Hi all.

I have a bit of a newbie question so sorry in advance if this is really obvious.

I have a project I am currently working on that requires both 12v dc battery input and 240v AC input.

The 240v AC input is currently going to a Mean Well transformer to supply my equipment with the needed 12v DC and i'm guessing I can connect the 12v DC straight to the output side of the transformer.

My only concern is that when i'm using the 240v mains supply, the battery connections will be live 12v DC supplied from the transformer so is there something I can use for auto switching between the inputs to resolve this?

 

Hi N60.
Have you considered using a couple of Schottky diodes, one in each 12V supply?
E

 

What's going to Charge your Battery ?

It's probably a better plan to use a proper Battery-Charger that can supply
more Current than your project requires.
This would keep the Battery fully Charged at all times.

Your current Meanwell SMPS ( Switch-Mode-Power-Supply ) may not have enough
Output-Voltage to be practically used as a Lead-Acid-Battery-Charger.

What this whole scheme amounts to is readily available, and not very expensive,
it's called a UPS ( Uninterruptible-Power-Supply ).
.
.
.

 

Hi N60.
Have you considered using a couple of Schottky diodes, one in each 12V supply?
E

Thanks for the reply.

I'm I correct in saying the Schottky diode will only allow a current to flow in one direction? and I need to attach one of these diodes to the positive from coming from the battery side and another one on the positive running from the 12v transformer (240v side) to the equipments PCB?

If it helps, the equipment is a 2A stepper motor driver and I need the option to supply this with 240v AC (mains) or a 12v DC (battery).

Sorry I know this is all very basic for most on here.

 

What's going to Charge your Battery ?

It's probably a better plan to use a proper Battery-Charger that can supply
more Current than your project requires.
This would keep the Battery fully Charged at all times.

Your current Meanwell SMPS ( Switch-Mode-Power-Supply ) may not have enough
Output-Voltage to be practically used as a Lead-Acid-Battery-Charger.

What this whole scheme amounts to is readily available, and not very expensive,
it's called a UPS ( Uninterruptible-Power-Supply ).
.
.
.

Sorry LowQCab I probably haven't explained properly.

The equipment is a 2a stepper driver and I need to have the option to supply it with 240v ac or off a 12v battery

 

I'm I correct in saying the Schottky diode will only allow a current to flow in one direction? and I need to attach one of these diodes to the positive from coming from the battery side and another one on the positive running from the 12v transformer (240v side) to the equipments PCB?

Correct, a diode allows current in only one direction. A Schottky diode is a nice refinement because it shows a lower voltage drop and thus wastes less power. But any properly rated (for current and voltage) diode would work.

I'm not sure what you're thinking about with the AC supply. You want to OR (logical operator) the power so that current arrives at the load from either power source but cannot flow back into either source. The diodes should be on the 12V side, not the 240V side.

 

Correct, a diode allows current in only one direction. A Schottky diode is a nice refinement because it shows a lower voltage drop and thus wastes less power. But any properly rated (for current and voltage) diode would work.

I'm not sure what you're thinking about with the AC supply. You want to OR (logical operator) the power so that current arrives at the load from either power source but cannot flow back into either source. The diodes should be on the 12V side, not the 240V side.

Thanks the reply.

Ok so do I need two diodes? both on the 12v side, one to protect the transformer when the battery is in use and another to protect the battery when the 240v supply is used?

Would this attached drawing be correct?

 

Hi N60.
That looks OK, how do you plan to charge the battery?
E

 

Hi N60.
That looks OK, how do you plan to charge the battery?
E

The batteries will not be within the equipment so charging isn't a problem. They will simply be connected when needed via a 2 pin weipu connector.

Would a these Schottky diodes be ok?

 

hi N60.
That diode looks OK, ~500mV drop at 3A.
E

 

hi N60.
That diode looks OK, ~500mV drop at 3A.
E

That's great. Thank you all so much for taking the time to help!

 

Hi all.

I ended up purchasing SB220-E3/54 Schottky diodes for this project as the SB340-E3/54 were out of stock and when I tested the diode with a 12v DC power supply and meter the reverse voltage reading is around 10.1v

For this project I need the diode to give me 0v reverse voltage.

Am I missing something? Have I purchased the wrong diodes?

 

I need the diode to give me 0v reverse voltage.

In that case it wouldn't be much use as a diode, since it couldn't block any reverse voltage! Perhaps you mean 0V forward voltage drop, i.e. an 'ideal diode'?

 

Hi N60.
The reverse leakage current is only 500uA at 20V.
At 12V it will be approx 50% that value
How are you measuring the 10.1V?
E

 

In that case it wouldn't be much use as a diode, since it couldn't block any reverse voltage! Perhaps you mean 0V forward voltage drop, i.e. an 'ideal diode'?

Ok I must be missing something here. As above thread shows. I needed a way of separating two different power supplies. 12v from a battery and 12v from a 240v transformer. Only one or the other will be used at the same time, I just don't want the connections to become live with 12v when not in use.

So... this is maybe where I am mistaken.

I thought the diode would allow a voltage to flow forwards but not allow ANY voltage to flow back (0v)?

 

Hi N60.
The reverse leakage current is only 500uA at 20V.
At 12V it will be approx 50% that value
How are you measuring the 10.1V?
E
View attachment 301705

Hi ericgibbs.

I have a bench power supply adjusted to supply 12v DC and a fluke meter attached to the negative side on the power supply.

 

I thought the diode would allow a voltage to flow forwards but not allow ANY voltage to flow back (0v)?

Voltage doesn't flow. It's current that flows.

 

Hi ericgibbs.

I have a bench power supply adjusted to supply 12v DC and a fluke meter attached to the negative side on the power supply.

 

Voltage doesn't flow. It's current that flows.

Ahh yes of course it does!

So this might be another stupid question but if I use these diodes, I will still be able to measure a voltage on the supply connection/pins that aren't being used. Is that normal for equipment with two separate power supply/inputs?

 

Hi N60,
Using a high impedance Fluke DVM will give a high reading when connected in that way.
E