I'm planning to build a 6G15 standalone reverb unit. I note in the the Weber schematic (5G15) there is a Thermistor between the bridge rectifier and the filter caps.
Can anyone tell me what value Thermistor I should use?
Thanks in advance.
Thermistor value for reverb unit
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This is an inrush current limiter, nothing more.
In truth you could probably survive without bothering, or could use a simple resistor. However a NTC resistor has some advantages. The idea is that it has a highish resistance when cold, which limits the inrush current, but it then heats up and the resistance drops. The steady state current then keeps it warm enough to maintain a low resistance. Of course this is a feedback system, so so long as you don't exceed the maximum steady state dissipation of the thermistor, and don't begrudge the slight voltage drop, the actual final values don't matter too much.
In modern switching PS applications special tough NTC resistors are used to limit the current feeding the capacitors. However in your application you don't really need a tough one, and indeed you have not a lot of current, so keeping it warm enough might even be an issue.
You need to work out the steady state current drawn by the circuit, which should be easy. Being a class-A driver there is not much in the way of surprises. This will define one of the parameters of the NTC resistor (that which keeps it warm enough.) Then work out what the maximum inrush current you want is. This would be based upon a safe fraction of the rating of the rectifier. So now you can work out the cold resistance needed. After that you simply need to work out the energy dissipated. Empirically it seems that a good guess is the energy that ends up in the capacitors - so 0.5*C*V^2 The thermistor must be rated for at least this.
However, as noted, the usual NTC resistors are designed for switching power supplies, and I suspect you won't find one designed for low enough current draw all that easily. But this is a least a start.
In truth you could probably survive without bothering, or could use a simple resistor. However a NTC resistor has some advantages. The idea is that it has a highish resistance when cold, which limits the inrush current, but it then heats up and the resistance drops. The steady state current then keeps it warm enough to maintain a low resistance. Of course this is a feedback system, so so long as you don't exceed the maximum steady state dissipation of the thermistor, and don't begrudge the slight voltage drop, the actual final values don't matter too much.
In modern switching PS applications special tough NTC resistors are used to limit the current feeding the capacitors. However in your application you don't really need a tough one, and indeed you have not a lot of current, so keeping it warm enough might even be an issue.
You need to work out the steady state current drawn by the circuit, which should be easy. Being a class-A driver there is not much in the way of surprises. This will define one of the parameters of the NTC resistor (that which keeps it warm enough.) Then work out what the maximum inrush current you want is. This would be based upon a safe fraction of the rating of the rectifier. So now you can work out the cold resistance needed. After that you simply need to work out the energy dissipated. Empirically it seems that a good guess is the energy that ends up in the capacitors - so 0.5*C*V^2 The thermistor must be rated for at least this.
However, as noted, the usual NTC resistors are designed for switching power supplies, and I suspect you won't find one designed for low enough current draw all that easily. But this is a least a start.
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