Samson
1 W
This unit may have been mentioned in another thread.
http://www.ebay.com/itm/Dc-10-90v-0-100a-dual-Digital-led-voltmeter-ammeter-voltage-amp-power-meter-12v-/181004140379?ssPageName=ADME:L:OC:US:3160
I wanted to use the Alarm functions of this device to protect my batteries from a "run away" charger. This unit can be set to shut off the charge when Volts, Amp, AmpHrs, Low Volts, or Time exceeds a preset alarm threshold. I designed for 16 Amps max at 60 Vdc though the unit can handle more current.
This Meter is designed to drive an external relay when an alarm set point is reached. I decided to use MosFets instead of a relay. The problem was that the Meter outputs 3.2 Volts and most Power Mosfets need much more than this to drive the gate hard enough to reduce Rds to a decent value, The PHP79NQ08LTs are described as Logic level Mosfets and produce an Rds of about .018 ohms at a gate voltage of 3.2 volts. I Paralleled five to produce an Rds of about .0046 Ohms which was verified in practice. At 16 Amps the total dissipation in the Mosfets is 1.2 watts. A plastic box was handy and has the advantage of being insulated. Plastic is not good at dissipating internal heat but a quick calculation suggested that the unit would only produce about 5 watts in total when running at 16 Amps and this was proven in practice. The side of the box that the heat sink is bonded to shows about 10 degrees C above ambient after 30 minutes at 16 Amps.
Suppression across the Drain Source pins of the Mosfet are needed to suppress high voltage spikes which would occur when driving an inductive load. I used a 10 uF electrolytic capacitor with a 4.7 ohm series resistor and a .15 uf 100 Vdc ceramic cap. An Oscilloscope was used to verify that this was effective at suppressing spikes from the fan motor in a 1 kW space heater. The intent though is not to use the unit with an inductive load. Most of the testing was done with a large array of incandescent lamps as a load.
One deficiency with this meter is that the displayed values reset when powered down, though the alarm presets do not. I added a large "tank" capacitor of 6800 uf to keep the unit powered up for about 20 seconds in the event power is removed from the meter. This helps if there is a dip in the AC line or a momentary break in the connections but if it is necessary to keep it powered indefinitely then a remote 9 V supply could be connected to the meter.
http://www.ebay.com/itm/Dc-10-90v-0-100a-dual-Digital-led-voltmeter-ammeter-voltage-amp-power-meter-12v-/181004140379?ssPageName=ADME:L:OC:US:3160
I wanted to use the Alarm functions of this device to protect my batteries from a "run away" charger. This unit can be set to shut off the charge when Volts, Amp, AmpHrs, Low Volts, or Time exceeds a preset alarm threshold. I designed for 16 Amps max at 60 Vdc though the unit can handle more current.
This Meter is designed to drive an external relay when an alarm set point is reached. I decided to use MosFets instead of a relay. The problem was that the Meter outputs 3.2 Volts and most Power Mosfets need much more than this to drive the gate hard enough to reduce Rds to a decent value, The PHP79NQ08LTs are described as Logic level Mosfets and produce an Rds of about .018 ohms at a gate voltage of 3.2 volts. I Paralleled five to produce an Rds of about .0046 Ohms which was verified in practice. At 16 Amps the total dissipation in the Mosfets is 1.2 watts. A plastic box was handy and has the advantage of being insulated. Plastic is not good at dissipating internal heat but a quick calculation suggested that the unit would only produce about 5 watts in total when running at 16 Amps and this was proven in practice. The side of the box that the heat sink is bonded to shows about 10 degrees C above ambient after 30 minutes at 16 Amps.
Suppression across the Drain Source pins of the Mosfet are needed to suppress high voltage spikes which would occur when driving an inductive load. I used a 10 uF electrolytic capacitor with a 4.7 ohm series resistor and a .15 uf 100 Vdc ceramic cap. An Oscilloscope was used to verify that this was effective at suppressing spikes from the fan motor in a 1 kW space heater. The intent though is not to use the unit with an inductive load. Most of the testing was done with a large array of incandescent lamps as a load.
One deficiency with this meter is that the displayed values reset when powered down, though the alarm presets do not. I added a large "tank" capacitor of 6800 uf to keep the unit powered up for about 20 seconds in the event power is removed from the meter. This helps if there is a dip in the AC line or a momentary break in the connections but if it is necessary to keep it powered indefinitely then a remote 9 V supply could be connected to the meter.
