3BABY
Member level 5
- Joined
- Jan 14, 2011
- Messages
- 91
- Helped
- 0
- Reputation
- 0
- Reaction score
- 0
- Trophy points
- 1,286
- Location
- New Zealand
- Activity points
- 2,252
i need some device that will convert the 150V pulse to between 5V-3.3V. the device needs to be fast.
Hi,
In simples case: two resistors as voltage divider.
If you just need a digital output, then I recommend to use a comparator for exact threshold.
If you need the analog signal then maybe an Opamp as buffer is useful.
It all depends on your specifications. Like timing, expected resolution and precision...
Klaus
Added:
Maybe you need additional protection against overvoltage and/or negative voltage.
Fast? Is this a jet engine with spark plugs and CDI?
Use a resistive divider or for better safety, use a resistor and Zener diode. The diode will protect against reverse voltage spikes (Zeners work like ordinary diodes in forward conduction mode) and it will clamp the voltage to a fixed level, removing any variation in amplitude.
Brian.
resistor value = 9.6K Ohm
resistor Power = 2.8W
Zener Voltage = 5V
Zener Power = 0.09W
So i could get away with a 3W resistor
Actually, roller-skate fast rather than jet fighter fast. The 74S and HC would be able to handle as much as 100,000,000 RPM. Your requirement is somewhat slower!i need a digital signal out the other side. Brian,. yes i get it, it does not need to be 40000rpm fast.. just 7400 S fast or around the 5uS mark.. at least HCmos fast, i should have elaborated on what i meant by fast
I would use a resistive divider but still add the Zener as a safety measure.
Actually, roller-skate fast rather than jet fighter fast. The 74S and HC would be able to handle as much as 100,000,000 RPM. Your requirement is somewhat slower!
I would urge caution on the resistor rating without knowing the exact design of the CDI unit. For example, is it possible for the 150V to be present while the engine is stopped for any length of time? If that could happen the rating would have to be enough to withstand full voltage continuously.
I'm a little concerned about the Zener current, it is possible (no CDI info) it is being drawn from the voltage used to charge the 'C' in the unit. More load on it may drop the CDI performance, in which case a lower current is preferable. If the 5V loading is small, for example an MCU pin or CMOS logic input, currents as low as 0.5mA may work just as well. I agree with Klaus about the pull-down issue, I think if I did this myself I would use a resistive divider but still add the Zener as a safety measure.
Brian.
Hi,
after reading Brian´s post I re-read my post.
The zener is urgent. I didn´t mean to omit the zener.
And additionally I forgot to add the zener´s capacitance.
Calculate with 300pF and get a tau of about 10us.
Klaus
This expression is worse then "fast"... ;-)i was simply meaning without delay
This is not my day... Somehow I calculated with 500V. But you need 150V.Klaus, using the 1.5M and 30K ohm resistors would give 2.94V.. im looking for around 5V
I would use a voltage divider AND a Zener diode. This is my reasoning:
My caution earlier about constant high voltage being present can be ignored under the circumstances. I was worried because some CDI units have a high voltage inverter circuit to charge the capacitor and it runs as long as the 12V supply is present. In some circumstance the 150V could be there all the time the ignition was turned on, even if the engine wasn't running.
I would use a voltage divider AND a Zener diode. This is my reasoning:
Capacitance is present in all components and indeed in the wiring around them. It is a property that can be ignored in most applications of this kind because it has little effect. Basically, it is just like a real capacitor wired across the component but is a consequence of the manufacturing process and device package. It draws a small current from a rising voltage and releases it as the voltage drops so in your application it results in a slightly 'rounded' edge on the rise and fall of the signal. If the voltage rises and falls very fast the effect can be significant, particularly if the current in to it and out of it is restricted so the charge and discharge is slower. I would be very suprised if it caused any problem at the speeds you are using though.
Hi,
He gave no values. Therefore my recommendation:
Now I recommend 150k for the upper resistor. This gives less than 150mW of power dissipation. (1mA)
Then use about 3,9k for the lower resistor.
Thresholds are (for TTL input): 28V @ 180uA / 80V @ 520uA with a source impedance of about 3.9k.
The zener maybe has about 300pF capacitance.
With the source impedance of the dvivder it generates a tau of 1.2us. (delay)
****
BOM:
1 x 150k 0.5W (like standard THM 0207)
1 x 3k9
1 x zener 4,7V or 5,1V
1 x HCT schmitt-trigger (like 74HCT1G14)
Klaus
Reducing the resistor from 150k to 87k just reduces the voltage_sensitivity.Klaus, how about 87K resistor and a 3K resistor
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?