This is just a note from someone to say that microbees are sensitive to the their input voltage.
This is because the input voltage is monitored on the Power on Jump and reset circuit using a voltage divider comprising of two resistors.
During the evolution of the microbee, different input voltages were used. The original kit microbees operated from 12VAC and subsequent models worked from a range of voltages from 9V through to 12V.
The 12VAC was originally used to provide the RS232 TXD pin with a negative voltage swing. It was subsequently removed and thus the most negative voltage the TXD was around 0V. (i.e. a voltage swing from +12V to - 12V)
It's removal also meant the omission a full wave diode bridge on the base board. Instead a single forward biased diode was used as voltage polarity protection (another place to waste energy as dissipated heat).
Because linear voltage regulators were used, providing the microbee with 12V just made them dissipate excessive heat. This was a reason why the plug pack specifications were altered so that less heat was dissipated. The nominal 10V voltage selected as a compromise between the original 12V, the voltage regulators' 7V drop out voltage (in combination with the polarity protection diode) and the TXD positive output voltage. (Technically the RS232 Mark and Space voltages are -15V to -3V and +3V-15V respectively - This was why the Alpha Plus model was designed to use a 751701 and ICL7660 but most were installed with the original PNP transistor design to save costs - It looks rather nastily implemented with the transistor strewn across IC23's pads. In later years the 751701 ICL7660 combination became the ubiquitous MAX232 IC.)
If you plan to change power supplies for a microbee, be mindful of the power supply's output voltage. It may not boot on a lowish 8-9V.
The solution is to either change to a power supply with higher voltage or to reconfigure the voltage detection resistor divider network.
The nice thing about using a lower voltage is that the voltage regulators don't get so hot.
Be aware of mainboards that have the protection diode replaced with a wire link. At some stage it was deemed unnecessary and the risk accepted.
(These days we can use a more effective P-FET voltage reversal protection circuit.)
The big input filter capacitor on the mainboard was reduced in capacity to cater for the introduction of switch mode power supplies. Early switch mode power supplies saw the large input capacitor as a short circuit and could cause excessive strain on them. When you see a microbee with a small capacitor where the big capacitor should go, you know it is configured for a switching power supply. Just be careful when connecting linear power supplies to these microbees because this smaller input capacitor may not be sufficient to filter the input ripple.
There was a short period where there was a stupid high wattage series resistor was connected to the power input has a poor attempt to limit the initial inrush current. This may have addressed the initial inrush current but at the cost of making heat and wasting energy. It's very easily identifiable.
If you're changing to LDO linear regulators or switching regulator replacements or just wondering why a microbee won't power up, someone hopes that this information helps.
This is because the input voltage is monitored on the Power on Jump and reset circuit using a voltage divider comprising of two resistors.
During the evolution of the microbee, different input voltages were used. The original kit microbees operated from 12VAC and subsequent models worked from a range of voltages from 9V through to 12V.
The 12VAC was originally used to provide the RS232 TXD pin with a negative voltage swing. It was subsequently removed and thus the most negative voltage the TXD was around 0V. (i.e. a voltage swing from +12V to - 12V)
It's removal also meant the omission a full wave diode bridge on the base board. Instead a single forward biased diode was used as voltage polarity protection (another place to waste energy as dissipated heat).
Because linear voltage regulators were used, providing the microbee with 12V just made them dissipate excessive heat. This was a reason why the plug pack specifications were altered so that less heat was dissipated. The nominal 10V voltage selected as a compromise between the original 12V, the voltage regulators' 7V drop out voltage (in combination with the polarity protection diode) and the TXD positive output voltage. (Technically the RS232 Mark and Space voltages are -15V to -3V and +3V-15V respectively - This was why the Alpha Plus model was designed to use a 751701 and ICL7660 but most were installed with the original PNP transistor design to save costs - It looks rather nastily implemented with the transistor strewn across IC23's pads. In later years the 751701 ICL7660 combination became the ubiquitous MAX232 IC.)
If you plan to change power supplies for a microbee, be mindful of the power supply's output voltage. It may not boot on a lowish 8-9V.
The solution is to either change to a power supply with higher voltage or to reconfigure the voltage detection resistor divider network.
The nice thing about using a lower voltage is that the voltage regulators don't get so hot.
Be aware of mainboards that have the protection diode replaced with a wire link. At some stage it was deemed unnecessary and the risk accepted.
(These days we can use a more effective P-FET voltage reversal protection circuit.)
The big input filter capacitor on the mainboard was reduced in capacity to cater for the introduction of switch mode power supplies. Early switch mode power supplies saw the large input capacitor as a short circuit and could cause excessive strain on them. When you see a microbee with a small capacitor where the big capacitor should go, you know it is configured for a switching power supply. Just be careful when connecting linear power supplies to these microbees because this smaller input capacitor may not be sufficient to filter the input ripple.
There was a short period where there was a stupid high wattage series resistor was connected to the power input has a poor attempt to limit the initial inrush current. This may have addressed the initial inrush current but at the cost of making heat and wasting energy. It's very easily identifiable.
If you're changing to LDO linear regulators or switching regulator replacements or just wondering why a microbee won't power up, someone hopes that this information helps.
