| Electronics > Beginners |
| Transistor selection confusion |
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| phaseform:
I'm using a current sensing board from ebay to trigger an SSR, although the 'digital' output from the sensor board isn't ideal for the SSR. I'm getting 0.2v low, and 2.2v high from the sensor board (with settings best I can figure to achieve), I want to use a transistor as a switch to send the full 5v through the SSR (3v min). The SSR draws ~5ma. I have a BC639 (datasheet) I've found, which seems like overkill (1A max). Just from looking at transistor equations to figure out a base resistor, and whether this transistor is suitable.. I've gotten a bit confused. I'll have another look at it tomorrow, and probably get a lower power transistor from Jaycar.. Anyone with actual electronics knowledge that can shed some light on this..? - I was confused with min Hfe values - wont the base current just be lower in this case with a very low collector current? |
| Benta:
For switching purposes, it does not help looking at the hFE figures, they are for linear operation. For a transistor of this size a good value for base current is IC / 25. Anything between 20 and 40 will also work. |
| Audioguru:
Why add a high current transistor? The current sensing board has a "TTL" output high which can drive the SSR without using an additional transistor and base resistor. With the (+) input of the SSR connected to +5V then the (-) input goes to 0.2V when the current sensor is triggered and has an input of 4.8V which is plenty. |
| phaseform:
I can change the sensitivity of the sensor to get these outputs: * 5.2v trigger off, 2.75v trigger on * 0.15v trigger off, 2.7v trigger onSo it seems I can't use these values alone to drive the SSR. I'm thinking a low power NPN transistor with the latter sensor settings. I'm not sure about application of IC/25.. but I've done some reading around and maybe I've got it right.... I found this page on transistor calculation... so using for example a BC546 NPN Transistor.. From the datasheet which has "Figure 2. “Saturation” and “On” Voltages" - not sure why different datasheets dont show this info, nor how the Ic/Ib = 10 is found in this case Ic = 5mA. Hence VBE = .75V VBE = .75V is perfect for the sensor output values, with 2.7V sensor output (with 2kW through the sensor), I get ~1.95 voltage drop across the base resistor, at .5mA = 0.0005A (IC/10). V=IR therefore R = 3900 Ohms I think this is correct.. |
| Benta:
"not sure why different datasheets dont show this info, nor how the Ic/Ib = 10 is found" Base current is set so high to make certain the transistor is driven 100% into saturation. Like I wrote, a ratio of 25 is normally enough. |
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