# AC-DC Powersupply voltage drop

I just bought this power supply for a project.

I just went in with a multimeter to test the voltage and pins, which gives me a 20V reading as expected.

However, after I remove the power supply from the wall, I still see 20V initially, and then I see the voltage dropping quite slowly, over a fairly long period; I'd say around half a volt per second, or even slower.

Is there any way to discharge this voltage this fast, or any other circuit to not have long discharge times from powersupply as it causes problems to switch to my battery?

• Of course there is. Put a load on the power supply and the output capacitor will discharge faster. – Justme Nov 18 '20 at 8:26
• you could open the psu and remove the output capacitor, or put a much smaller one in. – dandavis Nov 18 '20 at 19:17

You just need to add a discharge resistor to the power supply output to provide a controlled discharge path for the resevoir capacitors. In most cases the load itself will discharge the power supply for you when you disconnect the mains power.

Figure 1. RC discharge curve. Image source: School Physics.

The graph shows that the discharge time is a function of RLOAD × CRESEVOIR and this parameter is called the time-constant and uses the Greek symbol τ, "tau".

Some numbers worth remembering:

• At τ the capacitor will have discharged by 63%.
• At 3τ the capacitor will have discharged by 95%.
• At 5τ the capacitor will have discharged by 99%.

If you do add a discharge resistor then make sure that it doesn't increase the load on the PSU too much and that it can dissipate the continuous power safely.

• Hi, Thanks for the answer. Unfortunately, my load does not discharge the powersupply fast. It takes more than10seconds and this is causing the problem to switch to my battery for the continious running of my system. To add the discharge resistor to output of powersupply do you mean i should add a resistor on my board in between the power supply and my load? i am using an fuse in between powersupply and load already so is it good to add resistor before or after fuse? Thanks in advance. – user268884 Nov 18 '20 at 9:49
• The resistor would go between V+ and 0V terminals on the power supply or on the input terminals to the load. After the fuse means that it is protected by the fuse which would be better. – Transistor Nov 18 '20 at 10:09
• Hi, Tried connecting a resistor but it still does not work, takes same time. conencted 10k and 1M resistors in between +V and GND. – user268884 Nov 18 '20 at 13:10
• @user268884 Try much smaller resistors. Maybe hundreds of ohms. – Simon B Nov 18 '20 at 13:33
• @SimonB if i use below 5k ohms, the device does not turn on. :( – user268884 Nov 18 '20 at 14:04

Can I suggest a different approach to the problem?

It appears that the voltage from your power supply is dropping slowly, causing your device to malfunction. But it's not switching over to battery until the device has already failed.

You need to look again at whatever you are using to switch between the power supply and the battery, and make sure it switches over to battery before the voltage from the power supply is too low.

After that, it doesn't matter how long it takes for the rest of the voltage to decay.

• Hi, Thanks for reply. I am using BSS123 N-channel to switch between powersupply and battery and when i check with multimeter it looks it switches fast. Can you suggest anyother IC which can improve the switching speeds. – user268884 Nov 19 '20 at 8:54