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modify press-on-press-off circuit to higher current

Thread starter #1
I have this circuit for press-on-press-off. it is working with a resistor (and led) as R11=1500ohm.
1. How can i change the circuit if i replace the R11 from 1500ohm to 8 ohm?
2. the press-on-press-off works only in low frequency, how can i change the capacitor or other part to work on higher rate.(twice)
led.PNG
 

crutschow

Well-Known Member
Most Helpful Member
#3
If you reduce R11 to 8 ohms, the current through Q2 will be 0.75A, so Q2 will need to be a transistor that can carry at least 1A (the 2N5551 has a maximum of 600mA).
You will then need to reduce Q2's base resistance consisting of R6, R8, and R9 to provide at least 50 mA to the base of Q2.

Alternately you could change Q2 to a logic-level type N-MOSFET (maximum Vgs threshold <=2V) with the gate connected directly to the collector of T2.
 

Tony Stewart

Well-Known Member
Most Helpful Member
#4
The 2N5551 is not suitable to drive 8 Ohms with 6V with this circuit and even if all the base drive R's were reduced it is still not suitable for any current > 100mA for heat and insufficient current gain reasons. However a bigger Logic level NFET could work.

However a much simpler design can be made from a D flip flop with clock RC pulse input and feedback from Qbar to D input. then driving a FET from either output.
 
#5
if you need it for real application, a counter such as 4017 can do the job easily.
Alternatively you can use a relay to drive 8 ohms load.
 

Tony Stewart

Well-Known Member
Most Helpful Member
#6
if you need it for real application, a counter such as 4017 can do the job easily.
Alternatively you can use a relay to drive 8 ohms load.
A 4017 driving a Relay coil is almost like short circuit load then the back EMF will produce an unhealthy spike and latchup the chip like an SCR as it heats up.
 

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