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constant to momentary not pulsing


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oldspark 
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: February 05, 2011 at 4:03 AM / IP Logged  
Yes. A strange way of stating it (since 1k uF = 1mF), but it does reflect some capacitor markings = eg, 10, 100, 1k, 10k where the "stamped" size is "times uF".
1,000 uF is quite large, but 10,000uF is common for power supply filtration, and 1F and larger with >15V rating is often used by people that probably don't know better. (Those with 1F & 100F 5V caps usually do know enough.)
1F some decades back was considered impossible - at least in practicable terms.
Just make sure you aren't confusing resistors & capacitors.
And if it's for a timing constant (ie; T ~RC), you may be able to reduce C by increasing R; eg, RC = (Rx10)x(C/10) = (Rx1000)x(C/1000) - but that is usually only suitable for low current loads (transistors etc), not relays.
(Why? Because the "R" forms a voltage divider with the load (coil). The bigger R gets, the lower the coil-voltage gets.... Remember? A coil might need 10mA or 100mA, but a transistor (Base) might only need 100uA for its Collector-Emitter to pass 10mA to 100mA to turn the coil on. (IE - the transistor has a "gain" (aka Beta) of 100 to 1,000.)
chaminda 
Member - Posts: 8
Member spacespace
Joined: February 01, 2011
Location: New Zealand
Posted: February 14, 2011 at 1:17 AM / IP Logged  
I made the same circuit with a 1k Uf cap and a 10k resistor
According to the spec sheet my relay coil has a 360 ohm resistance.
It pulses reliably with these components but takes ages to discharge
So I'm going to try Joe's values as my relay has similair coil resistance.
I want to try to get my discharge time down to at least 2-3 seconds and have my output for around 1s.
Joebloe4 - how much was your output time? Closer to 1s or less?
thanks
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