Ultra small Blue LED Volt Meter DC100V ( 0V~99.9V ) | eBay
Monitor range : DC 0V ~ 99.9V
Working power : DC 4.5 ~ 28V
Current consumption : about 5mA
Accuracy : 0.1V
Monitoring frequency : 500ms
Led Size : 0.36″ LED
Meter Size : 29.3mm x 15.2mm x 10mm
Working temperature : -10℃~65℃
Basically, I have 45v DC that I want to monitor. I am building something which will utilize 45v dc, and want to put this led display to monitor the voltage in real time. Couple of questions:
1 - how would I connect this? there are 3 wires.
2 - Should I have a separate dc source for the required 4.5 - 28v needed to power this thing?
3 - It draws 5ma of current...if I have a separate dc power source to power this thing...lets say 9vDC...how long will it take to use up the 9v source?
thanks
Spiro
Monitor range : DC 0V ~ 99.9V
Working power : DC 4.5 ~ 28V
Current consumption : about 5mA
Accuracy : 0.1V
Monitoring frequency : 500ms
Led Size : 0.36″ LED
Meter Size : 29.3mm x 15.2mm x 10mm
Working temperature : -10℃~65℃
Basically, I have 45v DC that I want to monitor. I am building something which will utilize 45v dc, and want to put this led display to monitor the voltage in real time. Couple of questions:
1 - how would I connect this? there are 3 wires.
2 - Should I have a separate dc source for the required 4.5 - 28v needed to power this thing?
3 - It draws 5ma of current...if I have a separate dc power source to power this thing...lets say 9vDC...how long will it take to use up the 9v source?
thanks
Spiro
Your asumptions are correct. You need a fixed 4.5 to 28 V to power the meter. You could drop the 45V with a voltage devider and then regulate it with a LM7805 regulator to power the meter. Black is ground and red is working power. Put your 5V to red. Connect the white to the 45V that you want to monitor. Google the +5v regulator because you will need a cap or two described in the data sheet. Should work.
thanks. Only thing is that I want to keep this a simple as possible. I saw a meter that didnt require a voltage source to operate...but it would only read to 30v DC.
Any ideas on how long a 9vDC source will last with a draw of 5ma?
How to Connect a Voltage Regulator in a Circuit
I was reading this, and was wondering why would you need the caps to filter AC noise? Where is that coming from if the source is only DC?
Also, I would probably need a heat sink if Im feeding the reg. with a 45vdc input correct?
If this is the case, then its getting too complicated for my needs.
I was reading this, and was wondering why would you need the caps to filter AC noise? Where is that coming from if the source is only DC?
Also, I would probably need a heat sink if Im feeding the reg. with a 45vdc input correct?
If this is the case, then its getting too complicated for my needs.
Is the 45V fairly constant ?
The components values aren't critical.
You can increase R to 4K7 at 1W with no real detrimental effect.
(By the way the 3K3 only needs to be 1W - I got decimal point in wrong place)
0.6W in both instances will be sufficient but they will get warm.
The cap is only there to decouple the meter, it might not be necessary. 1uF/16V would be more than sufficient. It also decouples the noise from the zener. Depending on how well designed the meter is it probably wont be required.
You cannot use a 7805 as 45V will exceed its maximum input voltage.
The components values aren't critical.
You can increase R to 4K7 at 1W with no real detrimental effect.
(By the way the 3K3 only needs to be 1W - I got decimal point in wrong place)
0.6W in both instances will be sufficient but they will get warm.
The cap is only there to decouple the meter, it might not be necessary. 1uF/16V would be more than sufficient. It also decouples the noise from the zener. Depending on how well designed the meter is it probably wont be required.
You cannot use a 7805 as 45V will exceed its maximum input voltage.
Attachments
Last edited:
What is the capacity of your 9V DC source in mAh at 5mA current draw? If you know that then arithmetic can give you an answer. If you don't know that then nobody can give you an answer.
Why bother with the LM7812 when a simple zener will do here ? In SOFASPUD's circuit, if the 12V is not drawing any current for whatever reason, the 7812 will fail.
You could put a minimum load on it but that is just adding unnecessary complexity.
Last edited:
Wouldn't argue with that. I might dispute the need for a 5W resistor, though. 1W should be adequate, shouldn't it?
You're perfectly correct - if you read the text you will see that I corrected myself after I had posted the drawing.
The maths gives you about 1/3W but lets err on caution and go for at least 0.6W if not 1W.
Yes...the 45vdc is generated by alkaline batteries. Let me ask this...what if I were to put a momentary switch in the circuit, so the led only displays the voltage when pressed? Wouldnt that make the life of the 9v batt last longer? (I think yes would be the answer). So, if I did this... would there be a different way to handle this project?
Thanks guys for all your replies so far.
If you want to use the extra battery or use a slave supply from the 45V then a momentary switch will certainly preserve your battery life.
The circuit will work; the 2700R will drop sufficient voltage at 5 mA to ensure that the input voltage of the 7812 is within limits (35 V limit). However, what happens when the load is removed from the output of the 7812? The voltage will go up to ~ 45 V and you will fry the 7812. And don't forget the decoupling caps at input and output of the reg, it will likely oscillate without them, especially with the high input impedance.
Actually it can be made even simpler. Since the meter has such a wide working voltage range, adding a regulator is overkill.
If you look at the meter, you will see a simple 7805 (or similar) regulator on it. If you know that the current draw is about 5 mA, you can simply put a series resistor on the red line to drop the measured voltage (45 V) down into the range suitable for that regulator. Shoot for about 15 to 20 volts, so you need 25 to 30 V of drop. At 5 mA that is 5 to 6 kOhm. Try that and see how it works, maybe the thing draws a little more or less than 5 mA and the drop is too much (or too little). You could also add a zener (25 V or so) for "safety" just in case the current draw through the R is too little to have sufficient drop.
Yes, your 9V battery would last quite a while. And makes this thread irrelevant. 🙂
Actually, you might need to be concerned about the meter's settling time. It's not likely to provide an instant reading. Other than that, it's pretty cut and dried.
Actually, you might need to be concerned about the meter's settling time. It's not likely to provide an instant reading. Other than that, it's pretty cut and dried.
Im a moron...I should have mentioned that the 45vdc will be separate and not utilized along with the led meter. It needs to stay at 45vdc. So basically, what I was asking for was how to connect the led meter (which will be supplied by its own 9vdc source and a momentary switch) to the 45vdc part...so I can occasionaly monitor the 45vdc voltage?
- Status
- Not open for further replies.