Hi all,
I am trying to teach myself some basics to be less dependent on posted schematics... and not having much success so far.
I am trying to create a basic blinking led circuit using a relay, as described here: https://www.build-electronic-circuits.com/blinking-led-circuit/
I don't have a 5V relay so I am using one of those common 5V relay modules: http://www.techydiy.org/opto-isolated-dual-relay-module-arduino/
My module schematic looks like this:
I have a 1800 microF capacitor at hand, so I tried building an RC circuit like this (I have a separate 5V supply for the relay coil):
The voltage drops (1.06V and 1.78V across the two diodes) are what I measured with the largest R1 that still closes the relay. In that setup R1 = 3.2kohm and If = 0.5mA
I had two goals:
- estimate how long at most that capacitor can keep the relay closed
- find out what resistor will maximize that time
My problem is that I am not too sure how to reason about that circuit... here is what I came up with:
- I assume the tensions across the diodes will need to be at least the forward voltages I have measured above, 1.78V and 1.06V
- I assume the current If will need to be at least 0.5mA
... so I am using the equation for an RC discharge:
t = (R1 + 1kohm) * C * Ln( Vmin / V5)
where Vmin = 1.78V + 1.06V + (R1 + 1kohm) * 0.5mA
If I put the above in a spreadsheet I find a maximum t of about 0.95s for R1=1kohms; unfortunately experiene doesn't confirm that... when I disconnect Vcc the relay shuts down immediately (though the Led does decay in about 1s).
So my question is, how do you reason about that circuit? And how do you predict what capacitor you need to hold the relay for say 5s?
Thanks!
Franck
I am trying to teach myself some basics to be less dependent on posted schematics... and not having much success so far.
I am trying to create a basic blinking led circuit using a relay, as described here: https://www.build-electronic-circuits.com/blinking-led-circuit/
I don't have a 5V relay so I am using one of those common 5V relay modules: http://www.techydiy.org/opto-isolated-dual-relay-module-arduino/
My module schematic looks like this:
I have a 1800 microF capacitor at hand, so I tried building an RC circuit like this (I have a separate 5V supply for the relay coil):
The voltage drops (1.06V and 1.78V across the two diodes) are what I measured with the largest R1 that still closes the relay. In that setup R1 = 3.2kohm and If = 0.5mA
I had two goals:
- estimate how long at most that capacitor can keep the relay closed
- find out what resistor will maximize that time
My problem is that I am not too sure how to reason about that circuit... here is what I came up with:
- I assume the tensions across the diodes will need to be at least the forward voltages I have measured above, 1.78V and 1.06V
- I assume the current If will need to be at least 0.5mA
... so I am using the equation for an RC discharge:
t = (R1 + 1kohm) * C * Ln( Vmin / V5)
where Vmin = 1.78V + 1.06V + (R1 + 1kohm) * 0.5mA
If I put the above in a spreadsheet I find a maximum t of about 0.95s for R1=1kohms; unfortunately experiene doesn't confirm that... when I disconnect Vcc the relay shuts down immediately (though the Led does decay in about 1s).
So my question is, how do you reason about that circuit? And how do you predict what capacitor you need to hold the relay for say 5s?
Thanks!
Franck
R1 | R2 | C | RC | If | Vs1 | Vs2 | Vmin / 5 | t_on |
3000 | 1000 | 1800μF | 7.2 | 0.5mA | 1.06V | 1.78V | 0.968 | 0.23416698 |
2000 | 1000 | 1800μF | 5.4 | 0.5mA | 1.06V | 1.78V | 0.868 | 0.764443247 |
1000 | 1000 | 1800μF | 3.6 | 0.5mA | 1.06V | 1.78V | 0.768 | 0.950275965 |
500 | 1000 | 1800μF | 2.7 | 0.5mA | 1.06V | 1.78V | 0.718 | 0.894471417 |
0 | 1000 | 1800μF | 1.8 | 0.5mA | 1.06V | 1.78V | 0.668 | 0.72624079 |