How to have a certain volts range on a meter?

[AlainB]

Should be around 7 volts, like before.

Up to now, I did not find what is the "official" resistance of the thermistor at 180 c. I am looking for a good infrared thermometer to get this value. I will then tweek the circuit accordingly. But 7 volts is about that for the middle of the meter.

Thanks!

Alain

 

[Roff]

OK, here's the design methodology I used, with a circuit (lower right) for nominally 1V range, centered at 7V. I don't really show the resistor values in terms of full scale voltage and center voltage, but these could easily be derived.

 

[eblc1388]

@AlianB,

The circuit is working well. As is, the green zone of the meter is big enough. The needle is going nicely back and forth on about one third of the total range.

Looks like the fishing trip went very well.

Now you have the circuit working well, its time to sit back and think of what would happen if one of these component failed. In many electronic design, when parts do fail, it simply just doesn't work. However, situation becomes very serious in your case. Its very bad because it would just take a few seconds to ruin the fuser, shorter than the time one could possibly sense something wrong and react unless one is keeping a constant watch on the meter.

The TRIAC, the opto MOC3020 driver, the rest of the circuit, the NTC or its connection can become open circuited in operation. All these would result in a steady AC to the fuser, until it burnt out. This is an expensive part which we don't want to replace.

Knowing of the consequences, a designer should really do something to limit the damage that resulted.

Everyone would see this problem differently and it will be fruitful for forum members to suggest practical way of protection. Murphy visited once, sure he will come again.

 

[AlainB]

Well, what can I say Ron!!

You really make my day! Thank you!

The second box was a little bit hard to swallow and I did not assimilated all of it yet but I will.

But since you gave the final equations, it has been a piece of cake to make a spreadsheet to compute the values. Took me some time to figure out what was that bloddy 5,5V. I went to a false track assuming that it was the 6,5 volts minus the 1 volt of the wanted range. But wy! wy! wy!


Anyway! Let's see:

For the same meter, 392 Ohms, 0,842mA, Excel computing the voltage.

I want to monitor a range of 2 volts, from 7 to 9 volts. The Vin of the divider is still 12 volts and I want 0,5 volts as fine tuning (centering).

Without the centering Variable Resistor, R1 will be 3400 Ohms and R2 4760 Ohms.

The value computed for the VR is 680 Ohms. I dont know what is the nearest standard value so I will use a 500 Ohms for this example. This will lower my fine tuning to only 0,37 volts though.

Then, with this 500 Ohms VR, R1 will be 3150 Ohms and R2 4510 Ohms. So the nearest value would be 3,1K and 4,5K if these are standard values.

I you have a chance, could you confirm that. The values computed concord with the ones in your sheet when applied.

Thanks!

Alain

 

[Roff]

Well, what can I say Ron!!

You really make my day! Thank you!

The second box was a little bit hard to swallow and I did not assimilated all of it yet but I will.

But since you gave the final equations, it has been a piece of cake to make a spreadsheet to compute the values. Took me some time to figure out what was that bloddy 5,5V. I went to a false track assuming that it was the 6,5 volts minus the 1 volt of the wanted range. But wy! wy! wy!


Anyway! Let's see:

For the same meter, 392 Ohms, 0,842mA, Excel computing the voltage.

I want to monitor a range of 2 volts, from 7 to 9 volts. The Vin of the divider is still 12 volts and I want 0,5 volts as fine tuning (centering).

Without the centering Variable Resistor, R1 should be 3400 Ohms and R2 4760 Ohms.

The value computed for the VR is 680 Ohms. I dont know what is the nearest standard value so I will use a 500 Ohms for this example.

Then, with this 500 Ohms VR, the R1 give me 3150 Ohms and R2 4510 Ohms. So the nearest value would be 3,1K and 4,5K if these are standard values.

I you have a chance, could you confirm these figures.

Alain

Your calculations look good. The standard values depend on whether you are using 1%, 5%,... etc. values.

 

[AlainB]

The TRIAC, the opto MOC3020 driver, the rest of the circuit, the NTC or its connection can become open circuited in operation. All these would result in a steady AC to the fuser, until it burnt out. This is an expensive part which we don't want to replace.

Knowing of the consequences, a designer should really do something to limit the damage that resulted.

Hi,

There is a thermal fuse in the circuit of the lamp. It would open if the temperature was reatching 210 C. This fuse would have save my fuser had it been there. But of course, that was the first thing that I bowed up while testing the lamp the first minute that I start to play with that thing, a while ago. So, I bypassed it!

Unfortunately, there is no reset on those fuses. Once opened they are good for the garbage.

The fuse is an absolute must. Otherwise the fuser cannot be left working unattended for even only a few seconds.

Alain

Note: I opened this thread concerning the meter after we discussed about the fuser circuit in another thread. This was a mistake. I should have continued the fuser thread instead of opening this one. So if you want the whole story here is the link to the other thread.

https://www.electro-tech-online.com/threads/circuit-to-control-hp-laserjet-fuser.36293/

 

[eblc1388]

Unfortunately, there is no reset on those fuses. Once open they are good for the garbage.

Can you remove and replace the thermal fuse if it has gone open? If so, then it is an acceptable protection method.

 

[AlainB]

Yes, the fuse can be replaced. The new fuser that I have is not protected. I have the fuse somewhere but I can't find it. For now, most of my tests are done without the fuser. I use a 10K pot to replace the NTC. When I use the fuser for testing I am extra carefull.

Alain