Hi I'm wanting to switch a 12v power supply to my alarm module
But need to keep a feed to the module during the switch
I know I need a bypass / decoupling capacitor
But what value capacitor will I need

 

0

 

It looks like you are switching the "alarm module" on and off with the timer. The alarm module may well have an internal startup circuit. What makes you think you need a capacitor on its power supply? That will actually increase the load on the power supply since the capacitor will appear as a short at the instant power is applied to the alarm module.

 

It doesn't have a built in capacitor im sure because when you connect a live feed to the module the led lights up and then it goes out straight away which tells me there's no built in capacitor
Even if you leave power connected for a while then disconnect it the led goes out straight away

 

And so? What are you trying to accomplish?

 

I need to keep a supply voltage to the alarm module during switchover
So I proposed a capacitor would help in this

 

The current drawn by the alarm module and how long it will be unpowered will determine how large of a capacitor you need.

If you don't know one or the other, add a capacitor and keep increasing capacitance until it works.

 

The maximum draw is 300milli amps
And switch over is about 0.1 second
And maximum would be 0.5 seconds
Can you workout both switch over times please

 

Half a second at 300mA is 0.15 coulomb (since 1C = 1 Amp•sec). A 1F capacitor would lose 1V during that scenario (since a Farad is volts/coulomb). Any idea how much voltage drop you can tolerate?

A battery would be cheaper.

 

The current drawn by the alarm module and how long it will be unpowered will determine how large of a capacitor you need.

If you don't know one or the other, add a capacitor and keep increasing capacitance until it works.

And then double it, since it is unlikely that your test revealed the largest size that was needed under all conditions, but rather just one size that will work under one particular condition. Doubling the size will probably cover all the bases, but there is no guarantee of that. At the same time, doubling the size will be unlikely to move the solution from acceptable to infeasible.

 

Half a second at 300mA is 0.15 coulomb (since 1C = 1 Amp•sec). A 1F capacitor would lose 1V during that scenario (since a Farad is volts/coulomb). Any idea how much voltage drop you can tolerate?

A battery would be cheaper.

Why would a 1 F cap lose 1 V in half a second at a draw of 300 mA? Seems like it would lose 150 mV (since, as you said, 1 F is 1 V / coulomb, so 0.15 C would equate to 0.15 V, would it not?)

A small, rechargeable battery might well be a much better solution. Using something like is commonly used in computers will likely give many years of trouble free service at very little cost, possibly even less cost than a suitable capacitor.

 

Why would a 1 F cap lose 1 V in half a second at a draw of 300 mA? Seems like it would lose 150 mV (since, as you said, 1 F is 1 V / coulomb, so 0.15 C would equate to 0.15 V, would it not?)

A small, rechargeable battery might well be a much better solution. Using something like is commonly used in computers will likely give many years of trouble free service at very little cost, possibly even less cost than a suitable capacitor.

Battery would be better
What sort of battery do you recommend
Which will work in a 12v system

 

The maximum draw is 300milli amps
And switch over is about 0.1 second
And maximum would be 0.5 seconds
Can you workout both switch over times please

I calculate 25,000uF for one second; assuming the alarm module would tolerate a 10% voltage drop.

To determine an appropriate battery, we need to know the lowest voltage that the alarm module will operate at.

 

Minimum voltage is 9 volts

 

Why would a 1 F cap lose 1 V in half a second at a draw of 300 mA? Seems like it would lose 150 mV (since, as you said, 1 F is 1 V / coulomb, so 0.15 C would equate to 0.15 V, would it not?)

My bad, it would.

 

My bad, it would.

Easy mistake to make. No harm, no foul.