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I need to be able to out a voltage between 0-10VDC but I can't seem to figure out the best way. Any suggestions? I have a 12V source that I can use for this purpose without the need for a doubler. I just need a way to modulate the 12V line. I have tried using a transistor and the PWM output without success. The output from the pic works great but the output from the transistor doesn't seem to work.
Here's a voltage doubler... 0-5v input (use 0-100% PWM from PIC), will produce 0-10v Output. To calibrate, output 5v from PIC (100% PWM) and adjust R2 for exactly 10v Output. For cheapness and to dispense with calibration, replace R1 and R2 with a single 1K5 Resistor, but all Reistor tollerances should in that case be 1% or better. Can't really get any cheaper than this... op-amp is about as low-cost as you can get at around 20 cents. You end up with a spare Op-Amp, configure it as your 5v Regulator to supply the PIC and dispense with the 78L05 or whatever you were planning to use.
Melanie
Melanie, that looks almost the same as the one we're currently using. The biggest difference I see is that we originally used a CA5160E Op-Amp due to it high input impedance. We ended up switching to a CA3140E which was a lot cheaper. This circuit works good for us except we keep speratically frying Op-Amps. Not sure why but when the end product is about 2,000 miles away it makes for a tough on-site fix. Just wish we could fix out what was causing them to fail. They are decoupled with a 0.1uF capacitor, a diode on the output to prevent any kind of feedback and they still sometimes fail without warning.
Watch the supply to the Op-Amps - make sure it NEVER goes beyond the maximum device rating. For example, if your product is an idustrial device connected to 24vAC... many transformers have huge winding variations - I have seen transformers marked 24vAC producing 36v when only lightly loaded, you the recify this and have over 50vDC, way beyond the max rating limit of most Op-Amps, 78L05 and 7812 voltage regulators and the like. Many people forget this. Over-voltage on the supply is THE most common cause of Op-Amp fry-ups.
Another reason is short circuit outputs. If you have a PIC, then sample the output back from the op-amp thru a voltage divider chain to a spare ADC. If you have a short, then switch-off the PWM drive until the short has been removed.
Melanie
You might be on to something here. We do use a 24VAC transformer as this is an HVAC controller. I'm starting to wonder it we are generating a large spike on the ground side which will change the ground potential to the op-amp. The only problem with this theory though is that why wouldn't other components be affected as well? As Op-Amps more sensitive to transients like these. Next question would be how to protect the little sucker from this type of condition? Also why the LM324? What about the 741 or another single chip variation. I'm very limited for space.
It's not nescessarily spikes that kill your circuits... it's the fact you are CONSTANTLY running them beyond their maximum ratings - they are just going to die, if not today, then tomorrow.
A lot of people take the 24vAC, rectify and smooth it, then apply that straight to circuits and regulators. Take a selection of typical 24vAC transformers used in industry (eg HVAC) and measure their off-load terminal voltage - you might be shocked. The smaller the VA, the worse they are - 30% is quite typical from some famous 'trustworthy' manufacturers. Factor in for example a transformer designed for 220-230vAC input, then apply 240vAC which is in use in some countries like the UK, and again this adds to overvoltage on the secondary.
Remember, even if it is EXACTLY a picture-perfect 24vAC, rectifying and smoothing gives you almost 34vDC... now go look at the Datasheets for LM324, CA3140, CA516E and all the others you're using... then come back and tell me what their Vmax is. Now also tell me why you are still surprised that your designs are flaming out.
If the world was flat, you're sailing right on the edge. Simple choice - either select devices with a higher Vmax, or, add additional circuitry to ensure the maximum device tollerances are not exceeded.
Melanie
We are feeding the op-amp from a LM2574-12 switching regulator and the pic is fed from a LM2574-5.0 switching regulator. The power supply is a half-wave rectifier because we need to use a common ground. We also have a bi-directional zener on the input along with a MOV to help surpress any spikes. I'm not sure what else we could do to isolated this little puppy.
Can't comment on your switching regulators, other than saying we're selling about a thousand HAVC boards a week which have LM358 op-amps. Our circuits are also half-wave rectified and stabilised to approx 24vDC using a zener and a 1 cent transistor. I find simplicity is sometimes the key... (has to be with the clients we have)...
When we originally built this circuit we were experiencing a lot of negative generated spikes coming from the power relays that we were switching inside the HVAC unit. We combated this with a 51V Bi-Directional TVS. I now starting to wonder it maybe we should be using one in the range of 12-15V instead. This might be source of or problems. Out of about 1,000 units about 20 of them experience this op-amp problem. Most of them seem to be job specific so I'm thinking it might be a combination of things inside the units that are making this happen. It's become quite frustrating. I've tried to duplicate the problem without any luck though.
I'm still looking for other ideas for this one if anyone has any suggestions.
hello you ken use a power transistor with 50amp or less connect to de pic with 2 resistor one 12k to ground and the other to the gate 10k in the same line and work with the pwm 0 to 100 scale =0 to 255 the power transistor work from 0 to 30v ....
use one diode to the motor (50v)
I have this working in my Robot for long time no problem....
Hi, cocacolakid
0-10 v smells driving and regulating high Power units ... could you catch high voltage spikes on the AOP output , due to the drive line ...
Note, you surely drive a long line whose capacitance is not negligible nor. Have remarked output HF oscillations ???
This makes me think an AOP with full bipolar output ( 3140 is NOT full bipolar ...) and clamping diodes to VCC and GND has to be tested here .
I don't think 10^12 ohms input impedance is compulsory not to trouble the PWM to DC filter ...
The more I will know, the more I will help ...
Alain
There is nothing wrong with my original circuit suggestion (standard classic design), but everything also depends on your selection of op-amp. Many do not have output current limiting and will drive themselves to destruction if shorted or overloaded, even across what could be the same device type. For example, a favourite of mine is the LM358 (because it only costs about 10 cents!). An LM358 from one manufacturer is NOT the same as an LM358 from another. Compare say National Semiconductors internals with Texas Instruments. Nat Semi's tend to fry, TI's do not. It's all in the Datasheets...
And don't forget Microchip... they do a great line of Analogue products including op-amps.
Hi
i am using IRL3103 HexFet Work from 0v to 30v and
55a it work perfect ..
@Melanie,Quote:
Originally Posted by Melanie
Voltage Doubler working perfect for me, but i had to change R4(10k to 1k),PWM produces 3+ volts only from PIC if i use 10k as per your schematic..i dont know whats wrong with my circuit..
And would like to dim gradually a 5watt led x 2, could pls help me which power transistor needs to be used.
Thanks a lot.
Hello Melanie,
Perhaps can you post a link for image (circuit) so I can read the components value?
Thanks
Joseph
That post is 10 years old and Melanie hasn't been here in 2 years.
Save the picture and zoom in to see the values.
Robert
I have tried using a transistor and the PWM output without success. The output from the pic works great but the output from the transistor doesn't seem to work.
waleeed
a Mosfet driver circuit like a 4426/4428 would be the best here ...Quote:
Originally Posted by waleeed00
Alain