Negative voltages

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Hi Freaks,

Attached is a schematic from the STK500 kit.

Can I get negative 5V from this?

Thanks.

edit: Rephrasing the question: How can the above circuit be modified to give +5V and -5v? Will I have to change the transformer side? I already have the components and now I found that I need a +/-5V.

I found one circuit using the 555 and I am going to try and see if it works. Other ideas are welcome.

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It depends what You are connecting to the DC jack. If You are using DC (as the name suggests) then there is no way getting negative voltages easily.

BUT

If this thing is fed by AC (Never seen a STK500) it should be fairly simple to modify the circuit to make both positive and negative voltages.

1. Pin 4 of DF10S goes to ground
2. Pin 2 of DF10S is disconnected from ground as it becomes the negative supply
3. A bypass capacitor is needed on pin 2 of DF10S (Note the polarity)

This makes the thing a half wave rectifier which may DC bias the transformer and may eventually heat it up. So there is no guarantee that this actually works.

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Steal -7V from the RS232 converter

Imagecraft compiler user

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Thanks, eskilola and Bob.

That's a great idea, Bob. I found that pin 6 (V-) on my MAX232 IC is showing me -5.24V. Any idea of the max. current this pin can sink?

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Good question, and I suspect it depends on exactly which RS-232 converter chip is actually being used.

My copy of the Maxim Max220-249 chips says, under "Dual Charge-Pump Voltage Converter":

Quote:

A small amount of power may be drawn from the +10V
(V+) and -10V (V-) outputs to power external circuitry
(see the Typical Operating Characteristics section),
except on the MAX225 and MAX245–MAX247, where
these pins are not available. V+ and V- are not regulated,
so the output voltage drops with increasing load current.
Do not load V+ and V- to a point that violates the minimum
±5V EIA/TIA-232E driver output voltage when
sourcing current from V+ and V- to external circuitry.

The graph, which is not so easily copied, would seem to indicate that V- could source about 10 mA @ -8V, using 1 uF caps; about 5mA if using 0.1 uF caps.

This is without any load on the V+ part of the chip, and drawing the current will impact both the V+ charge pump, and the actual RS-232 Level outputs.

If you really need to have a significant V- supply, and need it regulated, it would be much better to add a true V- supply to the circuit. There are chips designed to take the V+ supply and give you a regulated V- output, with a known current capability.

If you need to work on your circuit while awaiting the arrival of newly ordered parts you might be able to use 3 x 1.5 V batteries for a -4.5 V supply. Just tie the Grounds together.

JC

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Thanks, JC that is certainly very useful info. I tried driving an LED with the V- and it seemed to glow. Why would changing capacitors change the current sinking? Is it something along the lines of a higher cap would need higher charging current (q = cv, so i = cv/t)? Just oiling the rusty brain cells.

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Perhaps.
It would be much easier to determine the cause if the internal circuitry of the Max chip were know.

JC

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Transmitting at 115200 bps needs a certain amt of coulohms of charge to keep the negative cap charged. Big caps and lo charge pump freq can pump x coulohms of charge. Smaller caps and faster freq can pump same amt of charge. Newer chips are faster.

Imagecraft compiler user

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In the case that an AC transformer is used instead of DC power supply,remove the bridge rectifier,connect one pin of transformer to ground of the circuit,in the other connect the anode of a diode and the cathode to the + of input capacitor.This is for positive supply.
For negative connect the cathode of a diode to the same pin of the transformer and the anode to - of a capacitor and the + of capacitor to ground.This is for negative voltage,use then an 7905 connected and get the -5V.

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If you work with step down regulators (upper figure) there is a trick to get negative voltage using a dual coil (lower figure). I have tested it and it works fine if you need some 10% current from the negative supply and tolerate +-10% voltage error.

Efficiency is better than using two converters, especially at quiescent current (idle).

Example of coil:

http://search.digikey.com/script...

The dual coil should have just the same inductance at each coil as the single.

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Interesting circuit,Heihopp. Thanks, I will take a look at it.