Hi everyone, I'd like your advice on powering a 12vDC computer fan from mains power (to act as a small fume extractor). I've had a quick look, but all the sources just attach the fan to a 12vDC power supply. However I'd like to accomplish this with bits I have lying around. My plan is to use 16V AC transformer (a Hornby C912 transformer), and I have a couple of questions.
Firstly (more a general transformer question): The transformer I want to use is rated at 16V (13VA) output. Am I correct in assuming that 16V is the RMS value, so once the voltage is rectified using a diode bridge (1n4001, so assume a 2V drop across the bridge) and a filtering capacitor, the voltage will be approximately 20.5v ([16V x √2] -2V)?
I've attached a diagram of my proposed circuit: it's the first time I've done something like this so it's a bit of a stab in the dark.
Capacitor values:
I usually see the 7812 used with C1=470μf and C2=0.1μf. I would guess that C1 smooths the rectified AC wave, so a bigger value is better (470μf seems to be a fairly common "large" value). C2 I assume filters the output of the regulator, and given the use of this circuit I would think it may be possible to omit it?
The transistor is there because I'd like to have a switch in this circuit, but the only one I have is a tiny one scavenged from a solar light, so I'm not sure what current it can take (and it seems I should minimise the current flowing through components I will be routinely touching). The value of resistor would probably be around 1-10K, giving a current into the base of around 1-10mA to give a base-collector current of a few 100mA. The transistors I have are BC337, which can take 800mA, although can only dissipate 625mW, not sure if this may be a problem (I think I have a higher power one lying around, I'm not sure what its part number is though).
Finally, a lot of the example circuits for the 7812 include a ground, but I'm not sure how I would incorporate that into this.
So what are your thoughts? I'm I going about this in a reasonable way, and are my assumptions correct?
Firstly (more a general transformer question): The transformer I want to use is rated at 16V (13VA) output. Am I correct in assuming that 16V is the RMS value, so once the voltage is rectified using a diode bridge (1n4001, so assume a 2V drop across the bridge) and a filtering capacitor, the voltage will be approximately 20.5v ([16V x √2] -2V)?
I've attached a diagram of my proposed circuit: it's the first time I've done something like this so it's a bit of a stab in the dark.
Capacitor values:
I usually see the 7812 used with C1=470μf and C2=0.1μf. I would guess that C1 smooths the rectified AC wave, so a bigger value is better (470μf seems to be a fairly common "large" value). C2 I assume filters the output of the regulator, and given the use of this circuit I would think it may be possible to omit it?
The transistor is there because I'd like to have a switch in this circuit, but the only one I have is a tiny one scavenged from a solar light, so I'm not sure what current it can take (and it seems I should minimise the current flowing through components I will be routinely touching). The value of resistor would probably be around 1-10K, giving a current into the base of around 1-10mA to give a base-collector current of a few 100mA. The transistors I have are BC337, which can take 800mA, although can only dissipate 625mW, not sure if this may be a problem (I think I have a higher power one lying around, I'm not sure what its part number is though).
Finally, a lot of the example circuits for the 7812 include a ground, but I'm not sure how I would incorporate that into this.
So what are your thoughts? I'm I going about this in a reasonable way, and are my assumptions correct?
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