Hi, will this circuit perform as intended ? . A normally closed transistor relay , that when a voltage is applied on the input it cuts the current flow to the Relay Coil .
The use of it is in a speaker DC detect / protect circuit , Instead of using two Relay's I was thinking this would work . Also for the Relay flyback , will a 1n4148 survive?. The relay is rather small ( 350mH inductance ). Thank you.
Mosfet I'll use something in D-Pak like IRFR2405 .
- Bruno.
The use of it is in a speaker DC detect / protect circuit , Instead of using two Relay's I was thinking this would work . Also for the Relay flyback , will a 1n4148 survive?. The relay is rather small ( 350mH inductance ). Thank you.
Mosfet I'll use something in D-Pak like IRFR2405 .
- Bruno.
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At 12V, a 400R relay would draw a current of 12/400 = 30mA, since the inductance is a short circuit at DC. However, when the MOSFET turns off, there's a commutation current but it starts with a peak value of just 30mA, vs. the 400mA rating of 1N4148. So, all is good.
The kV figures you show in the second picture simply do not occur with the diode in place and the max voltage at the drain of the MOSFET would only be Vcc+0.6V, never more than that.
If you use a IRFR2405 then you'll require V+ to be at least 20V with that circuit due to your divider. You seem to be using 12V supply - so why use a divider for the gate at all? In other words what is R4 doing?
It is OK to have R4 (10k) as it prevents spurious turn-on of the MOSFET but the value of the upper resistor at 10k is too high at 10k. Further, as already mentioned by @Mark Tillotson, it also ends up reducing the gate drive to half.
So that Q1 doesn't conduct much Current and won't heat up. Won't 10V Vgs be enough?. With 4.7k and 51k .
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Yes, you may skip that resistor if the transistor is driven hard into saturation (Vce < 0.2V). The main point was regarding the upper 10k being too high.
Skip the 51k and put the transistor close to the MOSFET's gate, that'll do. Vgs = 10V would be enough but the gate would take some time to charge if the pullup resistor is too high. Turn off would be quick since that's done by the transistor.