Hi
I've been trying to temperature stabilise a 555 square wave oscillator (astable) at around 20Hz.
I have chosen the SE555D as, I believe, it has the smallest negative temperature coefficient of all the 555s. I used 3 x 0.22 COG (NPO) SMD caps in parallel, a 39K fixed with a 3K3 and a 3K9 linear PTC thermistors in series. (see circuit diagram attached)
For a 50/50 square wave output Pins 6 and 2 are connected together the resistor chain is to pin 3 and the caps to earth, pins 7 and 4 are not connected, pins 5 and 1 have a 0.1 COG (NPO) SMD cap across and pin 5 has a 0.1 COG (NPO) SMD cap to earth. I have used a 1N4148WS in the power supply for reverse polarity protection.
This configuration gives me the stability I require - 20.05Hz at ambient, 19.92Hz, at +125 deg C. and 19.87Hz at -20 deg C on a single sided circuit board the size of a postage stamp.
I’m pretty sure if you play around with the PTC thermistor values you could get even better stability.
I've been trying to temperature stabilise a 555 square wave oscillator (astable) at around 20Hz.
I have chosen the SE555D as, I believe, it has the smallest negative temperature coefficient of all the 555s. I used 3 x 0.22 COG (NPO) SMD caps in parallel, a 39K fixed with a 3K3 and a 3K9 linear PTC thermistors in series. (see circuit diagram attached)
For a 50/50 square wave output Pins 6 and 2 are connected together the resistor chain is to pin 3 and the caps to earth, pins 7 and 4 are not connected, pins 5 and 1 have a 0.1 COG (NPO) SMD cap across and pin 5 has a 0.1 COG (NPO) SMD cap to earth. I have used a 1N4148WS in the power supply for reverse polarity protection.
This configuration gives me the stability I require - 20.05Hz at ambient, 19.92Hz, at +125 deg C. and 19.87Hz at -20 deg C on a single sided circuit board the size of a postage stamp.
I’m pretty sure if you play around with the PTC thermistor values you could get even better stability.
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