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3
Posts |
Posted
21/9/2005
Hi
My mate is a car electrical tech and wants to test temparature gauges, specific types, mainly VDO brand. He asked me to build one and I have nutted on it for a while but am stuck trying to get such a low voltage.
The principle is this, the analogue gauge reads a minimum of 100C which corresponds to a voltage of 4.04mV and maximum 900C which corresponds to 37.325mV
I need to have a variable supply, with a linear pot and an LCD display which can supply this low voltage. I have only seen some regulators which can control from -3 - 37V but I feel it is a bit of an overkill. It needs to be run from a 1.5V battery and all fit in a reasonable jiffy box, that part is probably easy. There is no current drain, except for what the active components will draw, so I feel the 1.5V AA will do the job.
Any help or pointers would be appreciated :)
Cheers
Richard
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Yang-Lit
63
Posts |
Posted
22/9/2005
I feel that it is pretty important to consider the current draw in this situation. At such low voltages, using an op amp to attenuate a rail voltage is probably a good way to generate your variable 0-50mV voltage. If your require substantial current (say, a few mA) you may need to buffer the output with a transistor.
ylp88
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Yang-Lit
63
Posts |
Posted
22/9/2005
In addition, most linear voltage regulators (such as the 317) have a rated output ripple of several mV, thus making them unsusable for your application. Not to mention that they will probably refuse to regulate to such a low voltage in the first place.
ylp88
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Yang-Lit
63
Posts |
Posted
22/9/2005
Try feeding your 1.5V to the non-inverting input and then create a voltage divider using your pot with the wiper connected to the inverting input. Positive or negative feedback can then be applied to create permanent attenuation - ie. limit the output voltage to between 0 to 50mV.
ylp88
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3
Posts |
Posted
23/9/2005
Hi there, thanks for the replyand the tip. I had not considered op amps and it has been a looong while since I did any theory on them, time for some boning up I feel.
Cheers
Richard
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3
Posts |
Posted
12/10/2005
Hi again
I have a solution and thought I may share it with you. I grabbed my old Op Amps book and did some boning up, but it looked like it was becoming too complicated and expensive to make something that would do the trick as well as have an LCD display, then make a circuit board etc.
The method I have chosen is this. I have a digital LCD multimeter which has diode test. That supplies 1.5V, if I put a 1k pot across the pos/neg terminals, which are in parallel with the thermocouple inputs I have a 0-1550 mV supply which should do the trick nicely. Very cheap and quick.
Cheers
Richard
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0 - 50mV linear power supply needed