battery connected to this circuit unattended. ... hydrogen gas can build up leading to an explosion. ... an experience that should be avoided.
Good advice well worth heeding. I would go so far as saying no battery should be left on a charger unattended, smart or otherwise.

I can show you photos of what happens to a SLAB when the "smart" charger fails to terminate charging.